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doi.org/10.26434/chemrxiv.9821846.v1

Efficient Z-Selective Semi-Hydrogenation of Internal Alkynes Catalyzedby Cationic Iron(II) Hydride ComplexesNikolaus Gorgas, Julian Brünig, Berthold Stöger, Stefan Vanicek, Mats Tilset, Luis F. Veiros, Karl Kirchner

Submitted date: 13/09/2019 • Posted date: 16/09/2019Licence: CC BY-NC-ND 4.0Citation information: Gorgas, Nikolaus; Brünig, Julian; Stöger, Berthold; Vanicek, Stefan; Tilset, Mats; Veiros,Luis F.; et al. (2019): Efficient Z-Selective Semi-Hydrogenation of Internal Alkynes Catalyzed by CationicIron(II) Hydride Complexes. ChemRxiv. Preprint.

We describe here the application of the well-defined bench-stable cationic aminoborane complex as highlyefficient pre-catalysts for the semi-hydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes withmolecular hydrogen under mild conditions.

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http://doi.org/10.26434/chemrxiv.9821846.v1

https://chemrxiv.org/authors/Nikolaus_Gorgas/6643973

https://chemrxiv.org/authors/Karl_Kirchner/5948816

https://chemrxiv.org/ndownloader/files/17617493

https://chemrxiv.org/articles/Efficient_Z-Selective_Semi-Hydrogenation_of_Internal_Alkynes_Catalyzed_by_Cationic_Iron_II_Hydride_Complexes/9821846/1?file=17617493





S1

Supporting Information

Efficient Z-Selective Semi-Hydrogenation of Internal Alkynes Catalyzed by

Cationic Iron(II) Hydride Complexes

Nikolaus Gorgas,† Julian Brünig,† Berthold Stöger,‡ Stefan Vanicek,§ Mats Tilset,§ Luis F. Veiros,# and Karl Kirchner*,†

†Institute of Applied Synthetic Chemistry and ‡Institute of Chemical Technologies and Analytics, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, AUSTRIA

§Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, NORWAY

#Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa, PORTUGAL

General Experimental Information ..................................................................................................................... S2

X-ray Structure Determination of Complex 1 .................................................................................................... S2

Computational Details .......................................................................................................................................... S2

Generation of Complex 3...................................................................................................................................... S3

T1 Measurements for Complex 3 ......................................................................................................................... S3

General Procedure for the Catalytic Semi-hydrogenation of Alkynes ............................................................ S3

Spectroscopic Data of Isolated Alkenes .............................................................................................................. S4

Kinetic Experiments ............................................................................................................................................ S7

References ............................................................................................................................................................. S8

NMR Spectra of the Isolated Compounds ......................................................................................................... S9

S2

General. All manipulations were performed under an inert atmosphere of argon by using Schlenk techniques or in a MBraun inert-gas glovebox. Hydrogen (99.999% purity) was purchased from Messer Austria and used as received. Solvents were purified according to standard procedures. Deuterated solvents were purchased from euriso-top and dried over 4 Å molecular sieves. All alkyne substrates were obtained from commercial sources and used as received. Complexes 1 and 2 were prepared according to literature procedures.1,2 HRMS spectra were recorded on an Agilent 7200B GC/Q-TOF (EI) or a Bruker maXis UHR-TOF (ESI) spectrometer. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on Bruker AVANCE-250, AVANCE-400, DRX-500 and ASCEND-600 spectrometers. 1H and 13C{1H} NMR spectra were referenced internally to residual protio-solvent, and solvent resonances, respectively, and are reported relative to tetramethylsilane (δ = 0 ppm). 31P{1H} NMR spectra were referenced externally to H3PO4 (85%) (δ = 0 ppm).

Crystallization and X-ray structure determination of [Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1). Crystalls suitable for X-ray diffraction could be obtained by slow evaporation of a concentrated solution of 1 (0.04 mmol / 0.5 mL) diluted with 3.0 mL of n-pentane.

X-ray diffraction data of 1 (CCDC 1951434) were collected at T = 100 K in a dry stream of nitrogen on a Bruker Kappa APEX II diffractometer system using graphite-monochromatized Mo-Kα radiation (λ = 0.71073 Å) and fine sliced φ- and ω-scans. Data were reduced to intensity values with SAINT and an absorption correction was applied with the multi-scan approach implemented in SADABS.3 The structures were solved by the dual space method implemented in SHELXT4 and refined against F2 with SHELXL.5 Non-hydrogen atoms were refined with anisotropic displacement parameters. The H atoms connected to C atoms were placed in calculated positions and thereafter refined as riding on the parent atoms. Hydride Hs were located in difference Fourier maps and refined freely. Molecular graphics were generated with the program MERCURY.6

Computational Details. The computational results presented have been achieved in part using the Vienna Scientific Cluster (VSC). All calculations were performed using the GAUSSIAN 09 software package,7 without symmetry constraints. The optimized geometries were obtained with the PBE0 functional. That functional uses a hybrid generalized gradient approximation (GGA), including 25 % mixture of Hartree-Fock8 exchange with DFT9 exchange-correlation, given by Perdew, Burke and Ernzerhof functional (PBE).10 The basis set used for the geometry optimizations (basis b1) consisted of the Stuttgart/Dresden ECP (SDD) basis set11 to describe the electrons of iron, and a standard 6-31G(d,p) basis set12 for all other atoms. Transition state optimizations were performed with the Synchronous Transit-Guided Quasi-Newton Method (STQN) developed by Schlegel et al,13 following extensive searches of the Potential Energy Surface. Frequency calculations were performed to confirm the nature of the stationary points, yielding one imaginary frequency for the transition states and none for the minima. Each transition state was further confirmed by following its vibrational mode downhill on both sides and obtaining the minima presented on the energy profiles. The electronic energies (Eb1) obtained at the PBE0/b1 level of theory were converted to free energy at 298.15 K and 1 atm (Gb1) by using zero-point energy and thermal energy corrections based on structural and vibration frequency data calculated at the same level.

Single point energy calculations were performed using the M06 functional and a standard 6-311++G(d,p) basis set,14 on the geometries optimized at the PBE0/b1 level. The M06 functional is a hybrid meta-GGA functional developed by Truhlar and Zhao,15 and it was shown to perform very well for the kinetics of transition metal molecules, providing a good description of weak and long range interactions.16 Solvent effects (benzene) were considered in the single point energy calculations using the Polarizable Continuum Model (PCM) initially devised by Tomasi and co-workers17 with radii and non-electrostatic terms of the SMD solvation model, developed by Truhler et al.18 The free energy values presented (Gb2

soln) were derived from the electronic energy values obtained at the M06/6-311++G(d,p)//PBE0/b1 level (Eb2

soln), according to the following expression: Gb2soln = Eb2

soln + Gb1 – Eb1.

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

A

R

HH+ N

PR2

PR2N

N

Fe

HMe

Me

+

R

HH

HH + H2

1-G (R = Me)

∆G = 3 kcal/mol (R = Me)

2-F (R = H)

∆G = −3 kcal/mol (R = H)

Figure S1. Free energy balance for the exchange between H2 and olefins in [Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3, A).

S3

Generation of [Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3). Inside a glovebox, a vial was charged with a freshly prepared solution of 1 (0.04 mmol) in THF (1.0 mL) and sealed with a septum screw cap. Nonafluoro-tert-butyl alcohol (94 mg, 0.40 mmol) was dissolved in THF (0.5 mL) and the solution was added in one portion via a syringe to the prepared sealed vial containing 1. Gas evolution took place and the colour of the solution changed from orange to yellow. In situ prepared 3 was kept under the atmosphere of H2 generated by alcoholysis in order to prevent its decomposition. For NMR experiments, the same procedure was applied by using a septum screw cap NMR-tube charged with a solution of 1 (0.04 mmol) in THF-d8 (0.5 mL) to which nonafluoro-tert-butyl alcohol (94 mg, 0.40 mmol) dissolved in C6D6 (0.5 mL) was added. 1H NMR (250 MHz, THF-d8/C6D6): δ 7.08 (t, J = 8.1 Hz, 1H, py4), 5.61 (d, J = 8.1 Hz, 2H, py3,5), 2.58 (s, 6H, NCH3), 2.39–2.21 (m, 4H, CH(CH3)2), 0.99–0.72 (m, 24H, CH(CH3)2), -14.68 (br t, J = 12.0 Hz, 5H, Fe-H) ppm. 1H{31P} NMR (250 MHz, THF-d8/C6D6): δ 7.08 (t, J = 8.1 Hz, 1H, py4), 5.61 (d, J = 8.1 Hz, 2H, py3,5), 2.58 (s, 6H, NCH3), 2.39–2.21 (m, 4H, CH(CH3)2), 0.91 (d, J = 6.8 Hz, 12H, CH(CH3)2), 0.81 (d, J = 6.6 Hz, 12H), -14.68 (br s, 5H, Fe-H) ppm. 31P{1H} NMR (101 MHz, THF-d8/C6D6) δ 187.3 ppm. The low stability of 3 prevented the recording of 13C{1H} NMR spectra.

Figure S2. Temperature-dependent T1 measurements of the Fe-H resonance in complex 3.

