The lithium salt of p-phenylisobutyrophenone (LiPhIBP) exists in tetrahydrofuran (THF) as a mixture of monomer and tetramer contact ion pairs (CIP). The equilibrium constant, K 1,4 ) 5.0 × 10 8 M -3 , indicates that the lithium enolate is primarily tetrameric at higher concentrations, but the monomer is still present in significant amounts even at concentrations typical of synthesis reactions. Alkylation reactions of LiPhIBP with various alkylating agents were investigated in THF at 25°C at concentrations of 10 -3 to 10 -2 M by using UV-vis spectroscopy. The kinetics followed rate laws of 0.50 to 0.30 order in the formal lithium enolate concentration but is first order in the monomer concentration. These rate studies provide direct evidence that the reactive species is the monomer, even when tetramer dominates the equilibrium.Reactions involving lithium enolates represent a large class of modern organic synthesis reactions and are important methods for C-C bond formation. 2-8 It is now well-known that these species, as well as other organolithium compounds (alkyl-and aryllithiums, lithium amides, etc.), exist generally as aggregates in ethereal solution and in the solid state. 9-19 What has not been clear is the actual role of such enolate aggregates in reactions with electrophiles. A better understanding of this subject is important in view of the possible influence of enolate aggregation and mixed aggregates on reactivity and regio-and stereoselectivity. [20][21][22][23][24][25] Jackman et al. have studied the lithium salt of isobutyrophenone by NMR and reported that it exists in tetrahydrofuran (THF) solution exclusively as a tetramer. 10,26,27 They concluded that these aggregates are directly involved in the alkylation reaction on the basis of the analysis of the product distribution (C-and O-alkylation) 28 and later hypothesized that dimers could also be involved. 29 This and other indirect evidence, especially the observation that lithium enolates crystallize generally as dimers, tetramers, or hexamers, led Amstutz et al. to propose that a tetrameric cubic structure is a reaction intermediate. 30,31 Although these mechanisms have not been confirmed, they are widely accepted and have been used as working hypotheses in the analysis of the reactivity and selectivity of lithium enolates. 4,16,17,[32][33][34] We recently showed for the lithium enolate of p-phenylsulfonylisobutyrophenone that the dimer and a mixed aggregate with LiBr are much less reactive in an alkylation reaction than the monomer in THF. 35,36 In the accompanying paper we showed that the cesium enolate of 1-(4-biphenylyl)-2-methyl-(1) Carbon Acidity. 102.(2) Evans, D. A.; Nelson, J. V.; Taber, T. R.

An ab initio study of ionic and ion pair displacement reactions involving allylic systems has been carried out at the RHF/6-31+G* level. The geometries and natural charges show the absence of conjugative stabilization in the ionic transition states, thus differing from traditional explanations. The high reactivity of allyl halides is explained by electrostatic polarization of the double bond. Substituent effects were also studied; in general, electron-withdrawing groups lower the barriers of the ionic S(N)2 reactions but increase the barriers of the ion pair reactions. The allylic reactions are compared with related benzylic systems. Hammett correlations give rho of opposite sign for the ionic and ion pair displacement reactions, in agreement with some experimental results.

Various disparate experimental results are explained by the hypothesis that reactions of anionic nucleophiles with allylic halides are generally S(N)2. The S(N)2' reactions that do occur proceed generally with anti stereochemistry. Reactions with ion pair nucleophiles occur preferentially as S(N)2' reactions with syn stereochemistry. This hypothesis is consistent with a variety of computations at the HF, B3LYP, mPW1PW91 and MP2 levels with the 6-31+G(d) basis set of reactions of Li and Na fluoride and chloride with allyl halides and 4-halo-2-pentenes. Solvation is considered by a combination of coordination of dimethyl ether to the lithium and sodium cations and "dielectric solvation" with a polarized continuum model.

No abstract

A delayed exotherm was recorded during the reduction of a carboxylic acid with BH 3 ‚THF. This article will describe our efforts to explain and control the observed exotherm using a reactor integrated with a mass-flow meter, a reaction calorimeter, and an in situ FT-IR probe. In the end, a kinetic model is proposed that allows control of the addition rate of the reagent based on the known cooling capacity of the reaction vessel.

No abstract

[formula: see text] Although phenyl esters react with both monomers and dimers or tetramers of two lithium enolates in THF, the reactions of phenyl thiobenzoates are relatively much faster with the monomers. Similarly, imidazole esters react primarily with the monomers but pyrazole esters react with monomers and aggregates. The results are rationalized by a mechanism in which coordination with two lithium cations within an enolate aggregate is required for the reaction of aggregates to compete with monomers.

No abstract