Butyltin hydride, reaction with halides

In an attempt to resolve this dilemma, the reaction rates were measured for a series of organic bromides with sodium anthracene (Table I) and correlated with two model systems. The model for the first possibility, a one-step process with bond dissociation, is the tri-n-butyltin hydride reaction with these same halides (6 ). Correlation of the reaction rates would indicate that the transition state for anthracene radical anion reduction is similar to the transition state for radical formation by tri-n-butyltin radical. On the other hand, the model for the second possibility, the organic halide reduction potential, is a measure of organic halide radical anion formation (7). 

RX RCHO. Alkyl halides can be converted directly into aldehydes in moderate to high yield by reaction with carbon monoxide (1-3 atm.) and tri-n-butyltin hydride catalyzed by the palladium(O) complex. The reaction involves insertion of carbon monoxide to form an acyl halide, which is known to be reduced to an aldehyde under these conditions (10, 411). Direct reduction of the halide can be minimized by slow addition of the tin hydride to the reaction and by an increase in the carbon monoxide pressure. 

Reduedon of M dts under radical conditions, Reduclion of halides with tr -n butyltin hydride under radical conditions is considered to proceed by the following chain reaction ... 

In a similar attempt to determine the nature of the rate-determining step in the reaction of aryl halides with magnesium, the kinetics of the reaction of substituted aryl bromides with magnesium and with tri-/i-butyltin hydride in ethereal solvents were examined [81c], using competitive kinetics techniques, and were correlated with the substituent effect using Hammett cr parameters. A Hammett plot log iK/K)=f(( x) then provided the value of the Hammett constant p for the reactions of aryl halides with magnesium and with tri-u-butyltin hydride in various solvents (Fig. 4 and Table 11). 

Inspection of Table 11 shows that p for the reaction of aryl halides with tri-u-butyltin hydride was not solvent-dependent, whereas p for the reaction with magnesium was. The small values of p for the reaction of aryl bromide with magnesium in a polar solvent were again interpreted [81c] in terms of a mass transport-limited process. Thus reactions of aryl bromides would be transport-limited in THF and more polar solvents, but not in diethyl ether and less polar solvent. On the other hand, the rate of reaction of aryl iodides with magnesium seemed to be transport-limited, even in diethyl ether, whereas the rate of reaction of chlorobenzene with magnesium, which was lO slower than that of bromobenzene, was not. 

Table 11 Hammett Values for Reactions of Aryl Halides with Magnesium and with Tri-/i-butyltin Hydride...

In contrast, for reaction of organic halides with metallic magnesium, Whitesides ( 7) found a poor correlation with the rates of tri-n-butyltin hydride, and a reasonable correlation, especially with primary bromides, with reduction potentials, suggesting the formation of RXT. Sodium naphthalene reductions also correlate with reduction potentials for primary halides (4b). Moreover, reaction rates are faster in solvents favoring loose ion pairs over tight, further evidence for an early transition state involving election transfer and little bond dissociation. (4c)... 

Reduction of the vinyl halide, j8-bromostyrene, at 100° C with tri-w-butyltin hydride provided only 35% of styrene, presumably because of hydrostannation of some of the styrene under the reaction conditions. Aryl halides can be reduced satisfactorily under fairly drastic conditions (66, 77). For example, 1- and 2-bromonaphthalenes have been reduced by triphenyl-tin hydride in 31% and 69% yields, respectively, at 150° C in 15 hours. Several -substituted halobenzenes have been studied, as shown in Table XII. There is an indication that electron-releasing substituents hinder reaction and electron-withdrawing substituents have the opposite effect. However the facilitation by the phenyl group complicates the picture. 

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