General procedure for the catalytic semi-hydrogenation of alkynes: All hydrogenation reaction were carried out in a 50 mL Fischer-Porter bottle (up to 5 bar) or a 50 mL Carl Roth stainless steel autoclave (for higher pressures). The pressure reaction vessels were evacuated and flushed several times with hydrogen gas prior to the addition of the reaction solution. For a typical experiment, the reaction solution was prepared previously inside a glovebox by mixing 2 (0.01 mmol), the respective alkyne substrate (1.0 mmol) and CD2Cl2 (1 mL). In cases where 3 was used as the catalyst, a freshly prepared stock solution of the in situ generated complex (40 mM in THF/C6D6 (1:1)) was added via a micro-syringe (50 µL, 0.002 mmol). The resulting solution was taken up in a syringe, removed from the glovebox and injected into the autoclave against a slight flow of hydrogen gas. The pressure was adjusted to the specified value and the reaction was run for the stated time. After 1 h reaction time, the reaction was quenched by exposure to air, the catalyst removed by filtration over short plug of silica (eluent: dichloromethane) and the solvent evaporated under reduced pressure.

Spectroscopic Data of Isolated Alkenes

A1:

Isolated as colourless liquid in 86% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.26 – 7.17 (m, 4H), 7.13 – 7.08 (m, 1H), 6.33 (dq, J = 11.6, 1.9 Hz, 1H), 5.70 (dq, J = 11.6, 7.2 Hz, 1H), 1.79 (dd, J = 7.2, 1.9 Hz, 3H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 137.7, 129.7, 128.8, 128.1, 126.8, 126.4, 14.4. HRMS (EI, MeOH/CH3CN): m/z 117.0698 [M-H]+ (calc. 117.0699).

10

12

14

16

18

20

22

24

26

28

30

-110 -90 -70 -50 -30 -10 10 30

T 1 (m

s)

temperature (°C)

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

T1 (min): 12 ms

(-65°C, 500 MHz)

S4

A2:

Isolated as a colourless oil in 83% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.33 – 7.19 (m, 10H), 6.66 (s, 2H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 137.8, 130.6, 129.3, 128.6, 127.5.

A3:

Isolated as a colourless liquid in 82% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 5.32 – 5.24 (m, 2H), 1.99 – 1.87 (m, 4H), 1.28 (tq, J = 7.4 Hz, 4H), 0.82 (t, J = 7.4 Hz, 6H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 129.8, 29.3, 22.9, 13.5. HRMS (EI, MeOH/CH3CN): m/z 112.1245 [M]+ (calc. 112.1252).

A4: TMS

Isolated as a colourless oil in 89% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.32 (d, J = 15.1 Hz, 1H), 7.29 – 7.17 (m, 5H), 5.79 (d, J = 15.1 Hz, 1H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 147.6, 141.2, 133.8, 129.2, 129.0, 128.4, 0.9. HRMS (EI, MeOH/CH3CN): m/z 104.0617 [M-CH3]+ (calc. 104.0621).

A5: TMS

Isolated as orange oil in 83% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 6.20 (dt, J = 14.0, 7.4 Hz, 1H), 5.35 (dt, J = 14.0, 1.3 Hz, 1H), 2.01 (qd, J = 7.4, 1.3 Hz, 2H), 1.24 – 1.15 (m, 8H), 0.80 – 0.76 (m, 3H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 150.4, 129.6, 34.6, 32.8, 30.8, 30.1, 23.7, 14.9, 0.9.

A6: TMSF

Isolated as orange oil in 91% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.27 (d, J = 15.1 Hz, 1H), 7.24 – 7.18 (m, 2H), 7.00 – 6.93 (m, 2H), 5.79 (d, J = 15.1 Hz, 1H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 163.3 (d, JCF = 245.7 Hz), 146.3, 137.5 (d, JCF = 3.2 Hz), 134.1 (d, JCF = 1.0 Hz), 130.9 (d, JCF = 8.0 Hz), 115.8 (d, JCF = 21.5 Hz), 0.9. 19F NMR (376 MHz, CD2Cl2, 20°C): δ -115.8. HRMS (EI, MeOH/CH3CN): m/z 194.0920 [M]+ (calc. 194.0927), m/z 179.0684 [M-CH3]+ (calc. 179.0687).

A7: TMS

F3C

Isolated as colourless oil in 87% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.53 – 7.51 (m, 1H), 7.50 – 7.46 (m, 1H), 7.42 (d, J = 1.2 Hz, 1H), 7.44 – 7.40 (m, 1H), 7.33 (d, J = 15.2 Hz, 1H), 5.93 (d, J = 15.2 Hz, 1H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 145.9, 142.2, 136.4, 132.8 (q, JCF = 1.2 Hz), 131.3 (q, JCF = 33.2 Hz), 129.8, 126.0 (q, JCF = 3.8 Hz), 125.6 (q, JCF = 273.4 Hz), 125.1 (q, JCF = 3.8 Hz), 0.9. 19F NMR (376 MHz, CD2Cl2, 20°C): δ -63.0. HRMS (EI, MeOH/CH3CN): m/z 244.0889 [M]+ (calc. 244.0895), m/z 229.0664 [M-CH3]+ (calc. 229.0655).

S5

A8: TMS

tBu

Isolated as colourless oil in 85% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.28 – 7.24 (m, 2H), 7.24 (d, J = 15.1 Hz, 1H), 7.17 – 7.14 (m, 2H), 5.71 (d, J = 15.1 Hz, 1H), 1.24 (s, 9H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 151.6, 147.4, 138.1, 132.7, 128.9, 125.9, 35.5, 32.1, 0.9. HRMS (EI, MeOH/CH3CN): m/z 232.1641 [M]+ (calc. 232.1647).

A9: O TMS

Isolated as yellow oil in 94% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.29 (s, 1H), 7.25 (s, 1H), 6.91 (d, J = 15.3 Hz, 1H), 6.35 (s, 1H), 5.61 (d, J = 15.3 Hz, 1H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 144.3, 143.0, 136.8, 132.6, 126.8, 111.7, 0.9.

A10: S TMS

Isolated as colourless oil in 92% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.18 – 7.15 (m, 1H), 7.14 (d, J = 15.1 Hz, 1H), 7.07 – 7.05 (m, 1H), 7.00 – 6.97 (m, 1H), 5.69 (d, J = 15.1 Hz, 1H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 143.0, 141.3, 133.2, 129.1, 126.4, 124.6, 0.9. HRMS (EI, MeOH/CH3CN): m/z 182.0580 [M]+ (calc. 182.0585).

A11:

O

TMS

Isolated as yellow oil in 90% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.11 (dd, J = 8.6, 7.4 Hz, 2H), 6.77 (t, J = 7.4 Hz, 1H), 6.73 (d, J = 8.6 Hz, 2H), 6.37 (dt, J = 14.6, 6.0 Hz, 1H), 5.69 (dt, J = 14.6, 1.5 Hz, 1H), 4.41 (dd, J = 6.0, 1.5 Hz, 2H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 159.9, 144.0, 134.5, 130.7, 122.0, 115.9, 69.2, 0.9. HRMS (EI, MeOH/CH3CN): m/z 205.1043 [M-H]+ (calc. 205.1043), m/z 191.0888 [M-CH3]+ (calc. 191.0887).

A12: TMSHO

Isolated as colourless oil in 87% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.29 (d, J = 15.2 Hz, 1H), 7.26 – 7.19 (m, 4H), 5.78 (d, J = 15.2 Hz, 1H), 4.58 (s, 2H), 1.84 (s, 1H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 147.2, 141.5, 140.4, 133.8, 129.3, 127.5, 65.8, 0.9.

A13:

Isolated as colourless liquid in 84% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 5.40 – 5.24 (m, 4H), 2.01 – 1.91 (m, 4H), 1.54 – 1.48 (m, 6H), 1.31 – 1.25 (m, 4H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 130.7, 123.6, 29.2, 26.7, 12.4. HRMS (EI, MeOH/CH3CN): m/z 109.1010 [M-C2H5]+ (calc. 109.1012), ), 96.0932 [M-C3H6]+ (calc. 96.0939), 81.0697 [M-C4H9]+ (calc. 81.0699), 67.0541 [M-C4H9]+ (calc. 67,0542).

S6

A14: TMS

TMS

Isolated as orange oil in 91% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.28 (d, J = 15.1 Hz, 2H), 7.17 (s, 4H), 5.77 (d, J = 15.1 Hz, 2H), 0.00 (s, 18H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 147.1, 140.3, 133.8, 128.7, 0.9.

A15:

TMS

Isolated as pale yellow oil in 90% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 6.57 (d, J = 14.7 Hz, 1H), 5.57 – 5.53 (m, 1H), 5.31 (d, J = 14.8 Hz, 1H), 2.03 – 1.89 (m, 3H), 1.58 – 1.42 (m, 5H), 0.00 (s, 9H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 150.3, 139.1, 128.8, 126.9, 28.3, 25.9, 23.1, 22.6, 0.9. HRMS (EI, MeOH/CH3CN): m/z 180.1324 [M]+ (calc. 180,1334), m/z 165.1090 [M-CH3]+ (calc. 165.1094).

