Silanes alkyl halides

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... 

Figure 15. Silane-mediated free-radical carbonylations of alkyl halides in scC02...

The reaction of thiyl radicals with silicon hydrides (Reaction 3.18) is the key step of the so called polarity-reversal catalysis in the radical-chain reduction of alkyl halides as well as in the hydrosilylation of olefins using silane-thiol couple (see Sections 4.5 and 5.1) [33]. The reaction is strongly endothermic and reversible (Reaction —3.18). 

A variety of alkyl halides have been reduced at room temperature, including benzyl halides, primary, secondary and tertiary alkyl halides. The reaction times depend on the halide, and vary between 20min (benzyl bromide, 0.5 mol% 28) up to several days (iodopentane, fluoropentane). The reactivity of alkyl halides decreases in the order R—Br > R—Cl > R—1 when reductions are performed in separate flasks. Several mechanistic details of the reaction have been uncovered by in situ monitoring of the reaction by NMR spectroscopy. The precatalyst 28 appeared to be activated by a rapid reduction of the coordinated acetone to PrO—SiEt3 and concomitant coordination of an alkyl halide (H Scheme 12.11). This complex represents a resting state that is in equilibrium with a o-silane... 

The same research group has further performed radical carbonylation reactions on the same microreactor system [36]. First, alkyl halides were initiated and effectively reacted with pressurized carbon monoxide to form carbonyl compounds. The principle was subsequently successfully extrapolated to the multicomponent coupling reactions. 1-Iodooctane, carbon monoxide and methyl vinyl ketone were reacted in the presence of 2,2 -azobis(2,4-dimethylvaleronitrile) (V-65) as an initiator and tributyltin hydride or tris(trimethylsilyl)silane (TTMSS) as catalyst (Scheme 15). 

Radical chain processes break down whenever the velocity of a termination reaction is comparable to the velocity of the rate-controlling step in a chain reaction. This situation would occur, for example, if one attempted to use EtsSiH as the hydrogen atom donor in the alkyl halide reduction sequence in Figure 4.6 and employed typical tin-hydride reaction conditions because the rate constant for reaction of the silane with an alkyl radical is 4 orders of magnitude smaller than that for reaction of Bu3SnH. Such a slow reaction would not lead to a synthetically useful nonchain sequence, however, because no radical is persistent in this case. In fact, a silane-based radical chain reduction of an alkyl halide could be accomplished successfully if the velocity of the initiation reaction was reduced enough such that it (and, hence, also the velocity of alkyl radical termination... 

An alternative approach to difluoroenoxy silanes developed by the Reims group was described in Sect. 2.3.1 (Eq. 16). Lewis acid-mediated reactions with aldehydes and with alkyl halides were also described, constituting a valuable complementary approach to the Ishihara chemistry. The third approach to the... 

Hydrosilylations of fluorine-containing alkenes are free radical reactions initiated by UV light or organic peroxides The direction of addition is the same as with fluonnated alkyl halides However, the reaction between hydrosilanes and fluorine-containing olefins catalyzed by platinum group metal complexes may result in bidirectional addition and/or formation of a vinylic silane, the latter by de hydrogenative silylation[/] The natures of both the silane and the catalyst affect the outcome of the reaction[7] A random selection of some typical new reactions of silanes are shown in Table 1 [1, 2, 3 4]... 

SILICONK RESINS. The chemistry of the silicones is based on the hydrides, or silanes, the halides, the esters, and the alkyls or aryls. The silicon oxides are composed of networks of alternate atoms of silicon and oxygen so arranged that each silicon atom is surrounded by four oxygen atoms and each oxygen atom is attached to two independent silicon atoms ... 

