Group transfer process

Appropriately substituted selenides can undergo cyclization reactions via a group transfer process. 

Group transfer processes are of particular importance in the production of telechelic or di-end functional polymers. 

This chapter will begin with a discussion of the role of chiral copper(I) and (II) complexes in group-transfer processes with an emphasis on alkene cyclo-propanation and aziridination. This discussion will be followed by a survey of enantioselective variants of the Kharasch-Sosnovsky reaction, an allylic oxidation process. Section II will review the extensive efforts that have been directed toward the development of enantioselective, Cu(I) catalyzed conjugate addition reactions and related processes. The discussion will finish with a survey of the recent advances that have been achieved by the use of cationic, chiral Cu(II) complexes as chiral Lewis acids for the catalysis of cycloaddition, aldol, Michael, and ene reactions. 

Hydrolytic enzymes may be regarded as catalysts of group transfer processes. It is often the case in such processes that the group being... 

The half-wave potential for the enzyme-bound Co VCo cobalamin couple of the methionine synthase from E. coli at 526 mV versus SHE is about 80 mV lower than that of the Co /Cokcobalamin couple in neutral aqueous solution. Access to the catalytic cycle of the enzyme by one-electron reduction of Co kcobalamin (and reactivation upon occasional adventitious formation of Co -cobalamin) is indicated to be accomplished by a unique mechanism. The (thermodynamically unfavorable) reduction with intermediate formation of the enzyme-bound Cokcobalamin is driven by a rapid methylation of the highly reduced Co -center of the reduced corrin with Y-adenosyhnethionine. The modular nature of methionine synthase allows for the control of the methyl-group transfer processes by modulating and alternating conformational equilibria. ... 

The poly(methyl methacrylate)s prepared in this experiment, as well as polymers formed in model reactions with silanes that contain bulky substituents (such as phenyldimethyl and diphenylmethyl groups), have predominantly syndiotactic structures identical to polymers prepared by the conventional group-transfer process. This result supports a two-step dissociative mechanism for a group-transfer process, because steric hindrance from bulky silyl groups should increase the proportion of isotactic triads in the hypothetical associative concerted propagation step. 

Labeling experiments performed by Carreira et al. showed that other metal triflates and the related Lewis acids, Yb(OTf)3, Sn(OTf)2, Zn(OTf)2, and LiC104 induced silicon group-transfer processes rather than not metal-catalyzed processes [28]. These observations confirm that 1 is a true Lewis acid catalyst, although the situation would be more complicated when Lewis bases trapping trialkylsilyl groups are contained in the system. 

A similar conjugate addition- silyl group transfer process was reported later by Danishefsky and co-workers for the synthesis of PGF2a (Scheme 52) (103). In this case, the silyl ketene acetal adds, under Hgl2 promotion, cis to the OTBS group in the optically pure enone 52.1 to provide silyl enol ether 52.2 as the exclusive product. The indicated aldol products are obtained from 52.2 in subsequent reactions with ( )- and (Z)-octenal using TiCl4 catalysis. 

The nucleophilic addition of enol silanes with aldehydes to produce P-silyloxy carbonyl adducts 47 is an example of a group-transfer process (Scheme 2), for applications in polymer synthesis, see [64a, 64b, 64cj. In its simplest mechanistic rendition the reaction proceeds upon coordination of the aldehyde to Lewis acid MX4 to afford an activated electrophilic species 42. Addition of the nucleophilic enol silane 43 to 42 leads to C-C bond formation and generation of the aldol adduct. Various intermediate structures 44,45,46 have been postulated to be formed concomitant with or following C-C bond formation. The generation of intermediates 45 and 46 necessitates subsequent silylation of the P-alkoxide furnishing aldol adduct 47 and regenerating catalyst MX4. 

NMR, and the involvement of ATP in phosphoryl-group transfer processes has been studied by NMR (see Figure 5 15). 

Viewing the reaction as a group-transfer process, we see that the PO3 group is transferred from a donor to an acceptor. Therefore, since PO3 is... 

Ulstrup J. Temperature dependence of the transfer coefficient in electron and atom group transfer processes. Electrochim Acta 1984 29 1377-80. 

Proton-transfer and group-transfer processes have much more in common with each other and with the general run of chemical reactions than electron-transfer processes have. Electron-transfers (Chapter 9) are unique in that the mass transferred is very small compared with the masses of the adjacent groups. This special feature (coupled with the large changes of charge distribution) sets electron-transfer reactions apart from all other types of reaction in important respects. These differences will become clear as we proceed. 

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