Titanium sulfonamide complexes

Figure 17 A titanium sulfonamidate complex for polylactide synthesis...

Doye s group [81] showed that a dinuclear titanium-sulfonamidate complex (Scheme 21), with a tetrahedral sulfur in the ligand backbone, can be used for intermo-lecular hydroaminoalkylation as well. This system gives mixtures of branched and linear products, although to date there has been no mechanistic rationale provided for the reduced regioselectivity of group 4 metal complexes in this transformation. There has been one report by Zi s group [44] that describes axially chiral bis(sulfonamidate) tantalum and niobium complexes for application as precatalysts for hydroamination and hydroaminoalkylation. Unfortunately, these complexes did not show any reactivity for either of these reactions. 

Scheme 21 Hydroaminoalkylation with a dinuclear titanium-sulfonamide complex...

Catalytic asymmetric vinylation of ketones has been achieved. Vinylzinc reagents have been generated by hydrozirconation of terminal alkynes which are then transmet- allated with zinc.199 A titanium(IV) complex of a tims-cyclohexane-bis(sulfonamide) provides chiral catalysis it also facilitates dienylation of ketones, with ees also >90% in this case. 

A combination of diethylzinc with sulfonamides 18 or 19 offers another possibility for the enantioselective acetate aldol reaction39,41. The addition of silyl enol ethers to glyoxylates can be directed in a highly enantioselective manner when mediated by the binaphthol derived titanium complex 2040. 

In addihon to the more generally reachve group 3 elements, examples of group 4 metals with amidate, pyridonate, and sulfonamidate ligands have been reported for ROP of cyclic esters. Such group 4 metals, and in particular titanium, are attractive due to their low cost, low toxicity, and high earth abundance. Furthermore, such complexes are known to be more robust than rare earth element complexes and thus less sensitive to the purity of the monomeric feedstock that is used for ROP. 

An even better nucleophile is nitrogen. The incompatibility of basic amines for almost every one of these reactions catalyzed by these coordinatively unsaturated Ru complexes led us to examine sulfonamides and carboxamides. However, no productive results ensued. A basic amino group was also examined to verify its incompatibility. In contrast to that expectation, cyclization proceeded without problems as summarized in Equation 1.70 [61]. A Lewis acid was required as a cocatalyst. For formation of pyrrolidines, titanium tetrachloride proved most efficacious whereas for formation of piperidines, methylaluminum dichloride proved best. In principle, any nucleophile, such as carbon, that satisfactorily reacts in ruthenium-catalyzed allylic alkylations should function here also. 

A-alkynylsulfonamides 174 are useful intermediates for diastereoselective synthesis <04OL727>. An efficient copper-promoted alkynylation of sulfonamide 173 has been developed to afford 174 with completely retained enantiomeric purity. The acetylene-titanium complexes 175, obtained from 174 upon treatment with titanium(II) alkoxide, react with aldehydes 176 to give alcohol 178, after hydrolysis, with virtually complete regio- and /Z-diastereoselectivity and also with high 1,5-diastereoselectivity (up to de = 98 2). The N-... 

In 2004, Walsh and coworkers demonstrated that titanium complexes of tra s-l,2-bis(hydroxy-camphorsulfonylamino) cyclohexane were excellent catEilysts for asymmetric ZnPh additions to ketones. The reactions showed excellent enantioselectivities (Scheme 7.49) [79]. The reaction employs the readily available bis(sulfonamide) diol ligand (Scheme 7.49), a sub-stoichiometric amount of titanium tetraisopropoxide, and commercially available diphenylzinc. The reactions were clean, affording high yields of tertiary alcohol in less than 24 h at room temperature. This, in fact, was an improvement over the results reported previously by Dosa and Fu [78]. Additionally, there was no need for methanol as an additive. 

Walsh developed an efficient protocol for the asymmetric catalytic vinylation of a variety of aldehydes and ketones, using the vinylzinc species generated in situ by treatment of terminal alkynes with Schwartz s reagent for hydrozirconation, followed by transmetallation to zinc via reaction with Me2Zn. Under the catalysis of titanium complex of the chiral dihydroxyl bis (sulfonamide) ligand, the reaction provides access to the corresponding secondary or tertiary allylic alcohols in high yields and excellent enantiomeric excesses [65] (Scheme 14.13). 

The reaction of 2-bromoacrolein with a-methyl buta-1,3-diene was demonstrated to be efficiently catalyzed by the chiral titanium complex derived from a sulfonamide of an amino alcohol, affording the DA product with high enantioselectivity [153] (Scheme 14.62). 

Scheme 14.138). A ligand accelerating effect has been observed with the titanium bis (sulfonamide)imido complex. This complex can promote the reaction at room temperature with reaction rate comparable to that of its precursor, supporting the hypothesis of a catalytically active titanium imido intermediate. 

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