A16:

HH

Isolated as yellow oil in 93% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.32 – 7.21 (m, 8H), 7.17 – 7.12 (m, 2H), 6.64 – 6.55 (m, 2H), 6.51 – 6.42 (m, 2H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 137.3, 132.0, 129.1, 128.2, 127.3, 126.4. HRMS (EI, MeOH/CH3CN): m/z 206.1087 [M]+ (calc. 206.1096).

A16-d2:

DD

Isolated as yellow oil in 94% yield. 1H NMR (600 MHz, CD2Cl2, 20°C): 7.33 – 7.29 (m, 4H), 7.28 – 7.22 (m, 4H), 7.18 – 7.13 (m, 2H), 6.62 (d, J = 11.6 Hz, 1H), 6.48 (d, J = 11.6 Hz, 1H). 13C{1H} NMR (151 MHz, CD2Cl2, 20°C): δ 137.4, 137.3, 132.1, 131.6 (t, JCD = 23.5 Hz), 129.2, 128.3, 127.3, 126.4, 126.0 (t, JCD = 23.5 Hz). HRMS (EI, MeOH/CH3CN): m/z 208.1211 [M]+ (calc. 208.1221).

A16-d4:

DD

DD

Isolated as white solid in 98% yield. 1H NMR (600 MHz, CD2Cl2, 20°C): δ 7.33 – 7.29 (m, 4H), 7.28 – 7.22 (m, 4H), 7.18 – 7.13 (m, 2H). 13C{1H} NMR (151 MHz, CD2Cl2, 20°C): δ 137.3, 131.6 (t, JCD = 23.5 Hz), 129.2, 128.3, 127.4, 126.0 (t, JCD = 23.5 Hz). HRMS (EI, MeOH/CH3CN): m/z 210.1339 [M]+ (calc. 210.1347).

A17:

HH

Isolated as yellow oil in 89% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.23 (d, J = 8.1 Hz, 4H), 7.10 (d, J = 8.1 Hz, 4H), 7.14 – 7.07 (m, 4H), 6.63 – 6.54 (m, 2H), 6.49 – 6.40 (m, 2H), 2.27 (s, 6H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 137.21, 134.52, 131.53, 129.02, 128.89, 125.90, 20.94. HRMS (EI, MeOH/CH3CN): m/z 234.1400 [M]+ (calc. 234.1409).

S7

A18:

HH

Isolated as pale orange solid in 89% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.35 – 7.22 (m, 4H), 6.66 – 6.56 (m, 2H), 6.49 – 6.40 (m, 2H), 1.24 (s, 18H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 150.4, 134.6, 131.5, 128.9, 126.1, 125.2, 31.1. HRMS (EI, MeOH/CH3CN): m/z 318.2337 [M]+ (calc. 318.2348).

A19:

HH

NH2

H2N

Isolated as off-white solid in 84% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.04 (t, J = 7.8 Hz, 2H), 6.69 (dt, J = 7.8, 1.2 Hz, 2H), 6.62 (t, J = 2.2 Hz, 2H), 6.64 – 6.56 (m, 2H), 6.49 (ddd, J = 8.0, 2.2, 1.2 Hz, 2H), 6.42 – 6.34 (m, 2H), 3.63 (s, 4H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 146.7, 138.3, 131.9, 129.1, 126.5, 119.4, 115.4, 114.0. HRMS (ESI, MeOH/CH3CN): m/z 237.1384 [M+H]+ (calc. 237.1386).

A20:

HH

O

O

O

O

Isolated as white solid in 87% yield. 1H NMR (400 MHz, CD2Cl2, 20°C): δ 7.91 (d, J = 8.4 Hz, 4H), 7.45 (d, J = 8.4 Hz, 4H), 7.08 – 6.98 (m, 2H), 6.76 – 6.66 (m, 2H), 3.81 (s, 6H). 13C{1H} NMR (101 MHz, CD2Cl2, 20°C): δ 166.5, 141.5, 133.1, 131.3, 129.9, 129.3, 126.4, 51.9. HRMS (EI, MeOH/CH3CN): m/z 322.1196 [M]+ (calc. 322.1205).

Kinetic Experiments.

Table S1. Kinetic experiments based on initial turnover frequencies (TOFinit) determined after 5 min reaction time. Conditions: (a) 2 (0.005 mmol), 1-phenyl-1-propene (1.0 mmol), CD2Cl2 (2 mL), H2 (5-20 bar), 5 min. (b) 2 (0.01 mmol), 1-phenyl-1-propene (0.5–2.0 mmol / 2.0 mL CD2Cl2), H2 (5 bar), 5 min.

1a

2a

3a

4b

5b

6b

49%

26%

14%

43%

> 99%

30%

p (H2) c (subst)yield TOFinit TOFinityield

5 bar

10 bar

20 bar

1032 h-1

2400 h-1

720 h-1

312 h-1

336 h-1

294 h-10.25 M

0.50 M

1.00 M

entry entry

S8 References 1 Gorgas, N.; Alves, L. G.; Stöger, B.; Martins, A. M.; Veiros, L. F.; Kirchner, K. Stable, Yet Highly Reactive

Nonclassical Iron(II) Polyhydride Pincer Complexes: Z-Selective Dimerization and Hydroboration of Terminal Alkynes. J. Am. Chem. Soc. 2017, 139, 8130–8133.

2 Gorgas, N.; Stöger, B.; Veiros, L. F.; Kirchner, K. Access to Fe(II) Bis(σ-B-H) Aminoborane Complexes via Protonation of a Borohydride Complex and Dehydrogenation of Amine-Boranes. Angew. Chem., Int. Ed. 2019, in press. DOI: 10.1002/anie.201906971

3 Bruker computer programs: APEX2, SAINT and SADABS (Bruker AXS Inc., Madison, WI, 2018). 4 Sheldrick, G. Acta Crytallogr. A 2015, 71, 3–8. 5 Sheldrick, G. Acta Crytallogr. C 2015, 71, 3–8. 6 Macrae, C. F.; Edgington, P. R.; McCabe, P.; Pidcock, E.; Shields, G. P.; Taylor, R.; Towler, M.; van de

Streek, J.J. Appl. Cryst. 2006, 39, 453–457. 7 Gaussian 09, Revision A.02, Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.;

Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, Jr., J. A.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö.; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian, Inc., Wallingford CT, 2009.

8 Hehre, W. J.; Radom, L.; Schleyer, P. v.R.; Pople, J. A. Ab Initio Molecular Orbital Theory, John Wiley & Sons, NY, 1986.

9 Parr, R. G.; Yang, W. Density Functional Theory of Atoms and Molecules; Oxford University Press: New York, 1989.

10 (a) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1997, 78, 1396. (b) Perdew, J. P. Phys. Rev. B 1986, 33, 8822.

11 (a) Haeusermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Mol. Phys. 1993, 78, 1211-1224. (b) Kuechle, W.; Dolg, M.; Stoll, H.; Preuss, H. J. Chem. Phys. 1994, 100, 7535-7542. (c) Leininger, T.; Nicklass, A.; Stoll, H.; Dolg, M.; Schwerdtfeger, P. J. Chem. Phys. 1996, 105, 1052-1059.

12 (a) McLean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639-5648. (b) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A. J. Chem. Phys. 1980, 72, 650-654. (c) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033-1036. (d) Hay, P. J. J. Chem. Phys. 1977, 66, 4377-4384. (e) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062-1065. (f) Binning Jr., R. C.; Curtiss, L. A. J. Comp. Chem., 1990, 11, 1206. (g) McGrath, M. P.; Radom, L. J. Chem. Phys. 1991, 94, 511-516.

13 (a) Peng, C.; Ayala, P.Y.; Schlegel, H.B.; Frisch, M.J. J. Comp. Chem. 1996, 17, 49-56. (b) Peng, C.; Schlegel, H.B. Israel J. Chem. 1993, 33, 449-454.

14 (a) McClean, A. D.; Chandler, G. S. J. Chem. Phys. 1980, 72, 5639-5648. (b) Krishnan, R.; Binkley, J. S.; Seeger, R. Pople, J. A. J. Chem. Phys. 1980, 72, 650-654. (c) Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033-1036. (d) Hay, P. J. J. Chem. Phys. 1977, 66, 4377-4384. (e) Raghavachari, K.; Trucks, G. W. J. Chem. Phys. 1989, 91, 1062-1065. (f) Binning Jr., R. C.; Curtiss, L. A. J. Comp. Chem. 1990, 11, 1206-1216. (g) McGrath, M. P.; Radom, L. J. Chem. Phys. 1991, 94, 511-516. (h) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comp. Chem. 1983, 4, 294-301. (i) Frisch, M. J.; Pople, J. A.; Binkley, J. S. J. Chem. Phys. 1984, 80, 3265-3269.

15 Zhao, Y.; Truhlar, D. G. Theor. Chem. Acc., 2008, 120, 215-241. 16 (a) Zhao, Y.; Truhlar, D. G. Acc, Chem. Res. 2008, 41, 157-167. (b) Zhao, Y.; Truhlar, D.G. Chem. Phys.

Lett. 2011, 502, 1-13. 17 (a) Cancès, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032-3041. (b) Cossi, M.; Barone, V.;

Mennucci, B.; Tomasi, J. Chem. Phys. Lett. 1998, 286, 253-260. (c) Mennucci B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151-5158. (d) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999-3094.

