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Advances in transition metal (Pd, Ni, Fe)-catalyzed cross-coupling reactions using alkyl-organometallics as reaction partners.

Sterically encumbered Lewis acid and Lewis base combinations do not undergo the ubiquitous neutralization reaction to form "classical" Lewis acid/Lewis base adducts. Rather, both the unquenched Lewis acidity and basicity of such sterically "frustrated Lewis pairs (FLPs)" is available to carry out unusual reactions. Typical examples of frustrated Lewis pairs are inter- or intramolecular combinations of bulky phosphines or amines with strongly electrophilic RB(C(6)F(5))(2) components. Many examples of such frustrated Lewis pairs are able to cleave dihydrogen heterolytically. The resulting H(+)/H(-) pairs (stabilized for example, in the form of the respective phosphonium cation/hydridoborate anion salts) serve as active metal-free catalysts for the hydrogenation of, for example, bulky imines, enamines, or enol ethers. Frustrated Lewis pairs also react with alkenes, aldehydes, and a variety of other small molecules, including carbon dioxide, in cooperative three-component reactions, offering new strategies for synthetic chemistry.

Frustrated Lewis pairs: metal-free hydrogen activation and more.

Chem. Pharm. Bull.(オピニオン)

Alan Ford,† Hugues Miel, Aoife Ring,† Catherine N. Slattery,† Anita R. Maguire,*,†,‡ and M. Anthony McKervey* †Department of Chemistry and ‡School of Pharmacy, Analytical and Biological Chemistry Research Facility, Synthesis and Solid State Pharmaceutical Centre, University College Cork, Cork, Ireland Almac Discovery Ltd., David Keir Building, Stranmillis Road, Belfast BT9 5AG, United Kingdom Almac Sciences Ltd., Almac House, 20 Seagoe Industrial Estate, Craigavon BT63 5QD, United Kingdom

Modern Organic Synthesis with α-Diazocarbonyl Compounds

Transition metal chemistry of cyclopropenes and cyclopropanes.

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†

A convenient and highly efficient method for the Lewis acid-catalyzed trans-selective hydrosilylation of alkenes has been developed. The mechanism of this novel protocol operates via direct additio...

Highly Efficient B(C6F5)3-Catalyzed Hydrosilylation of Olefins†

2,2-Disubstituted cyclopropyl boronates have been synthesized with high degrees of diastereo- and enantioselectivity via the rhodium-catalyzed asymmetric hydroboration of 3,3-disubstituted cyclopropenes. A strong directing effect of ester and alkoxymethyl substituents has been demonstrated. The directing effect was found to be necessary in achieving high degrees of enantiomeric induction. Selected cyclopropylboronic derivatives were successfully employed in the Suzuki cross-coupling reaction to produce the corresponding optically active aryl- and vinylcyclopropanes in good yields.

Catalytic Enantioselective Hydroboration of Cyclopropenes

The aliphatic carboxylic group was efficiently reduced to the methyl group by HSiEt3 in the presence of catalytic amounts of B(C6F5)3. To the best of our knowledge, this is the first example of a direct exhaustive reduction of aliphatic carboxylic function. Aliphatic aldehydes, acyl chlorides, anhydrides, and esters also underwent complete reduction under similar reaction conditions. Aromatic carboxylic acids, as well as other carbonyl functional equivalents, underwent smooth partial reduction to the corresponding TES-protected benzylic alcohols. It was shown that, unlike the reduction of aliphatic substrates, the exhaustive reduction of aromatic substrates was not straightforward:  a concurrent Friedel−Crafts-like alkylation process competed with the reduction yielding trace to notable amounts of dimeric products, thus decreasing the overall selectivity of the reduction process.

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Michael Rubin

Papers

Transition metal chemistry of cyclopropenes and cyclopropanes.

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

Highly Efficient B(C6F5)3-Catalyzed Hydrosilylation of Olefins†

Catalytic Enantioselective Hydroboration of Cyclopropenes

A direct reduction of aliphatic aldehyde, acyl chloride, ester, and carboxylic functions into a methyl group.