Nucleophilic addition to the η2-alkyne ligand in [CpFe(CO)2(η2-R-CC-R)]+.: Dependence of the alkenyl product stereochemistry on the basicity of the nucleophile

Reaction of the cationic eta (2)-alkyne iron complex, [Fp( eta (2)-Ph-C drop CPh)]BF4 (1) [Fp = (eta (5)-C5H5)-Fe(CO)(2)], with various O-, N-, and C-nucleophiles afforded products containing the Ph-C= C-Ph linkage. i.e., cis- (3) and trans-alkenyl complexes (4), Fp-C(Ph)=C(Ph)-Nu, and metallacycles, Cp(CO)Fe-C(Ph)=C(Ph)-C(=O)-Nu [coordinated at =O (5) or Nu (6)]. Crystallographic determination of the configuration of the C=C moiety in 3 and 4 and analysis of the product distribution revealed that basic nucleophiles bearing alkyl substituents produced the cis-alkenyl complex (3) and/or the metallacycles (5, 6), whereas less basic nucleophiles bearing aryl substituents afforded the trans-alkenyl complex (4). Correlation between the product distribution and pK(b) values of the nucleophiles leads to the conclusions that (1) products 3, 5, and 6 arise from acyl intermediates formed by initial nucleophilic addition of basic nucleophiles at the CO ligand, (2) trans-alkenyl complexes 4 result from addition of less basic nucleophiles to the eta (2)-alkyne ligand from the exo-side, and (3) the equilibrium of the CO addition governed by the basicity of the nucleophile is the key step of the diversity of the stereochemistry of the alkenyl products. Reactions of 2-hexyne complex, [Fp(eta (2)-Et-C dropC-Et)]BF4 (2), were also examined.