DOI: 10.1002/adsc.201000781

N-Heterocyclic Carbene Piano-Stool Iron Complexes as EfficientCatalysts for Hydrosilylation of Carbonyl Derivatives

Fan Jiang,a David B�zier,a Jean-Baptiste Sortais,a,* and Christophe Darcela,*a UMR 6226 CNRS “Sciences Chimiques de Rennes”, Universit� de Rennes 1, Equipe “Catalyse et Organom�talliques”,

Campus de Beaulieu, Bat 10C, Avenue du G�n�ral Leclerc, 35042 Rennes Cedex, FranceFax: (+33)-2-2323-6939; e-mail: jean-baptiste.sortais@univ-rennes1.fr or christophe.darcel@univ-rennes1.fr

Received: October 15, 2010; Published online: January 19, 2011

Supporting information for this article is available on the WWW underhttp://dx.doi.org/10.1002/adsc.201000781.

Abstract: Hydrosilylation with well defined piano-stool iron(II) complexes bearing both N-heterocy-clic carbene and cyclopentadienyl ligands was ac-complished with both aldehydes and ketones. Typi-cally, the reduction of aldehydes exhibited good ac-tivities (full reduction at 30 8C in 3 h), whereas forketones, the reaction need 16 h at 70 8C to obtaingood conversions. Of notable interest is the use ofvisible light irradiation to generate one of the activecatalyst.

Keywords: aldehydes; hydrosilylation; iron cataly-sis; ketones; N-heterocyclic carbene (NHC) ligands;reduction

Since the discovery of stable N-heterocyclic carbenes(NHCs) by Arduengo,[1] which followed the pioneer-ing reports by �fele and Wanzlick,[2] and the earlystudies by Lappert[3] on the coordination to late tran-sition-metal complexes, these ligands have found awide range of applications in coordination chemistryand catalysis during the last two decades.[4] Further-more, despite recent impressive advances in iron ho-mogeneous catalysis,[5] iron is still one of the raretransition metals which has not been intensively stud-ied with NHCs. Indeed, the number of applicationsusing the combination iron salt/NHC ligands in homo-geneous catalysis is relatively low. They covered thefields of polymerization,[6] intermolecular ring expan-sion of epoxides,[7] cyclotrimerization,[8] C�C bondformation,[9] allylation reactions,[10] and very recentlyaromatic esterification of aldehydes in the presence ofboronic acids.[11]

In the area of well defined iron complexes, com-pared to other metals, NHC-iron ones are still lessrepresented. A small number of examples of bis-car-

bene iron complexes[12] has been synthesized. The firstexamples of mono-carbene complexes were reportedin 2003 by Guerchais[13] which was followed by exam-ples from Albrecht[14] , Llewellyn,[15] and more recentlythe groups of Cesar and Lavigne[16] and Ohki andTat-sumi.[17] Up to now, they were used in only very fewexamples as catalysts.[17,18]

Our interest in iron catalysis[19] and especially hy-drosilylation[20,21] prompted us to examine the possibil-ity of using NHC well-defined iron complexes in thehydrosilylation reaction. The report by Nikonov[22]

using cationic [CpFe(phosphine)] complexes and thepioneer works from Brunner[23] oriented us towardspiano-stool iron carbene complexes to catalyze suchreduction reactions.

To start our investigation, we have chosen to studywell-defined iron complexes containing the IMes N-heterocyclic carbene ligand prepared by Guerchais:[13]

the cationic complex 1 with one iodine as the counter-ion, and the neutral complex 2 obtained by photoirra-diation of 1 in CH2Cl2 (Figure 1). Herein, we reportthe efficient reduction of carbonyl compounds by hy-drosilylation with these NHC-carbene piano-stooliron complexes.

Initial studies on the catalytic activities of the Guer-chais� iron piano-stool complexes 1 and 2 were per-

Figure 1. Guerchais� cationic and neutral iron piano-stoolcomplexes bearing N-heterocyclic carbene ligands.

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formed to explore their potential in the hydrosilyla-tion reaction of aldehydes (Scheme 1, Table 1).

