Alkyl transfer reaction

Sn2 reactions can be thought of as alkyl transfer reactions, and Sn2 characteristics can be anticipated by examining analogous proton transfer reactions. 

C(3)-Zn-N(21) [133.1(3)°] in the molecular structure of 15 and implies a possible stereo-selective induction for future alkyl transfer reactions. 

Transition State Models. The stoichiometry of aldehyde, dialkylzinc, and the DAIB auxiliary strongly affects reactivity (Scheme 9) (3). Ethylation of benzaldehyde does not occur in toluene at 0°C without added amino alcohol however, addition of 100 mol % of DAIB to diethylzinc does not cause the reaction either. Only the presence of a small amount (a few percent) of the amino alcohol accelerates the organometallic reaction efficiently to give the alkylation product in high yield. Dialkyl-zincs, upon reaction with DAIB, eliminate alkanes to generate alkylzinc alkoxides, which are unable to alkylate aldehydes. Instead, the alkylzinc alkoxides act as excellent catalysts or, more correctly, catalyst dimers (as shown below) for reaction between dialkylzincs and aldehydes. The unique dependence of the reactivity on the stoichiometry indicates that two zinc atoms per aldehyde are responsible for the alkyl transfer reaction. 

Since the original report, about 12 other reagents have been reported to catalyze this reaction, but this reagent of Noyori (1) and that of Oguni (15,268) seem to be the most effective. In addition, both can effect chiral amplification, an increase of enan-tioselectivity over that of the catalyst. Noyori suggests that the alkyl transfer reaction involves a dinuclear Zn complex such as 3, whose structure has been established by... 

The rate was increased by the presence of hydrogen. A rather complex polymerization mechanism is proposed in which the propagation step is regarded as an alkyl transfer reaction, followed by realkylation with monomer of the metal hydride so formed. 

Using a curve fitting procedure from the experimental data propagation and alkyl transfer reaction rates and activation energies were calculated. Values for fep and fetr.A 40°C are included in Tables 6 and 8. The catalyst efficiency varied from 0.016 to 0.10 with most of the data in the range 0.01 to 0.04. 

A typical investigation to prove kinetically the formation of a sulfurane as an intermediate in the alkyl transfer reactions at the sulfonium sulfur has been demonstrated by Young and co-workers [33]. They used S-substituted benzyl-S-methyl-S-(substituted)phenylsulfonium salts (3) with amines as nucleophiles. The Hammett per relationship and the / -values of the amines suggest that the reactions are involved in equilibrium formation of a sulfurane 4 from which ligand coupling takes place to give the final N-benzylamines and thioanisole derivatives, as shown in Scheme 2. 

Influence of dehydrogenating component zinc on product pattern of light naphtha conversion over the Zn/HZSM-5 catalyst is similar as it is observed in case of n-heptane (Table 7). Increase in aromatic yield with enhanced selectivity towards toluene and decreased selectivity to C9+ aromatics observed over Zn/HZSM-5 catalyst can be explained by the additional path ways Km2, Km4 provided by zinc. In addition to this, an interesting change in selectivity for benzene was observed in light naphtha aromatization. Benzene yield has decreased from 6.8 wt % to 3.6 wt % over HZSM-5 catalyst, while it has increeised from 6.8 wt % to 8.1 wt % over the Zn/HZSM-5 catalyst. The decrease in benzene concentration over the HZSM-5 catalyst may be due to the alkylation of benzene facilitated in presence of olefmic intermediates formed during the reaction. It appears that, acid catalyzed alkyl transfer reactions are reduced over Zn/HZSM-5, presumably due to modifying effect of Zn on HZSM-5. This assumption explains, why the concentration of benzene is more in the product formed over Zn/HZSM-5. 

In similar experiments, the activation energies of the propagation reaction and of the A1 alkyl transfer reaction were determined to be 29 and 58 kJ mol , respectively. 

In order to learn more about the chemistry of alkyl transfer reactions, we have carried out kinetic studies on several non-enzymic model reactions (1). Our most recent efforts have involved the study of reactions (1) and (2), as models for the enzyme-catalyzed methylation of 2 -hydroxyl groups of tRNA (15) and the phenolic hydroxyl groups of catecholamines (16) respectively. The rate-pH profiles for reactions (1) and (2) are shown in Fig. 1 (T=40 , y=1.0 M). The values for k are buffer-independent rates obtained by extrapolating linear plots of k gj total buffer concentration ([B.j,]) to [Bj] = 0. Buffer catalysis of reactions at sp carbon is rare (1), and the buffer catalysis observed in reactions (1) and (2) will be discussed in detail below. 

The authors reported copper-mediated highly diastereoselective alkyl transfer reaction of 5-hydroxy-4,4-difluoro-2-alken-l-ol (7) with trialkylaluminum (Scheme 6.3)... 

Since organoaluminums are highly oxophilic, the reaction of organoaluminum compounds and various carbonyl compounds gives 1 1 Lewis acid-base complexes. In the cases of alkylaluminums and electrophilic carbonyl compounds, alkyl transfer reactions to carbonyl carbon proceed via Lewis acid-base complexes at high temperature [37, 38]. 

Since acetals are more reactive than the corresponding carbonyl compounds under Lewis acid-catalyzed conditions, alkyl transfer reaction of acetals provides alkylated ethers in high yield. Recently, combined use of AlBrs/MesAl (10 1) and CuBr was reported as an excellent catalytic system for allylation reactions of dimethyl acetals with allyltrimethylsilane (Scheme 6.34) [40]. Catalytically active species are believed to be a mixed Al-Cu species. MesAl acts as a desiccant scavenging harmful HBr from the reaction mixture. 

Pontoni G, Coward JK. Stereochemical studies of enzyme-catalyzed alkyl-transfer reactions. An NMR method for distinguishing between the two prochiral hydrogens at C-1 of spermidine. Tetrahedron Lett. 1983 23 151-154. 

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