No abstract

Vinyl pivalate in the presence of the titanocene(II) reagent Cp 2 Ti[P(OEt) 3 ] 2 reacts both with nonpolar CϵC triple bonds and with polar C=O double bonds, to produce conjugated dienes and allylic alcohols, respectively. Similar alkenylationWe recently reported two methods for the transformation of thioacetals into cyclopropanes. Alkyl-substituted cyclopropanes are produced through reactions between thioacetals and vinyl pivalate (2) promoted by the titanocene() reagent Cp 2 Ti[P(OEt) 3 ] 2 (1), whilst vinylcyclopropanes are obtained by treatment of thioacetals with the titanocene() reagent 1 and buta-1,3-dienes.[1] During this study we found that the vinylcyclopropane 3 was produced when the pivalate 2 (3 equiv.) was treated successively with excess quantities of 1 (7 equiv.) and the thioacetal 4 at 25°C for 2 h (Scheme 1). The formation of 3 indicates that buta-1,3-diene is formed by the self-coupling of 2, probably via the titanacyclopentane intermediate 5, in which equilibrium of the formation of titanacycle 5 would be favored by elimination of titanocene dipivalate (Scheme 2). This result prompted us to investigate a new titanocene()-promoted intermolecular cross-coupling of unsaturated compounds via a similar titanacycle intermediate. Suzuki coupling, [3] Stille coupling, [4] and Hiyama coupling [5] -are frequently employed for carbon-carbon bond formation between two sp 2 carbon atoms. A major synthetic route to the alkenylmetal compounds used in these reactions is hydrometallation of alkynes. Direct cross-coupling between alkenes containing a leaving group and alkynes, promoted by a low-valent metal species 1 as depicted in Scheme 3, would therefore be advantageous: the procedure should give conjugated dienes, the products of the above conventional cross-couplings, without the need for prior transformation of alkynes into any alkenylmetal species. Another advantage of this methodology is that it should also be applicable to reactions of compounds possessing a carbon-heteroatom multiple bond. In this context, we have studied titanocene()-promoted cross-couplings between vinyl pivalate (2) and unsaturated compounds, and here we describe the preparation of conjugated dienes 6 and allylic alcohols 7 through reactions between 2 and alkynes 8 or carbonyl compounds 9, via the titanacycle intermediates 10 and 11 (Scheme 4). These reactions are useful in that vinyl pivalate (2), a poor substrate for transition-metal-catalyzed cross-couplings, is available as a coupling component.Treatment of alkynes 8 with vinyl pivalate (2) (3 equiv.) in the presence of titanocene() reagent 1 (3 equiv.) at 0-25°C for 2.5 h produced the conjugated dienes 6 in good yields (Table 1). Although the reactions of unsymmetrical alkynes generally proceeded with mixed regioselectivity, high selectivity was observed when phenylacetylene deriva-

Titanocene(II)-promoted cross coupling of alkynyl- and (Z)-alkenyl sulfones affords alpha-(phenylsulfonyl)alkenyltitanium species. Further treatment of these species with the titanocene(II) reagent generates titanium vinylvinylidene complexes, which react with carbonyl compounds in one pot to produce substituted vinylallenes with complete stereoselectivity. By using alpha,beta-unsaturated ketones, 1,3,4,6-tetraenes are also obtained stereoselectively.

Page 3821. In Scheme 7, the structure 7B was incorrect. The corrected scheme is shown below.

The stereoselective alkenylation of unsaturated compounds by means of a (Z)-alkenyl sulfone-titanocene(II) system is described. Treatment of alkynes and (Z)-alkenyl methyl sulfones with the titanocene(II) reagent Cp2Ti[P(OEt)3]2 produced conjugated dienes. This alkenylation system is also applicable to polar C=O bonds; the simple mixing of carbonyl compounds, (Z)-alkenyl methyl sulfones, and the titanocene(II) reagent formed allylic alcohols. The advantages of alkenylation are that it requires no prepreparation of the alkenylmetal reagent and that it proceeds with complete stereoselectivity.