The optimization of the hydrosilylation reactionwas carried out with benzaldehyde 3 as the modelsubstrate. As illustrated in Table 1, the preliminarysurvey was carried out using 1 mol% of[Cp ACHTUNGTRENNUNG(IMes)Fe(CO)2]I complex 1 as the catalyst, and1 equivalent of diphenylsilane as the reducing agent,under exposure to visible light. After 1 h of reaction,a full conversion was observed and the benzyl alcohol4 was obtained as the single product after a basiccleavage of the silyl ether intermediate (Table 1,entry 1). Interestingly, when the reaction was per-formed in the absence of visible light irradiation, noconversion was observed which shows the crucial roleof the light activation process in the generation of theactive catalytic species from the pre-catalyst 1(Table 1, entry 2). With PhSiH3 as the hydride source,a full conversion was obtained at 30 8C in only 3 hunder irradiation, whereas no reaction occurred in theabsence of light (Table 1, entries 3 and 4). As thecomplex 2 [CpACHTUNGTRENNUNG(IMes)Fe(CO)(I)] is obtained by pho-toirradiation of 1, we have tested this catalyst withoutvisible light irradiation to promote the hydrosilylationreaction (Table 1, entries 5–11). Full conversions wereobtained in toluene using diphenylsilane, (EtO)3SiHand environmentally friendly PMHS, but Ph3SiH wasnot reactive even at 100 8C (Table 1, entries 5–9). Wefound, eventually, that the reaction could be carried

out at 30 8C in 3 h successfully using PhSiH3 as thesilane and toluene as the solvent (Table 1, entries 11and 12).

To investigate the scope of this iron-catalyzed hy-drosilylation reaction, a variety of aldehydes was thentested using the optimized conditions (PhSiH3, tolu-ene, 3 h at 30 8C, 1 mol% of catalyst 2, conditions A).

Several aldehydes were reduced with good to excel-lent yields (Table 2). Interestingly, when the reactionwas carried out with aldehydes bearing substituents inthe ortho-position of the aryl ring, the correspondingprimary alcohols were obtained in 65% isolated yield,which shows that ortho substituents did not hamperthe reaction, but had a significative impact on theyield (Table 2, entries 2 and 3). The electronic effectson the reactivity were limited. Electron-deficient aswell as electron-withdrawing groups on the aryl ringdid not show any significant influence on the activityof the iron catalyst. Interestingly, functional groupssuch as CN or Br remained unchanged under such re-action conditions (Table 2, entries 4 and 7). On theother hand, using p-Me2N- and p-MeO-substitutedbenzaldehydes under these conditions, no conversionwas observed. This issue was circumcised by using thecationic catalyst 1, in THF at 70 8C under visible lightirradiation (conditions B). Under these conditions,both p-subtituted aldehydes were fully converted intotheir corresponding alcohols (Table 2, entries 8 and9). The chemoselectivity of the hydrosilylation of al-dehyde versus alkene was also demonstrated: the cat-alyzed hydrosilylation of enal derivatives such as (�)-myrtenal led exclusively to the corresponding allyl al-cohol resulting in an exclusive 1,2-addition with noGC-detectable amounts of 1,4-addition products(Table 2, entry 10). With undecen-10-ylaldehyde, onlyreduction of the aldehyde was observed and the unsa-turated alcohol was obtained with 99% isolated yield

Scheme 1. Hydrosilylation of benzaldehyde 3.

Table 1. Optimization for the reduction of benzaldehyde with catalysts 1 and 2.[a]

Entry Catalyst Silane (equiv.) Solvent Temperature [8C] Time Conversion [%][c]

1[b] 1 Ph2SiH2 (1) toluene 100 1 h 992 1 Ph2SiH2 (1) toluene 100 1 h 03[b] 1 PhSiH3 (1) THF 30 3 h 994 1 PhSiH3 (1) THF 30 3 h 05 2 Ph2SiH2 (1) toluene 120 5 min 996 2 Ph2SiH2 (1) toluene 70 1 h 997 2 ACHTUNGTRENNUNG(EtO)3SiH (1) toluene 100 2 h 91[d]

8 2 Ph3SiH (1) toluene 100 2 h 0[d]

9 2 PMHS (2) toluene 100 2 h 9110 2 Ph2SiH2 (1) toluene 30 3 h 2211 2 PhSiH3 (1) toluene 30 3 h 99[d]

12 2 PhSiH3 (1) THF 30 3 h 90[d]

[a] Typical procedure: benzaldehyde (0.5 mmol), catalyst (1 mol%), solvent (2.5 mL) and silane are stirred under argon.[b] Under visible light irradiation.[c] Conversion determined by 1H NMR after methanolysis (MeOH, 2 N NaOH).[d] Conversion determined by GC.

