Details are given for the preparation of "uranocene," U(C8H8)2, from UC14 and CSH82-in THF. Uranocene reacts only slowly with water or acetic acid but decomposes with oxygen, bases, and strong acids. It does not undergo electrophilic substitutions or metallation reactions. Its chemistry is discussed in terms of a simple theory of the electronic structure that involves covalent interaction of the highest occupied e2u ligand MO's with metal fx yi and f(z= _ AO's.

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.

The rate of deuterium exchange of methane with caesium cyclohexylamide in cyclohexylamine a t 50° is 3.1 x 10-2 that of cyclopropane, on a perhydrogen-atom basis.THE relative acidity of the simplest hydrocarbon, methane,

The one-electron MO description in ref 4 should be regarded as one of the several configurations because of the mixing of f orbitals resulting from spin-orbit coupling.

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