Alkyl chlorides, reduction

Direct Alkyl Chloride Reductions. It was previously pointed out that at 40° in HF-TaF5 methyl- and ethyl-chloride do not form the expected alkylation product with alkanes, expected that is, assuming traditional carbenium ion chemistry, but rather underwent an independent reaction. The second part of this paper will describe in more depth what reactions do take place when alkyl chlorides react in the presence or absence of the lower alkanes under our reaction conditions of 40°C. 

The yield of the more active RRR-a-tocopherol can be improved by selective methylation of the other tocopherol isomers or by hydrogenation of a-tocotrienol (25,26). Methylation can be accompHshed by several processes, such as simultaneous halo alkylation and reduction with an aldehyde and a hydrogen haUde in the presence of staimous chloride (27), amino alkylation with ammonia or amines and an aldehyde such as paraformaldehyde followed by catalytic reduction (28), or via formylation with formaldehyde followed by catalytic reduction (29). 

Reduction of simple alkyl halides to alkanes by ethylenediamine complexes of Cr(II), denoted Cr"(en) occurs readily , e.g. for isopropyl chloride in aqueous dimethylformamide at 25 °C simple second-order behaviour is found with ki dependent on [en]/[Cr(II)] but reaching a limiting value of 1.6x 10 l.mole . sec . Competition studies between a mixture of two alkyl chlorides for Cr(II) was achieved by estimating alkane products by gas-liquid chromatography and... 

Alkyl Halides. Commonly, reductions with liquid silanes and liquid alkyl halides do not require the use of a solvent.186 When the alkyl halide is a solid, either pentane186 or dichloromethane may be used as solvent.192 No significant difference in reactivities is observed between alkyl chloride and bromide substrates,186 but allyl halides are more reactive than 2-halopropanes, which, in turn, are more reactive than 1-halopropanes.190,146... 

Reduction to Halocarbons. The best conditions for the reductive chlorination of ketones use the reagent combination Me2ClSiH/In(OH)3 (Eq. 241).331 Examples include conversions of aryl ketones to benzyl chlorides, ethynyl ketones to propargyl chlorides, and alkyl ketones to alkyl chlorides (Eq. 242).331 Addition of lithium iodide to the reaction mixture yields the corresponding iodide product. The combination of TMDO/I2 reductively iodinates aryl ketones and aldehydes in good yields (Eq. 243).357... 

Allyl halides, reduction reactions, 31 Aluminum chloride reagent/catalyst alkyl halide reduction, 30-31 secondary alkyl alcohol reduction, 14-15... 

The direct reduction of haloalkynes using either mercury or vitreous carbon as the cathode has been examined in considerable detail [80-84] one example is portrayed in Eq (77). The influence of reduction potential, current consumption, proton donor, electrode, and substrate concentration on the course of the process has been examined. Vitreous carbon electrodes are preferred, though mercury has been used in many instances. Unfortunately, these reactions suffer from the formation of diorganomercurials. While both alkyl iodides and bromides can be used, the former is generally preferred. Because of their higher reduction potential, alkyl chlorides react via a different mechanism, one involving isomerization to an allene followed by cyclization [83]. 

Alkyl chlorides are with a few exceptions not reduced by mild catalytic hydrogenation over platinum [502], rhodium [40] and nickel [63], even in the presence of alkali. Metal hydrides and complex hydrides are used more successfully various lithium aluminum hydrides [506, 507], lithium copper hydrides [501], sodium borohydride [504, 505], and especially different tin hydrides (stannanes) [503,508,509,510] are the reagents of choice for selective replacement of halogen in the presence of other functional groups. In some cases the reduction is stereoselective. Both cis- and rrunj-9-chlorodecaIin, on reductions with triphenylstannane or dibutylstannane, gave predominantly trani-decalin [509]. 

Reduction of alkyl and benzyl halides proceeds in two one-electron addition steps. The first detectable product is the alkyl or benzyl radical and this is reduced further to the carbanion. Some alkyl iodides show two polarographic waves corresponding to the two steps. Alkyl bromides show only one two-electron wave and alkyl chlorides are not reducible in the available potential window. Benzyl halides also show only one wave and benzyl chlorides are reducible in the available potential range. Reduction potentials measured in dimethylformamide are collected in... 

The synthesis of organozinc compounds by electrochemical processes from either low reactive halogenated substrates (alkyl chlorides) or pseudo-halogenated substrates (phenol derivatives, mesylates, triflates etc.) remains an important challenge. Indeed, as mentioned above, the use of electrolytic zinc prepared from the reduction of a metal halide or from zinc(II) ions does not appear to be a convenient method. However, recent work reported by Tokuda and coworkers would suggest that the electroreduction of a zinc(II) species in the presence of naphthalene leads to the formation of a very active zinc capable of reacting even with low reactive substrates (equation 23)11. 

Protodehalogenation.1 This hydride is the reagent of choice for hydrogenolysis of alkyl halides. Lithium aluminum hydride is somewhat less powerful, particularly for reduction of alkyl chlorides. 

Unsubstituted hydrazones of aromatic ketones and aldehydes have been converted in high yield to alkyl chlorides under Swern oxidation conditions, although the substrate actually undergoes a net reduction.93 When the hydrazone is dideuterated, a deuterium ends up on the carbon, supporting the proposed intermediacy of cation (34), which tautomerizes and loses N2, to give a carbocation which combines with the chloride. 

The unsubstituted hydrazones derived from aromatic ketones and aldehydes are converted to the corresponding alkyl chlorides, in high yield, under Swern oxidation conditions. In this unusual oxidation/reduction sequence, the substrate undergoes a net reduction. Unsubstituted hydrazones derived from cyclohexyl ketones yielded elimination products. The mechanism in Scheme 7 has been postulated.111... 

Reduction of RX to RH.1 In the presence of di-r-butylhyponitrite (initiator) and a thiol,2 triethylsilane reduces alkyl chlorides, bromides, or iodides to alkanes in >91% yield by a chain reaction in which the thiol effects transfer of H from the silane to an alkyl radical. This reduction is generally effected with a R3SnH, which is toxic and more difficult to remove from the products. 

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