Radical Chain Reduction of Alkylboron Compounds with Catechols

Abstract

The conversion of alkylboranes to the corresponding alkanes is classically per-formed via protonolysis of alkylboranes. This simple reaction requires the use of severe reaction conditions, that is, treatment with a carboxylic acid at high temperature (>150 °C). We report here a mild radical procedure for the transformation of organoboranes to alkanes. 4-tert-Butylcatechol, a well-established radical inhibitor and antioxidant, is acting as a source of hydrogen atoms. An efficient chain reaction is observed due to the exceptional reactivity of phenoxyl radicals toward alkylboranes. The reaction has been applied to a wide range of organoboron derivatives such as B-alkylcatecholboranes, trialkylboranes, pinacolboronates, and alkylboronic acids. Furthermore, the so far elusive rate constants for the hydrogen transfer between secondary alkyl radical and catechol derivatives have been experimentally determined. Interestingly, they are less than 1 order of magnitude slower than that of tin hydride at 80 °C, making catechols particularly attractive for a wide range of transformations involving C-C bond formation.

Enantiospecific Trifluoromethyl‐Radical‐Induced Three‐Component Coupling of Boronic Esters with Furans

“…[12] Thec oupling of tertiary boronic ester 1p proved to be more challenging,t he increased steric hindrance imposing anegative effect on the final oxidation step.Specifically,only one of the two diastereomers of 2p was converted into furan product 3p (32 %y ield from RBpin) under the standard reaction conditions.W et herefore investigated alternative oxidants and found that Cu(OAc) 2 was able to oxidize both diastereomers of 2p and gave the product 3p in an improved yield (41 %). [13] Thec ontrasting behavior of the two diastereomers warranted further investigation, which was conducted with the achiral tert-butyl-Bpin. As before,o ne diastereomer was rapidly oxidized to the desired product 3q leaving behind the unreactive diastereomer 2q,w hich was fully characterized.…”

Synthesis of hydroxyphthioceranic acid using a traceless lithiation–borylation–protodeboronation strategy

“…Although this strategy had been applied to benzylic carbamates, the resulting benzylic boronic esters (where boron is adjacent to functionality) are very easily protodeboronated simply with TBAF·3H 2 O (TBAF, tetra-N-butylammonium fluoride), so the strategy is not generally applicable 9-12 . The major challenge in realizing this strategy is the protodeboronation of the more-stable secondary alkyl pinacol esters, remote from any functionality (for example, 3 → 4), as there were no known methods for achieving such a transformation directly, although there were methods for carrying out the overall transformation indirectly 13,14 . In this paper we describe a new protocol for protodeboronation and its tactical use in conjunction with lithiation-borylation as a new strategy for assembling molecules without trace of the functional groups used in their genesis.…”

Antioxidative Polyketones from the Mangrove-Derived Fungus Ascomycota sp. SK2YWS-L

“…The oxidation of emodin ( 9 ) gave the benzophenone intermediate (structure i ), which could further transform into diphenyl ether derivatives ( 7 and 8 ) via grisendience1516. Generated by i and 8 17, intermediate ii could afford compound 1 − 3 through an aldol condensation as well as the additional esterification. In addition, 4 and 5 were transformed from intermediate iii by esterification and reduction, which was a reductive product of intermediate i .…”