Allylation of Nitrosobenzene with Pinacol Allylboronates. A Regioselective Complement to Peroxide Oxidation

Abstract

Addition of nitrosobenzene to pinacol allylboronates leads to oxidation of the organoboron with concomitant rearrangement of the substrate alkene. This reaction appears to proceed by allylboration of the nitroso group in analogy to carbonyl and imine allylation reactions. Remarkably, the N-O bond is cleaved during the reaction such that simple alcohols are the final reaction product.Due to the utility of allylboron reagents in organic synthesis, their preparation has been studied intensely.1 Along these lines, recent efforts from our laboratory have focused on the development of the catalytic hydroboration of dienes2 and the catalytic diboration of both allenes3 and dienes.4 These reactions convert simple hydrocarbon building blocks into substituted pinacolato allylboronates. These types of allylmetal reagents participate in a wide range of allylations with carbonyl and imine derivatives.5 However, the only other reactions that have been developed for these species are a narrow range of oxidation,6 crosscoupling,7 conjugate addition,8 and homologation reactions.9 To expand the utility of allyl boronates in organic synthesis we have begun to study other reactions that might apply to these reagents. Considering the isoelectronic relationship between the nitroso group and carbonyl groups, we were prompted to study the reaction between nitrosobenzene and allyl boronates. While a single example by Bubnov describes the reaction between nitrosobenzene and highly reactive triallyborane, a number of critical issues remain unaddressed.10 First, it is not clear whether the diminished electrophilicty of boronic esters relative to boranes will impede the reaction. Second, with substituted allylic boronates, it is not readily apparent whether the reaction with nitrosobenzene will occur with allylic transposition as occurs with carbonyls, or whether it will occur by coordination and subsequent 1,2 alkyl migration as occurs in the reaction between organoboranes and many reagents.11 Lastly, while Bubnov has demonstrated that triallylborane and nitrosobenzene react with relatively non-selective formation of both N-O and C-O bonds, the issue of Nversus O-allylation with allylic boronates was uncertain.12Two preliminary experiments revealed much about the reaction between allylboronates and nitrosobenzene. In the first (eq 1, Scheme 1), 1,3-decadiene (1) was subjected to Nicatalyzed 1,4-hydroboration, a reaction that delivers cis allyl boronate 2.2a After diluting the reaction mixture with THF it was treated with 1.05 equivalents of nitrosobenzene. Subsequent oxidative treatment furnished allylic alcohols 3 and 4 in 44% and 23% yield, morken@bc.edu. Supporting Information Available. Complete experimental procedures, characterization data ( 1 H and 13 C NMR, IR, and mass spectrometry). This material is free of charge via the internet at http://pubs.acs.org. respectively. In the second experiment (eq 2, Scheme 1), intermediate allyl boronate 2 was treated with nitrosobenzene for 13 hours at −78 °C prior to work-up with brine ...

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