A Brief Computational Study of Decamethyldizincocene Formation via Diethylzinc and Decamethylzincocene

Abstract

We have used density functional theory and ab initio methods to study different mechanistic possibilities for the formation of decamethyldizincocene from the reaction between decamethylzincocene and diethylzinc. Our results suggest that decamethyldizincocene could form from the combination of two pentamethylcyclopentadienylzinc radicals. More importantly, our data show that homolytic dissociation of decamethylzincocene into pentamethylcyclopentadienylzinc and pentamethylcyclopentadienyl radicals is 6.7 kcal/mol less costly than the analogous dissociation of zincocene. If such an energy difference is coupled with the fact that the activation barrier to form the half-sandwich product pentamethylcyclopentadienylethylzinc is 11.8 kcal/mol more costly than to form cyclopentadienylethylzinc, we can rationalize why dizincocene does not form experimentally.

Zincocene and Dizincocene N‐Heterocyclic Carbene Complexes and Catalytic Hydrogenation of Imines and Ketones

“…The formation of 2 is commonly believed to proceed by dimerization of [ZnCp*] ⋅ radicals. This view was supported by the detailed work of Carmona and co‐workers,20d reactivity studies of the hydride complexes [ArMH] 2 (M=Zn, Cd; Ar=C 6 H 3 ‐2,6‐(C 6 H 2 ‐2,4,6‐ i Pr 3 ) 2 , C 6 H 2 ‐2,6‐(C 6 H 2 ‐2,4,6‐ i Pr 3 ) 2 ‐4‐SiMe 3 ) reported by Power and co‐workers25 and theoretical calculations 26…”

“…This view was supported by the detailed work of Carmona and co-workers, [20d] reactivity studies of the hydride complexes [ArMH] 2 (M = Zn, Cd; Ar = C 6 H 3 -2,6-(C 6 H 2 -2,4,6-iPr 3 ) 2 , C 6 H 2 -2,6-(C 6 H 2 -2,4,6-iPr 3 ) 2 -4-SiMe 3 ) reported by Power and co-workers [25] and theoretical calculations. [26] In the case of the reaction of 1 with H 2 , the stability of 1 precludes a pathway involving the homolytic dissociation into [ZnCp*] · and Cp* · radicals in a pre-equilibrium. This view was further supported by calculations by Hepperle and Wang that showed this dissociation is uphill by 32.3 kcal mol À1 , [26] although 6.7 kcal mol À1 less costly than the analogous dissociation for [ZnCp 2 ].…”

“…[26] In the case of the reaction of 1 with H 2 , the stability of 1 precludes a pathway involving the homolytic dissociation into [ZnCp*] · and Cp* · radicals in a pre-equilibrium. This view was further supported by calculations by Hepperle and Wang that showed this dissociation is uphill by 32.3 kcal mol À1 , [26] although 6.7 kcal mol À1 less costly than the analogous dissociation for [ZnCp 2 ]. A more likely pathway involves heterolytic cleavage of H 2 to eliminate Cp*H and generate [Cp*ZnH].…”

Comprehensive Computational Study of Decamethyldizincocene Formation. 1. Reaction of ZnR2 Reagents with Decamethylzincocene

“…On the other hand, if this difference in dissociation is coupled with the large difference in the symmetric activation barriers in the formation of 4 (R = Et) and 7 (R = Et) (13.2 vs 3.1 kcal/mol), it is possible that during the formation of 3 , the homolytic dissociation of 1 competes with the formation of the symmetric products (36.1 vs 13.2 kcal/mol). Further, the formation of 6 would not be possible due to a prohibitive energy difference between these two processes (43.0 vs 3.1 kcal/mol) …”

Comprehensive Computational Study of Decamethyldizincocene Formation. 2. Reaction of KH/ZnCl2 with Decamethylzincocene

“…Our proposed formation route of 3 from 1 and KH is fairly straightforward to understand, but it still involves free radicals; radical processes are difficult to control, and yields can be inconsistent. − The addition of one equivalent of ZnCl 2 into the reaction mixture does not change the basic reaction scheme all that much since Steps 1a, 1b, 2b, and 2c remain, but there is one major difference: the radical dissociation and reassociation steps (Steps 2a and 3a in Scheme ) become highly unfavorable and are bypassed by the reaction of Me 5 CpZnK with ZnCl 2 to form Me 5 CpZn 2 Cl and KCl (Step 2e, Scheme ).…”