Asymmetric Cross-coupling Of Non-activated Secondary Alkyl Halides.
Published 2008 · Medicine, Chemistry
Modern transition-metal-catalyzed cross-coupling reactions have altered organic synthesis enormously, and the coupling of aryl and alkenyl electrophiles has become a routine transformation in natural product and fine-chemical synthesis. For a long time, equivalent reactions of alkyl electrophiles, especially those with non-activated b hydrogen atoms, remained elusive. The difficulties encountered in attempts to carry out such reactions have been ascribed to a slow oxidative-addition step and undesired side reactions, such as b-hydride elimination. Palladium complexes of electron-rich phosphines, in particular P(c-Hex)3 and PtBu2Me, were eventually found to be competent catalysts for the Suzuki–Miyaura coupling and related reactions of primary alkyl halides under mild conditions. 3] Even more desirable and challenging is the coupling of secondary alkyl halides: challenging, as the increased steric demand and electron richness of these substrates leads to a reduced rate of oxidative addition; desirable, as a new stereogenic center is often formed. Whereas palladiumcatalyzed methods are limited to primary alkyl halide substrates, cheaper nickel complexes were found to be uniquely suited to the catalysis of the cross-coupling of secondary alkyl halides. 5] Biand tridentate nitrogen ligands (Scheme 1), many of which are commercially available, were found to be key to success of these nickel-catalyzed reactions. These chelate ligands favor the cis orientation of the coupling partners at the nickel center. Thus, the rate of reductive elimination is increased relative to that of undesired b-hydride elimination. Recently, Fu and co-workers developed a number of impressive highly enantioselective nickelcatalyzed Negishi, Hiyama, and Suzuki–Miyaura crosscoupling reactions of secondary alkyl halides. The last of these methods is the only asymmetric cross-coupling described to date in which non-activated secondary alkyl halides can be used and thus paves the way for exciting future developments. In 2003, Zhou and Fu reported the high catalytic activity of nickel–pybox complexes formed in situ in the Negishi coupling of non-activated secondary alkyl bromides and iodides with organozinc reagents at ambient temperature. Their study served as a starting point for the development of an impressive series of asymmetric Negishi coupling reactions of secondary alkyl halides. Activated racemic alkyl halides, such as a-bromoamides, benzylic chlorides and bromides, and allylic chlorides, were coupled successfully as electrophiles with alkyl zinc reagents with high enantioselectivity (Schemes 2 and 3). The reactions were even carried out below ambient temperature. The reaction conditions were optimized carefully, and it was found that the optimal combination of a solvent, nickel precursor, and additive differed for each class of substrate. Interestingly, in each of these studies, nickel–pybox complexes formed in situ were found to be the most active and most selective catalysts; furthermore, donor solvents, such as 1,3-dimethyl-2-imidazolidinone (DMI), N,N-dimethylacetamide (DMA), and N,Ndimethylformamide (DMF), were found to be crucial. The exact mechanism of these transformations still remains to be elucidated. However, in view of the high levels of enantioScheme 1. Ligands employed in nickel-catalyzed cross-coupling reactions of secondary alkyl halides (Bn = benzyl, pybox= bis(oxazolinyl)pyridine).