18 Marenich, A. V.; Cramer, C. J.; Truhlar, D. G. J. Phys. Chem. B, 2009, 113, 6378-6396.

S9

-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1012345678910

4.9

25.6

4.2

6.5

2.1

0.8

-14.

68

0.77

0.80

0.82

0.86

0.88

0.92

0.95

2.30

2.58

5.59

5.62

7.04

7.08

7.11

0.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5

25.6

4.2

6.5

2.1

0.8

0.77

0.80

0.86

0.89

0.92

0.96

2.31

2.59

5.59

5.63

7.05

7.08

7.11

-15.0-14.8-14.6-14.4

-14.

73-1

4.68

-14.

63

C6D6

H2

THF-d8

(CF3)3COH

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

3 (R = iPr)

Figure S6. 1H NMR spectrum of 3 (250 MHz, THF-D8/C6D6).

-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1012345678910

4.8

12.0

12.6

4.0

5.9

1.7

1.1

-14.

68

0.18

0.80

0.82

0.89

0.92

2.26

2.29

2.31

2.34

2.37

2.59

5.59

5.62

7.04

7.08

7.11

0.51.01.52.02.53.03.54.04.55.05.56.06.57.07.5

12.0

12.6

4.0

5.9

1.7

1.1

0.80

0.82

0.89

0.92

2.26

2.29

2.31

2.34

2.37

2.59

5.59

5.62

7.04

7.08

7.11

C6D6

H2

THF-d8

(CF3)3COH

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

3 (R = iPr)

Figure S7. 1H{31P} NMR spectrum of 3 (250 MHz, THF-D8/C6D6).

S10

-50-40-30-20-100102030405060708090100110120130140150160170180190200210220230240250

187.

30

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

3 (R = iPr)

Figure S8. 31P{1H} NMR spectrum of 3 (250 MHz, THF-D8/C6D6).

-17-16-15-14-13-12-11-10-9-8-7-6-5-4-3-2-1012345678

100

110

120

130

140

150

160

170

180

190

200

210

220

230

240

250

260

270

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

3 (R = iPr)

Figure S9. 1H,31P-HMBC NMR spectrum of 3 (250 MHz, THF-D8/C6D6).

S11

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0

3.3

1.0

1.0

1.0

4.0

1.78

1.78

1.80

1.80

5.65

5.67

5.68

5.69

5.70

5.71

5.72

5.74

6.31

6.32

6.32

6.33

6.34

6.35

6.35

6.35

7.09

7.10

7.12

7.19

7.21

7.23

7.25

5.65.75.85.96.06.16.26.36.4

1.0

1.0

5.65

5.67

5.68

5.69

5.70

5.71

5.72

5.74

6.31

6.32

6.32

6.33

6.34

6.35

6.35

6.35

CD2Cl2

Figure S10. 1H NMR spectrum of A1 (400 MHz, CD2Cl2).

-100102030405060708090100110120130140150160170180190200

14.4

3

126.

4212

6.78

128.

1112

8.81

129.

74

137.

67

CD2Cl2

Figure S11. 13C{1H} APT NMR spectrum of A1 (101 MHz, CD2Cl2).

S12

-2.5-2.0-1.5-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.510.010.511.011.512.012.5

2.0

10.1

6.66

7.23

7.24

7.25

7.26

7.27

7.27

7.28

7.28

7.28

7.29

CD2Cl2


0102030405060708090100110120130140150160170180190200

127.

5112

8.61

129.

2613

0.64

137.

76

CD2Cl2


S13

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

6.2

4.1

4.1

2.0

0.80

0.82

0.83

1.23

1.25

1.27

1.29

1.31

1.33

1.90

1.91

1.92

1.92

1.92

1.93

1.93

1.94

1.94

1.95

5.27

5.27

5.28

5.28

5.29

5.30

5.30

5.055.105.155.205.255.305.355.405.455.50

2.0

5.27

5.27

5.28

5.28

5.29

5.30

5.30


-100102030405060708090100110120130140150160170180190200

13.5

22.9

29.2

129.

8

CD2Cl2


S14

TMS

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0

9.3

1.0

4.7

1.0

-0.0

0

5.77

5.81

7.24

7.24

7.25

7.30

7.34

5.55.65.75.85.96.06.16.26.36.46.56.66.76.86.97.07.17.27.37.47.5

1.0

4.7

1.0

5.77

5.81

7.24

7.24

7.25

7.30

7.34

CD2Cl2


-20-100102030405060708090100110120130140150160170180190200

0.90

128.

4012

8.99

129.

1913

3.84

141.

23

147.

56

TMS

CD2Cl2


S15

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

9.0

3.3

8.4

2.0

0.9

0.9

-0.0

0

0.76

0.78

0.79

1.16

1.26

1.98

2.00

2.02

2.04

5.33

5.33

5.34

5.37

5.37

5.37

6.16

6.18

6.20

6.20

6.21

6.23

5.35.45.55.65.75.85.96.06.16.26.30.

9

0.9

5.33

5.33

5.34

5.37

5.37

5.37

6.16

6.18

6.20

6.20

6.21

6.23

TMS

CD2Cl2


-100102030405060708090100110120130140150160170180190200

0.90

14.8

6

23.6

5

30.0

530

.77

32.8

134

.60

129.

55

150.

38

TMS

CD2Cl2


S16

-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5.0

18.0

2.0

3.5

3.6

1.9

-0.0

0

5.79

6.97

7.21

7.25

7.29

5.75.85.96.06.16.26.36.46.56.66.76.86.97.07.17.27.37.4

2.0

3.5

3.6

1.9

5.79

6.97

7.21

7.25

7.29

TMSF

CD2Cl2


-100102030405060708090100110120130140150160170180190200

0.90

115.

7211

5.93

130.

8513

0.93

134.

0613

4.07

137.

4813

7.52

146.

32

162.

1016

4.54

TMSF CD2Cl2


S17

-160-150-140-130-120-110-100-90-80-70-60-50-40-30-20-1001020

-115

.75

TMSF

Figure S22. 19F{1H} APT NMR spectrum of A6 (376 MHz, CD2Cl2).

-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0

9.5

1.0

1.0

1.8

0.9

0.8

0.9

0.00

5.91

5.95

7.31

7.31

7.34

7.34

7.40

7.42

7.47

7.49

7.52

5.85.96.06.16.26.36.46.56.66.76.86.97.07.17.27.37.47.57.6

1.0

1.0

1.8

0.9

0.8

0.9

5.91

5.95

7.31

7.31

7.34

7.34

7.40

7.42

7.47

7.49

7.52

TMS

F3C

CD2Cl2


S18

-100102030405060708090100110120130140150160170180190

0.90

121.

5312

4.23

125.

0612

5.10

125.

1412

5.18

125.

9512

5.99

126.

0312

6.06

126.

9412

9.64

129.

8013

0.85

131.

1713

1.49

131.

8313

2.77

132.

7813

2.79

132.

8013

6.43

142.

16

145.

93TMS

F3C

CD2Cl2


-160-150-140-130-120-110-100-90-80-70-60-50-40-30-20-1001020304050

-63.

03

TMS

F3C

Figure S25. 19F{1H} APT NMR spectrum of A7 (376 MHz, CD2Cl2).

S19

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

9.4

9.4

1.0

1.9

1.0

1.9

0.00

1.24

5.69

5.73

7.14

7.16

7.23

7.25

7.28

5.65.86.06.26.46.66.87.07.27.4

1.0

1.9

1.0

1.9

5.69

5.73

7.14

7.16

7.23

7.25

7.28

TMStBu

CD2Cl2


-100102030405060708090100110120130140150160170180190200

0.90

32.1

135

.48

125.

8612

8.91

132.

69

138.

09

147.

39

151.

60

TMStBu

CD2Cl2


S20

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0

9.3

1.0

0.9

1.0

1.7

0.00

5.59

5.63

6.35

6.89

6.93

7.24

7.29

5.45.65.86.06.26.46.66.87.07.27.4

1.0

0.9

1.0

1.7

5.59

5.63

6.35

6.89

6.93

7.24

7.29

CD2Cl2

O TMS


-100102030405060708090100110120130140150160170180190200

0.90

111.

65

126.

78

132.

62

136.

79

143.

0114

4.29

CD2Cl2

O TMS


S21

-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

8.2

1.0

0.9

0.8

1.8

-0.0

0

5.67

5.71

6.98

6.98

6.98

6.99

6.99

6.99

7.06

7.06

7.06

7.07

7.13

7.16

7.16

7.18

5.65.75.85.96.06.16.26.36.46.56.66.76.86.97.07.17.27.3

1.0

0.9

0.8

1.8

5.67

5.71

6.98

6.98

6.98

6.99

6.99

6.99

7.06

7.06

7.06

7.07

7.13

7.16

7.16

7.18

CD2Cl2

S TMS


-100102030405060708090100110120130140150160170180190200

0.90

124.

5812

6.38

129.

1413

3.21

141.

