Free-Radical Hydroxylation Reactions of Alkylboronates

Abstract

The radical hydroxylation of B-alkylcatecholboranes, easily prepared by hydroboration of olefins, has been investigated. When molecular oxygen was used as oxidizing agent, the corresponding alcohols were obtained directly without alkaline treatment. The presence of Lewis base additives such as Et3N or DABCO has a benefic effect on the selectivity and yield. Alternatively, 2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) reacts cleanly with B-alkylcatecholboranes to afford alkyl radicals that can be trapped by a second equivalent of TEMPO to give alkoxyamines. Reduction of the alkoxyamines with zinc in acetic acid affords the desired alcohols. The whole procedure is particularly mild and does not require any basic condition. The two approaches presented in this paper are valuable and represent mild alternatives to the classical alkaline oxidation of organoboranes to alcohols.

B‐Alkylcatecholborane‐Mediated Radical Reactions

“…In this context, we recently reported a valuable very mild alternative to the classical oxidative treatment to convert olefins into alcohols via the corresponding B-alkylcatecholboranes. [16,17] In this procedure, the B-alkylcatecholboranes, prepared by hydroboration of the corresponding alkenes with N,N-dimethylacetamide as catalyst, [18] were treated with 2 equiv. of TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl), a stable persistent radical (Scheme 7).…”

“…For example, the hydroboration of 2-carene with catecholborane followed by treatment with H 2 O 2 /NaOH afforded a bicyclic alcohol according to Scheme 8 [Equation (a)]. [21] Interestingly, the TEMPO oxidation of the same intermediate followed by [16,17] zinc-mediated reduction of the alkoxylamine give the monocyclic tertiary alcohol resulting from the ring-opening of the intermediate cyclopropylalkyl radical [Scheme 8, Equation (b)]. [16,17] Scheme 8.…”

“…[21] Interestingly, the TEMPO oxidation of the same intermediate followed by [16,17] zinc-mediated reduction of the alkoxylamine give the monocyclic tertiary alcohol resulting from the ring-opening of the intermediate cyclopropylalkyl radical [Scheme 8, Equation (b)]. [16,17] Scheme 8. Classical (H 2 O 2 , NaOH) versus radical oxidation of Balkylcatecholboranes [16,17] Alkyl radicals generated from B-alkylcatecholboranes and TEMPO have also been used for conjugate addition.…”

“…[16,17] Scheme 8. Classical (H 2 O 2 , NaOH) versus radical oxidation of Balkylcatecholboranes [16,17] Alkyl radicals generated from B-alkylcatecholboranes and TEMPO have also been used for conjugate addition. This tandem process requires again the use of 2 equiv.…”

Synthesis of Unusual Oxime Ethers by Reaction of Tetranitromethane with B‐Alkylcatecholboranes

“…This step is closely related to the fast reaction observed between nitroxides, such as 2,2,6,6-tetramethylpiperidine N-oxide (TEMPO), and B-alkylcatecholboranes. [21,22] The radical process may be initiated by traces of oxygen or by a singleelectron-transfer process between tetranitromethane and the B-alkylcatecholborane, which leads to an alkyl radical, a boron nitronate of nitroform and nitrogen dioxide.…”