Titanium complexes allyl

The titanium complex 4, prepared from (A./ )-2,3-0-isopropylidenc-1,1.4,4-tetraphenyl-1,2,3,4-butanetetrol7,113 and chlorotriisopropoxytitanium or tetraisopropoxytitanium, is treated with 2-propenylmagnesium bromide. The resulting titanate affords, with benzaldehyde, ( —)-(5)-l-phenyl-3-butenol. Several further attempts, which do not include allylation, have also been reported113, as have examples using the dichloride114. 

Table 8. 1-Substituted 3-Butenols from Enantiosclcctive Allylation of Aldehydes by Chiral Titanium Complexes...

Allyltrialkylsilanes add to aldehydes in the presence of a Lewis acid." The mechanism of this reaction has been examined." " When chiral titanium complexes are used in the reaction, allylic alcohols are produced with good asymmetric... 

Figure 1.25 Minimum-energy diastereoisomeric monomer free intermediates for butadiene polymerization catalyzed by titanium complexes presenting Cp group as ancillary ligand. Chiralities of coordination of allyl groups (assumed to be si) and back-biting double bonds (si or re) are indicated, in order to easily visualize possible stereoregularity (iso or syndio) of model chains. In fact, like and unlike chiralities would possibly lead to isotactic and syndiotactic enchainments, respectively.

Reactions of aldehydes with complexes 13—17 provide optically active homoallylic alcohols. The enantioselectivities proved to be modest for 13—16 (20—45% ee). In contrast, they are very high (> 94% ee) for the (ansa-bis(indenyl))(r]3-allyl)titanium complex 17 [32], irrespective of the aldehyde structure, but only for the major anti diastereomers, the syn diastereomers exhibiting a lower level of ee (13—46% ee). Complex 17 also gives high chiral induction (> 94% ee) in the reaction with C02 [32], in contrast to complex 12 (R = Me 11 % ee R = H 19% ee) [15]. Although the aforementioned studies of enan-... 

The oxygen that is transferred to the allylic alcohol to form epoxide is derived from tert-butyl hydroperoxide. The enantioselectivity of the reaction results from a titanium complex among the reagents that includes the enantiomerically pure tartrate ester as one of the ligands. The choice whether to use (+) or (-) tartrate ester for stereochemical control depends on which enantiomer of epoxide is desired. 

We were particularly interested to see whether a regio- and stereoselective hy-droxyalkylation and amrnoalkylation of 1 and 2 with aldehydes and imino esters, perhaps by choice of the substituent X at the Ti atom, with formation of the corresponding sulfonimidoyl-substituted homoallyl alcohols 4-7 and the homoallyl amines 8-11 (Fig. 1.3.3) could be achieved. Reggelin et al. had already demonstrated that the sulfonimidoyl-substituted mono(allyl)titanium complexes 3, the... 

The enantiopure acyclic and cyclic allyl sulfoximines 13 and 14, respectively, required for the synthesis of the corresponding titanium complexes 1 and 2, are available from sulfoximine 12 [13] and the corresponding aldehydes and cycloal-kanones by the addition-elimination-isomerization route, which can be carried... 

The lithiation of the T-configured acyclic allyl sulfoximines T-13 with n-BuLi gave the corresponding lithiated allyl sulfoximines -15 [15] which upon treatment with 1.1 equiv ofClTi(OiPr)3 at-78 to 0 °C in THF furnished the bis (allyl) titanium complexes -16, admixed with equimolar amounts of Ti(OiPr)4, in practically quantitative yields (Scheme 1.3.5) [14, 16]. Surprisingly the bis (allyl) titanium complexes -16 together with Ti(OiPr)4 and not the corresponding mono (allyl) titanium complexes were formed. 

