Katsuki, sharpless process

A particular class of modified electrodes consists of those containing a layer of asymmetric compounds, and such electrodes are termed chiral. If one uses these electrodes in organic synthesis, the compound produced may also be asymmetric and optically active. One of the better-known examples of such phenomena is called the Sharpless process (Finn and Sharpless, 1986 Katsuki, 1996). In such processes, the electrode is modified by asymmetric compounds that lead to epoxidation and dihy-droxylation of olefenic compounds, but in an asymmetric form. An example is shown in Fig. 11.5, in which the hydroxylation occurs either on the top or the bottom of the enantiomorphic surface. 

As far as the epoxidation of enantiomerically pure acyclic allylic alcohols is concerned, the Katsuki-Sharpless enantioselective epoxidation process can be applied (see Section 4.5.2.4.1. and Houben-Weyl, Vol. E13/2, p 1219, and Table 148, pp 1226-1230). If matched substrate/ catalyst combinations are employed, many, otherwise unselective, epoxidations may be rendered highly diastereoselective. 

Indeed, several interesting procedures based on three families of active catalysts organometallic complexes, phase-transfer compounds and titanium silicalite (TS-1), and peroxides have been settled and used also in industrial processes in the last decades of the 20th century. The most impressive breakthrough in this field was achieved by Katsuki and Sharpless, who obtained the enantioselective oxidation of prochiral allylic alcohols with alkyl hydroperoxides catalyzed by titanium tetra-alkoxides in the presence of chiral nonracemic tartrates. In fact Sharpless was awarded the Nobel Prize in 2001. 

Katsuki and Sharpless reported the new process of asymmetric epoxidation8 using a complex of titanium tetraisopropoxide and diethyl tartrate (DET), and t-butyl hydroperoxide (equation 18). 

The potential of a catalytic process for use on a large scale can be a good indication of its efficiency. During recent decades there has been an increasing tendency to apply asymmetric catalytic processes in industry [1], The asymmetric Noyori hydrogenation [2] and the Sharpless and Jacobsen-Katsuki epoxidation [3] are representative examples of impressive developments in this field [1]. 

Catalytic, asymmetric epoxidations are one of the most important asymmetric processes. In 1980 Katsuki and Sharpless reported a stoichiometric asymmetric epoxidation of allylic alcohols, a method that was later improved to become a catalytic process.9 Moreover, catalytic asymmetric epoxidations of unfunctionalized olefins using salen-manganese complexes have been reported independently by several groups.10-12 In striking contrast to these successful achievements, an efficient catalytic asymmetric epoxidation of enones with broad generality has not been developed.13-22... 

The mechanism of the asymmetric epoxidation of allylic alcohols with the Sharpless-Katsuki catalyst is assumed to be very similar to the one described for the Halcon-ARCO process in Section 2.5. The key point is that the chiral tartrate creates an asymmetric environment about the titanium center (Figure 18). When the allylic alcohol and the t-butyl hydroperoxide bind through displacement of alkoxy groups from the metal, they are disposed in such a way as to direct oxygen transfer to a specific face of the C=C double bond. This point is crucial to maximize enantioselectivity. 

Imido and 0x0 compounds are intermediates in many of the transfers of oxygen atoms and nitrene units to olefins to form epoxides and aziridines, and they are intermediates in many of the insertions of oxygen atoms and nitrene units into the C-H bonds of hydrocarbons to form alcohols and amine derivatives. The enantioselective epoxidation of allylic alcohols (Scheme 13.22) " is the most widely used epoxida-tion process, and the discovery and development of this process was one of the sets of chemistry that led K. Barry Sharpless to receive the Nobel Prize in Chemistry in 2001. The mechanism of this process is not well established, despite the long time since its discovery and development. Nevertheless, most people accept that transfer of the oxygen atom occurs from a titanium-peroxo complex - rather than from an 0x0 complex. Jacobsen s and Katsuki s - manganese-salen catalysts for the enantioselective epoxidations of unfunctionalized olefins, which were based on Kochi s achiral chromium- and manganese-salen complexes, are a second set of... 

While Table 4 describes the situation in 2001 and many additional processes have been reported. The following analysis is still valid. A look at the processes listed in Table 4 shows that hydrogenation of C=C and C=0 is by far the most predominant transformation applied for industrial processes, followed by epoxidation and dihydroxylation reactions. On the one hand, this is due to the broad scope of cataljrtic hydrogenation and on the other hand it could be attributed to the early success of Knowles with the L-dopa process, because for many years, most academic and industrial research was focused on this transformation. The success with epoxidation and dihydroxylation can essentially be attributed to the efforts of Sharpless, Katsuki, and Jacobsen. If one analyzes the structures of the starting materials, it is quite obvious that many of these compounds are often complex and multifunctional, that is, the successful catalytic systems are not only enantioselective but tolerate many functional groups. 

The process is called the Sharpless oxidation, after Sharpless, K. B., Professor, Massachusetts Institute of Technology (MIT). The first descriptons of the process appeared in 1980 (Katsuki.T. Sharpless, K. B. J. Am. Chem. Soa, 1980,102,5974). 

Scheme 8.19. A cartoon representation of the process by which stereospeciflc introduction of an oxygen (as an oxirane) is accomplished in the Sharpless oxidation. Diethyl (2R,3R)-2,3-dihydroxybutane-l,4-dicarboxylate [L-(+)-diethyl tartrate],Et = CHjCHj forms a complex with titanium(IV) Mopropoxide,TiL4, where l = OCH(CHj)2. In the process, the two hydroxyl groups at C2 and C3 of the tartrate displace (replace) two of the four isopropoxy groups originally attached to titanium (see Katsuki,T. Sharpless, K. B. /. Am. Chem. 5oc., 1980,102, 5974).

This Section would be incomplete without a mention of the dramatic breakthrough achieved by Sharpless and Katsuki in the field of asymmetric epoxidation of olefins. Their beautifully simple process (outlined in Scheme 15) provides a timely solution to this problem and will undoubtedly be widely exploited in the near future. 

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