Competing Methods for the Cyclization of Dienes

The initial H transfer will occur to the a-substituted acrylate, giving radical 13. 

The 6-endo-trig cyclization of 13 gives tertiary radical 14, which forms the decaUn 15 by intramolecular addition to its E double bond. 

Initial coordination to the nitrogen of the heterocycle guides insertion into the alkenyl C-H bond to form the transition metal hydride. Insertion into the pendant olefin gives the metallobicycle, which reductively ehminates to regenerate the catalytic species and form the cycloalkane. 

Rh(I) catalysts under H2 effect the reductive cychzation of diynes and enynes [119]. In the example below (1.39), Rh(C0D)20Tf (COD = 1,5-cyclooctadiene) is converted by H2 gas to a Rh hydride, which is thought to undergo oxidative cycliza-tion to produce a rhodacyclopentene intermediate. Cleavage of the Rh-C bonds by reductive elimination to form a vinyl rhodium species, and hydrogenolysis of that intermediate, forms the Rh hydride and the cychzed product. 

Steric as well as electronic factors affect the rate at which H is transferred between a transition metal and the carbon of a double bond. However, the weak M-H bonds of the first-row metals, particularly vanadium, make them uniquely effective in this regard. 

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