N-Methylsulfonimidoyl-Substituted (2-Alkenyl)titanium Complexes: Application to the Synthesis of β- and δ-Sulfonimidoyl-Substituted Chiral Homoallylic Alcohols, X-ray Crystal Structure Analysis, and Fluxional Behavior

Abstract

Enantiopure acyclic (E)- and (Z)-configured allylic sulfoximines have been synthesized from N,S-dimethyl-S-phenylsulfoximine and aldehydes by the addition− elimination−isomerization route through the intermediate generation of the corresponding (E)-configured vinylic sulfoximines. Isomerization of the vinylic sulfoximines with DBU preferentially afforded the corresponding (Z)-configured allylic sulfoximines, which were subsequently isomerized by DBU to preferentially yield the (E)-isomers. Titanation of lithiated (E)-configured allylic sulfoximines with ClTi(OiPr)3 furnished the corresponding bis(2-alkenyl)diisopropyloxytitanium(IV) complexes, which reacted with aldehydes in the presence of ClTi(OiPr)3 with high regio- and diastereoselectivities at the γ-position to give the corresponding (Z)-anti-configured δ-N-methylsulfonimidoyl-substituted homoallylic alcohols in good yields. In the absence of ClTi(OiPr)3 at low temperatures, only one allylic moiety of the bis(alkenyl)diisopropyloxytitanium complex is transferred to the aldehyde. In this way, a cyclic lithiated allylic sulfoximine has been converted with high regio- and diastereoselectivity to the corresponding homoallylic alcohols bearing a vinylic sulfonimidoyl group. Titanation of lithiated (E)- and (Z)-configured allylic sulfoximines with ClTi(NEt2)3 afforded the corresponding mono(2-alkenyl)tris(diethylamino)titanium(IV) complexes, which reacted with aldehydes with moderate to high regioselectivities and high diastereoselectivities preferentially at the α-position to give the corresponding syn-configured β-N-methylsulfonimidoyl-substituted homoallylic alcohols along with the (Z)-anti-configured δ-N-methylsulfonimidoyl-substituted homoallylic alcohols in good yields. In this way, the cyclic lithiated allylic sulfoximine was converted with high regio- and diastereoselectivity to the corresponding isomeric homoallylic alcohols bearing an allylic sulfonimidoyl group. In the case of mono(alkenyl)tris(diethylamino)titanium(IV) complexes, the regioselectivity of their reactions with aldehydes has been found to depend on the size of the substituent at the CC double bond and the aldehyde, as well as on the configuration of the double bond. Reaction of racemic lithiated N-methyl-S-(3,3-diphenyl-2-propenyl)-S-phenylsulfoximine with ClTi(OiPr)3 afforded the corresponding bis(alkenyl)diisopropyloxytitanium(IV) complex. X-ray structure analysis revealed a distorted octahedral cis,cis,cis-configured bis(2-alkenyl)diisopropyloxytitanium(IV) complex, in which the allylic moieties are coordinated in a bidentate fashion through C-α and the N atom to the Ti atom, both having the relative configuration RSSC. In solution, the titanium complex shows fluxional behavior, which is characterized by topomerization of the isopropyloxy groups and allylic moieties. The exchange of the latter occurs with retention of the configuration at C-α.