Tris(pentafluorophenyl)boron-Catalyzed Hydrosilation of Aromatic Aldehydes, Ketones, and Esters

The strong organoborane Lewis acid B(C6F5)3 catalyzes the hydrosilation (using R3SiH) of aromatic and aliphatic carbonyl functions at convenient rates with loadings of 1−4%. For aldehydes and ketones, the product silyl ethers are isolated in 75−96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45−70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quantitative kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concentrations of substrate have an inhibitory effect on the observed reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition experiments. The borane thus must dissociate from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, whic...

Studies on the Mechanism of B(C6F5)3-Catalyzed Hydrosilation of Carbonyl Functions

The bond activation chemistry of B(C6F5)3 and related electron-deficient boranes is currently experiencing a renaissance due to the fascinating development of frustrated Lewis pairs (FLPs). B(C6F5)3's ability to catalytically activate Si–H bonds through η1 coordination opened the door to several unique reduction processes. The ground-breaking finding that the same family of fully or partially fluorinated boron Lewis acids allows for the related H–H bond activation, either alone or as a component of an FLP, brought considerable momentum into the area of transition-metal-free hydrogenation and, likewise, hydrosilylation. This review comprehensively summarises synthetic methods involving borane-catalysed Si–H and H–H bond activation. Systems corresponding to an FLP-type situation are not covered. Aside from the broad manifold of CX bond reductions and CX/C–X defunctionalisations, dehydrogenative (oxidative) Si–H couplings are also included.

/pdf/a-unified-survey-of-si-h-and-h-h-bond-activation-catalysed-1ix9iq1qpj.pdf

A unified survey of Si–H and H–H bond activation catalysed by electron-deficient boranes

In the last decade, an increasing number of useful catalytic reductions of carboxylic acid derivatives with hydrosilanes have been developed. Notably, the combination of an appropriate silane and catalyst enables unprecedented chemoselectivity that is not possible with traditional organometallic hydrides or hydrogenation catalysts. For example, amides and esters can be reduced preferentially in the presence of ketones or even aldehydes. We believe that catalytic hydrosilylations will be used more often in the future in challenging organic syntheses, as the reaction procedures are straightforward, and the reactivity of the silane can be fine-tuned. So far, the synthetic potential of these processes has clearly been underestimated. They even hold promise for industrial applications, as inexpensive and readily available silanes, such as polymethylhydrosiloxane, offer useful possibilities on a larger scale.

Selective reduction of carboxylic acid derivatives by catalytic hydrosilylation.

The primary alcohols 1a−e and ethers 4a−d were effectively reduced to the corresponding hydrocarbons 2 by HSiEt3 in the presence of catalytic amounts of B(C6F5)3. To the best of our knowledge, this is the first example of catalytic use of Lewis acid in the reduction of alcohols and ethers with hydrosilanes. The secondary alkyl ethers 4j,k enabled cleavage and/or reduction under similar reaction conditions to produce either the silyl ethers 3m−n or the corresponding alcohol 5a upon subsequent deprotection with TBAF. It was found that the secondary alcohols 1g−i and tertiary alcohol 1j, as well as the tertiary alkyl ether 4l, did not react with HSiEt3/(B(C6F5)3 reducing reagent at all. The following relative reactivity order of substrates was found:  primary ≫ secondary > tertiary. A plausible mechanism for this nontraditional Lewis acid catalyzed reaction is proposed.

A Novel B(C6F5)3-Catalyzed Reduction of Alcohols and Cleavage of Aryl and Alkyl Ethers with Hydrosilanes†

Reduction of aldehydes and ketones to alcohols and hydrocarbons through use of the organosilane-boron trifluoride system

A convenient one-step synthesis of hydrocarbons from alcohols through use of the organosilane-boron trifluoride reducing system

Asymmetric reductions of carbocations by chiral organosilicon hydrides. The stereochemical nature of the carbocation captured

Additions and Corrections - Asymmetric Reductions of Carbocations by Chiral Organosilicon Hydrides. The Stereochemical Nature of the Carbocation Captured

Merwyn G. Adlington

Papers

Reduction of aldehydes and ketones to alcohols and hydrocarbons through use of the organosilane-boron trifluoride system

A convenient one-step synthesis of hydrocarbons from alcohols through use of the organosilane-boron trifluoride reducing system

Asymmetric reductions of carbocations by chiral organosilicon hydrides. The stereochemical nature of the carbocation captured

Additions and Corrections - Asymmetric Reductions of Carbocations by Chiral Organosilicon Hydrides. The Stereochemical Nature of the Carbocation Captured

Pyrolysis of N,N,N-trimethyladamantylammonium hydroxides. Unusual stevens rearrangements