Group organotin hydrides

Clive and coworkers have developed a new domino radical cyclization, by making use of a silicon radical as an intermediate to prepare silicon-containing bicyclic or polycyclic compounds such as 3-271 and 3-272 (Scheme 3.69) [109], After formation of the first radical 3-267 from 3-266, a 5-exo-dig cyclization takes place followed by an intramolecular 1,5-transfer of hydrogen from silicon to carbon, providing a silicon-centered radical 3-269 via 3-268. Once formed, this has the option to undergo another cyclization to afford the radical 3-270, which can yield a stable product either by a reductive interception with the present organotin hydride species to obtain compounds of type 3-271. On the other hand, when the terminal alkyne carries a trimethylstannyl group, expulsion of a trimethylstannyl radical takes place to afford vinyl silanes such as 3-272. 

Organotin hydrides R SnI L- , where R represents an alkyl or aryl group, are generally prepared quite easily by reduction of the corresponding organotin halides using either LiAlFLt or NaBFL in ether or dioxane ... 

Many other groups can be selectively reduced by organotin hydrides. Although these reactions have not been used to date to form labelled compounds, the reactions are included here using tributyltin deuteride as the reducing agent to illustrate the synthetic possibilities for this reagent. 

Although these groups can be reduced with organotin hydrides, John and coworkers78 found that cyclic products accompanied the reduced (hydrocarbon) products in some reactions see Schemes 16 and 17 (vide supra). 

Finally, some important functional groups, notably the keto group and the carboxylic acid group, cannot be reduced with organotin hydrides. However, these groups can be reduced provided they are converted into a more reactive group. 

All of these syntheses begin with the addition of an organotin hydride to a carbon-carbon it bond. Therefore, it is important to understand the scope of these addition reactions. Organotin hydrides add without initiators to olefinic bonds, which are activated with an electron-withdrawing group. The hydride of the organotin hydride always adds to the more positive carbon in the addition reaction (equation 87). 

Organotin hydrides can also be added to unactivated olefinic (without an electron-withdrawing group on one carbon) bonds provided the reaction is initiated by UV radiation, AIBN or y-rays113-116. In one instance, an organotin hydride has even been added to an unactivated olefin without an initiator. This unexpected addition reaction occurred at high pressure117. 

Another application of the direct alkylation of metal-14 anions is the synthesis of polymer-supported organotin hydrides. These were prepared by the reaction of stannyl group was separated from the phenyl ring of polystyrene by two, three or even four carbon spacers. These polymers were found to contain 0.8-1.4 mmol of Sn-H per gram. The reducing ability of the polymer-supported organotin hydrides was monitored by reactions with haloalkanes (Scheme 22)142. 

Organotin hydrides add to alkenes and alkynes (equations 22 and 23). Unless strongly electron-withdrawing groups are present in the unsaturated species, the additions have radical-chain mechanisms (Scheme 2). For l-aUcenes, tin usually adds predominantly to the terminal carbon (equations 24 and 25). 

Organotin hydrides can undergo either heterolytic or homolytic reactions. Reactions with carboxylic acids and with R)SnX (X = OR, NR2, or other nucleophilic groups) are considered to be heterolytic reactions. Other heterolytic reactions of triorganotin hydrides are shown in Scheme 12. 

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