Titanocene(II)‐Promoted Cross‐Coupling of Unsaturated Compounds

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 Stereoselective Alkenylation Utilizing (Z)-Alkenyl Sulfones

“…Reaction Pathway. As we described in the previous paper, the titanocene(II)-promoted cross-coupling of unsaturated compounds is expected to proceed via the formation of five-membered titanacycles (path A in Scheme ). Compounds 2 react with unsaturated compounds 3 or 4 in the presence of 1 to form two titanacycle intermediates 7a and 7b , which exist in equilibrium.…”

“…Recently we reported the titanocene(II)-promoted (Cp 2 Ti[P(OEt) 3 ] 2 , 1 ) vinylation of alkynes and carbonyl compounds with vinyl pivalate . We also found that various alkynyl phenyl sulfones reacted with ( Z )-alkenyl methyl sulfones ( 2 ) in the presence of 1 to form the alkenylation products, 1-(phenylsulfonyl)-1,3-dienyltitanium compounds.…”

Cross-Dimerization of 2,5-Dihydrofuran with Conjugated Dienes Catalyzed by (Chiral Diene)ruthenium(0) Complexes and Origins of the Enantioselectivity

“…15 Ph 2 -bod* was purchased from a commercial supplier. 2-Phenylbutadiene (5f), 28 1,2dihenylbutadiene (5g), 28 2-methyl-1-phenylbutadiene (5h), 29 3methyl-1-phenylbutadiene (5i), 30 2-(2-naphthyl)butadiene (5j), 31 and 2-ferrocenylbutadiene (5k) 31 were prepared according to literature methods. All other reagents and solvents were obtained from commercial suppliers and used as received.…”

The Society for Cardiovascular Angiography and Interventions' 27th Annual Scientific Sessions

“…Based on these observations, a new route to vinylvinylidene complexes 64 was developed, which involves the formation of α‐heteroatom‐substituted vinyltitanium species. It was found that the cross‐coupling between ( Z )‐alkenyl sulfones 71 and alkynes was promoted with the titanocene(II) reagent 8 to produce dienyltitanium species 59. Therefore, it is reasonable to assume that a similar reaction of 71 with alkynyl sulfones 72 produces α‐phenylsulfonyl dienyltitanium 73 , which affords the vinylvinylidene complexes 64 on treatment with another equivalent of 8 (Scheme ).…”