Radicals reaction with silanes

Table 1 shows the kinetic data available for the (TMSjsSiH, which was chosen because the majority of radical reactions using silanes in organic synthesis deal with this particular silane (see Sections III and IV). Furthermore, the monohydride terminal surface of H-Si(lll) resembles (TMSjsSiH and shows similar reactivity for the organic modification of silicon surfaces (see Section V). Rate constants for the reaction of primary, secondary, and tertiary alkyl radicals with (TMSIsSiH are very similar in the range of temperatures that are useful for chemical transformations in the liquid phase. This is due to compensation of entropic and enthalpic effects through this series of alkyl radicals. Phenyl and fluorinated alkyl radicals show rate constants two to three orders of magnitude... 

The majority of sequential radical reactions using silanes as mediators for the C—C bond formation deal with (TMS)3SiH. Nevertheless, as we shall see there are some interesting applications using other silanes, too. 

Radical reactions with acyl radicals sometimes involve decarbonylation as side-reactions, especially when stabilized secondary or tertiary radicals can be formed. These side-reactions can be suppressed using low-temperature reaction conditions together with different reducing agents such as tris(trimethylsilyl)silane.254... 

Functionally substituted benzylic, allylic, and vinylic compounds containing alkoxides, esters, ethers, nitriles, or amides can be reacted with halosilanes under Barbier conditions using HMPT to yield C- and O-silylated products, 1,2- or 1,4-addition products, as well as reductive dimers. Radical and anionic intermediates are postulated, based on SET reactions from the metal, and multiple silated species can be obtained. The use of the TMSCl-Mg-HMPT system has been extensively investigated by Galas group [85] at the University of Bordeaux, and their work has greatly advanced the science of the Barbier reaction with silanes. 

Most alkyl radical clocks with rate eonstants smaller than 1 x 10 s at ambient temperatures are ultimately calibrated against LFP-determined BuaSnH trapping kinetics [17, 29] this includes cases where the second-order competition studies were performed with tra-(trimethylsilyl)silane, (Me3Si)3SiH, because rate constants for alkyl radical reactions with the silane were determined via clocks that were eali-brated against tin hydride [30]. The rate constants for radical 1-10 depend on multiple methods, those for 1-11 depend on nitroxyl trapping kinetics, and those for... 

Synthetic strategies based on multistep radical reactions have steadily grown in popularity with time. The knowledge of radical reactivity has increased to such a level as to aid in making the necessary predictions for performing sequential transformations.Silanes, and in particular (TMSlsSiH, as mediators have contributed substantially in this area, with interesting results in terms of reactivity and stereoselectivity. ... 

The reaction between MAH, PE, clay and the catalyst must take place essentially simultaneously. If the clay is first reacted with MAH at elevated temperatures, analogous to the treatment of clay with silane coupling agents, a clay maleate half ester or diester is probably formed. The latter does not readily react with PE even in the presence of a free radical catalyst. 

The review is divided into sections according to the type of metal hydride for convenience in discussing the information systematically. At one extreme, kinetic studies have been performed with many types of silicon hydrides, and much of the data can be interpreted in terms of the electronic properties of the silanes imparted by substituents. At the other extreme, kinetic studies of tin hydrides are limited to a few stannanes, but the rate constants of reactions of a wide range of radical types with the archetypal tin hydride, tributylstannane, are available. Kinetic isotope effects for the various hydrides are collected in a short section, and this is followed by a section that compares the kinetics of reactions of silicon, germanium, and tin hydrides. 

Although we will deal with organic radicals in solution, it is worth mentioning that the reactivity of atoms and small organic radicals with silanes in the gas phase has been studied extensively. For example, the bond dissociation energies of a variety of Si-H bonds are based on the reaction of iodine or bromine with the corresponding silanes.1... 

It is noteworthy that the absolute rate constants for the reaction of the benzophenone triplet with Et3SiH, n-C5HnSiH3, PhSiH3, and Cl3SiH have been measured by LFP,56 and comparison of the kinetic data with corresponding data for reactions of /-BuO radicals shows that these two transient species have a rather similar reactivity toward silanes. Furthermore, the xanthate and the p-methoxyacetophenone triplets were found to be more and less reactive, respectively, than the benzophenone triplet with Et3SiH.56 Similar behavior of excited states in reactions with tin hydrides is discussed in Section V. 

Several reactions of halogen-substituted carbon-centered radicals with silanes have been studied, but limited kinetic information is available for reactions of halogen-substituted radicals with tin hydrides. A rate constant for reaction of the perfluorooctyl radical with Bu3SnH was determined by competition against addition of this radical to styrenes, reactions that were calibrated directly by LFP methods.93 At ambient temperature, the n-C8F17 radical reacts with tin hydride two orders of magnitude faster than does an alkyl radical, consistent with the electron-deficient nature of the perflu-oroalkyl radical and the electron-rich character of the tin hydride. Similar behavior was noted previously for reactions of silanes with perhaloalkyl radicals. 

In theoretical work, the initial steps in the polymerization of 1,1-dicyano-, 1,1-difluoro-, and 1,1-dimethyl-cyclopropanes by reaction with H, OH, and Me have been modelled by ab initio methods. " Other ab initio MO calculations for the reactions of H, Me, Ft, j-Pr, and r-Bu with a variety of silanes and germanes have been carried out. The results indicate that the attacking and leaving radicals adopt an almost co-linear arrangement. Bond distances and energy barriers were predicted for the reactions studied. 

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