Concerning the Si—Li bonding in phenylsilyllithiums as studied by variable temperature Li nuclear magnetic resonance

Abstract

A variable temperature 7Li nrnr study of equimolar mixtures of two structurally related phenylsilyllithiums supports a bimolecular exchange mechanism between monomeric structures as being responsible for the collapse of the 7~i signals of the two species. The different exchange rates indicated in MTHF and THF parallel the observations from a Si-Li coupling study. The experimental data provide support for a mainly ionic Si-Li bond, although some orbital interaction is indicated.ERWIN BUNCEL, T. KRISHNAN VENKATACHALAM et ULF EDLUND. Can. J. Chem. 64, 1674 (1986.Les rCsultats d'une Ctude des spectres rmn du 7~i , effectuk a des tempkratures variables sur des mklanges CquimolCculaires de deux phCnyl silyl lithium relikes par leur structure, sont en accord avec la suggestion qu'un mkanisme d'Cchange bimolCculaire entre les structures monombres est responsable de la coalescence des signaux 7~i des deux espkes. Les diverses vitesses dlCchange dans le MTHF et le THF sont parallbles celles obtenues lors d'une ktude du couplage Si-Li. Les donnCes expkrimentales fournissent un support a llhypoth&se d'une liaison Si-Li qui serait principalement ionique, mCme s'il semble exister quelques interactions entre orbitales.[Traduit par la revue] IntroductionThe precise nature of bonding in organolithium compounds has been a topic of considerable controversy (1). On the basis of recent ab initio calculations it has been argued that the carbon-lithium bond is predominantly ionic, i.e. ca 80% ionic character, and in fact more ionic than the Li-F bond (2).

Proton and metal‐ion activation of C–H exchange in five‐membered azoles

“…Furthermore, CH 3 substitution in 1a and 2a to give 1b and 2b, respectively, results in 2-3-fold increase in C(2)-H exchange rate. Interestingly, the results of benzo annelation and CH 3 substitution run contrary to expectations based solely on the inductive/ field effects of these groups but have been satisfactorily explained by Buncel and co-workers 21,22 using FMO-PMO theory. The ratio of 1.6 obtained when C(2)-H exchange rates in histamine and 1a are compared, 26,30 favouring the former, is amenable to a similar interpretation.…”

“…Simplification of Equation (2) to reflect values of [H + ] and [OH À ] in the different pH regions 22 yields kinetic equations consistent with pH-rate profiles which generally provide unambiguous information regarding the identities of exchanging species as well as unreactive species along the reaction coordinate. 9,22 Addition of M n+ to the reaction medium maintained at a pH in which the substrate is completely protonated ensures exchange only via Paths A and C (Scheme 1), 9 such that Equation (3) obtains when both protonated and metal-coordinated substrate forms are the reactive species in H/D or H/T exchange. Formation of kinetically unproductive metal-azole complexes (i.e.…”

“…The reactivity order C(2)-H4C(5)-H>C(4)-H, and C(2)-H>C(5)-H have been reported for 1 29 and 3 25 , respectively. The reactivity sequence for 1 is attributed to a combination of C(2) flanking by heteroatoms 22,29 and the adjacent lone pair (ALP) phenomenon. 29b The same order of reactivity is obtained in protonated 1 as in the neutral substrate; conceivably 2 follows the same trend.…”

“…The superiority of Path A over Path B has been demonstrated in several studies. 9,21,22,29 The ratio k H+ /k, otherwise known as the proton activating factor (paf) 12 , is typically in the range 10 2 -10 9 , reflecting the combined effects of ground-state acidification of ring hydrogens consequent upon N(3) protonation and transition state stabilization in the formation of the derived intermediate which overwhelmingly favour exchange from the protonated azole over the neutral substrate. 9,18,21.26,29 The same magnitude of rate enhancement obtained upon protonation at N(3) is also induced by N(3) methylation (alkylation).…”

“…8 Ground-and transition-state effects in neutral and protonated azoles which determine observed reactivity trends, positional reactivities, and preferential exchange at C(2) have received considerable attention. 18,19,[21][22][23]31,32 Metal-ion activation A rational expectation, based on the effect of H + on C-H exchange, is that large rate enhancements would result upon placement of multiple positive charges at N(3) of azoles through M n+ (n>1) coordination. 11,23,33,34 Literature results, however, show that relative to the proton, metal ions actually depress C-H exchange rates in these substrates.…”