3414

2.97

CD2Cl2

S TMS


S22

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

9.4

2.2

1.0

1.0

2.0

1.0

2.1

0.00

4.40

4.40

4.41

4.42

6.33

6.35

6.36

6.37

6.39

6.40

6.72

6.74

6.75

6.77

6.79

7.09

7.10

7.11

7.13

4.55.05.56.06.57.0

2.2

1.0

1.0

2.0

1.0

2.1

4.40

4.40

4.41

4.42

6.33

6.35

6.36

6.37

6.39

6.40

6.72

6.74

6.75

6.77

6.79

7.09

7.10

7.11

7.13

CD2Cl2

O

TMS


-100102030405060708090100110120130140150160170180190200

0.90

69.1

6

115.

90

122.

01

130.

68

134.

53

144.

03

159.

92

CD2Cl2

O

TMS


S23

-1.-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

9.0

1.1

2.4

1.1

4.5

1.3

-0.0

0

1.84

4.58

5.76

5.80

7.20

7.22

7.23

7.25

7.27

7.31

2.02.53.03.54.04.55.05.56.06.57.07.5

1.1

2.4

1.1

4.5

1.3

1.84

4.58

5.76

5.80

7.20

7.22

7.23

7.25

7.27

7.31

CD2Cl2

CD2Cl2

TMSHO


-100102030405060708090100110120130140150160170180190200

0.90

65.8

5

127.

5212

9.30

133.

78

140.

3814

1.48

147.

18

CD2Cl2

TMSHO


S24

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

3.1

6.3

4.4

4.0

1.28

1.50

1.50

1.52

1.52

1.95

1.95

1.96

1.97

5.33


-30-20-100102030405060708090100110120130140150160170180190200210220230240250

12.4

4

26.6

729

.17

123.

57

130.

67

CD2Cl2


S25

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0

18.4

2.0

4.0

2.1

0.00

5.75

5.79

7.17

7.26

7.30

5.65.86.06.26.46.66.87.07.27.47.6

2.0

4.0

2.1

5.75

5.79

7.17

7.26

7.30

TMSTMS


0102030405060708090100110120130140150160170180190200

0.90

128.

74

133.

84

140.

25

147.

13

CD2Cl2

TMSTMS


S26

-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.0

9.8

4.3

4.5

1.0

1.0

1.0

-0.0

0

1.44

1.55

1.94

1.99

5.29

5.33

5.55

6.55

6.59

1.52.04.55.05.56.06.5

4.3

4.5

1.0

1.0

1.0

1.44

1.55

1.94

1.99

5.29

5.33

5.55

6.55

6.59

TMS


-100102030405060708090100110120130140150160170180190200

0.90

22.5

923

.07

25.9

028

.31

126.

9312

8.75

139.

13

150.

27

CD2Cl2

TMS


S27

-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

2.0

1.9

1.9

7.8

6.45

6.46

6.47

6.48

6.58

6.59

6.60

6.61

7.13

7.15

7.17

7.23

7.25

7.26

7.29

7.30

6.46.56.66.76.86.97.07.17.27.37.47.5

2.0

1.9

1.9

7.8

6.45

6.46

6.47

6.48

6.58

6.59

6.60

6.61

7.13

7.15

7.17

7.23

7.25

7.26

7.29

7.30

CD2Cl2


0102030405060708090100110120130140150160170180190200

126.

4112

7.26

128.

2112

9.11

131.

9713

7.31

CD2Cl2


S28

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

1.0

1.0

2.0

3.9

3.9

6.46

6.49

6.60

6.63

7.14

7.16

7.17

7.23

7.25

7.27

7.30

7.32

6.26.36.46.56.66.76.86.97.07.17.27.37.47.57.6

1.0

1.0

2.0

3.9

3.9

6.46

6.49

6.60

6.63

7.14

7.16

7.17

7.23

7.25

7.27

7.30

7.32

CD2Cl2

HH

DD

Figure S44. 1H NMR spectrum of 16-d2 (600 MHz, CD2Cl2).

0102030405060708090100110120130140150160

125.

8912

6.05

126.

2012

6.42

127.

3512

8.31

129.

1913

1.42

131.

5813

1.74

132.

0713

7.30

137.

38

121122123124125126127128129130131132133134135136137138139140

125.

8912

6.05

126.

2012

6.42

127.

35

128.

31

129.

19

131.

4213

1.58

131.

7413

2.07

137.

3013

7.38

HH

DD

CD2Cl2

Figure S45. 13C{1H} APT NMR spectrum of 16-d2 (151 MHz, CD2Cl2).

S29

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

2.0

4.1

4.0

7.15

7.16

7.18

7.24

7.26

7.27

7.31

7.32

6.26.36.46.56.66.76.86.97.07.17.27.37.47.57.6

2.0

4.1

4.0

7.15

7.16

7.18

7.24

7.26

7.27

7.31

7.32

CD2Cl2

DD

DD

Figure S46. 1H NMR spectrum of 16-d4 (600 MHz, CD2Cl2).

0102030405060708090100110120130140150160

125.

8312

5.99

126.

1412

7.35

128.

3212

9.19

131.

4313

1.58

131.

7413

7.32

119120121122123124125126127128129130131132133134135136137138139140141142143144

125.

8312

5.99

126.

14

127.

35

128.

32

129.

19

131.

4313

1.58

131.

74

137.

32

DD

DD

CD2Cl2

Figure S47. 13C{1H} APT NMR spectrum of 16-d4 (151 MHz, CD2Cl2).

S30

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.03.

2

1.0

1.0

1.9

2.0

2.27

6.43

6.46

6.57

6.60

7.09

7.11

7.22

7.24

6.36.46.56.66.76.86.97.07.17.27.37.4

1.0

1.0

1.9

2.0

6.43

6.46

6.57

6.60

7.09

7.11

7.22

7.24

CD2Cl2


-100102030405060708090100110120130140150160170180190200

20.9

125.

912

8.9

129.

013

1.5

CD2Cl2


S31

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

9.4

0.9

1.0

4.1

1.24

6.43

6.44

6.45

6.46

6.60

6.60

6.62

6.63

7.25

7.27

7.29

7.31

6.26.36.46.56.66.76.86.97.07.17.27.37.47.5

0.9

1.0

4.1

6.43

6.44

6.45

6.46

6.60

6.60

6.62

6.63

7.25

7.27

7.29

7.31

tBu

tBu

CD2Cl2


-100102030405060708090100110120130140150160170180190200

31.1

34.5

125.

212

6.1

128.

913

1.5

134.

6

150.

4

tBu

tBu

CD2Cl2


S32

-1.0-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.5

4.2

2.0

2.0

2.0

2.0

2.0

2.0

3.63

6.38

6.49

6.60

6.62

6.69

7.02

7.04

7.06

6.36.46.56.66.76.86.97.07.1

2.0

2.0

2.0

2.0

2.0

2.0

6.38

6.49

6.60

6.62

6.69

7.02

7.04

7.06

NH2

NH2

CD2Cl2


0102030405060708090100110120130140150160170

113.

9811

5.40

119.

42

126.

5112

9.08

131.

94

138.

32

146.

72

NH2

NH2

CD2Cl2


S33

O

MeO

O

OMe

CD2Cl2

-0.50.00.51.01.52.02.53.03.54.04.55.05.56.06.57.07.58.08.59.09.5

6.2

2.0

2.1

3.9

4.0

3.81

6.67

6.69

6.70

6.72

6.73

6.75

6.99

7.01

7.02

7.04

7.05

7.07

7.44

7.46

7.90

7.92

6.56.66.76.86.97.07.17.27.37.47.57.67.77.87.98.08.1

2.0

2.1

3.9

4.0

6.67

6.69

6.70

6.72

6.73

6.75

6.99

7.01

7.02

7.04

7.05

7.07

7.44

7.46

7.90

7.92


0102030405060708090100110120130140150160170180190200

51.9

3

126.

3512

9.26

129.

8613

1.25

133.

06

141.

51

166.

54

CD2Cl2

O

MeO

O

OMe


download fileview on ChemRxivFe PNP Alkyne hydrogenation ESI.pdf (2.81 MiB)



1

Efficient Z-Selective Semi-Hydrogenation of Internal Alkynes Catalyzed by Cationic Iron(II) Hydride Complexes Nikolaus Gorgas,† Julian Brünig,† Berthold Stöger,‡ Stefan Vanicek,§ Mats Tilset,§ Luis F. Veiros,# and Karl Kirchner*,† †Institute of Applied Synthetic Chemistry and ‡X-Ray Center, Vienna University of Technology, Getreidemarkt 9, A-1060 Vienna, AUSTRIA; §Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, N-0315 Oslo, NORWAY; #Centro de Química Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais No. 1, 1049-001 Lisboa, PORTUGAL

ABSTRACT: The bench-stable cationic bis(σ-B-H) aminoborane complex [Fe(PNPNMe-iPr)(H)(η2-H2B=NMe2)]+ (2) efficiently catalyzes the semi-hydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. Moreover, selective incorporation of deuterium was achieved in the case of 1,3-diynes and 1,3-enynes. The catalytic reaction takes place under mild conditions (25oC, 4-5 bar H2 or D2) in 1 h and alkenes were obtained with high Z-selectivity for a broad scope of substrates. Mechanistic insight into the catalytic reaction, explaining also the stereo- and chemoselectivity, is provided by means of DFT calculations. Intermediates featuring a bis-dihydrogen moiety [Fe(PNPNMe-iPr)(η2-H2)2]+ are found to play a key role. Experimental support for such species was unequivocally provided by the fact that [Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3) exhibited the same catalytic activity than 2. The novel cationic bis-dihydrogen complex 3 was obtained by protonolysis of [Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1) with an excess of nonafluoro-tert-butyl alcohol.

