Cobalt‐Catalyzed Asymmetric Hydrovinylation

Abstract

,3-dienes · enantioselectivity · homogeneous catalysis · hydrovinylationThe transition-metal-catalyzed codimerization of 1,3-dienes with alkenes, also called hydrovinylation, offers great potential for practical synthetic applications. The hydrovinylation reaction was first reported by Alderson et al. [1] in 1965. They used rhodium and ruthenium salts under high ethene pressure with a range of substrates. Since then, a number of other metals such as iron, cobalt, nickel, and palladium have been used.[2] Most studies have focused on nickel and palladium. Styrene has been used as a benchmark substrate in many studies, especially for asymmetric variants of the reaction, which until very recently were almost exclusively carried out with nickel and palladium catalysts.[2]The asymmetric nickel-catalyzed hydrovinylation reaction was pioneered by Wilke, Bogdanovic, and co-workers. In these early studies 1,3-cyclooctadiene, norbornene, and norbornadiene were codimerized with ethene by using [{(h 3 -allyl)NiCl} 2 ]/Et 3 Al 2 Cl 3 in combination with chiral monodentate phosphines derived from monoterpenes.[3] Enantioselectivity of up to 70 % ee was reached with cyclooctadiene. A real breakthrough was achieved by Wilke with the introduction of the azaphospholene ligand (R,R)-1, which gives rise to high ee values combined with high reaction rates for a range of vinylarene substrates. Styrene was converted into (R)-3-phenyl-1-butene in up to 95 % ee at À72 8C with a turnover number of 1650 (Scheme 1).[4] It was noted early on that the counterion used in the active cationic nickel complexes has a profound influence on the performance of the catalyst. The most active and enantioselective nickel catalysts known to date are based on [{(allyl)NiX} 2 ] (X = Cl, Br), activated by NaBAr F (BAr F = tetrakis-[3,5-bis(trifluoromethyl)phenyl]borate), in combination with monodentate phosphoramidite ligands. These systems were introduced by Leitner and coworkers [5] and further optimized by Smith and RajanBabu, [6] which gave rise to ee values of > 95 % for a range of substrates. The highly active nickel catalysts have a high tendency for double-bond isomerization, thus lowering the selectivity of the chiral primary product, especially at high conversion. For this reason, reactions are typically carried out at low temperature. The corresponding cationic palladium catalysts are usually derived from the dimeric [{(allyl)PdX} 2 ] complexes by halide abstraction with a silver salt of a weakly coordinating anion. Those systems show a much lower activity than the nickel catalysts. This fact was used to advantage in the first example of an asymmetric palladium-catalyzed hydrovinylation, since the isomerization of the double bond in the chiral codimers occurred only after full conversion of the starting alkene. This led to high chemoselectivity also at room temperature. The application of a bulky, monodentate Pstereogenic ligand led to 3-phenyl-1-butene being obtained in up to 86 % ee.[7] Bulky phosphine ligands with planar chirality also gave hig...

Low‐Pressure Cobalt‐Catalyzed Enantioselective Hydrovinylation of Vinylarenes

“…Until today, nickel is the most important metal used in the asymmetric hydrovinylation, and especially the groups of RajanBabu [7] and Leitner [8] have demonstrated that high chemo‐ and enantioselectivities (≥90 % ee ) can be achieved, also in synthetically relevant cases, [9] employing chiral phosphoramidites as (monodentate) ligands [10] . However, the Ni‐catalyzed hydrovinylation methodology has found little application in other (nonspecialized) laboratories, probably because of the demanding experimental requirements associated to the high air sensitivity of the precatalysts (which have to be freshly prepared in a glove box from [Ni(COD) 2 ] via [{Ni(allyl)Br} 2 ] and subsequent ligand exchange in the presence of NaBARF).…”

Enantioselective Cobalt-Catalyzed Transformations

“…The transition-metal-catalyzed codimerization of ethene with alkenes, also called the hydrovinylation reaction, is the addition of the elements of ethene (vinyl and hydrogen) across the double bond of a second alkene, offering a great potential for practical synthetic applications . Only moderate success has been reported in the first cobalt-catalyzed hydrovinylation reactions, and most of the time, this process is catalyzed by nickel or palladium complexes, which are limited to the use of monodentate ligands . Inspired by the work of Hilt et al reported in 2001 dealing with the cobalt-catalyzed codimerization of a range of 1,3-dienes and alkenes, Vogt et al have explored the asymmetric cobalt-catalyzed hydrovinylation of styrene with ethene, providing the corresponding chiral 3-phenyl-1-butene .…”

Cobalt-Catalysed Bond Formation Reactions; Part 2

“…In this respect, significant advances in scope and application have been reported by RajanBabu and Vogt for the asymmetric 1,4-HV of 1,3-dienes (Scheme 22). [52][53][54] Acyclic 1,2-disubstituted 1,3-dienes as well as vinyl-substituted cyclic alkenes were used in the 1,4-HV with ethene at moderate pressures to afford the desired 1,4-dienes. A prototype example is the synthesis of the chiral 1,4-diene 30, which was formed in excellent yield, an outstanding regioselectivity of >99:1 and an excellent enantioselectivity.…”