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(Table 2, entry 11). Extension of the procedure to het-erocyclic aromatic aldehydes such as 3-pyridylcarbox-aldehyde was also possible (Table 2, entry 12)

The reduction of ketones was then investigated,starting with acetophenone as the model substrate(Scheme 2, Table 3).

With the neutral catalyst 2, no conversion was de-tected using various silanes, at 100 8C as illustrated inTable 3, entries 1 and 2. We found that the reductionof the ketone was possible at 70 8C, using Ph2SiH2 asthe silane and 2 mol% of catalyst 1 under visible lightirradiation, even if the conversions were moderate(Table 3, entry 3). The combination of PhSiH3 and tol-uene as the solvent, allowed us to obtain the desired

alcohol 6 in good conversion (78%) after 16 h at70 8C (Table 3, entries 4 and 5). The reaction proceedsmore slowly than with aldehydes, but the reductioncould still be achieved at 30 8C after 72 h (Table 3,entry 7). Once again, the irradiation activation of thecatalyst 2 is crucial to perform the hydrosilylation(Table 3, entry 6 vs. entries 5 and 7).

The optimized conditions (2 mol% of catalyst 1,1.2 equivalents of phenylsilane in toluene at 70 8C for16 h under visible light irradiation) have been em-ployed for the hydrosilylation of a variety of ketones.(Table 4) The reduction of activated aryl ketones withelectron-withdrawing substituents gave good to excel-lent yields (Table 4, entries 2, 4 and 5). In contrast,ketones bearing electron-donating substituents werereduced with moderate conversions (Table 4, en-tries 6–8). We have also demonstrated that alkyl ke-tones such as cycloheptanone could be reduced usingthis methodology (Table 4, entry 10).

Even if the hydrosilylation of ketones seems moredifficult in comparison with aldehydes, this represents,to the best of our knowledge, the first example usingNHC-iron complexes as catalyst.

In conclusion, we have developed an efficient iron-catalyzed hydrosilylation of both aldehydes and ke-

Table 2. Scope of iron-catalyzed hydrosilylation of aldehydesusing Guerchais� complexes 1 and 2.

Entry Substrate Conditions[a] Conversion(Yield)[b]

1 A >99 (80)

2 A >99 (86)

3 A >99 (65)

4 A >99 (88)

5 A >99

6 A >99

7 A 97

Table 2. (Continued)

Entry Substrate Conditions[a] Conversion(Yield)[b]

8 B >99 (70)

9 B >99

10 B 94 (75)

11 B >99 (99)

12 B 88[c]

[a] Typical conditions – A : aldehyde (0.5 mmol), catalyst 2(1 mol%), PhSiH3 (1 equiv.), toluene, 30 8C, 3 h; B : cata-lyst 1 (1 mol%), 1 equiv. of PhSiH3, THF, 70 8C, 1 h,under visible light irradiation.

[b] Conversion determined by 1H NMR after methanolysis,in parenthesis, isolated yield after purification by columnchromatography.

[c] 15 h 30 min, conversion determined by GC.

Scheme 2. Hydrosilylation of acetophenone 5.

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tones using two-well defined Fe(II) complexes con-taining cyclopentadienyl and N-heterocyclic carbeneligands. Good to excellent conversions and yieldswere observed for a variety of aldehydes, althoughlower activities were obtained in this preliminarystudy with ketones. Notwithstanding, this NHC-Fe(II)-catalyzed hydrosilylation of ketones constitutesthe first example described in the literature. Of nota-ble interest is the use of visible light activation to gen-erate the catalyst from [CpFe ACHTUNGTRENNUNG(NHC)(CO)2]I whereas[CpFeACHTUNGTRENNUNG(NHC)(CO)I] works at 30 8C without any acti-vation. Moreover, the interesting difference of behav-iour between the two catalysts in this reaction, espe-cially for neutral catalyst 2 (for aldehydes and ketoneshydrosilylation) prompts us to start further mechanis-tic studies to understand this phenomenon.

