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In the presence of Ti(Oi-Pr) 4 reaction of EtMgBr with allylic alcohols and allylic ethers affords the products of formal S N 2' substitution of hydroxy or alkoxy groups with ethyl groups in moderate to good yields and excellent regioselectivity. The mechanism of the reaction includes the formation of titanacyclopropanes as the key organometallic intermediates. Promoting action of EtMgBr on the process of C-C bond formation in the transformation of titanacyclopropane-olefin complex 5 into the titanacyclopentane ate complex 6 is proposed.Alkene complexes of the Group IV transition metals show properties of metallacyclopropane reagents in the reactions with electrophiles. 1 Until now, the properties of titanocene and zirconocene derivatives are the most studied. Some years ago, we found that the reaction of ethylmagnesium bromide with carboxylic esters in the presence of titanium alkoxides smoothly results in formation of 1-substituted cyclopropanols. 2 It was supposed that the key organometallic intermediate in this reaction is diisopropoxytitanacyclopropane (diisopropoxytitanium-ethylene complex) which acts like a 1,2-dicarbanionic equivalent. To the best of our knowledge, it was the first synthetically useful transformation for the generation of titanacyclopropane reagents where inexpensive and easyto-use titanium alkoxides were used instead of titanocene derivatives. Later, it was shown that dialkoxytitanacyclopropane reagents react with N,N-dialkylcarboxamides to form the corresponding aminocyclopropanes, 3 and are involved in the ligand exchange 4 as well as in inter-and intramolecular olefins and acetylenes coupling reactions. 5 Dialkoxytitanacyclopropane 1,2-dicarbanionic equivalents have also been successfully used in consecutive reactions with two different electrophiles. 6 We report herein the titanium alkoxide mediated reaction of ethylmagnesium bromide with allylic alcohols and allylic ethers, resulting in the formation of the products of formal S N 2' allylic ethylation. 7 As shown in Scheme 1, when 2-undecen-4-ol (1a) or its methyl (1b) and THP (1c) ethers are treated with 3-4 equiv EtMgBr in the presence of 1 equiv Ti(Oi-Pr) 4 at room temperature in Et 2 O, alkene 2 is obtained in good yields. 8 Carbon-carbon bond formation occurred exclusively at the g-carbon and no formation of regioisomeric olefins, the a-attack products, was detected by GC-MS analysis.As illustrated in the Table, the reaction of the allylic alcohol 1a with EtMgBr in the presence of equimolecular amounts of Ti(Oi-Pr) 4 provided much better yields of substitution product 2 in comparison with catalytic conditions (entry1-4). Full consumption of substrate occurred after addition of 4 equiv of the Grignard reagent. When the ethers 1b and 1c were used, 3 equiv of EtMgBr were sufficient for their full involvement in the reaction (entries 5-11). Under similar reaction conditions, the primary allylic alcohol with disubstituted double bond (entry 12) as well as primary and tertiary allylic alcohols and 2-tetrahydropyranyl ethers with mono...

Interaction of titanium(IV) isopropoxide with ethylmagnesium bromide and its higher homologs leads to the generation of alkoxytitanacyclopropane species, which are able to act in the reactions with unsaturated compounds as 1,2-dicarbanionic alkylating agents. The present review is focused on the processes of intermolecular regio- and stereoselective alkylation of carboxylic esters and allylic alcohol derivatives with alkoxytitanacyclopropane reagents. The oxophilicity of the titanium atom and its tendency to form sterically crowded ate complexes are probably the main factors providing a high level of selectivity of these transformations.

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