INTRODUCTION

The semi-hydrogenation of alkynes to alkenes is an important transformation for the industrial manufacture of bulk and fine chemicals.1 Since the reduction of C≡C triple bonds to alkenes may potentially lead to the formation of (E)- or (Z)-alkenes as well as saturated hydrocarbons, it remains a challenging task to control the chemo- and stereoselectivity of this reaction.2 For example, (Z)-olefins are traditionally obtained by heterogenous hydrogenation with the Lindlar catalyst being the most prominent example.3 However, these systems require a careful control of the reaction conditions in order to avoid isomerization or overreduction of the products.

Although significant progress could be achieved in the field of heterogeneous catalysis and homogeneous transfer hydrogenations, the number of molecular defined catalysts that operate under an atmosphere of H2 is surprisingly low.4-6 Concerning the stereoselectivity of the reaction, recent examples were shown to deliver (E)-alkenes which, except in some cases7, are formed due to secondary isomerization processes.8 Consequently, the development of new and selective homogeneous semi-hydrogenation catalysts that are able to produce (Z)-alkenes without isomerization or overreduction would still be of great advantage.9

Within this context, homogeneous catalysts based on first row transition metals attracted particular interest. Apart from obvious advantages such as their low price and high abundance, it is their intrinsic properties that may provide new opportunities paving the way to unprecedented reactivities and selectivities in catalytic transformations.10 Iron may be considered as a particularly promising candidate in this respect.11 For example, much progress could be achieved in the field of iron catalyzed hydrogenations of olefins.12 Concerning the selective reduction of alkynes, some iron complexes could successfully be applied that operate via hydrofunctionalization or transfer hydrogenation procedures.13 However, only two examples are currently known that can promote this transformation using hydrogen gas as the reductant (Scheme 1).8a,14

In 1989, Bianchini et al. discovered that the non-classical polyhydride [Fe(PP3)(H)(η2-H2)]+ is capable of catalyzing the semi-hydrogenation of terminal alkynes.14 More recently, the group of Milstein reported on a novel acridine based pincer type complex that bears an imino borohydride co-ligand.8a This complex was found to reduce internal alkynes selectively to the respective (E)-olefins. The reaction requires several hours and rather high reaction temperatures (90 °C) to achieve high yields

R

R

R R

H H

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

N

PR2

PR2N

N

Fe

H

H

Me

Me

H

HN

PR2

PR2N

N

Fe

H

H

H

Me

Me BN

Me

Me

+

not active active

unstable

active

stable

- fast semi-hydrogenation

- highly (Z)-selective

- bench-stable catalyst precursor

catalyst design:

H2 (5 bar), r.t.

Fe-cat.

- selective deuterium incorporation

2

and selectivities. Additional studies revealed that the hydrogenation initially affords (Z)-alkenes which, however, are isomerized to the respective (E)-olefins by the same catalyst.

Scheme 1. Iron Catalysts for the Semi-Hydrogenation of Alkynes

N

H

H

P'

P'Fe

N

NH

Me

BMe HHH

P

P'

P'P'

FeH

HH

+

N

P'

P'N

N

Fe

HMe

Me

HH

HH

+

H

H BN Me

Me

N

P'

P'N

N

Fe

HMe

Me

+

or

R

H

RR

RR

R

R

R

R

RH

H

H

H

H

HH

restricted to terminal alkynes

Bianchini 1989

(E)-selective due to isomerization

Milstein 2013

this work:

high (Z)-selectivity / no isomerizationP'

= PiPr22

a)

b)

H2 (1 bar)

H2 (4-10 bar)

90°C

r.t.

r.t.

H2 (5 bar)

P' = PPh2

P' = PPh2

P' = PiPr23

We describe here the application of the well-defined bench-stable cationic aminoborane complex [Fe(PNPNMe-iPr)(H)(η2-H2B=NMe2)]+ (2) described recently15 as highly efficient pre-catalysts for the semi-hydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes with molecular hydrogen under mild conditions. We take advantage of the fact that the aminoborane ligand, which is coordinated to the metal center via two weak σ-B-H bonds in η2-fashion, is substitutionally labile and upon dissociation readily provides two vacant coordination sites to bind dihydrogen and alkynes.

RESULTS AND DISCUSSION

The aminoborane complex [Fe(PNPNMe-iPr)(H)(η2-H2B=NMe2)]+ (2) was tested as pre-catalyst for the hydrogenation of a variety of different alkyne substrates in order to examine the general applicability and functional group tolerance of this novel system. Apart from 1-phenylpropene, also dialkyl and diphenyl substrates could be reduced to the respective (Z)-alkenes (Table 1, A1-A3) without isomerization of the products. Neither an increase of the catalyst loading nor higher pressures led to a further reduction of the alkene. Terminal alkynes, however, are further hydrogenated to yield the saturated alkanes. Overreduction could be prevented by the introduction of trimethylsilyl (TMS) moieties as protecting groups which were tolerated throughout without cleavage of the C–Si bond. Even those substrates were quantitatively reduced affording the respective alkenes with excellent (Z)-selectivity (except for the TMS-protected phenylacetylene A4 for which minor amounts of the trans-isomer were found in the isolated product). The reaction proceeds well also in case of diynes (A13, A14) which could successfully by converted to the corresponding (Z,Z)-dienes.

Encouraged by these results, we extended the substrate scope to 1,3-diynes (A16a) as well as 1,3-enynes (A16b-A20). In any case, the respective (Z,Z)-butadienes could be obtained with excellent stereoselectivity. Functional groups such as esters or amine groups are tolerated whereas the C=C double bond of the enyne substrates remained unaffected.

This circumstance was further examined by conducting the hydrogenation in presence of D2 (Scheme 2) The reaction of 1,3-enyne A16b with D2 catalyzed by 2 resulted in complete and selective deuterium incorporation at the former alkyne carbons, whereas an isotope scrambling into the existing olefin C–H positions was not observed. On the other hand, the respective 1,3-diyne A16a gave the fully deuterated butadiene product under the same reaction conditions. Thus, the novel iron hydride complex represents an interesting catalytic system that allows for the selective hydrogen isotope labelling of butadiene derivatives.

Based on the experimental observations, it is likely that the reaction proceeds via a classical insertion mechanism. Due to the labile nature of the dihydrogen and bis(σ-B-H) aminoborane ligands, the pre-catalyst gets activated by the addition of the

3

substrate in the presence of dihydrogen. In contrast, dissociation of the aminoborane ligand in 2 under an atmosphere of dihydrogen to form 3 was not be observed.15

Table 1. Semi-hydrogenation of Alkynes Catalyzed by 2.

R RR R

H H

S

2 (1.0 mol%)

CH2Cl2, r.t.1h

H2 (5 bar)

O

TMS

TMS

TMS TMStBu

TMSO TMS

TMSTMS

Z/E > 99>99% (86%)

A1a

Z/E > 99>99% (83%)

A2a

Z/E > 99>99% (82%)

A3a

Z/E > 99>99% (83%)

A5a

Z/E = 89/11>99% (89%)

A4a

Z/E > 99>99% (91%)

A6a

Z/E > 99>99% (87%)

A7a

Z/E > 99>99% (85%)

A8a

Z/E > 99>99% (94%)

A9a

Z/E > 99>99% (92%)

A10a

Z/E > 99>99% (90%)

A11b

Z,Z/E,E > 99>99% (84%)

A13a

Z,Z/E,E > 99>99% (91%)

A14a

Z/E > 99>99% (90%)

A15a

TMS

F3C

TMS

TMSHO

TMSF

H

H

HH

H

H

H

HH

overreduction in case of terminal alkynes

Z/E > 99>99% (87%)

A12c

Conditions: (a) alkyne (1.0 mmol), 2 (1.0 mol%), CH2Cl2 (1 mL), 25 oC, 1 h, conversion and selectivity determined by 1H NMR, isolated yields given in parenthesis. (b) 3 (2.0 mol%), 2h reaction time. (c) 7 (1.5 mol%), MeOH /CH2Cl2 (1:9, 1 mL), 1h.

The hydrogenation of A1 was conducted at different hydrogen pressures as well as substrate concentrations and the initial turnover frequencies were determined after a reaction time of five minutes. A significant increase of the initial TOFs could be observed at higher hydrogen pressures (see supporting information, Table S1). At 20 bar H2, after 5 min the product was obtained in >99% yield corresponding to a TOF of 2400 h-1. In contrast, essentially no acceleration took place when the

4

substrate concentration was raised. These results indicate that the rate determining step of the mechanism should be loss of the aminoborane ligand and formation of the active species being in line with the a very small energy barrier (11 kcal/mol) calculated for the catalytic reaction (Scheme 4).

