Dimerization allyl cyanide

One potential problem in the reactions of stabilized allylic or propargylic carb-anions is the dimerization of the starting material if the carbanions are not formed stoichiometrically. Alkenes substituted with electron-withdrawing groups are good Michael acceptors, to which nucleophiles will undergo conjugate addition. For instance, the Baylis-Hillman reaction of allyl cyanide with benzaldehyde requires careful optimization of the reaction conditions to avoid dimerization of the nitrile (Scheme 5.12). This problem is related to a common side reaction of Michael additions reaction of the product with the Michael acceptor (Scheme 10.21). 

Butenenitrile (allyl cyanide) is isomerized by the CU2O—CNC complex to 2-butenenitrile and its dimer (248). The key intermediate is... 

The low-temperature reaction of allyl cyanide with the Ni hydrido dimer [NiH(dippe)]2 gives the Ni(0) complex Ni(77 -CH2=CHCH2CN)(dippe), which rearranges to the nickel(ii) allyl(cyano) complex 71 (via C-CN bond activation) and the olefin-isomerization product Ni(77 -CH3CH=GHGN)(dippe) (via C-H bond activation).The kinetics of these bond-activation processes has been studied and the role of Lewis acids in these competitive reactions has been clarified. 

Structural characterization of these species has revealed that complex 69 adopts a trigonal-bipyramidal geometry in which the methylallyl moiety occupies the apical position (Ni-GN 189 pm), whereas complex 71 adopts a square-pyramidal structure with the cyanide ligand at the apical position at a relatively long distance from the Ni center (Ni-CN 199 pm). The latter structure is similar to the bromo analog NiBr(allyl)(dippe), prepared from the reaction of the nickel(i) hydrido dimer [NiH(dippe)]2 and allyl bromide. The involvement of allyl cyano species such as 69 and 71 in the catalytic hydrocyanation of butadiene is supported by the following observations (i) complex 69 catalyzes the isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile (ca. 100 turnovers at 100 °G), (ii) complex 71 decomposes slowly to give Ni(0) complexes of cis- and // 77i--crotonitrile (Scheme 20). 

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