Aldehydes, reduction with silanes

Another variation of the Pauson-Khand is the cyclization of w-alkenyl-aldehydes 249 in the presence of HSi(OEt)3 resulting in siloxycyclopentadienes (Scheme 40)187-189 The observed products, 250, arise from functionalization of the intermediate titanacycle with silane followed by reductive elimination of product. 

Silanes normally reduce aldehydes or ketones under catalytic conditions to the silyl ethers. However, with certain catalysts such as nickel sulfide,Co2(CO)8, or (Ph3P)3RhCl, carbonyl compounds react with silanes to yield an equilibrium mixture of enol silyl ethers (Scheme 17). In a similar vein, the silyl-hydroformylation reaction of cycloalkenes with CO and silanes may be a practical way to prepare enol silyl ethers. An example is the preparation of compound (49). Catalytic 1,4-hydrosilation of a, -un-saturated ketones or aldehydes gives the corresponding enol silyl ethers. The reaction is similar to the reductive silylation referred to previously, but the reaction conditions are neutral and milder. The formation of the enol silyl ether (50) is outlined below. ... 

Polymethylhydrosiloxane (PMHS), a safe and inexpensive polymer co-product of the silicon industry, is an efficient alternative reducing agent for C=0 and C=N bonds when associated with catalysts (1). Mimoun et al. recently reported a new system based on zinc hydride catalysts which enables the chemoselective reduction of unfunctionalized and a,/ -unsaturated- aldehydes, ketones and esters (2). Because gummy silicon residues, which are usually associated with silane reductions, do not form, this PMHS system is attractive for synthetic / industrial purposes. Nevertheless, in contrast to tin-catalyzed reductions of ketones with PMHS... 

In 1992 the Bullock and Norton groups reported the reduction of aldehydes and ketones under similar conditions, and isolated an isopropyl alcohol complex formed thereby (eq 44) [75] similar results have recently been reported with H2Re(NO)(CO)(PR3)2/CFjC02H by Bakhmutov and co-workers [76]. These reactions resemble the ionic hydrogenations long known with silanes as hydride donors [77],... 

The regiocontrol of nickel-catalyzed reductive couplings of aldehydes and alkynes is often dictated by biases of the substrate, particularly the alk)me. However, the procedure involving iV-heterocyclic carbene ligands with silane reductants is... 

Morken and coworkers [39b] developed the first asymmetric reductive aldol reaction with silanes as reductants in combination with a chiral rhodium catalyst. a,P-Unsaturated esters were reacted with several aldehydes to provide the corresponding aldol products 79 in good yields and enantio- and diastereoselectivities (Scheme 8.23). Both aliphatic and aromatic aldehydes could be converted into aldol products 79 under these conditions. Furthermore, the group reported an iridium-catalyzed asymmetric version that tolerated various protected hydroxyaldehydes [39aj. On the basis of this precedence, a highly enantio- and diastereoselective... 

Ruthenium dihydride complex [Ru(H)2(CO)(PPh3)3], activated by treatment with st3rrene, catalyzes several hydrosilylation polymerization processes, such as the reaction of tereftaldehyde with 1,3-tetramethyldisiloxane and hydrosilylation of poly(l-hydrido-l,3,3,5,5-pentamethyltrisiloxane) with benzophenone (221,222). [Ru3(CO)i2] was found catalytically active in the hydrosilylation of acetophenone with HSi(OEt)3 (222). The same cluster and several ruthenium carbonyl complexes efficiently catalyze the reduction of linear and cyclic amides with trisubstituted silanes to give the corresponding amines (224). Activated diruthenium and triruthenium carbonyls catalyze the hydrosilylation of ketones, as well as that of aldehydes wdth different silanes (225). First-generation Grubbs... 

Hypercoordinate silicates formed by reaction of PMHS with KF, TBAF, or other nucleophiles, can reduce ketones, aldehydes, and esters (eq 12). With proper nucleophile choice, aldehydes can be reduced selectively over ketones, and ketones over esters (eq 13). Halides, nitriles, nitro groups, and olefins survive these conditions, but enones often undergo both 1,2- and 1,4-reductions. PMHS is worse than other silanes for performing enantioselective reductions with chiral fluoride sources, affording alcohols in only 9-36% ee. ... 

Other important molecules that are useful intermediates in the synthesis of natural products are chiral diols. anli-l,2-Diols of type 30 were obtained in good yields (75-85%) and moderate to good diastereoselectivity (76-96% de) by a nickel-catalyzed three-component addition of a-silyloxy aldehydes 27, alkynyl silanes 28a, and reduction with triisopropyl silane (29a) (Scheme 11.11) [31]. The diastereoselectivity of this process could be explained by the Felkin model. Alternatively, a chiral alkynyl derivative can control the outcome of the reaction. Thus, the coupling of optically active, oxazolidinone-derived ynamides, aldehydes, and silane as reducing agent led to the formation of y-siloxyenamide derivatives with diastereoselectivities up to 99% [32]. 

Reductive aldol reactions could also be catalyzed by In(III) salts and Shibata and Baba et al. reported the use of InBr3 (10 mol%) in catalyzing the reaction of a,P-unsaturated ketones with aldehydes using trialkyl silanes as hydride source... 

The Houk group reported a theoretieal study of the Ni-catalyzed alkyne-aldehyde reductive coupling reactions with silane as reducting agent. They showed that the selectivity of the [5+2] cycloaddition is controlled by the anisotropic steric environment of the NHC ligands. 

Enones and enoates undergo 1,2-reduction [115, 191]. Lipshutz et al. reported the effective protection of carbonyl functions by the triisopropylsilyl acyl silane group (TIPS), which allowed the selective conversion of alkenes or alkynes to the corresponding zirconocene complexes [24]. The aldehyde could subsequently be regenerated by desilylation with TBAF [186]. 

Although the titanium-based methods are typically stoichiometric, catalytic turnover was achieved in one isolated example with trialkoxysilane reducing agents with titanocene catalysts (Scheme 28) [74], This example (as part of a broader study of enal cyclizations [74,75]) was indeed the first process to demonstrate catalysis in a silane-based aldehyde/alkyne reductive coupling and provided important guidance in the development of the nickel-catalyzed processes that are generally more tolerant of functionality and broader in scope. 

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