Epoxidation titanium-catalyzed

The titanium-catalyzed AE reaction is a fairly robust system and it can be performed on substrates containing a wide range of different functional groups (FGs) (Table 6.3) [13]. However, it is important to point out that an intramolecular reaction with the formed epoxide is possible whenever the FG present in the molecule has a favorable position to facilitate such a transformation. An illustration of this phenomenon is presented in Eq. (1) [28]. 

While the titanium-catalyzed epoxidation reaction was flourishing, the pioneer version of the reaction remained undeveloped and it was not until 1999 that new... 

This approach provides a new method for carbohydrate synthesis. In the synthesis of tetritols, pentitols, and hexitols, for example, titanium-catalyzed asymmetric epoxidation and the subsequent ring opening of the thus formed 2,3-epoxy alcohols can play an essential role. 

SCHEME 59. Titanium-catalyzed asymmetric epoxidation of both enantiomers of aUyUc alcohol 39j... 

SCHEME 62. Titanium-catalyzed enantioselective epoxidation with different chiral hydroperoxides... 

In 2003, Lattanzi and coworkers reported on the use of the tertiary camphor-derived hydroperoxide 61 in the titanium-catalyzed asymmetric epoxidation of allylic alcohols (equation 39f. Yields were moderate and ranged between 30 and 59%. Also, the enantio-selectivities obtained were only moderate (24-46%). After the reaction, the enantiomeri-cally pure camphor-derived alcohol can be recovered in good yields by chromatography without loss of optical purity and can be reconverted into the corresponding hydroperoxide. 

SCHEME 70. Titanium-catalyzed epoxidation of ene-diols 145 by, 6-hydroperoxy alcohols 148... 

Asymmetric epoxidation. Sharpless et a .1 have reviewed the numerous applications of titanium-catalyzed asymmetric epoxidations developed in their own and other laboratories. All the reactions conform to the enantiomeric selectivity first observed and formulated as in Scheme (I). 

Since the introduction of the titanocene chloride dimer 67a to radical chemistry, much attention has been paid to render these reactions catalytic. This field was reviewed especially thoroughly for epoxides as substrates [123, 124, 142-145] so only catalyzed reactions using non-epoxide precursors and a few very recent examples of titanium-catalyzed epoxide-based cyclization reactions, which illustrate the principle, will be discussed here. A very useful feature of these reactions is that their rate constants were determined very recently [146], The reductive catalytic radical generation using 67a is not limited to epoxides. Oxetanes can also act as suitable precursors as demonstrated by pinacol couplings and reductive dimerizations [147]. Moreover, 5 mol% of 67a can serve as a catalyst for the 1,4-reduction of a, p-un saturated carbonyl compounds to ketones using zinc in the presence of triethylamine hydrochloride to regenerate the catalyst [148]. 

I to 5. 0 mol per mol of hydroperoxide. The presence of sodium naphtheoate, by prevenling side reaction, helps to reduce the excess propylene required (from lO/l to 2/1 in moles). In the Shell technology, epoxidation is catalyzed by metallic oxides (molybdenum, vanadium, titanium, etc.) supported on sih cau The liighiy exothe c reaction takes place around 100 to 130 at 3.5.10 Pa absolute. Hydroperoxide conver> sion is very hi (> 97 per cent). Propylene oxide molar selectivity exceeds 70 per cent and that of the styrene precursors 93 per cent As for propylene, its once-through conversion is about 15 per cent, for a oxide molar selectivity greater than 90 per cent, and the main by-products are dimers and heavier hydrocarbons. 

Many other reports of ligand libraries for specific catalytic applications have been reported. Among them, Gilbertson and co-workers reported a chiral phosphine library, tested in the rhodium-catalyzed asymmetric hydrogenation of an enamide (158,159), and a similar library for the palladium-catalyzed allylation of malonates (160, 161) Hoveyda and co-workers (162, 163) reported a chiral Schiff base library, screened in the titanium-catalyzed opening of epoxides with (TMSCN) (trimethyl silyl cyanide) ... 

The literature has been reviewed through 1989 for the purposes of preparing this chapter but the documentation herein is not intended to be comprehensive. Other reviews have covered various aspects of asymmetric epoxidation including synthetic applications through 1984, a thorough compilation of uses through early 1987 and an extensive discussion of the mechanism of the reaction. Use of homochiral epoxy alcohols in the synthesis of polyhydroxylated compounds, e.g. sugars, and for the preparation of various synthetic intermediates has been reviewed.A personal account of the discovery of titanium-catalyzed asymmetric epoxidation has been recorded." A comprehensive review of titanium-catalyzed asymmetric epoxidation is planned."... 

The essence of titanium-catalyzed asymmetric epoxidation is illustrated in Figure 1. As shown there, the four essential components of the reaction are the allylic alcdiol substrate, a titanium(IV) alkoxide, a chiral tartrate ester and an aUcyl hydroperoxide. The asynunetric complex formed from these reagents de-... 

This section presents a summary of the currently preferred conditions fw perfrmning titanium-catalyzed asymmetric epoxidations and is derived primarily from the detailed account of Gao et al We wish to draw the reader s attention to several aspects of the terminology used here and throughout this chapter. The terms titanium tartrate complex and titanium tartrate catalyst are used interchangeably. The term stoichiometric reaction refers to the use of the titanium tartrate complex in a stoichiometric ratio (100 mol %) relative to the substrate (allylic alcohol). The term catalytic reaction (or quantity) refers to the use of the titanium tartrate complex in a catalytic ratio (usually S-10 mol %) relative to the substrate. 

K. Barry Sharpless and Tsutomu Katsuki Titanium-catalyzed asymmetric epoxidation... 

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