Alkyl benzyl carbonates, reduction

The efficiency of reduction of benzophenone derivatives is greatly diminished when an ortho alkyl substituent is present because a new photoreaction, intramolecular hydrogen-atom abstraction, then becomes the dominant process. The abstraction takes place from the benzylic position on the adjacent alkyl chain, giving an unstable enol that can revert to the original benzophenone without photoreduction. This process is known as photoenolization Photoenolization can be detected, even though no net transformation of the reactant occurs, by photolysis in deuterated hydroxylic solvents. The proton of the enolic hydroxyl is rapidly exchanged with solvent, so deuterium is introduced at the benzylic position. Deuterium is also introduced if the enol is protonated at the benzylic carbon by solvent ... 

In the asymmetric reduction of ketones, stereodifferentiation has been explained in terms of the steric recognition of two substituents on the prochiral carbon by chirally modified reducing agents40. Enantiomeric excesses for the reduction of dialkyl ketones, therefore, are low because of the little differences in the bulkiness of the two alkyl groups40. In the reduction of ketoxime ethers, however, the prochiral carbon atom does not play a central role for the stereoselectivity, and dialkyl ketoxime ethers are reduced in the same enantiomeric excess as are aryl alkyl ketoxime ethers. Reduction of the oxime benzyl ethers of (E)- and (Z)-2-octanone with borane in THF and the chiral auxiliary (1 R,2S) 26 gave (S)- and (R)-2-aminooctane in 80 and 79% ee, respectively39. 

Another case of an apparent 1,5-C-H activation is the aryl to homobenzylic migration mentioned before in Sect. 2.4.2 [67], When both the alkyl substitutions on the benzylic carbon are secondary alkyl groups, the steric bulkiness leads to a reduction in homobenzylic 1,4-C-H activation and affords instead a product apparently arising by a 1,5-C-H activation on the bis(homo)benzylic positions (8 and 9). Again, the exact nature of the mechanism has yet to be investigated. 

Cu(II) to yield a paramagnetic Cu(II)-thiolate complex/ Carbon dioxide coordinated to Ni(0) may be reduced to CO by R—SH (R = H, alkyl, benzyl, phenyll, or substituted phenyl) providing a model for carbon monoxide dehydrogenase enzyme. The disulfide generated in this reaction reacts further with the Ni(0) complex to give Ni(SR)2/ The kinetics of the reduction of Cr(VI) by l-methionine (represented by RSCH3) provides the rate law (45), with values for /c2... 

The p-chloro analog of phentermine has much the same activity as the parent compound, with perhaps a somewhat decreased activity on the central nervous system. Alkylation of p-chloro-benzyl chloride with the carbanion obtained from treatment of 2-nitropropane with strong base affords the compound containing the required carbon skeleton (74). Catalytic reduction of the nitro group yields chlorphentermine (75). ... 

Although the tin hydride reductions of alkyl halides seem simple, one must be careful because these reactions occur by a free radical mechanism. This is important, because the carbon radical produced in the reaction can isomerize68,78 and one often obtains two different stereoisomers from the synthesis. Another problem is that chiral centres can be lost in tin hydride reductions when an optically active halide is reduced. One example of this is the reduction of benzyl-6-isocyanopenicillanate with tributyltin deuteride78 (Scheme 14). The amount of isomerization depends on the temperature, the concentration of the tin hydride and the presence of and /-substituents78-82. However, some authors have reported tin hydride reductions where no racemization was observed78. 

The cation 18 did not show any DNA cleavage, due probably to its inherent stability (18 would be more stable than a benzyl cation [66], relatively stable ions that do not alkylate the heterocyclic bases in DNA). The 9-diazofluorene 19 would not undergo reduction like the corresponding diazonium salts (17) because of the presence of a negative charge on the fluorenyl carbon. 

Catalytic processes based on the use of electrogenerated nickel(O) bipyridine complexes have been a prominent theme in the laboratories of Nedelec, Perichon, and Troupel some of the more recent work has involved the following (1) cross-coupling of aryl halides with ethyl chloroacetate [143], with activated olefins [144], and with activated alkyl halides [145], (2) coupling of organic halides with carbon monoxide to form ketones [146], (3) coupling of a-chloroketones with aryl halides to give O -arylated ketones [147], and (4) formation of ketones via reduction of a mixture of a benzyl or alkyl halide with a metal carbonyl [148]. 

Reduction potentials for alkyl and benzyl halides measured by cyclic voltammetry at 0.1 or 0.2 V s" with a glassy carbon electrode in dimethylformamide. 

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