Abstract
The mechanism of the oxidation of arylsilanes to phenols has been investigated using 19F NMR spectroscopy. The formation of silanols in these reactions results from a rapid background equilibrium between silanol and alkoxysilane; the relative rates of reaction of these species was evaluated by modeling of concentration profiles obtained through 19F NMR spectroscopic reaction monitoring. Combining these results with a study of initial rates of phenol formation, and of substituent electronic effects, a mechanistic picture involving rapid and reversible formation of a pentavalent peroxide ate complex, prior to rate-limiting aryl migration, has evolved.
Highlights
We describe an experimental study of the mechanism of the H2O2-mediated oxidation of arylalkoxysilanes, a reaction that proceeds in the presence or absence3g,5a,10 of fluoride
In addition to shedding new light on the dynamic behavior of alkoxysilanes, this work explores the effects of low concentrations of fluoride ion on the oxidation, which can be rationalized by consideration of the relative reactivity of different organosilane intermediates
In earlier work,5a,b we had found that the addition of a fluoride source was not essential to achieve full conversion of arylsilanes to phenols, but that even substoichiometric amounts increase the overall efficiency of oxidation. This observation is mirrored in studies by Phillips and co-workers on the deprotection of silyl ethers using catalytic amounts of TBAF (0.1 equiv),[13] which demonstrate that fluoride ion can be recycled from fluorosilane byproducts.[14]
Summary
Arylsilanes are highly versatile intermediates in organic synthesis, with a number of methods having recently been disclosed for both their preparation[1] and transformations.[2]. Mader and Norrby subsequently calculated (Scheme 1b) that anionic substitution of anionic pentavalent silicates (e.g., 6) is rapid,[12] and that hydroperoxide anion is more likely to attack pentacoordinate silicon (6) than the neutral species (7).[8,9] They proposed that addition of fluoride ions is not essential for the oxidation to proceed, but increases the rate of hydroperoxide attack at silicon via formation of a greater proportion of pentavalent trifluorosiliconate species, which are more reactive toward substitution of F− by HOO− than neutral tetravalent difluorosilanes These authors concluded that alkyl migration from conformer 8b, and not peroxide ion attack to generate hexacoordinate species such as 3 or 4, is the rate-limiting step. We disclose investigations of arylsilane oxidation which unite the various experimental and computational studies in the field, and provide a comprehensive picture of the equilibria that are at play during oxidation, as well as the influence of fluoride
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