Organolithium compounds stereochemistry

A mechanistic study28,31 on deuterated compounds showed that stereoselective deprotonation occurs to give the (S.Sj-configurated lithio compound with a selectivity of about 6 1. This is of no consequence for the overall alkylation process, since its stereochemistry is determined by the reaction of the equlibrated organolithium compounds. This conclusion was drawn from the result, that the same stereochemical outcome is observed in reactions starting with an a 1 1 mixture of (R,S)- and (S,S )-configurated lithio compound as from the case with selectively formed (S,S)-lithio compound only. Hence the (R,S)- and (5,5)-lithio compounds equilibrate rapidly on the timescale of the experiment and the question as to whether the (R,S)- or ( S iSj-lithio compound is the actual reacting species cannot be answered. 

Lewis bases exert dramatic effects on the rate, stereochemistry, and reaction pathway in organolithium chemistry 4). A partial explanation for these observations can be deduced from the effects of Lewis bases on the degree of association of organolithium compounds as shown in Table 3. In general, the presence of basic molecules tends to decrease the average degree of association of organolithium compounds. Thus, simple alkyllithiums which are hexameric in hydrocarbon solution... 

To evaluate conclusively the hypotheses presented here, additional solution and solid-state studies are needed, particularly for trisolvated organolithium compounds, solvent-separated complexes, and unsaturated complexes of the heavier alkali metals. The results do, however, furnish the first glimpse of the stereochemistry of a chemically important class of compounds and should also provide working models for understanding the role of Group la metals in polymerization reactions and in their reactions with electrophilic reagents. 

Most a-thio carbanions are configmationally labile and racemize rapidly even at low temperature as shown in the preceding section. This may be the reason why the asymmetric reaction of a-lithiated thiocarbamates reported by Hoppe and coworkers does not show such high enantioselectivity [34] as attained in the reaction of similar dipole-stabilized a-oxy organolithium compounds [Eq. (4)] [2, 39]. Note that the stereochemistry of the products obtained from a-lithiated thiocarbamates is different from that obtained in the a-lithiated carbamate [Eq.(5)]. 

Stereochemistry — Stereochemical Aspects of Organolithium Compounds, Vol. 26, Wiley-VCH Weinhem (2010), p 93. Chapter 3. 

MI135 V. Capriati, S. Florio, and A. Salomone, In R.E. Gawley and J.S. Siegel, editors Topics in Stereochemistry — Stereochemical Aspects of Organolithium Compounds, Vol. 26, Wiley-VCH Weinhem (2010), p 135. Chapter 4. 

Kizirian, J.-C. (2010) Mechanism and Stereochemical Eeatures in Asymmetric Deprotonation Using RLi/ (—)-Sparteine Bases, in Topics in Stereochemistry, Stereochemical Aspects of Organolithium Compounds (eds. Gawley, R. E., Siegel, J. S.), Verlag Helvetica Chimica Acta, Zurich, pp. 189—251. 

Retention in the reactions of 15 is established both from presumed retention in the Sn-Li exchange step of a stannylation-destannylation sequence and by evidence that the s-BuLi-(-)-sparteine complex used to make the organolithium reliably removes the pro-/ proton adjacent to a carbamate (see below for crystallographic evidence involving a similar compound).11 The stereochemistry of the products 16, almost all formed essentially in enantiomerically pure form, was proved for the C02 adduct and the Mel adduct by comparison with known compounds. The only electrophiles for which incomplete retention of stereochemistry has been observed are the benzylic or allylic halides. These probably react in part by single electron transfer SE1 mechanisms, rather than by partial SE2inv.15 For example, 15 reacts with allyl bromide to give 16 (E = allyl) with only 42% ee. 

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