A broad survey of both the simple and double decomposition modes of transition metal alkyls has been undertaken. From the two main processes of C–M bond rupture, homolysis, and β‐metal hydride elimination, the following modes of decomposition and types of useful reagents generated can be distinguished: 1) reductive decomposition of RnMEm–n to yield subvalent, carbene‐ or free‐radical‐like MEm–n, which can cleave π‐ or σ‐bonds by direct epimetallation; 2) β‐metal hydride elimination to produce hydrometallating agents; 3) combined reductive and β‐metal hydride eliminations to produce anionic, and hence more nucleophilic, epimetallating agents; 4) α‐metal hydride elimination thought to be involved in the transition state leading to C–C bond coupling from (RR'HC)2ME2 derivatives; 5) α,μ‐elimination from lithiated RHC(Li)–MEn–1, which produces alkylidenes, RHC=MEn–2, capable of ROMP catalysis; 6) metal–hydrogen metathesis, whereby sufficiently acidic C–H bonds become C–M bonds; 7) metal–metal metathesis, whereby RR'ME2 undergoes an equilibrating redistribution reaction to form R2ME2 and R'2ME2; 8) transfer epimetallation, in which epimetallation (point 1) is achieved with R2ME2 in one step by transfer of ME2 to the substrate with the concomitant loss of R groups; and 9) transfer epimetallation of olefins by R2TiE2 in hydrocarbons to produce titana(IV)cyclopropanes, which serve as active sites in Ziegler–Natta polymerization. In each situation, the reaction‐mechanistic features are analyzed in terms of existing experimental data and evidence. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005)

An investigation of the Kulinkovich cyclopropanol synthesis, the interaction of esters with 2:1 or 3:1 mixtures of alkyl Grignard reagents and Ti(OiPr) 4 at low temperatures, has been conducted, in order to ascertain which reactive intermediates are involved and how they are interconverted. Because of the nature of the ultimate product, one of the most obvious intermediates is the 1,1-diisopropoxy-1-titanacyclopropane stemming from the epititanation of the alkene set free from the alkyl Grignard reagent employed. A search for the formation of such a titanocycle by warming an ethereal solution of either Et 2 Ti(OiPr) 2 or iPr 2 Ti(OiPr) 2 between −78°C and +25°C was attempted by chemical trapping with either an ester or nitrile. In this manner it was shown that such a titanocycle was formed in the case of Et 2 Ti(OiPr) 2 but not with iPr 2 Ti(OiPr) 2 . As to the role of two other potential intermediates, Ti(OiPr) 2 and R 2 Ti(OiPr) 2 , it was demonstrated that preformed Ti(OiPr) 2 in the presence of ethylene and an ester does not form the corresponding cyclopropanol. Thus, under the reaction conditions Ti(OiPr) 2 cannot perform the direct epimetallation necessary to produce the requisite titanacy-

The interaction of the Group 4 metal chlorides, MCl 4 , where M = Ti, Zr, Hf, with two equiv. of methyllithium at -78°C in toluene has led to the corresponding methylidene-metal complex, H 2 C=MCl 2 , possibly complexed with LiCl. That this complex was stable to at least -40°C was demonstrated by adding chemical trapping agents at this temperature: 1) benzophenone was quantitatively converted into 1,1-diphenylethylene when M = Ti or Zr; with the Hf analogue, a 1:3 mixture of 1,1-diphenylethylene and 1,1-diphenylethanol was produced; 2) propiophenone reacted with the methylidene complex to yield 2-phenyl-1-butene, 38 % (M = Ti) or 25 % (M = Zr), with the majority of the propiophenone being recovered (the unreacted ketone is attributed to enolate salt formation competing with the methylidenation); 3) diphenylacetylene reacted with H 2 C=TiCl 2 by cycloaddition in 50 % yield, ultimately to give a 1:1 mixture of α-methyl-cis-stilbene and α-methyl-trans-stilbene; and finally 4) norbornene was

For the first time the unstable titanocene(II) has been directly synthesized by the Wilkinson metallocene approach, namely the interaction of a THF-soluble form of titanium(II) chloride with two equivalents of cyclopentadienylsodium in THF solution at 0°-25°C. Because of the transient existence of the titanocene(II) thereby obtained, it could only be chemically trapped in high yield as 1,1-bis(cyclopentadienyl)-2,3,4,5-tetraphenyltitanacyclopentadiene by two equivalents of diphenylacetylene, if the acetylene was added at 25°C, without removal of the by-product LiCl and NaCl. If the addition of the acetylene was delayed, in order to filter off the LiCl and NaCl from the reaction mixture, then no trace of the tit-

Efficient transfer‐epimetallations of simple olefins and acetylenes by R2TiL2 reagents (R = Bun, But; L = X) are readily achieved in THF at −78°C to generate titanacyclopropa(e)ne intermediates, readily capable of inserting various unsaturated addends (olefin, acetylene, nitrile). Analogous epimetallations conducted in hydrocarbons lead to the isotactic stereoselective polymerization of 1‐alkenes and the cyclotrimerization of acetylenes. In place of the widely accepted Arlman‐Cossee model for the active catalytic site, namely a Ti‐C bond on the TiCl3 crystal lattice, the 2‐substituted‐1‐halotitanacyclopropyl cation formed in hydrocarbon media is proposed as the active site for stereoselective olefin polymerization.