[87370-10-3]  · C11H12I2TiZn  · Bis(h5-cyclopentadienyl)(diiodozinc)(m-methylene)titanium  · (MW 511.28)

(reagent for methylenation of ketones; methylation of nitriles and alkynes; coupling reagent for conversion of benzyl halides to bibenzyl)

Physical Data: dH (toluene-d8) 8.37 (2H), 5.92 (10H).

Solubility: sol THF and toluene at rt; precipitates from THF at -78 °C to give a deep-red Cp2TiCH2.ZnI2.(THF)x complex.

Analysis of Reagent Purity: concentration can be determined by reaction of originally prepared THF solution with benzophenone to form 1,1-diphenylethylene.

Preparative Methods: Cp2TiCH2.ZnI2 is readily generated by the reaction of Cp2TiCl2 with CH2(ZnI)2 in THF.1

Handling, Storage, and Precautions: moisture sensitive; must be manipulated under a nitrogen atmosphere; presumably pyrophoric.

Methylenation and Other Reactions.1

Cp2TiCH2.ZnI2 has been compared1,2 to the structurally related Tebbe reagent (m-Chlorobis(cyclopentadienyl)(dimethylaluminum)-m-methylenetitanium) and reacts in an analogous way with both cyclic and acyclic ketones to give the corresponding alkene (eqs 1 and 2).

However, unlike the Tebbe reagent, Cp2TiCH2.ZnI2 does not appear to form the methyl ketone, to any significant extent, when it reacts with acid chlorides. For example, reaction of the title reagent with benzoyl chloride affords only small amounts (ca. 20%) of acetophenone, the main product arising from ring-opening of the complexed THF which yields the benzoate of 4-chloro-1-butanol (eq 3). The reagent effects the reductive coupling, presumably via an electron transfer process, of active halides such as benzyl bromide or chloride, and bibenzyls are produced (eq 4).

While Cp2TiCH2.ZnI2 fails to react with imines, treatment of the reagent with benzonitrile gives acetophenone upon aqueous workup. The observation that a-deuteroacetophenone is formed when D2O is used in the quenching step supports the intermediacy of a metallocycle (eq 5) in this process.

Analogous metallocycles are formed when the reagent is allowed to react with alkynes, and hydrolytic cleavage of such intermediates then leads to net syn addition of the elements of methane across the original triple bond. The reaction of phenylacetylene with Cp2TiCH2.ZnI2 then D2O is representative (eq 6). However, methylenative dimerization is the major reaction pathway, and this process involves initial self-condensation of the metallocycle to produce a second intermediate which upon reaction with D2O affords, when phenylacetylene is used as substrate, a 3:1 mixture of diphenyl-1,5-hexadienes (eq 6).

The reaction of 1-trimethylsilyl-1-octyne with Cp2TiCH2.ZnI2 (eq 7) differs in two respects from that of the preceding alkyne in that: (i) small amounts of an allene are formed (presumably by a b-hydride elimination) and (ii) the principal methylenated dimers have the structures shown, and these probably arise by a process involving insertion into an allylic C-Zn bond. Deuterium-labeling studies are consistent with such mechanistic proposals which involve the intermediacy of a common metallocycle. Some 32% of the product mixture consisted of the same methylated vinylsilane as formed from reaction of the substrate with Cp2TiCl2 and MeAlCl2.

1. Eisch, J. J.; Piotrowski, A. TL 1983, 24, 2043.
2. Kelly, S. E. COS 1991, 1, 729.

Martin G. Banwell

University of Melbourne, Parkville, Victoria, Australia

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