In alkene-directed, nickel-catalyzed coupling reactions of 1,3-enynes with aldehydes and epoxides, the conjugated alkene dramatically enhances reactivity and uniformly directs regioselectivity, independent of the nature of the other alkyne substituent (aryl, alkyl (1°, 2°, 3°)) or the degree of alkene substitution (mono-, di-, tri-, and tetrasubstituted). These observations are best explained by a temporary interaction between the alkene and the transition metal center during the regioselectivity-determining step. The highly substituted 1,3-diene products are useful in organic synthesis and, in conjunction with a Rh-catalyzed, site-selective hydrogenation, afford allylic and homoallylic alcohols that previously could not be prepared in high regioselectivity (or at all) with related Ni-catalyzed alkyne coupling reactions.
Enantioselective nickel/phosphine-catalyzed reductive coupling of alkynes and aldehydes provides rapid access to synthetically useful allylic alcohols with high enantiomeric excess. A related reaction involving epoxides is enantiospecific, transforming alkynes and chiral terminal epoxides into enantiomerically pure homoallylic alcohols containing a trisubstituted olefin of defined geometry. A gram-scale example of each of these processes is described.
Nickel-catalyzed, intramolecular and intermolecular reductive coupling of alkynes and epoxides affords synthetically useful homoallylic alcohols of defined alkene geometry. Very high regioselectivity is generally observed, and cyclizations proceed with complete selectivity for endo epoxide opening. This catalytic reaction represents the first use of a non-π-based electrophile in a growing class of nickel-catalyzed, multicomponent coupling reactions, and is the first catalytic method of reductive coupling of alkynes and epoxides that is effective for both intermolecular and intramolecular cases, and mechanistically distinct from these, possibly involving a nickella(II)oxetane.
An unusual degree of functional group compatibility for imine addition reactions was observed in the assembly of allylic amines from alkynes, imines, and organoboron reagents (boronic acids or boranes) by using a catalyst derived from [Ni(cod)2] and (c-C5H9)3P—this catalytic three-component process (see scheme) is tolerant of ketones, esters, and hydroxylic solvents.
A highly enantioselective method for catalytic reductive coupling of alkynes and aldehydes is described. Allylic alcohols are afforded with complete E/Z selectivity, generally 95:5 regioselectivity, and in up to 96% ee. In conjunction with ozonolysis, this process is complementary to existing methods of enantioselective α-hydroxy ketone synthesis.
Eight P-chiral monodentate ferrocenyl phosphines (1a−h) were prepared in high enantiomeric excess (95% ee in most cases) by way of an ephedrine-based oxazaphospholidine borane complex. Primary alkyl, secondary alkyl, and substituted aromatic substituents were successfully introduced at the phosphorus center, along with ferrocenyl and phenyl groups, generating phosphines of the general structure FcP(Ph)(R) (Fc = ferrocenyl, R = aryl, alkyl). The synthetic route employed provides facile access to a previously undeveloped class of chiral monophosphines. These compounds were evaluated as ligands in asymmetric catalytic reductive coupling of alkynes and aldehydes and were found to provide the desired chiral allylic alcohols with good regioselectivity and ee in many cases and with complete (E)-selectivity (98:2) in all cases.
Alkynes (internal and terminal) and aldehydes (aromatic and aliphatic) are reductively coupled in a single catalytic reaction to yield di- and trisubstituted allylic alcohols with high stereoselectivity and regioselectivity. In most cases, a 1:1 ratio of alkyne to aldehyde is sufficient for efficient coupling. The yield and regioselectivity are strongly dependent on the phosphine ligand, but the allylic alcohols formed are invariably the products of cis addition to the alkyne.