palladium catalyzed hydrostannation of alkynes and palladium-catalyzed hydrostannolysis of propargyl...

Tetrahedron Letters,Vol.29,No.6,pp 619-622,1988 0040-4039/88 $3.00 + .oo Printed in Great Britain Pergamon Journals Ltd.

PALLADIUM CATALYZED HYDROSTANNATION OF ALKYNES AND PALLADIUM-CATALYZED HYDROSTAN- NOLYSIS OF PROPARGYL OR PROPARGVLOXYCARBONYL DERIVATIVES OF VARIOUS FUNCTIONAL GROUPS

Zhong H.X., F. Guibe’ and G. Balovoine

Instltut de Chimie Moleculoire d’0rsay. Loboratoire de Chimie Organique des Elements de Transition UA-CNRS no 255, Bat. 420.

91405 ORSAY cedex. France

Abstract : PdC12(PPh3) catalyses, under very mild conditions, the cls-hydrostonnation of oce- tylenic compounds by trlbutyltin hydride and the hydrostannolytic cleavage of proporgyl car- boxylates, phosphates, carbonates and corbamotes. The latter reactlon should find use in protective group chemistry.

In the past few years. we have shown that the ally1 derivatives of carboxyllc acids or

phenols and the ollyloxy car-bony1 derivatives of alcohols and amlnes may be deprotected In o

straightforward and very selective manner by palladium-catalyzed cleavage with tributyltln

hydride (equations l-a and l-b)T-4.

Pd cat. X-A + Bu3SnH t HV -> XH (t Bu3SnY + \\/-” ) Cl-01

X = RCO2-. ArO-

s Pd cat.

X1-C-s, t Bu3SnH t HY -_) X’H (t CO2 t Bu3SnY + N-H)

X’ = RO-, RN”- : YH = H20, pNO2-PhOH. AcOH...

[T-b]

We have applied this procedure to the preparation of trlbutyltln B-ketoesters from ally1

s-ketoesters2, In the selective protection-deprotection of various ally1 and allyloxycorbonyl

derivatives of amino-acids3 and in a solid-phase synthesis of the undecapeptlde substance P

In which the ollyloxycarbonyl group was used for the temporary protection of the a-amino

functlons4. In this communication we report : 1) that the catalytic hydrostonnolytlc depro-

tectlon procedure also applies to the proporgyl and propargyloxycorbonyl groups ; 2) that

palladium complexes catalyze the trlbutyltin hydride hydrostannotlon of simple ocetylenic

compounds ; 3) that the hydrostannatlon of the triple bond probobly represents the first step

of the hydrostannolytlc cleavage of propargylic esters mentlonned above.

When 2 to 2.2 equivalents of tributyltln hydride were added dropwlse over a period of o

619

620

few minutes to a benzene. THF or CH2C12 solution (5 mL) of propargyl acetate (1 mmole) ond of

dichloro-bis(triphenylphosphine)palladium(II) (2.10s2 equivalents) an immediate and exother-

mic reaction ensued. After 10 min, IR analysis (CC14) showed a total conversion of propargyl

acetate (uCO = 1730 cm-l) into tributyltin acetate ((uCO = 1635 cm-‘). Quantitative deprotec-

ti0fl (NMR, GC) was also achieved, in CH2C12. starting from prapargyl p-methylbenzoate, die-

thylpropargyl phosphate, propargyl benzyl carbonate and benzylamine propargyl carbamate

4 2 Bu3SnH HY Z-/ > Z-SnBu3 -_) ZH + tin by-products

[Pd] cat

Z = (EtO)p P(O)-0-. pMe-Ph-CO2-

4 ibid R-Z ’ -CO2 -/ -_) -> R-Z’H + CO2 + tin by-products

R-Z’ - PhCH20, PhCH2-NH

No formation of allenic or propargylic amlnes or ethers could

D-al

[2-b]

be detected in the hydro-

stonnolytic deprotection of propargyl carbonate or carbamate5. Apart from the tributyltin

salts Bu3SnY -whose nature depends on the final work-ups- and a small amount of hexabutyldis-

tannanelb. the tin by-products of these reactions consist mainly of 2-tributylstannylpropene

1 (X = H) together with 3-tributylstannylpropene 2 and small quantities of Z and E l-

tributylstannyl propene J (X = H)7.

Bu3Sn H H H H \ 0

= \ 0 SnBu3

X-CH2’ ’ H = \=/

Bu3Sn-CH2’ \H X-CH2/ ‘Ii

When applied to propargylphenoxide. the catalytic reaction with tributyltin hydride gave

different results. Upon addition of one equivalent of trlbutyltln hydride, propargyl phenoxl-

de wos essentially converted to a co. 95/5 mixture of 2-trlbutylstannyl- and (presumably) E-

1-tributylstannyl-3-phenoxy propene 1 and J (X = OPh)7*B. Further addition of tributyltin

hydride resulted only In partial release (ca 40 %) of trlbutyltln phenoxide. Extensive decom-

position of tributyltln hydride into hexabutyldistannanelb and formation of ally1 phenoxide

(up to 20 f6) were also observed.