Reaction of the bis (allyl) titanium complexes -16 with saturated and unsaturated aldehydes at -78 °C in the presence of 1.1 equiv of ClTi(OiPr)3 afforded the corresponding Z-anti-configured homoallyl alcohols 4 with >98% regioselectivity and >98% diastereoselectivity in good yields (Scheme 1.3.6) [14]. 

Scheme 1.3.5 Synthesis of cyclic and acyclic chiral sulfonimidoyl-substituted bis(allyl)titanium complexes.

Reaction of the bis(allyl) titanium complexes 16 and 18 with aldehydes occurs in a step-wise fashion with intermediate formation of the corresponding mono (allyl) titanium complex containing the alcoholate derived from 4 and 5 as a ligand at the Ti atom. Then the mono(allyl)titanium complexes combine with a second molecule of the aldehyde. Both the bis (allyl) titanium complexes and the mixed mono(allyl)titanium complexes react with the aldehydes at low temperatures with high regio- and diastereoselectivities. Interestingly, control experiments revealed that for the reaction of the bis (allyl) titanium complexes with the aldehyde to occur the presence of Ti(OiPr)4 is required, and for that of the intermediate mono(allyl)titanium complexes the addition of ClTi(OiPr)3 is mandatory (vide infra). 

Sulfonimidoyl-Substituted Mono(allyl)tris(diethylamino)titanium Complexes... 

The treatment of the lithiated allyl sulfoximines E-15 with 1.1-1.2 equiv of ClTi(NEt2)3 at -78 to 0°C in THF or ether afforded the corresponding mono (allyl) titanium complexes E-19 in practically quantitative yields (Scheme 1.3.7) [14, 16]. Similarly the Z-configured complexes Z-19 were obtained from the Z-configured allyl sulfoximines Z-15. Reaction of the titanium complexes E-19 with aldehydes at -78 °C took place at the a-position and gave the corresponding homoallyl alcohols 6 with >98% diastereoselectivity in medium to good yields (Scheme 1.3.8) [14, 16]. 

However, a more detailed study of the reaction of the mono(allyl)titanium complexes -19 carrying different alkyl groups at the double bond with different aldehydes revealed in some cases the highly diastereoselective (>98%) formation of significant amounts of the isomeric homoallyl alcohols 4 besides 6 (Table 1.3.1). 

Scheme 1.3.7 Synthesis of chiral sulfonimidoyl-substituted mono(allyl)titanium complexes.

Table 1.3.1 Reaction of the acyclic mono(allyl)titanium complexes -19 with aldehydes.

Table 1.3.3 Reaction of the cyclic mono(allyl)titanium complexes 20 (n = 1) with aldehydes.

The reaction of the acyclic bis(allyl)titanium complexes 16 with the imino esters 23a-c in the presence of Ti(OiPr)4 and ClTijOiPrjs at low temperatures proceeded with >98% regioselectivity and >98% diastereoselectivity and gave the corresponding T-syn-configured unsaturated a-amino acid derivatives E-24 in good yields (Scheme 1.3.11) [21, 22]. 

It is noteworthy that stereoselectivities were high, even with sterically demanding substituents at the double bond. Similarly, the treatment of the cyclic bis (allyl) titanium complexes 18 with the imino esters 23a-c afforded the corresponding B-syn-configured cyclic unsaturated amino acid derivatives -25 and the Z-syn-configured isomers Z-25 with >98% regioselectivity and >98% diaste-reoselectivity in good yields. 

Surprisingly, the mono (allyl) titanium complexes 19 reacted with the imino ester 23c also at the y-position with high diastereoselectivities and gave the unsaturated... 

Fig. 1.3.4 Structure of the sulfonimidoyl-substituted bis(allyl)titanium complex 28 in the crystal. Selected bond lengths Ti—C 244 and 229 pm, Ti—N 209 and 221 pm.

L = O/Pr, = aiiyisuifoximine Scheme 1.3.13 Reactivity scheme for the reactions of the bis(allyl)titanium complexes 16 with aldehydes. 

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