A series of DFT calculations were carried out in order to gain more detailed insight into the catalytic cycle as well as the origin of the observed selectivity.16 A simplified catalytic cycle for the semi-hydrogenation of internal alkynes is depicted in Scheme 3. The cycle starts with an alkyne dihydrogen complex of the type [Fe(PNPNMe-iPr)(H)(η2-H2)(η2-MeC≡CPh)]+ as the active species. The η2-coordinated alkyne inserts into the iron hydride bond resulting in a dihydrogen vinyl species (I). After coordination of a second equivalent of H2 (II), this intermediate split the H-H bond under release of the alkene product and recovery of the initial hydride dihydrogen complex (III).

Table 2. Hydrogenation of 1,3-Diynes and 1,3-Enynes Catalyzed by Complex 2.

tButBu

O

OMeOMeO

H2N

R

R

R

Ror

R

RH

H

HH

R

RH

H

or

HH

H H

2 (1.0 mol%)

CH2Cl2, r.t.1h

H2 (5 bar)

H2N

Z,Z/E,E > 99>99% (91%)

A16aa

Z,Z/E,E > 99>99% (89%)

A18b

Z,Z/E,E > 99>99% (84%)

A19b

Z,Z/E,E > 99>99% (87%)

A20a

Z,Z/E,E > 99>99% (89%)

A17b

Z,Z/E,E > 99>99% (93%)

A16ba

Conditions: (a) alkyne (1.0 mmol), 2 (1.0 mol%), CH2Cl2 (1 mL), r.t., 1 h, conversion and selectivity determined by 1H NMR, isolated yields given in parenthesis. (b) 10 bar H2.

Scheme 2. Selective Deuterium Incorporation through Hydrogenation of 1,3-Diynes and 1,3-Enynes by 2 with D2

DD

DD

D D

H H

HH

2 (2.0 mol%)

CH2Cl2, r.t.1h

D2 (4 bar)

2 (2.0 mol%)

CH2Cl2, r.t.1h

D2 (4 bar)

A16-d4

A17-d2

A more detailed picture is provided by the free energy profile in Scheme 4 employing 1-phenylpropyne as the substrate. Insertion of the alkyne into the iron hydride bond proceeds easily with an activation barrier of 1 kcal/mol in an exergonic step

5

(ΔG = –11 kcal/mol). This occurs with a H-shift between two adjacent coordination positions and results in intermediate 1-B with trans dihydrogen and vinyl ligands. Coordination of a H2 molecule to the resulting iron vinyl complex affords intermediate 1-D in which there are two dihydrogen ligands parallel to the P-Fe-P axis. 1-D formation overcomes a barrier of only 3 kcal/mol and that step is exergonic with ΔG = –5 kcal/mol. From 1-D, a second H-transfer with concomitant re-orientation of the H2 ligand produces 1-E, where the recently formed olefin is loosely coordinated as a C–H σ-complex. Requiring 11 kcal/mol, this step represents the highest barrier in the catalytic cycle. The overall free energy balance of the cycle is favorable with ΔG = –22 kcal/mol and closing the cycle with liberation of the olefin and addition of a new alkyne molecule regenerates the initial species (1-A) in a slightly exergonic process (ΔG = –3 kcal/mol).

Regarding the chemoselectivity of the reaction, we also considered a second hydrogenation step to the respective alkane in our calculations (right side of the profile in Scheme 4). Here, the loosely bonded olefin in intermediate 1-E rearranges to η2-coordinated (in 1-G). This process occurs in two steps. First there is H-exchange, bringing the H2 ligand back to its original position, i.e., opposite to the Npy-atom, in intermediate 1-F. Then, a reorientation of the olefin brings it to the η2-coordination mode present in intermediate 1-G. These steps have negligible barriers (≤ 1 kcal/mol) and 1-G is 7

Scheme 3. Simplified Catalytic Cycle for the Semi-hydrogenation of Internal Alkynes Catalyzed by 2.

N

PR2

PR2N

N

Fe

HMe

Me

H

H

+

Me

N

PR2

PR2N

N

Fe

Me

Me

+

Me

H

HH

N

PR2

PR2N

N

Fe

Me

Me

+

MeH

HH

HH

Me

H H

Me

N

PR2

PR2N

N

Fe

H

H

H

Me

Me BN

Me

Me

+

I

HH

II

III

+ PhC

CMe / + H2- H2B=NMe2

2

kcal/mol more stable than 1-E, reflecting the additional stability associated with stronger coordination of the olefin. From 1-G, there is H-transfer to the inner C-atom of the C=C double bond with formation of an alkyl complex (1-H). The process occurs with concomitant H-transfer between the two adjacent coordination sites and, thus, both the H2 as well as the alkyl ligands occupy two apical positions trans to each other, in 1-H. This step corresponds to a formal olefin insertion in a Fe–H bond and parallels the first step of the entire path (alkyne insertion, over 1-TSAB). This is an endergonic process with ΔG = 6 kcal/mol and the associated barrier (13 kcal/mol) is 2 kcal/mol higher than the one found for alkyne insertion, indicating that the second hydrogenation is less favorable than the first one, in agreement with the experimental results. However, the small difference between the calculated barriers may suggest the possibility of competitive processes. After H2 coordination (from 1-H to 1-I) a final H-transfer from the dihydrogen ligand to the terminal alkyl C-atom produces the final intermediate, 1-J, with an alkane bonded as a C–H σ-complex. This is a facile process with a barrier of 6 kcal/mol and, also, clearly favorable with ΔG = –27 kcal/mol.

6

The hydrogenation of phenylacetylene was also considered by means of DFT calculations and the energy profile obtained for the corresponding reaction is depicted in Scheme 5. The mechanism is entirely equivalent to the one discussed above for the hydrogenation of 1-phenylpropyne. Thus, it also comprises two stepwise H-atom additions to CC unsaturated bonds. The first can be viewed as an acetylene insertion into a hydride bond and results in a vinyl complex (2-B). Then, there is H2 coordination followed by another H-transfer producing the olefin (styrene, in this case) and regenerating a hydride ligand (intermediate 2-E). A repetition of this sequence leads to the hydrogenation of styrene and the formation of the alkane (ethylbenzene) in the final intermediate, 2-K. The most important difference between the two paths is the relative value of the highest barrier for each H2 addition. In the case of the internal alkyne (1-phenylpropyne, Scheme 4) the barrier for the second addition is the higher one, justifying the observed semi-hydrogenation of the substrate. However, in the case of the terminal alkyne (phenylacetylene, Scheme 5) the opposite occurs. That is, the barrier for olefin hydrogenation (12 kcal/mol, 2-TSFH) is lower than the one calculated for the hydrogenation of the alkyne (15 kcal/mol, 2-TSAB). Both processes correspond to the first H-transfer, from the H2 ligand to the corresponding CC unsaturation. The results above indicate that in the case of phenylacetylene the reaction is expected to go all the way until the saturated product.

The observed chemoselectivity can also be related to the stability of the η2-olefin complexes in each case (1-G and 2-F). In fact, those are the initial species in the second H2 addition, from olefin to alkane. Considering the competition between H2 and the olefin for the six coordination position (Figure S1, supporting information), it becomes clear that the olefin complex is more stable in the case of the terminal olefin (styrene, in 2-F), while the opposite happens in the case of the internal olefin (1-phenylpropene, in 1-G). Therefore, the existence of the initial species for the second hydrogenation process is favorable in the case of substrates with a terminal C=C double bond, contrarily to what occurs for internal ones. Scheme 4. Free Energy Profile for the Hydrogenation of 1-Phenylpropyne. Free Energies (kcal/mol) refer to Intermediate 1-A.

N

PR2

PR2N

N

Fe

H

H

Me

Me

+

Me

H

N

PR2

PR2N

N

Fe

Me

Me

+

Me

H

N

PR2

PR2N

N

Fe

Me

Me

+

MeH

N

PR2

PR2N

N

Fe

Me

Me

+

Me H

H

H

N

PR2

PR2N

N

Fe

Me

Me

+HH

Me

H

H

H

1-A

1-B 1-C

1-D

1-E

1-F 1-G

N

PR2

PR2N

N

Fe

Me

Me

+

MeH

HH

H

H

HH

HH

HH

HH

N

PR2

PR2N

N

Fe

HMe

Me

+

Me

HH

HH

N

PR2

PR2N

N

Fe

HMe

Me

+

Me

HH

HH

1-H 1-I

2-J

0 1

-11 -11

-8

-16

-5

-22 -22

-29 -28 -29

-16

-23 -23

-17

-50

N

PR2

PR2N

N

Fe

Me

Me

+

Me

HH

H

HH

N

PR2

PR2N

N

Fe

Me

Me

+

Me HH H

H

H

H

H

N

PR2

PR2N

N

Fe

HMe

Me

+

Me

H

H

HH

H

H

N

PR2

PR2N

N

Fe

HMe

Me

H

H

+

Me

1-TSAB

+ H2

+ H2

1-TSCD

1-TSDE

1-TSEF

1-TSFG

1-TSGH 1-TSIJN

PR2

PR2N

N

Fe

Me

Me

+

Me

H

HH

HH N

PR2

PR2N

N

Fe

HMe

Me

+

Me

HH

HH

N

PR2

PR2N

N

Fe

HMe

Me

+

Me

HH

H

H

N

PR2

PR2N

N

Fe

Me

Me

+

Me HH H

H H

H

H

∆G = -3 kcal/mol Me

Me

HH

11

13

N

PR2

PR2N

N

Fe

Me

Me

+HH

Me

H

H

H

7

Scheme 5. Free Energy Profile for the Hydrogenation of Phenylacetylene. Free Energies (kcal/mol) refer to Intermediate 2-A.

