Selective exclusion/transfer

Much use has been made of micellar systems in the study of photophysical processes, such as in excited-state quenching by energy transfer or electron transfer (see Refs. 214-218 for examples). In the latter case, ions are involved, and their selective exclusion from the Stem and electrical double layer of charged micelles (see Ref. 219) can have dramatic effects, and ones of potential imfKntance in solar energy conversion systems. 

Figure 4.2 Selective exclusion and selective transfer temperatures for long-chain alkanes plotted against boiling points. Data supplied by and reproduced with permission of Anatune Ltd.

The role of GTP in EF-Tu-promoted binding of aminoacyl-tRNA could be formulated as follows. First, a unique conformation of EF-Tu induced by GTP can select exclusively the aminoacylated form of tRNA in preference to its deacylated form. Second, the ternary complex, aminoacyl-tRNA-EF-Tu-GTP, is transferred to a precise location on the 508 ribosomal subunit through the conformation of EF-Tu-GTP favorable for interaction with ribosomes. The conformational change of aminoacyl-tRNA induced by complexing with EF-Tu-GTP may also serve for this interaction. Third, after the transfer of aminoacyl-tRNA to the A site of ribosomes, EF-Tu is to be released from ribosomes to reinitiate a new cycle of reactions. This could be accomplished by the hydrolysis of bound GTP to GDP. An additional advantage of the split of GTP is to shift the equilibrium irreversibly... 

The reaction of dihalocarbenes with isoprene yields exclusively the 1,2- (or 3,4-) addition product, eg, dichlorocarbene CI2C and isoprene react to give l,l-dichloro-2-methyl-2-vinylcyclopropane (63). The evidence for the presence of any 1,4 or much 3,4 addition is inconclusive (64). The cycloaddition reaction of l,l-dichloro-2,2-difluoroethylene to isoprene yields 1,2- and 3,4-cycloaddition products in a ratio of 5.4 1 (65). The main product is l,l-dichloro-2,2-difluoro-3-isopropenylcyclobutane, and the side product is l,l-dichloro-2,2-difluoro-3-methyl-3-vinylcyclobutane. When the dichlorocarbene is generated from CHCl plus aqueous base with a tertiary amine as a phase-transfer catalyst, the addition has a high selectivity that increases (for a series of diolefins) with a decrease in activity (66) (see Catalysis, phase-TRANSFEr). For isoprene, both mono-(l,2-) and diadducts (1,2- and 3,4-) could be obtained in various ratios depending on which amine is used. 

Diastereoselective hydroformylation can be achieved in special cases through passive substrate control in which conformational preferences are transferred in the corresponding selectivity-determining hydrometalation step [4-6]. A recent example is the highly diastereoselective hydroformylation of a kainic acid derivative (Scheme 17) [64], The selective formation of the major diastereomer has been explained via a reactive substrate conformation in which allylic 1,2-strain has been minimized. In this situation the czs-positioned methylene carbonylmethoxy group controls the catalyst attack to occur from the si face exclusively. 

The isopropyl group discourages P-H transfer, leading to the exclusive formation of Al-PEs. The Al-PEs can be readily transformed to a variety of functionalized PEs and to PE-based and polar polymer-based block and graft copolymers, using established methods. The selective synthesis of vinyl- and Al-terminated PEs with Zr-FI catalysts shows the critical importance of the substituent on the imine-N for polymerization catalysis. 

Complex 9 (Scheme 43.3) can be reduced by different redox equivalents to the active rhodium(I) species 10 namely, by electrons, formate [37, 38], and hydrogen. This hydrido complex then transfers the hydride ion onto the nicotinamide. In electrochemical applications, TOFs in the range of 5 to 11 h-1 have been reported [31, 39]. It is noteworthy that this complex accepts NAD+ and NADP+ as substrates with the same efficiency and almost exclusively produces the 1,4-reduced cofactor (selectivity >99%). 

This system fulfills the four above-mentioned conditions, as the active species is a rhodium hydride which acts as efficient hydride transfer agent towards NAD+ and also NADP+. The regioselectivity of the NAD(P)+ reduction by these rhodium-hydride complexes to form almost exclusively the enzymatically active, 1,4-isomer has been explained in the case of the [Rh(III)H(terpy)2]2+ system by a complex formation with the cofactor[65]. The reduction potentials of the complexes mentioned here are less negative than - 900 mV vs SCE. The hydride transfer directly to the carbonyl compounds acting as substrates for the enzymes is always much slower than the transfer to the oxidized cofactors. Therefore, by proper selection of the concentrations of the mediator, the cofactor, the substrate, and the enzyme it is usually no problem to transfer the hydride to the cofactor selectively when the substrate is also present [66]. This is especially the case when the work is performed in the electrochemical enzyme membrane reactor. 

The selective 1,4-reduction of a,p-unsaturated carbonyl compounds is always a challenge, but it has been met successfully by the use of dithionite under phase-transfer conditions. Reduction proceeds in high yield to the total exclusion of saturated or allylic alcohols (Table 11.10) [5, 6], Exocyclic and endocyclic conjugated C=C double bonds are reduced with equal ease, whereas non-conjugated double bonds remain intact. The predominant reduction pathway for conjugated dienoic... 

Unsubstituted cycloamyloses have been used to catalyze a number of reactions in addition to acyl group transfer. Brass and Bender (8) showed that cycloamyloses promoted phenol release from diphenyl and bis(p-nitro-phenyl) carbonates and from diphenyl and bis(m-nitrophenyl)methyl phos-phonates. Breslow and Campbell (10,11) showed that the reaction of anisole with HOCL in aqueous solution is catalyzed by cyclohexaamylose and cycloheptaamylose. Anisole is bound by the cyclodextrins and is chlorinated exclusively in the para position while bound. Cycloheptaamylose has been used to promote regiospecific alkylation followed by the highly selective oxidation shown in reaction (3) (95). In addition cycloheptaamylose effec-... 

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