REARRANGEMENT, HYDRIDE ABSTRACTION, AND RETRO ENE REACTIONS OF DICARBONYL(ETA-5-CYCLOPENTADIENYL)IRON VINYLIDENE TRIFLATES

Several (CO)2CpFe (i.e. Fp) eta(1)-polymethylenevinylidene triflates, (CH2)nC = C = Fp+TfO- (3a-e, n = 4-7, 10), were prepared at -78-degrees-C from the corresponding Fp cycloalkylacyls (CH2)nCHC(O)Fp (1a-e, n = 4-7, 10) and triflic anhydride. Solutions of 3c-e in CH2Cl2, when warmed to 25-degrees-C and reacted completely before being diluted with diethyl ether, are converted exclusively to Fp eta(2)-homocycloalkyne triflate, {Fp-[eta(2)-(CH2)n(C = C)]}+TfO- (5c-e, n = 7, 8, 10). This rearrangement, the organometallic analogue of an ''end-to-end'' alkyl shift in a vinyl cation, has not been previously observed in transition-metal vinylidenes. In contrast to 3c-e, solutions of 3a,b (and also 3c at 0-degrees-C) in CH2Cl2, when warmed to 25-degrees-C and diluted with diethyl ether, react with the diluent to yield mixtures of Fp eta(2)-methylenecycloalkane triflates, {Fp-[eta(2)-(CH2)nC = CH2]}+TfO- (9a-c, n = 4-6), and Fp polymethyleneethoxycarbene triflates. {Fp-[eta(1)-C-(OEt)CH(CH2)n]}+TfO- (12a-c, n = 4-6). 9b formed from 1b and triflic anhydride in perdeuteriodiethyl ether contains approximately 1.6 atoms of deuterium per molecule, suggesting that, in the absence of a viable course of rearrangement or a better nucleophile, 3a-c produce 9a-c by abstracting a hydride from diethyl ether.