Methylcarbene complexes L(n)M=C(OEt)CH3 1 [L(n)M=(CO)5Cr, (CO)5W] react with tropylium tetrafluoroborate (2a) or coordinated olefins of [C6H7Fe(CO)3]BF4 (2c), [C7H9Fe(CO)3]BF4 (2d), and [C7H7Fe2(CO)6]BF4 (2d) in the presence of Et3N to give complexes L(n)M=C(OEt)CH2R 3a-e and L(n)M=C(OEt)CHR2 7a-d by substitution of one or two alpha-hydrogen atoms. The trimetallic WFe2 carbene complexes 7c, d derived from lb with 2c, d form three stereoisomers each, which are fully characterized spectroscopically, (R*,R*,S*)-7c additionally also by an X-ray structure analysis. Ligand elimination from 3 and 7 may involve alpha- and/or beta-hydrogen transfer reactions: Thus enol ethers (Z/E)-6 are eliminated from alpha-monosubstituted methylcarbene complexes 3a, c, d in yields of 62-90% by a pyridine-induced transfer of an alpha-hydrogen to the carbene carbon atom; with increasing steric demand of R the elimination of the (Z) isomer (Z)-6 from 3 is favoured over the formation of the (E) isomer (E)-6. The enol ether 10 becomes a minor side product in the pyridine-induced ligand elimination from alpha-disubstituted complexes 7a, b. In these cases the spirocyclopropane 8 is formed as the main product (60-82%) besides the 4,5-homotropylidene 9 by a conformationally directed beta-hydrogen transfer reaction.
