Cross-Conjugated Triene Formation

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. 

Evidence for this mechanism includes (1) an experiment showing a 4.5% nOe between and of the triene in Eq. (14), validating the -stereochemistry of the exo-cyclic olefin and (2) preparation of a hexadeuterated allenyne that was subjected to the reaction conditions which gave complete transfer of a deuterium atom to the exocyclic double bond of the triene (Eq. 15). 

Moreover, in Zhang s systems cycloisomerization of an unprotected secondary amide in the enyne Alder-ene reachon was not possible. 

The allylic sulfonyl allene 35, possessing unsymmetrical substitution at the distal double bond of the allene, was examined. Since there are two possible sites for y5-hy-dride eliminahon, a variety of isomers are possible. Treatment of allene 35 with [Rh(CO)2Cl]2 gave a 3 5 1 raho of the three possible products 36, 37, and 38, which translates into an 8 1 constitutional group selectivity in the y5-hydride elimination step ((364-37) 38) and an T/Z-selechvity of 1 2 (Tab. 8.8, entry 1). 

The ratio of the isomeric trienes (36, 37, and 38) is governed by the nature of the rhodium catalyst. For example, when the catalyst was changed to a cationic rhodium(I) species ([Rh(COD)Cl]2/AgBF4), the /Z-selectivity reversed and gave exclusively the H-isomer, while the constitutional group selechvity was dramatically decreased to a 1 1 ratio (entry 3). Furthermore, lower reachon temperatures and changing the catalyst to a cahonic iridium(I) species furnished the -isomer 36 selectively (entries 4—6). Clearly, additional work is required to increase and understand the selechvity issues [33]. 

---

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)). 

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. 

---