Diisopropoxytitanium dihalide

Mikami and Nakai et al. have developed a chiral titanium catalyst for the glyoxylate-ene reaction, which provides the corresponding a-hydroxy esters of biological and synthetic importance [7] in an enantioselective fashion (Scheme 8C.3) [8,9]. Various chiral titanium catalysts were screened [ 10]. The best result was obtained with the titanium catalyst (1) prepared in situ in the presence of MS 4A from diisopropoxytitanium dihalides (X2Ti(OPr,)2 X=Br [11] or Cl [12]) and enantiopure BINOL or 6-Br-BINOL [13], The remarkable levels of enantiose-lectivity and rate acceleration observed with these BINOL-Ti catalysts (1) [14] stem from the... 

In our research on the asymmetric catalysis of the carbonyl-ene reaction, we found that the BINOL-Ti complexes (1) [30], prepared in situ, in the presence of 4-A molecular sieves, from diisopropoxytitanium dihalides (X2Ti(OPr )2 X = Br [31] or Cl [32]) and optically pure BiSfOL (vide infra), catalyze [33], rather than promote stoichiome-trically, the carbonyl addition reaction of allylic silanes and stannanes [34]. The addition to glyoxylate of ( )-2-butenylsilane and -stannane proceed smoothly to afford the syn product in high enantiomeric excess (Sch. 5). The s yn-product thus obtained could be readily converted to the iaetone portion of verrucaline A [35]. 

The catalysed carbonyl-ene reaction frequently employs reactive aldehydes, especially glyoxalate esters. Mikami s group has studied the titanium/BINOL catalysed carbonyl-ene reaction in considerable detail. Typically, the catalyst is prepared in situ from diisopropoxytitanium dihalide and BINOL in the presence of 4A molecular sieves. Thus, alkenes (7.179) and (7.180) are converted into the homoallylic alcohols (7.181) and (7.182) with high enantioselectivity. Typical examples use up to 10 mol% of catalysts, but variation in the catalyst preparation allows the use of only 0.2 mol%. ... 

Me02CCH0/H+ -7.90 eV, vide infra), by the BINOL-Ti catalyst (1) (Table 1). The reaction was carried out by simply adding an olefin and tfien freshly dehydrated and distilled fluoral (2a) at 0 C to the solution of the chiral titanium complex (1) prepared from (/ )- or (5)-BINOL and diisopropoxytitanium dihalide in the presence of molecular sieves MS 4A as described for die glyoxylate-ene reaction (5). The reaction was completed widiin 30 min. The ene-type product, namely homoallylic alcohol (3) was obtained along with the allylic alcohol (4) (entries 1-4). The enantiomeric purities of both products were determined to be more than 95% ee by 1h NMR analysis after transformation to the (5)- and (/ )-MTPA ester derivatives. Thus, the absolute configuration of the products was determined by the Mosher method (8). The sense of asymmetric induction is, therefore, exactly the same as observed for the glyoxylate-ene reaction the (/ )-catalyst provides the (/ )-alcohol products (5). 

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