Cyclization versus Pd-H elimination-readdition: Skeletal rearrangement of the products of Pd-C6F5 addition to 1,4-pentadienes

The reactions of [Pd(C6F5)Br(CH3CN)(2)] with 1,4-pentadiene or 3-methyl-1,4-pentadiene at low temperature afford, after insertion of one double bond into the Pd-C6F5 bond, two types of eta(1)-eta(2)-pentenylpalladium derivatives: the 4.5-membered palladacycles [Pd-2(mu-Br)(2)(5-Pf-3-R-1,2,4-eta(1)-eta(2)-pentenyl)(2)] (Pf = C6F5; R = H (2a), Me (2b)) and the 5.5-membered metallacycles [Pd-2(mu-Br)(2)(5-Pf-3-R-1,2,5-eta(1)-eta(2)-pentenyl)(2)] (R = H (3a), Me (3b)). Both enyls isomerize to eta(3)-allylic derivatives following two competitive pathways, i.e. (i) Pd migration or (ii) cyclopropane formation and reopening, which lead to the isomeric eta(3)-allylpalladium complexes [Pd-2(mu-Br)(2)(5-Pf-3-R-1-3-eta(3)-pentenyl)(2)] (R = H (5a), Me (5b)) and [Pd-2(mu-Br)(2)(4-Pf-3-(CH(2)R)-1-3-eta(3)-butenyl)(2)] (R = H (6a), Me (6b)), respectively. After the cyclopropane formation-reopening process, [Pd-2(mu-Br)(2)(3-CH(2)Pf-1,2,5-eta(1)-eta(2)-pentenyl)(2)] (8) is also formed which isomerizes to 6b. The reaction of a mixture of the eta(1)-eta(2)-pentenyl complexes at low temperature with CO/NaOMe gives methyl ester derivatives. Again, the formation of the major ester CH2=CH(CH(2)Pf)-CHRCOOMe involves cyclopropane formation-reopening in the enyl derivative 2. An unusual acyl complex, [Pd(mu-Br)(eta(1)-eta(2)-3-CH(2)Pf-1-oxo-4-pentenyl)](2) (15) containing a chelating eta(2) eta(1)-olefin acyl moiety has been characterized.