Retro-hydrocyanation

Hydrocyanation of butadiene is more complicated than that of ethene it requires two hydrocyanation steps and several isomers can be observed. The isomers obtained in the first step of the HCN addition to butadiene are shown in Figure 11.3. The addition first leads to compounds 1 and 2, in a 1 2 ratio, but they equilibrate to a favourable 1 9 ratio via the retro-reaction. The retro reaction involves a C-C bond breaking reaction, which is rare, but in this case the intermediate is a Tt-allyl species and a stable, anionic cyanide group. Electron-rich nickel species (Ni-dippe) can cleave aromatic nitrile C-C bonds... 

Meerwein-Ponndorf-Verley-type hydrocyanation was explored in hie presence of catalyhc amounts of DIBAL (Scheme 6.68) [91]. The retro-hydrocyanahon reachon proceeded from acetone-cyanation products, with concomitant cyanahon of aldehyde under equilibrium condihons. 

Since there is great interest in compounds bearing quaternary stereocenters, soluble catalyst 25 was also applied to keto-imines in the presence of in situ generated HCN (Scheme 30.6) [17]. Whereas hydrocyanation adducts of N-allyl protected ketimines were prone to decompose via a retro-Strecker reaction, N-benzylated Strecker adducts 28 were obtained in mostly excellent yields and very high ee s. In general, acetophenone imines 27 were suitable substrates whereas aliphatic ketimines 27 showed a lowered optical purity. Although the benzyl protection group was necessary to obtain stable Strecker adducts 28, transformation of these adducts into amino acids was made more difficult. It could also be shown that... 

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