Lithiation/Electrophilic Quench Sequence of “Benzylic” Position of (η⁵-Methylcyclohexadienyl)Mn(CO)₃ Complexes

Abstract

-2 complexes were prepared and reacted with nucleophiles to provide neutral exo-substituted (η 5 -polymethylcyclohexadienyl)Mn(CO) 3 complexes 3-6. To study the regioselectivity of the deprotonation at a "benzylic" position of (η 5 -methylcyclohexadienyl)Mn(CO) 3 complexes, compounds 3-6 were submitted to a lithiation/electrophilic quench sequence, and functionalized complexes 9-18 were obtained in good yields and with a total regioselectivity. A second sequence gave rise to the formation of unprecedented bifunctionalized (η 5 -1,2,4,5-tetramethylcyclohexadienyl)Mn(CO) 3 and (η 5pentamethylcyclohexadienyl)Mn(CO) 3 complexes 19-22.

A combined experimental–theoretical study on the lithiation/electrophilic quench reaction of benzylic position of (η5-tetramethylcyclohexadienyl)-Mn(CO)3

“…À complex 1, obtained in 90% yield using the ''silver (I) method'' with BrMn(CO) 5 and AgBF 4 À and the corresponding free arene, 10,11 treated with LiAlH 4 afforded the (Z 5 -tetramethylcyclohexadienyl)Mn(CO) 3 complex 7 in 66% yield. Pleasingly, monocrystals of complex 7 were readily obtained by slow evaporation of a diethylether/pentane solution of the complex.…”

“…In both cases, as evidenced by NMR and X-ray analyses, we observed a regioselective deprotonation at the methyl groups C9 or C10 (Scheme 3) substituting the C2 or C4 carbons of the cyclohexadienyl ring eclipsed by a Mn-CO bond by trapping the lithiated product with different electrophiles (ClSiMe 3 , PPh 2 Cl and DMF). 10 Scheme 1 Addition of Nu À to (a) (Z 6 -arene)Cr(CO) 3 Here we report on the synthesis and structure of the (Z 5 -1,2,4,5-tetramethylcyclohexadienyl)Mn(CO) 3 complex 7 bearing two hydrogen atoms at the sp 3 carbon and the study of its deprotonation. The results obtained are then analyzed on a theoretical basis using computations on the lithiation products of compound 7 compared to those of pentamethyl derivative 8 and non-methylated compound 9 (Scheme 4).…”