Formation of Linear Oligomers and Telomers

The linear dimers 89-91 are formed by Ni [32], Co [33], Fe [34] and Pd [35] catalysts. Linear dimers 90 and 91 are produced via the formation of metal M—H, accompanying migration of hydrogen. The formation of 89 is discussed later. The mechanism of the formation of 91 was studied by an experiment using butadiene 92 deuterated at the terminal carbons. In the formation of the branched dimer 91 from the deuterated butadiene 92, catalysed by Co or Fe complexes, insertion of the second butadiene occurs at the substituted side of the 7r-allyl complex 93 to give 94. Finally, the triene 96 is formed from 95 and Fe—H(D) is regenerated. 

Dimerization of isoprene, if carried out regioselectively to give the head-to-tail dimer 97, would be useful for terpene synthesis. So far few reports on successful regioselective dimerization of isoprene have been given, and this remains an unsolved problem. 2,6-Dimethyl-1,3,6-octatriene (97), the head-to-tail dimer, can be prepared with high selectivity using Zr [36], Ni [37,38] and Pd [39] catalysts. However, selective functionalization of this dimer to the terpene alcohol 98 is not easy. 

Several linear cooligomers of butadiene are prepared with alkenes and alkynes. Commercially important 1,4-hexadiene (103) is prepared by the reaction of ethylene and butadiene catalysed by Ni [40], Fe [41] and Rh [42], The experiment carried out using deuterated ethylene (100) supports the mechanism that the insertion of butadiene to M—H forms the 7i-allyl complex 99. Insertion of ethylene (100) to 99 gives 101, and its -elimination affords the cooligomer 102, tetradeuterated at C-1,1,2,6 of 103. 

Functionalized alkenes are used for the cooligomerization. Phenyl-1,4,8-decatriene (104) is obtained by the Ni-catalysed 1 2 addition of styrene and butadiene [42a], Pd catalyst affords the 1 1 adduct [43], Co or Fe catalyst gives the 1 1 adducts 105 and 106 of methyl acrylate and butadiene [44,42a], The 1 1 adducts 107 and 108 are obtained by the Ru-catalyzed coupling of butadiene and acrylamide [45]. Reaction of methyl methacrylate affords the 1 2 adduct 109 with Ni—PhjP catalyst at 0°C, whereas the oligomer 110 is obtained at higher temperature [46], 

Ene-type products are obtained by Co- and Fe-catalysed reaction of dienynes. Cocatalysed cyclization of substrate 111 proceeds smoothly with respect to the diene, acetylene and allylic ether moiety to afford 114. In this cyclization, the 7i-allyl complex 112 is formed by insertion of the diene to Co—H, followed by domino insertions of the triple and double bonds to give 113. The final step is the elimination of the /J-alkoxidc group from 113 to form 114 [47], The six-membered ene-type products 117 and 118 are obtained from the reaction of 115 catalysed by an Fe bipyridyl complex. The reaction seems to involve oxidative cyclization to form 116. Subsequent -elimination and reductive elimination provide 117 and 118. As another possibility, insertion of the diene to Fe—H gives a 7i-allyl complex. Then double bond insertion and -elimination should give 117 and 118 [48], 

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