Reactions of electrophiles with neutral cyclic carbene complexes of iron

Article abstract of DOI:10.1021/om00052a032

Reactions of the two benzannelated ferracyclopentadienes 2 and 3 with two electrophiles - anhydrous protic acid and trimethylsilyl triflate - have been studied. Reaction of 2 with either fluoroboric acid or trifluoromethanesulfonic acid under an atmosphere of CO leads to simple metal-carbon cleavage to give the new carbene complex 6. Similarly, in the presence of CO, 3 gives mostly the metal-alkyl cleavage product 10; but, in this case, a small amount (about 10%) of a new ring-opened product 13 is also formed. In the absence of CO, 13 becomes the predominant product Structures of 6 and 10 were confirmed by independent syntheses. Structure assignment (including stereochemistry) of 13 is based on elemental analysis, NMR spectroscopy, and NOE difference experiments. The position of the deuterium of 13 from reaction of 3 with D⁺ implicates an agostic intermediate (9) that reacts with CO to give 10 and reverts to 8 but does not equilibrate the methyl hydrogens. Treatment of 2 or 3 with trimethylsilyl triflate at -85°C under an atmosphere of CO gives the known carbene complex 24. Two reasonable mechanisms for formation of 24 - one involving a small-ring carbyne - are briefly discussed.