Fig, 9.34, Apparatus used for the preparation and sampling of Grignard reagents for NMR spectroscopy. The apparatus was evacuated through J2 and the Mg baked. Excess ether and alkyl halide were distilled onto the metal, and the apparatus was sealed off at S3. Upon warming to 0°C and shaking, a rapid reaction occurred. The apparatus was evacuated at Jj, the break-seal B was broken, and ether and excess alkyl halide were removed. Fresh dry ether and the letramethyl silane standard were distilled in, the apparatus was sealed off at S, and a small portion of the solution was filtered through the frit F into the NMR tube, which was sealed off at S2. (Adapted from D. F. Evans and J. P. Mahler, / Chem. Soc.t 1962, 5125.)... 

A much more general method for acyl silane synthesis involving silyl diazo intermediates is illustrated in Scheme 1688. The lithiated derivative of trimethylsilyl diazomethane reacts smoothly with alkyl halides in THF solution to give a-trimethylsilyl diazoalkanes in good yields. Oxidative cleavage of the diazo moiety is effected using 3-chloroperbenzoic acid in benzene solution, to give access to a wide variety of acyl silanes in yields of up to 71%. A phosphate buffer (pH 7.6) is used to prevent side reactions. Aromatic acyl silanes clearly cannot be prepared by this chemistry since an aromatic nucleophilic substitution reaction would be required. 

Radical reductions with [(CH ) Si]3SiH.1 In combination with an initiator, this silane (I) reduces not only alkyl halides, but also thionoesters (used for deoxygenation of secondary alcohols) and selenides. However, cleavage of C-S bonds is inefli-... 

Catalytic reduction of RX.2 This silane can be used in catalytic amounts in the radical reduction of alkyl halides if NaBH4 is added to regenerate the silane from the [(CH3)3Si]3SiX formed on reduction of RX by 1. Yields in the catalytic hydrodehalo-genation process are only slightly lower than those obtained by use of an equimolar amount of the silane. 

Several authors have demonstrated the feasibility of the alkylation of a MSMA substrate to obtain the homologous RSMA via reaction of the corresponding a-silylcarbanion with an alkyl halide. Treatment with LDA of (allyl)(pyrrazolylmethyl) silane gives a carbanion that on trapping with methyl iodide, leads to the unique formation of the corresponding RSMA derivative without traces of compounds that could have resulted from the allylic system. However, when. v-butyllithium is used instead of LDA, partial methylation of this system also occurs.66... 

Allyl[mA(trimethylsilyl)]silane (87) and allyl alkyl sulfone (93), instead of toxic allyltributyltin, can be used for the allylation of alkyl halides under the same conditions as shown below (eqs. 4.34a-4.34c) [96-104]. 

The radical-initiated allylation of alkyl halides with allyltris(trimethylsilyl)silanes proceeds via an SH2 process mediated by a tris(trimethylsilyl)silyl radical.225 The radical-allylating agents react with alkenes, alkynes, and aldehydes via a radical chain process to give the corresponding allylsilylation products.226... 

A general method for a-hydroxyalkylation involves addition of a Grignard reagent or an alkyllithium to a vinyl(alkoxy)silane. Metal-catalyzed coupling of the resulting a-silycarbanion with an alkyl halide (or epoxide) followed by oxidation results in a secondary alcohol. 

Ketone synthesis. The intermediate a in the synthesis of aldehydes can be alkylated in the presence of TMEDA to give the ketone equivalent 2. The silanes are converted into ketones (3) by a sila-Pummerer rearrangement (10, 314). Unfortunately, only primary alkyl halides give satisfactory yields. 

Alkyl halides, including bcnzylic and allylic halides, do not react with trimethyl(trifluoro-methyl)silane. even with molar amounts of tetrabutylammonium fluoride. A convenient method has been developed for the perfluoroalkylation of organic (aryl, vinyl, benzyl and allyl) halides, e.g. formation of 1. using a mixture of trialky (perfluoroalkyl)silane/potas-sium fluoride/copper in dimethylformamide. Under these conditions perfluoroalkylcopper RpCu reagents are produced in situ. 

Tris(thioalkyl)silanes may be prepared by reaction between ClsSiH and thiols in a manner similar to the preparation of alkoxysilanes and are useful reagents for radical reductions of alkyl halides or alkenes in organic chemistry (Scheme... 

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