The Decomposition of Transition Metal Alkyls Revisited: Surprising Wellspring of Novel Reagents for Organic Synthesis

Abstract

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)

Vanadium(I) Chloride and Lithium Vanadium(I) Dihydride as Selective Epimetallating Reagents for π‐ and σ‐Bonded Organic Substrates

“…This requisite process has proved to be alkylative reduction. [6] Illustrated in Equation (1) for TiCl 2 (3), [7] it appears to be generally applicable to early transition metal salts, such as Ti(i-OPr) 2 , [8] ZrCl 2 , [9] Zr(OEt) 2 , [10] HfCl 2 [9] and CrCl. [11] The reaction occurs by the partial alkylation of TiCl 4 (1) in THF at -78°C by n-butyllithium and the subsequent loss of butyl radicals from 2 above 0°C.…”

The Synthesis and Reactivity of Group 4 Zwitterionic Complexes of the Type Mt+CH2AlCl3−: One‐Component Stereoselective Polymerization and Oligomerization Catalysts for Olefins and Acetylenes

“…When Hf complex 26 was used as the catalyst in the cyclotrimerization of 1-hexyne under similar conditions, the reaction did not reach completion (42%) and the 1,2,4/1,3,5 isomer ratio was modestly reversed, 54:46 [Equation (7), R ϭ n-butyl]. (7) On the other hand, reaction of the aryl-substituted acetylene phenylacetylene (31) with these Group 4 zwitterions was interestingly different. The use of titanium analog 9 exclusively led to a 1,2,4/1,3,5 mixture of triphenylbenzenes through a cyclotrimerization reaction, while zirconium analog 25 gave a mixture of 64% of the isomeric cyclotrimers (45:55) and 36% of a poly(phenylacetylene) 32, while hafnium analog 26 gave poly(phenylacetylene) (32) only.…”

“…After initiating the reaction between 4 and 5 at Ϫ78°C (Scheme 2, reaction 1), bringing the mixture to 20°C and collecting the black precipitate, we found by Ti and Cl analyses that this black solid was not TiCl 2 (7), but rather the analytical data agreed with a composition corresponding to TiCl 2 ·Me 2 AlCl (8). The fact that TiCl 2 (7) underwent complexation with Me 2 AlCl (6) (reaction 2) was also re-flected in the finding that the toluene filtrate contained only half the expected amount of Me 2 AlCl (6).…”

“…( [2,5] HfCl 2 [2,5] CrCl, [6] VCl, [2,7] and their alkoxide derivatives [8,9] in THF solution. The method was applied to the alkylation of TiCl 4 (4) with 2 equiv.…”

α‐ und β‐Hydridabstraktion in Gold(I)‐alkylkomplexen

“…[2] Beiden Prozessen ist gemein, dass ein freies Orbital am Metallzentrum verfügbar sein muss. [3][4][5] Kürzlich berichteten Kçppel, Hashmi und Mitarbeiter in einer experimentellen und theoretischen Arbeit über thermisch induzierte b-H-Eliminierungen in d 10 -Gold(I)-alkylkomplexen (A; Schema 1). [6] Sie schlussfolgerten, dass ein solcher Prozess, der zu einem Gold(I)-hydridkomplex und freiem Alken führt, durch die hohe Energie der dreifach-koordinierten Gold(I)-alkenhydridspezies B, die durch die gefüllten d-Orbitale des Gold(I)-Zentrums bedingt ist, gehindert wird.…”