Titanium-catalyzed reactions

For the construction of oxygen-functionalized Diels-Alder products, Narasaka and coworkers employed the 3-borylpropenoic acid derivative in place of 3-(3-acet-oxypropenoyl)oxazolidinone, which is a poor dienophile in the chiral titanium-catalyzed reaction (Scheme 1.55, Table 1.24). 3-(3-Borylpropenoyl)oxazolidinones react smoothly with acyclic dienes to give the cycloadducts in high optical purity [43]. The boryl group was converted to an hydroxyl group stereospecifically by oxidation, and the alcohol obtained was used as the key intermediate in a total synthesis of (-i-)-paniculide A [44] (Scheme 1.56). 

A chiral titanium complex with 3-cinnamoyl-l,3-oxazolidin-2-one was isolated by Jagensen et al. from a mixture of TiCl 2(0-i-Pr)2 with (2R,31 )-2,3-0-isopropyli-dene-l,l,4,4-tetraphenyl-l,2,3,4-butanetetrol, which is an isopropylidene acetal analog of Narasaka s TADDOL [48]. The structure of this complex was determined by X-ray structure analysis. It has the isopropylidene diol and the cinnamoyloxazolidi-none in the equatorial plane, with the two chloride ligands in apical (trans) position as depicted in the structure A, It seems from this structure that a pseudo-axial phenyl group of the chiral ligand seems to block one face of the coordinated cinnamoyloxazolidinone. On the other hand, after an NMR study of the complex in solution, Di Mare et al, and Seebach et al, reported that the above trans di-chloro complex A is a major component in the solution but went on to propose another minor complex B, with the two chlorides cis to each other, as the most reactive intermediate in this chiral titanium-catalyzed reaction [41b, 49], It has not yet been clearly confirmed whether or not the trans and/or the cis complex are real reactive intermediates (Scheme 1.60). 

To improve this procedure a highly enantioselective process employing only a truly catalytic amount of the chiral Lewis acid was desirable. It was found that the presence of Molecular Sieves 4A (MS 4A) in the reaction mixture enhances the enantioselectivity in the chiral titanium-catalyzed reaction.(18)... 

The low regioselectivity of the enc reaction of singlet oxygen with mono-alkenes can also be improved by incorporation of trialkylsilyl groups at a vinylic position. Thus, simple vinyl-silanes afforded, after photooxygenation and reduction, regioselectively, /1-silyl allylic alcohols35. The diastereoseleclivity of the titanium-catalyzed reaction of the /1-silyl allylic hydroperoxides is described in Section 4.9.4.2. 

Titanium-Catalyzed Reactions Miscellaneous Mclal-Catalyzcd Reactions Glossary References... 

Ishihara, T., Yamanaka, T., Ando, T. New low-valent titanium catalyzed reaction of chlorodifluoromethyl ketones leading to a,a-difluorinated P-hydroxy ketones. Chem. Lett. 1984,1165-1168. 

Molecular sieves, acting as a cosolvent, have recently been shown to greatly improve the stereoselectivity in some titanium - catalyzed reactions (22). Usually the zeolite selectively traps small molecules like water or alcohol to avoid the destruction of the catalyst or to form the chiral catalyst more efficiently. 

In this chapter only titanium-catalyzed reactions with organic peroxides are compared. 

Schliiter et al. described the synthesis of soluble PPV derivatives from substituted aromatic dialdehydes via McMurry-type polycondensation reaction. With this low valent titanium catalyzed reaction (see Scheme 12), the obtained products are characterized by a double bond cisjtrans ratio of about 0.4 and an average degree of polymerization of about 30 [43]. 

Odom and coworkers [10]. The titanium-catalyzed reaction cascades give rise to highly functionalized substrates, in particular to several types of N-heterocycles (Scheme 13.3). 

When 1,5-diynes were subjected to the titanium-catalyzed reaction with aniline, regioselective hydroamination and subsequent 5-exo-dig cyclization resulted in the formation of the corresponding N-phenyl 2,5-substituted pyrroles ... 

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