The formation of trlbutyltln-substituted allylic phenoxides 1 and J (X = OPh) suggested

that PdC12(PPh3)2 could act OS a catalyst In the hydrostannation of carbon-carbon triple

bonds. We were able to confirm this property with a series of simple acetylenlc compoundsg.

From the preliminary results listed in the table, the reaction seems to be stereospecific

(cis-hydrostannatlon leading to E-tributylstannyl olefinic compounds, entries 1, 2, 4

and 5)TO-‘2 but not regiospecific (entries 1. 2) unless electronic factors are involved

(entries 3. 6).

From the above results, the following mechanism for the hydrostonnalytic deprotection of

propargylic esters may be tentatively proposed. In a first step. a palladium-catalyzed cis

hydrostannation of the triple bond would lead to the tributyltin-substituted allYliC interme-

diotes 1 ond 2 E (X = leaving group : LO) ; in a fost step, these intermediates would

undergo a pollodium-cotolyzed hydrostonnolytic cleovoge -probably through the formotion

621

then

of 0

i-ally1 pollodium complex13- leoding to the isomerlc tributylstonnylpropenes l_. 2 and J

(X = H) (Scheme I).

Scheme I : 1

= LO)

(X = LG)

BuSSnH > 1

[Pd] cot (X = H)

BuSSnH

[Pd] co: 2. + 2 (E +Z)

(X = H)

In the case of proporgyl phenoxide, due to the poor leaving group obility of the phenoxy

group. the intermediate 1 ond J (X = OPh) ore stoble enough to be characterized upon odditlon

of one equivalent of tributyltin hydride ; further addition would then lead not only to

hydrostonnolytic cleovoge but also to other side-reoctions14.

Toble : Pollodium-cotolyzed hydrostonnotion of ocetylenic compounds(o)

Entry Starting compounds Products (per cent)(b) Overall

H-2-R HWR BUSS-R HWR isolated

Bu3Sn _H H-H H-SnBu3 yieti\‘)

R = CH20H 45 % (c) 55 $ 41(e)

R = CH2OTHP 32 % (c) 68 % 68(e)

R = C02Me 100 $ (c) (c) 90

4 MeO2C-=-C02Me MeO$, ,C02Me

= Bu3Sn’ ’ H

BujSn,_,C02Me

Me02C /-xH

83 $ 17 z

5 Ph-=-CH3 Ph \_/CH3

,- ’ H 8u3Sn

8u3Sn, =

, CH3

Ph’ ‘H

Ph. _ ,SnBu3

H/--CH 79

3

100 $ (f) (cl (cl (0) 8u3SnH 1 to 1.2 equivalents, PdCl or THF (entries 3, 4, 5 OS the solven z

(PPh3)2 2.10e2 equlvolents, benzene (entries 1, 2) : room temperature, 0 few minutes.

Determined by NMR 250 MHz) on the crude reoctlon mixture. Undetected. Analytical over011 yields. OS determined by NMR on the crude reoctlon mixture using on

internal stondord. were found to be In 011 cases quantitative within experiment01 errors. Loss of product during purificotlon by column chromotogrophy is mainly due to slllco-indu- ted protodestonnylotlon. All purified compounds gave sotisfoctory element01 onolysis.

assignment based on NMR spectroscopy12.

In conclusion, the ally1 ond proporgyloxycorbonyl groups con be removed under very

mild conditions

conditions con

ollyloxycorbonyl

OS shown in the

chemistry.

by use of the hydrostonnolytic system 8u3SnH/Pd cotolyst. The fact that

often be found for the cotolytic selective cleovoge of the 011~1 and

groups in the presence of the proporgyl and proporgyloxycorbonyl groups,

occomponylng poper. should make these groups useful in protective group

622

REFERENCES

I) a) Guibe F.. Saint M’Leux Y. ; Tetrahedron Lett.. 1981, 20% 3591. b) Four P., Guibe F. ; Ibid, 1982. 21, 825.

2) Guibd F., Yang T.X.. Zigna A.M., Balavoine G. : Tetrahedron Lett.. 1985, 26. 3559 Guibe F.. Zhang H.X., Balavoine G. ; Nouv. J. Chim., 1986, TO, 697.