∆G = -18 kcal/mol H

H

HH

N

PR2

PR2N

N

Fe

HMe

Me

+

HH

H

HH

N

PR2

PR2N

N

Fe

Me

Me

+

H

HH

H

HH

N

PR2

PR2N

N

Fe

Me

Me

+

H HH H

HH

H

H

N

PR2

PR2N

N

Fe

Me

Me

+

HH

HH

HH

N

PR2

PR2N

N

Fe

HMe

Me

H

H

+

H

N

PR2

PR2N

N

Fe H

Me

Me

+

H

HH

N

PR2

PR2N

N

Fe

H

H

Me

Me

+

H

H

N

PR2

PR2N

N

Fe

Me

Me

+

H H

H

H

HH

2-A 2-B

2-C

2-D 2-E

2-F

N

PR2

PR2N

N

Fe

Me

Me

+

H

H

HH

2-G

2-H

2-I

2-J

0

15

1

-4 -3

-14

-5

-14 -14

-27

-15 -16 -15

-22-25

-20

-34

HH

N

PR2

PR2N

N

Fe

Me

Me

+HH

H

H

H

H

H

H

2-TSAB

2-TSCD2-TSDE

2-TSEF 2-TSFG 2-TSGH

2-TSIJ

+ H2

+ H2

N

PR2

PR2N

N

Fe

Me

Me

+

H

H

HH

HH

N

PR2

PR2N

N

Fe

Me

Me

+

HH

HH

HH

N

PR2

PR2N

N

Fe

Me

Me

HH

H

H

H

H

+

N

PR2

PR2N

N

Fe

Me

Me

+HH

H

H

H

H

N

PR2

PR2N

N

Fe H

Me

Me

+

H

HH

HH

N

PR2

PR2N

N

Fe H

Me

Me

+

H HH

HH

12

15

N

PR2

PR2N

N

Fe

Me

Me

+

H HH H

H H

H

H

In order to obtain support for the mechanistic studies from an experimental point of view, in particular the existence of bis-dihydrogen intermediates, we prepared the novel cationic bis-dihydrogen complex [Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3). This complex was readily obtained by reacting [Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1)15 with an excess of nonafluoro-tert-butyl alcohol (Scheme 6) in THF at room temperature. The coordinated [AlH4]- anion is protonated by the acidic alcohol thereby liberating H2 with concomitant formation of a mixture of several poorly coordinating counterions of the types [Al(OC(CF3)3)4-nHn]- as detected by 19F{1H} NMR spectroscopy and ESI-MS.16

The 1H NMR spectrum of 3 in THF-d8 features a broadened triplet resonance at -14.68 ppm that integrates to five hydrogen atoms, while a singlet at 187.3 ppm could be observed in the 31P{1H} NMR spectrum. Owing to fast exchange between classical and non-classical hydrides, it was not possible to determine separate proton resonances for the individual hydride ligands. Even at -100 °C only a slight broadening of the hydride signal was observed. 1H NMR spectra recorded at variable temperatures revealed an extremely short relaxation time T1(min) of 12 ms (-65 °C, 500 MHz)17 which is characteristic of coordinated dihydrogen molecules.18,19

The experimental observations are further supported by DFT calculations. The cis- and trans-isomers of the bis(dihydrogen) complex differ merely by 2.0 kcal/mol and their interconversion requires an activation energy of just 5.0 kcal/mol being in line with the experimentally observed fluxional behaviour (Scheme 7).

Complex 3 was also applied as pre-catalyst for the hydrogenation of alkynes. Experiments were conducted in C6D6 under an H2 pressure of 5 bar at 25 oC using 1-phenylpropyne as test substrate. By employing 1 mol% of in situ prepared 3 the alkyne could be quantitatively reduced to the corresponding alkyne within 30 min. The product was formed with >99% Z-selectivity and no hydrogenation to the respective alkane could be observed even when the reaction time was extended to several hours. Accordingly, complexes 2 and 3 are obviously synthons for the active catalyst which reacts with alkynes in the presence of H2 to form [Fe(PNPNMe-iPr)(H)(η2-H2)(η2-RC≡CR’)]+ thereby initiating the catalytic cycle as shown in Schemes 3-5. It has to be noted that the neutral non-classical iron(II) polyhydride complex [Fe(PNP)(H)2(η2-H2)] did not catalyzed the hydrogenation of alkynes, but the dimerization of alkynes to give 1,3-eneynes with high Z-selectivity. 20

8

8

Scheme 6. Preparation of a Cationic Non-Classical Fe(II) Polyhydride Complex via Protonolysis of 1a

N

PR2

PR2N

N

Fe

H

H

H

Me

MeAl

H

H

1

N

PR2

PR2N

N

Fe

HMe

Me

H

H

HH

+

3OF3C

CF3F3CH

THF, r.t.

(10 equiv.)2

a Selected bond distances (Å) and angles (o): Fe1-N1 1.9867(9), Fe1-P2 2.1443(3), Fe1-P1 2.1461(3), Fe1-Al1 2.3507(4), Fe1-H1 1.46(2), Fe1-HAL1 1.48(2), Fe1-HAL2 1.55(2), Al1-HAL1 1.82(2), Al1-HAL2 1.73(2), Al1-HAL3 1.65(2), Al1-HAL4 1.54(2), P2-Fe1-P1 157.22(1).

Scheme 7. Free Energy Profile (kcal/mol) Calculated for the Interconversion of cis- and trans-Isomers of 3. Inset: 1H NMR Spectrum of 3 (Hydride Region, 250 MHz, THF-d8, 20oC).

-14.8-14.7-14.6

Fe

H

HH

N

PR2

PR2N

N

Me

Me

+

HH

Fe

H

HH

N

PR2

PR2N

N

Me

Me

+

H

H

Fe HN

PR2

PR2N

N

Me

Me

+HH

HH

Fe

HH

HN

PR2

PR2N

N

Me

Me

+HH

2.0

5.0

1.01.0

0.0

A

B

C

TSAB

TSBC

CONCLUSION

The bench-stable cationic bis(σ-B-H) aminoborane complex [Fe(PNPNMe-iPr)(H)(η2-H2B=NMe2)]+ (2) turned out to be an efficient pre-catalyst for the semi-hydrogenation of internal alkynes, 1,3-diynes and 1,3-enynes. With 1,3-diynes and 1,3-enynes deuterium could be selectively incorporated in the presence of D2. This was exemplarily shown with 1,4-diphenylbuta-1,3-diyne and (Z)-but-1-en-3-yne-1,4-diyldibenzene were the isotopomeres (Z,Z-buta-1,3-diene-1,4-diyl-1,2-d2)dibenzene and (Z,Z-buta-1,3-diene-1,4-diyl-d4)dibenzene, respectively, were obtained. The catalytic reaction takes place under mild conditions (1 h, 25oC, 4-5 bar H2 or D2) and all alkenes were obtained with high Z-selectivity for a broad scope of substrates. Mechanistic insight into the catalytic reaction is provided by means of DFT calculations. The stereo- and chemoselectivity was fully explained in agreement with the experimental observations. Intermediates featuring a bis-dihydrogen moiety [Fe(PNPNMe-

9

9

iPr)(η2-H2)2]+ are found to play a key role. Experimental support for such species was provided by the fact that [Fe(PNPNMe-iPr)(H)(η2-H2)2]+ (3) exhibited the same catalytic activity than 2. The novel cationic bis-dihydrogen complex 3 was obtained by protonolysis of [Fe(PNPNMe-iPr)(H)(η2-AlH4)]2 (1) with an excess of nonafluoro-tert-butyl alcohol. Thus, complexes 2 and 3 are apparently synthons for the active catalyst [Fe(PNPNMe-iPr)(H)]+ which reacts with alkynes in the presence of H2 to form [Fe(PNPNMe-iPr)(H)(η2-H2)(η2-RC≡CR’)]+ thereby initiating the catalytic cycle.

ACKNOWLEDGMENT NG and KK gratefully acknowledge the Financial support by the Austrian Science Fund (FWF) (Project No. P29584-N28). NG thanks the COST-CHAOS for granting a STSM to the University of Oslo. SV thanks the Austrian Science Fund (FWF) for an Erwin Schrödinger Postdoctoral Fellowship (Project No. J4158). LFV thanks Fundação para a Ciência e Tecnologia, UID/QUI/00100/2013. The Research Council of Norway supported this work through the Norwegian NMR Platform, NNP (226244/F50). Senior Engineer Dirk Petersen (University of Oslo NMR center) is thanked for variable temperature NMR assistance. REFERENCES (1) Kluwer, A. M.; Elsevier, C. J. Homogeneous Hydrogenation of Alkynes and Dienes. In The Handbook of Homogeneous

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