Butylstannanes

Ketones can be prepared by trapping (transmetallation) the acyl palladium intermediate 402 with organometallic reagents. The allylic chloride 400 is car-bonylated to give the mixed diallylic ketone 403 in the presence of allyltri-butylstannane (401) in moderate yields[256]. Alkenyl- and arylstannanes are also used for ketone synthesis from allylic chlorides[257,258]. Total syntheses of dendrolasin (404)f258] and manoalide[259] have been carried out employing this reaction. Similarly, formation of the ketone 406 takes place with the alkylzinc reagent 405[260],... 

C Vinyl stannane coupling 9 3-Bromo-l-(methanesulfonyl)indole Pd(OAc)2, chloranil Z-(2-Etho,xy vinyl)tri-n-butylstannane, Pd(PPh3)2Cl2 83 [9]... 

Radicals react at the sulfur, and decomposition generating an acyloxy radical ensues. The acyloxy radical undergoes decarboxylation. Usually, the radieal then gives produet and another radical which can continue a chain reaction. The process can be illustrated by the reactions with tri-w-butylstannane and bromotrichloromethane. 

If selenide additions are carried out in the presence of tri- -butylstannane, the radical generated by addition can be reduced by hydrogen abstraction. The chain is then continued by selenide abstraction by the stannyl radical. This leads to nonselenated addition and cyclization products. 

Irradiations of the unsaturated epoxy ketone (92) in the presence of tri- -butylstannane at 2537 A (in dioxane) or >3100 A (in dioxane or benzene) produce qualitatively similar mixtures containing products (71), (101),... 

Reduction by hydrogen atom donors involves free radical intermediates and usually proceeds by chain mechanisms. Tri-n-butylstannane is the most prominent example of this type of reducing agent. Other synthetically useful hydrogen atom donors include hypophosphorous acid, dialkyl phosphites, and tris-(trimethylsilyl)silane. The processes that have found most synthetic application are reductive replacement of halogen and various types of thiono esters. 

Tri-rc-butylstannane is able to reductively replace halogen by hydrogen. Mechanistic studies indicate a free radical chain mechanism.199 The order of reactivity for the halides is RI > RBr > RC1 > RF, which reflects the relative ease of the halogen atom abstraction.200... 

Scheme 5.9 illustrates some of the conditions that have been developed for the reductive deoxygenation of alcohols. Entries 1 to 4 illustrate the most commonly used methods for generation of thiono esters and their reduction by tri-M-butylstannane. These include formation of thiono carbonates (Entry 1), xanthates (Entry 2), and thiono imidazolides (Entries 3 and 4). Entry 5 is an example of use of dimethyl phosphite as the hydrogen donor. Entry 6 uses r .s-(trimethylsilyl)silane as the hydrogen atom donor. 

Procedures for the synthesis of ketones based on coupling of organostannanes with acyl halides have also been developed.211 The catalytic cycle is similar to that involved in coupling with aryl halides. The scope of compounds to which the reaction is applicable includes tetra-u-butylstannane. This example indicates that the reductive elimination step competes successfully with (3-elimination. 

More remote oxygen substituents can also influence stereochemistry. 4-Benzyloxy-2-pentenyl tri-n-butylstannane exhibits excellent enantioselectivity in reactions with aldehydes.186 This reaction is believed to involve chelation of the... 

Entry 23 was part of a synthesis of the pancratistatin structure. The lactone ring was used to control the stereochemistry at the cyclization center. Noncyclic analogs gave a mixture of stereoisomers at this center. In this reaction, triphenylstannane gave much better yields than tri-n-butylstannane. 

By studying the initial copolymerization rate as a function of the initial concentrations of monomer and initiator(azo-bis-isobutyric acid, (I)) it was found that the order of di-n-butylstannane with respect to (I) is 0.5 for the monomers and 2.46 with respect to the initiator. On the basis of these data, the copolymerization rate equation can be expressed as follows ... 

The results of this study are presented in Table 4.7. As can be seen from the data in Table 4.7, decarbonylation with hydrogen or deuterium transfer to the resulting radical is a relatively efficient process. The failure to observe this reaction using acetone or acetophenone as photosensitizer would suggest a singlet pathway for the direct photolysis of the aldehyde. In agreement, decarbonylation could not be quenched by naphthalene, piperylene, or 1,3-cyclohexadiene when the aldehyde was excited directly. The reaction could, however, be somewhat quenched by the addition of tri-n-butylstannane. The products in this case were... 

It is mentioned in an early paper on the effect of water on Heck vinylations [62] that 2,4-dimethoxy-5-iodopyrimidine reacted with 1-(ethoxyethenyl)-tri-n-butylstannane to afford an acylated pyrimidine derivative in 83 % yield (via in situ hydrolysis of the intermediate enol ether) (Scheme 6.28). 

These reactions result in iodine-atom transfer and introduce a potential functional group into the product. This method of radical generation can also be used in conjunction with either tri-n-butylstannane or tris(trimethylsilyl)silane, in which case the reaction is terminated by hydrogen-atom transfer. 

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