β-Hydrogen Elimination of Ionic Butylzinc Complexes

Abstract

Gas-phase experiments on deuterium-labeled BuZn-(TMEDA) + (TMEDA = N,N,N′,N′-tetramethylethylenediamine) prove that the loss of butene from this species corresponds to a β-H elimination. Quantum-chemical calculations corroborate this finding and furthermore predict that the fragmentations of related cationic butylzinc complexes as well as of Bu 3 Zn − follow similar mechanisms.

Propylene Hydrogenation and Propane Dehydrogenation by a Single-Site Zn2+ on Silica Catalyst

“…Because there is no Al present in our catalyst, the Zn–O–Si site serves as the H + adsorption site. Zinc alkyls are known to be particularly resistant to β-hydride elimination; however, the catalysis temperature is well above the documented reaction for zinc alkyls . A separately computed pathway for C–C cracking was found; however, the transition state for C–C cracking, which involved β-methyl transfer to Zn, is at least 10 kcal/mol higher in energy than β-H transfer for the dehydrogenation rate-limiting step.…”

“…Zinc alkyls are known to be particularly resistant to β-hydride elimination; however, the catalysis temperature is well above the documented reaction for zinc alkyls. 27 A separately computed pathway for C−C cracking was found; however, the transition state for C−C cracking, which involved β-methyl transfer to Zn, is at least 10 kcal/mol higher in energy than β-H transfer for the dehydrogenation ratelimiting step. This is consistent with the observed high selectivity of Zn/SiO 2 for alkane dehydrogenation over C−C bond cleavage.…”

Molecular Main Group Metal Hydrides

“…In this process, the lithio-zincate intermediate ( 360 ) was formed by lithium chloride addition to [(To M )­Zn­{N­(SiHMe 2 ) 2 }] ( 361 ), which releases one arm of the To M chelate from zinc, facilitating the subsequent transfer of a hydride . Notably, β-hydride elimination from ZnEt 2 and Na­[ZnEt 3 ] can occur to yield Zn–H bonds, however only at very high temperatures (600 °C) or in the presence of strong reducing agents, such as Na metal. , The propensity for β-hydride elimination is greatly increased in cationic complexes, as exemplified by the gas phase observation of β-hydride elimination from labelled [TMEDA·Zn­{CH 2 C­(H/D) 2 CH 2 CH 3 }] + ( 362 ) (TMEDA = N , N , N ′, N ′-tetramethyl­ethylenediamine) …”

“…482,483 The propensity for βhydride elimination is greatly increased in cationic complexes, as exemplified by the gas phase observation of β-hydride elimination from labelled [TMEDA•Zn{CH 2 C(H/ D) 2 CH 2 CH 3 }] + (362) (TMEDA = N,N,N′,N′-tetramethylethylenediamine). 484 The hydride bound to zinc in (To M )ZnH (359) was located by single crystal X-ray crystallography, leading to a refined Zn− H single bond length of 1.52(2) Å. 462,481 Interestingly, 359 remains intact upon exposure to O 2 (up to 7 atm), to the radical initiator 2,2′-azobis(2-methylpropionitrile) (AIBN), to light, or to elevated temperatures (120 °C).…”