Molybdenum(0) Dihapto-Coordination of Benzene and Trifluorotoluene: The Stabilizing and Chemo-Directing Influence of a CF3 Group.

The preparation of the complexes TpMo(NO)(DMAP)(η2-PhCF3) (5) and TpMo(NO)(DMAP)(η2-benzene) (3) is described. The CF3 group is found to stabilize the metal-arene bond strength in 5 by roughly 3 kcal/mol compared to that in 3, allowing the large-scale synthesis and isolation of the trifluorotoluene analogue (5, 37 g, 70%). When a benzene solution of 5 is allowed to stand, clean conversion to the benzene analogue 3 occurs, and this complex may be precipitated from solution upon the addition of pentane and isolated. The trifluorotoluene complex is shown to be a synthetic precursor to functionalized cyclohexadienes: In solution, it selectively protonates at the ortho position, and the resulting η2-arenium species undergoes reactions with nucleophiles at the adjacent meta carbon. Thus, reactions of 5, triflic acid, and either N-methylpyrrole or 1-methoxy-2-methyl-1-(trimethylsilyloxy)-1-propene result in 5-substituted-1,3-cyclohexadienes after removal of the metal.