Enynes titanium complex

Equation 9.9 shows a remarkable example of the simultaneous asymmetric construction of three stereogenic centers by the aforementioned reaction of an enyne—titanium complex (see Scheme 9.4) [25], using imines derived from optically active phenylethylamine as the electrophile. 

Enyne titanium complexes are synthesized when enynes are treated with Ti(OPr1)2( 72-propene). These species add aldehydes, ketones, and chiral imines to generate multiple stereogenic centers in an acyclic system. Numerous methods for the stereoselective construction of two stereogenic centers are known, although those for more than three in one asymmetric procedure are less common.195... 

Scheme 2.56 Synthesis of functionalized allenes by electrophilic trapping ofthe enyne-titanium alkoxide complex 177.

Subsequently, direct incorporation of GO by titanocene(ii) catalyst, Gp2Ti(GO)2, under a GO atmosphere was reported.This catalytic system showed substantially higher TON and broader functional group compatibility. However, this catalyst fails to react with sterically hindered olefins and alkynes. In a recent contribution from the same group, a series of aryloxide titanium complexes 22 (figure 4) are prepared and shown to promote PKR with some sterically hindered enynes." ... 

In Section 9.2, intermolecular reactions of titanium—acetylene complexes with acetylenes, allenes, alkenes, and allylic compounds were discussed. This section describes the intramolecular coupling of bis-unsaturated compounds, including dienes, enynes, and diynes, as formulated in Eq. 9.49. As the titanium alkoxide is very inexpensive, the reactions in Eq. 9.49 represent one of the most economical methods for accomplishing the formation of metallacycles of this type [1,2]. Moreover, the titanium alkoxide based method enables several new synthetic transformations that are not viable by conventional metallocene-mediated methods. 

The syntheses and structures of similar dipyrrolylmethane complexes have been reported. Alkynes are rapidly hydroaminated by primary amines catalyzed by these Ti derivatives.711 Hydroamination of enynes to generate a,/ -unsaturated imines can be produced using titanium pyrrolyl complexes.712... 

The Pauson-Khand reaction is also mediated by complexes of other transition metals including those of titanium, rhodium and iridium. A number of complexes of these metals have been used to catalyse the intramolecular asymmetric Pauson-Khand reaction of 1,6-enynes to give bicyclo [3.3.0] octenes. The... 

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