Bis-metallation, Carbometallation and Hydrometallation

The Co reagent 192, prepared by the reaction of Co2(CO)8 with sodium, is reactive, and the acylcobalt complex 193 is formed by the reaction of acyl halides. Insertion of butadiene at the Co-acyl bond generates the 7r-allylcobalt complex 194, from which the acylbutadiene 195 is formed by deprotonation with a base [82]. Based on this reaction, various acyldienes are prepared by Co2(CO)8-catalysed reaction of active alkyl halides, conjugated dienes and CO. The Co-catalysed reaction can be carried out smoothly under phase-transfer conditions. For example, 6-phenyl-3,5-hexadien-2-one (197) was prepared in 86% yield by the reaction of Mel, 1-phenylbutadiene (196) and CO in the presence of cetyltrimethylammonium bromide [83]. 

Conjugated dienes undergo metallation to give the 1,4-adduct 198 and the dimerization-1,8-addition product 199 with main group metal compounds. The reaction proceeds by oxidative addition of main group metal compounds to transition metal complexes. Reactive allylmetal compounds 198 and 199 as useful synthetic intermediates are prepared by this methods. 

Addition of bis(pinacolate)diboron (206) to dienes is catalysed by Pt complexes. The 1 1 adduct 207 is obtained with Pt(Ph3P)4, whereas Pt(dba)2 without phosphine is a very active catalyst and the 1,8-adduct 208 is obtained at room temperature [90], 

Silaboration of diene with the silylborane 209 is also catalysed by Pt complexes to give the 1,4-adduct 211 as an E/Z mixture which reacts with benzaldehyde to afford the homoallyl alcohol 212 having a silyl group [91], However, the silaborative 

As an example of carbometallation, the 1,4-carbosilylation product 218 is obtained by the reaction of dienes, disilanes and acid chlorides of aromatic and a,/i-unsaturatcd acids at 80 °C. The phenylpalladium 216 is formed by the oxidative addition of benzoyl chloride, followed by facile decarbonylation at 80 °C, and reacts with butadiene to generate the benzyl-7i-allylic complex 217. Then, transmetallation with the disilane and reductive elimination afford 4-silyl-2-butenylbenzene 218 [92], Regioselective carbomagnesation of isoprene with allylic magnesium bromide 219 catalysed by Cp2TiCl2 gives 220, which is useful for terpene synthesis [93,94], 

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