3) Guibd F, Dangles 0.. Balavoine G. ; Tetrahedron Lett., 1986, 27. 2368. 4) Dangles O., Guibc! F., Balavoine G., Lavielle S., Marquet A. ; J. Org. Chem.. 1987. 52,

4984. 5) In palladium-catalyzed deallylatlon reactions performed on ally1 carbamates.

allylamine formation may be a serious side reaction3,4 and ref. competitive

therein.

stannolytic procedure33 4

In the hydro- the formation of allylamlne is completely suppressed in the

presence of weak acidic species. 6) Hydrostonnolytlc deprotection of benzyl propargyl carbonate leads directly to the tri-

butyltin salt of benzyl alcohol which is readily hydrolyzed in water. The deprotection of benzylamine prapargyl carbamate yields the corresponding benzylamlne tributyltin carba- mate (uC0 = 1630 cm-l. Conversion to free amine upon pratalysis (aq. HCl, CH3CO2H) is imnediate. Tin carboxylates are converted into their sodium salt or Into carboxylic acids by treatment with aqueous sodium carbonate or with various protonating agents HY.

7) TH NMR characteristics (250 MHz, CDC13) of main products : d (ppm). 1 (X = H) : 5.7 (dq.

JI ‘J2 :I-2 Hz, 1H) and 5.1 (dq. IH, vlnyllc H) ; 1.95 (app. t. 3 : 1.5 Hz. 3H, CH3). 2

(X = H) : 6.1-5.85 (m, IH, internal vinylic H) ; 4.80 (dm. Jtrans = 16 Hz, 1H) and 4.65

(dm. Jcis = 10 Hz, IH. termlnal vinylic H) ; allylic H : masked. 1 (X = OPh) : 6.05 (dt,

JI f J2 5 1.5-2 Hz. IH) and 5.45 (dt, lH, vinylic H) : 4.60 (app. t. J f 5 Hz, 2H,

allylic H). 8) By contrast upon addition of one equivalent of tributyltin hydride, propargyl carboxy-

lates or phosphate gave a l/l mixture of unreacted material and of tributyltin carboxy- lates or phosphate.

9) For a related palladium-catalyzed reaction of silylstannanes with alkynes see : Chenard

B.L., Zan Vyl C.M. ; J. Org. Chem., 1986. 5l_, 3561.

IO) Nan catalytic polar or radical-Initiated hydrastannation of alkynes usually results in trans addition : a) Leuslnk A.J., Budding H.A. ; J. Organamet. Chem.. 1968, 11. 533. b) Leusink A.J.. Budding H.A., Drenth W. ; Ibid, 1967, 9, 295. c) Leusink A.J.. Budding H.A., Marsman J.W. ; Ibid, 1967. 9, 285. d) Jung M.E.. Light L.A. ; Tetrahedron Lett., 1982. 23, 3851. e) Ensley H.E., Buescher R.R., Lee K. : J. Org. Chem.. 1982, c, 404. Tin hydride addition may also be induced by Et38 : f) Nozaki K., Oshima K., Utlmata K. ; J. Am. Chem. Sot., 1987, 109, 2547.

11) The formation of small amounts of trans addition product in the case of diethyl ethyne dicarboxylate (entry 4) may very likely be ascribed to competitive spontaneous (non catalytic) reaction (ref. 9c).

12) Most compounds have already been characterized by NMR spectroscopy (ref. 1Oc.d). E-BujSn(Ph)C=CH(CH3) : 6 (ppm, CDC13) : 5.88 (q. J = 7 Hz, IH) ; 1.67 (d, J = 7 Hz. 3H). The J3 (l17Sn-H) and J3 (llgSn-H) values (respectively 63 and 67 Hz) for tin and vlnylic hydrogen are consistent with a cis relationship (expected values for trans relation-

ship : ca 110-130 Hz. ref 10~). 13) Greenspoon N.. Keinan E. ; Tetrahedron Lett., 1982, 21, 241.

Guibd F., Zigna A.M., Balavoine 0. : J. Organomet. Chem.. 1986, 306. 257. Guibe F., Yang T.X.. Balavoine G. : J. Organomet. Chem.. 1986. Jo6, 267.

14) Other mechanistic pathways are conceivable such as the direct hydrostannalysis of o- allenic or o-propargylic palladium entitiesT5*16. However the need of two equivalents of trlbutyltin hydride to complete the cleavage of propargyl carboxylates and phosphates Is more in favor of a sequential hydrostannation-hydrostannolysis process. We defer a mare detailed mechanistic study of this reaction as well as a fuller investigation of the catalytic hydrostannation of alkynes to another report.

15) Elsevier C.J.. Kleijn H.. Baersma J.. Vermeer P. : Organometollics, 1986, 2, 716. 16) For reactions involving such intermediates, see inter 0110 : Colas Y., Cases B., Gore

J. ; Tetrahedron Lett.. 1984, 25, 845 and Bull. Sot. Chim. Fr.. 1987. 165. Kelnan E.. Bosch E. ; J. Org. Chem., 1986, 5l, 4006. Tsuji J., Sugiura T.. Yuhara M.. Minami I. ; J. Chem. Sot. Chem. Commun.. 1986, 922.

(Received in France 13 October 1987)