Cyclization versus Pd−H Elimination−Readdition: Skeletal Rearrangement of the Products of Pd−C6F5 Addition to 1,4-Pentadienes

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