Nickel carbometalation

Trost reported the synthesis of 1,4-dienes with ruthenium catalysis through regioselective carbometallation of alkynes with alkenes.51 Di- and trisubstituted olefins can also be obtained with arylboronic acids through an intermolecular process under rhodium,30 52 55 nickel,56 and palladium catalysis.57 Recently, Larock has reported an efficient palladium-catalyzed route for the preparation of tetrasubstituted olefins.58,59... 

Two catalytic cycles are proposed to explain the difference in selectivity. In both cases, catalytic cycle is initiated by the oxidative addition of an alkynylstannane to nickel(O) species, leading to the formation of alkynylnickel(ll) complex 77 (Scheme 24).92 Then, an allene is inserted into the nickel(ll) complex in a manner which avoids steric repulsion with the butyl group to afford the anti-ir-a y complex 80. The carbometallation of the terminal alkyne can take place at the non-substituted allylic carbon of the corresponding syn-Ti-a y complex 78. The stereoselectivity is determined by the relative rate of the two possible insertion modes which depend on the ligand used. A bidentate... 

Radical cyclization of polyfunctional 5-hexenyl halides mediated by Et2Zn and catalyzed by nickel or palladium salts has been demonstrated to produce stereoselectively polyfunctional 5-membered carbo- and heterocycles [56, 57]. Based on this strategy a formal synthesis of methylenolactocin (11) was achieved (Scheme 20). The acetal 130, readily being built up by asymmetric alkylation of aldehyde 127 followed by reaction with butyl vinyl ether and NBS, served as the key intermediate for the construction of the lactone ring. Nickel(II)-catalyzed carbometallation was initiated with diethylzinc to yield exclusively the frans-disubstituted lactol 132, which could be oxidized directly by air to 134. Final oxidation under more forcing conditions then yielded the lactone (-)-75 as a known intermediate in the synthesis of (-)-methylenolactocin (11) [47aj. 

Diyne cyclization/hydrosilylation catalyzed by 4 was proposed to occur via a mechanism analogous to that proposed for nickel-catalyzed diyne cyclization/hydrosilylation (Scheme 4). It was worth noting that experimental evidence pointed to a silane-promoted reductive elimination pathway. In particular, reaction of dimethyl dipropargylmalonate with HSiMc2Et (3 equiv.) catalyzed by 4 led to predominant formation of the disilylated uncyclized compound 5 in 51% yield, whereas slow addition of HSiMe2Et to a mixture of the diyne and 4 led to predominant formation of silylated 1,2-dialkylidene cyclopentane 6 (Scheme 5). This and related observations were consistent with a mechanism involving silane-promoted G-H reductive elimination from alkenylrhodium hydride species Id to form silylated uncyclized products in competition with intramolecular carbometallation of Id to form cyclization/hydrosilylation products (Scheme 4). Silane-promoted reductive elimination could occur either via an oxidative addition/reductive elimination sequence involving an Rh(v) intermediate, or via a cr-bond metathesis pathway. 

Mori has reported the nickel-catalyzed cyclization/hydrosilylation of dienals to form protected alkenylcycloalk-anols." For example, reaction of 4-benzyloxymethyl-5,7-octadienal 48a and triethylsilane catalyzed by a 1 2 mixture of Ni(GOD)2 and PPhs in toluene at room temperature gave the silyloxycyclopentane 49a in 70% yield with exclusive formation of the m,//7 //i -diastereomer (Scheme 14). In a similar manner, the 6,8-nonadienal 48b underwent nickel-catalyzed reaction to form silyloxycyclohexane 49b in 71% yield with exclusive formation of the // /i ,// /i -diastereomer, and the 7,9-decadienal 48c underwent reaction to form silyloxycycloheptane 49c in 66% yield with undetermined stereochemistry (Scheme 14). On the basis of related stoichiometric experiments, Mori proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of dienals involving initial insertion of the diene moiety into the Ni-H bond of a silylnickel hydride complex to form the (7r-allyl)nickel silyl complex li (Scheme 15). Intramolecular carbometallation followed by O-Si reductive elimination and H-Si oxidative addition would release the silyloxycycloalkane with regeneration of the active silylnickel hydride catalyst. 

Carbometalation of silylalkynes. In the presence of this nickel-aluminum catalyst, methylmagnesium bromide adds to 1-trimethylsilyI-l-octyne (1) to give the cis-addition product 2, which isomerizes slowly to 3, The use of ethylmagnesium bromide results in reduction mainly to cis-1 -trimethylsilyl-1 -octene. The products 2 and 3, as expected, react with a variety of electrophiles to give di- and trisubstituted... 

For two representative reviews see Negishi, E. Bimetalhc Catalytic Systems Containing Titanium, Zirconium, Nickel and Palladium. Their Apphcations to Selective Organic Syntheses Pure Appl. Chem. 1981, 53,2333-2356. Fallis, A. G. Forgione, P. Metal Mediated Carbometallation of Alkynes and Alkenes Containing Adjacent Heteroatoms Tetrahedron 2001, 57, 5899-5913. 

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