Experimental Section

General Procedure for the Iron-CatalyzedHydrosilylation of Aldehydes

Procedure A: A 10-mL oven-dried Schlenk tube containinga stirring bar, was charged with [CpFe(CO) ACHTUNGTRENNUNG(IMes)I] 2(2.9 mg, 0.005 mmol). After purging with argon (argon-

Table 3. Optimization for the reduction of acetophone 5.[a]

Entry Catalyst (mol%) Silane (equiv.) Solvent Temperature [8C] Time Conversion[b]

1 2 (1) Ph2SiH2 (1) toluene 100 1 h 0[d]

2 2 (1) ACHTUNGTRENNUNG(EtO)3SiH (1) toluene 100 1 h 0[d]

3[c] 1 (2) Ph2SiH2 (1.2) toluene 70 16 h 35[d]

4[c] 1 (2) PhSiH3 (1.2) THF 70 16 h 325[c] 1 (2) PhSiH3 (1.2) toluene 70 16 h 786 1 (2) PhSiH3 (1.2) toluene 70 16 h 37[c] 1 (2) PhSiH3 (2.4) toluene 30 72 h 78

[a] Typical procedure: acetophenone (0.5 mmol), catalyst, solvent (2.5 mL) and silane are stirred under argon.[b] Conversion determined by 1H NMR after methanolysis (MeOH, 2 N NaOH).[c] Under visible light irradiation.[d] Conversion determined by GC after methanolysis (MeOH, 2 N NaOH).

Table 4. Scope of iron-catalyzed hydrosilylation of ketonesusing the cationic complex 1.[a]

Entry Substrate Conversion (Yield)[b]

1 78 (50)

2 95 (90)

3 94

4 88 (82)

5 98 (97)

6 66 (58)

7 61 (61)

8 50

9 61

Table 4. (Continued)

Entry Substrate Conversion (Yield)[b]

10 72[c]

[a] Typical conditions – A : ketone (0.5 mmol), catalyst 1(2 mol%), PhSiH3 (1.2 equiv.), toluene, 70 8C, 16 h, undervisible light irradiation.

[b] Conversion determined by 1H NMR after methanolysis,in parenthesis, isolated yield after purification by columnchromatography.

[c] Conversion determined by GC.

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vacuum three cycles) distilled toluene (2.5 mL) was addedfollowed by aldehyde (0.5 mmol) and PhSiH3 (62 mL,0.5 mmol). The reaction mixture was stirred in a preheatedoil bath at 30 8C for 3 h.

Procedure B: A 10-mL oven-dried Schlenk tube contain-ing a stirring bar, was charged with [CpFe(CO)2 ACHTUNGTRENNUNG(IMes)]I 1(3.0 mg, 0.005 mmol). After purging with argon (argon-vacuum three cycles) distilled THF (2.5 mL) was added fol-lowed by aldehyde (0.5 mmol) and PhSiH3 (62 mL,0.5 mmol). The reaction mixture was stirred in a preheatedoil bath at 70 8C for 1 h under the presence of visible light.

For both procedures, 1 mL of MeOH and 1 mL of 2 MNaOH solution were then successively added under vigo-rous stirring for 1 hour at room temperature. Conversionwas determined by 1H NMR. The reaction mixture was ex-tracted with diethyl ether (2 �10 mL). The combined organiclayers were washed with brine (3 � 10 mL), dried over anhy-drous MgSO4, filtered and concentrated under vacuum. Theresidue was purified by silica gel column chromatographyusing ethyl acetate-petroleum ether mixture (10 to 50%) toafford the desired product.

Acknowledgements

We are grateful to Universit� de Rennes 1, CNRS, RennesM�tropole and Minist�re de l’Enseignement Sup�rieur et dela Recherche for support, the latter for a PhD grant to D.B.,and the European Union through network IDECAT.

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