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

Abstract

The preparation of the complexes TpMo(NO)(DMAP)(η-PhCF) (5) and TpMo(NO)(DMAP)(η-benzene) (3) is described. The CF 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 η-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.

Experiments and Direct Dynamics Simulations That Probe η2-Arene/Aryl Hydride Equilibria of Tungsten Benzene Complexes

“…The further stabilization of the ground state for the tungsten system raises the barrier for the purported η 2 -arene → sigma-complex transition state that is likely on the path to ring-walk (see Scheme ). For both the molybdenum and rhenium systems mentioned above, any purported C–H insertion would likely result in a face-flip isomerization, which is not observed in the corresponding NOESY data. , Thus, C–H insertion is significantly slower than the ring-walk for both the Mo and Re complexes.…”

“…In contrast, the previously reported MoTp(DMAP)(NO)(η 2 -3,4-trifluorotoluene) complex (DMAP = 4-(dimethylamino)pyridine) is able to interconvert between the analogous coordination diastereomers predominantly through an intrafacial mechanism (ring-slip) that is operative at ambient temperatures. 46 We speculate that this difference in the predominant isomerization pathway between the Group 6 congeners is due to the more π-basic nature of the tungsten fragment. The further stabilization of the ground state for the tungsten system raises the barrier for the purported η 2arene → sigma-complex transition state that is likely on the path to ring-walk (see Scheme 6).…”

“…The spin-transfer exchange observed with the trifluorotoluene system 4 is also consistent with an interfacial isomerization (“face-flip”) mechanism operating through an aryl hydride intermediate. In contrast, the previously reported MoTp­(DMAP)­(NO)­(η 2 -3,4-trifluorotoluene) complex (DMAP = 4-(dimethylamino)­pyridine) is able to interconvert between the analogous coordination diastereomers predominantly through an intrafacial mechanism (ring-slip) that is operative at ambient temperatures . We speculate that this difference in the predominant isomerization pathway between the Group 6 congeners is due to the more π-basic nature of the tungsten fragment.…”

“…For both the molybdenum and rhenium systems mentioned above, any purported C−H insertion would likely result in a face-flip isomerization, which is not observed in the corresponding NOESY data. 41,46 Thus, C−H insertion is significantly slower than the ring-walk for both the Mo and Re complexes.…”

Asymmetric organic synthesis with stoichiometric transition metal complexes

“…In contrast to electron-deficient aromatics (e.g. trifluoromethyl benzene) that stabilize the  complexes by significant metal to ligand -back bonding, 55 selective dearomatization of benzene, proved to be quite challenging. 56 The challenge has been effectively addressed using the chiral-at-tungsten, enantiopure complexes that added sequentially first a proton, and then a nucleophile, across the uncoordinated double bond distal to the phosphine ligand, both from the face opposite to the metal fragment.…”