Cross-conjugated trienes, formation

Malacria and co-workers76 were the first to report the transition metal-catalyzed intramolecular cycloisomerization of allenynes in 1996. The cobalt-mediated process was presumed to proceed via a 7r-allyl intermediate (111, Scheme 22) following C-H activation. Alkyne insertion and reductive elimination give cross-conjugated triene 112 cobalt-catalyzed olefin isomerization of the Alder-ene product is presumed to be the mechanism by which 113 is formed. While exploring the cobalt(i)-catalyzed synthesis of steroidal skeletons, Malacria and co-workers77 observed the formation of Alder-ene product 115 from cis-114 (Equation (74)) in contrast, trans-114 underwent [2 + 2 + 2]-cyclization under identical conditions to form 116 (Equation (75)). 

Brummond [28] was the first to illustrate that cross-conjugated trienes could be obtained via an allenic Alder-ene reaction catalyzed by [Rh(CO)2Cl]2 (Eq. 14). Selective formation of the cross-conjugated triene was enabled by a selective cycloisomerization reaction occurring with the distal double bond of the aUene. Typically directing groups on the allene, differential substitution of the aUene termini, or intramolecularization are required for constitutional group selectivity. However, rhodium(f), unlike other transition metals examined, facihtated selective cyclization with the distal double bond of the allene in nearly aU the cases examined. 

Dimerization of alkylallene 160 proceeded regioseleetively at each C-2 carbon, and the cross-conjugated triene 161 was obtained in high yield using P(< -Tol)3 as a ligand and p-nitrophenol as an additive [53]. In the presenee of formic acid, reductive dimerization at C-2 carbons occurred to give the conjugated dienes 164 and 165. The dimerization is explained by the formation of the palladacycle 163, formed by oxidative cyclization, as an intermediate [54]. 

A variety of allenynamides undergo cycloisomerization reactions in the presence of silver triflate leading to the formation of A-containing heterocycles incorporating cross-conjugated trienes (Scheme 160). ... 

Quantum yield for the formation of the cross-conjugated trienes 33 or 35. 

When perflluoro(2-methylpropene) reacts with tributylphosphane in acetonitrile at — 30"C, the reaction does not stop with the formation of fluoro-2 -phosphanes, and, as a result of a series of transformations, perfluorodiene 3 and conjugated cross-triene 4 are formed. When the reaction is carried out with triphenylphosphane in acetonitrile without cooling triene 4 is obtained in 67 % yield.On the other hand, the reaction of perfluoro(2-methylpropcnc) with tributylphosphane in diethyl ether at — 70 C leads to the corresponding fluoro-A -phos-phane treating this compound with potassium fluoride dihydrate in tetraglyme gives 1,1,1,3-tetrafluoro-2-(trifluoromethyl)hept-3-ene in 75% yield,... 

Poly(vinyl acetate) (3.38) undergoes simultaneous chain scission and cross-linking when UV irradiated (254 nm). At the same time its absorption spectrum (Fig. 3.23) shows the formation of short sequences of conjugated double bonds, i.e. trienes. 

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