Novel Synthesis of Alkenes via Triethylborane-Induced Free-Radical Reactions of Alkyl Iodides and β-Nitrostyrenes

Abstract

Reactions of (E)-beta-nitrostyrenes 1 and triethylborane 2 or tricyclohexylborane 4 in THF solution at room temperature in the presence of oxygen in the air as radical initiator generate high yields of trans-alkenes (E)-3 or (E)-5. Medium to high yields of different (E)-alkenes (E)-5, 7, 10, 12, and 14 also can be prepared when 1 reacts with different radicals, prepared from secondary alkyl iodides 6 and 8 or tertiary alkyl iodides 9, 11, and 13, in the presence of 2 and air as radical initiator. The generation of the only product (E)-alkenes can be explained by the generation of the benzylic radical A and/or B as the intermediate only and the mechanism is similar to Scheme 1. Both (E)- and (Z)-16a-c are generated when (E)- and (Z)-15a-c are used to react with adamantyl radical under similar conditions. Only (Z)-16d was observed when either (E)- or (Z)-15d was used to react with adamantyl radical. The generation of the (E)- and/or (Z)-alkenes can be explained by the free rotation of the A and/or B to generate A' and/or B' and vice versa, and the mechanism is proposed to be a free-radical reaction via NO2/alkyl substitution and is shown as Scheme 2.

Denitrative Cross-Couplings of Nitrostyrenes

“…Although each proposal differs in detail, the main three characteristic steps feature in all proposals. Firstly, carbon- or heteroatom-centered radical 4 is generated in situ by various methods from precursors 6 – 33 [ 17 , 18 , 19 , 20 , 21 , 22 , 23 , 24 , 25 , 26 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 , 43 , 44 , 45 , 46 , 47 , 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 ]. These include a thermal homolytic cleavage of labile bonds, metal-catalyzed and photochemically initiated processes.…”

“…Yao’s enhancement reported in 2001 took advantage of the abilities of trialkylborane to promote the alkyl radical formation from alkyl iodides. Thus, various alkyl iodides 12 reacted with nitrostyrenes 1 in the presence of triethylborane, producing alkenes 3 under very mild conditions, at an impressive reaction rate ( Scheme 8 , method A) [ 24 ]. Due to the better availability of alkyl iodides, such modification of the original reaction conditions allowed a significant expansion of the original method scope.…”

Decarboxylative and Denitrative Trifluoromethylation for the Synthesis of CvinylCF3 Compounds with Togni (II) Reagent

“…Nitroolefins are useful intermediates in organic synthesis and are important structural units that can be used as starting materials for many classes of compounds 20. Denitrative reactions of β‐nitrostyrenes with halides,21a,b thiols,21c and olefins,21d generating alkenes or vinyl sulfides had been achieved until now. Denitrative trifluoromethylation of β‐nitrostyrenes under Togni (II)/DMF conditions has been also achieved by our group which is a new protocol for the synthesis of C vinyl CF 3 compounds.…”

Radical‐Mediated Alkenylation, Alkynylation, Methanimination, and Cyanation of B‐Alkylcatecholboranes

“…The mechanism proposed at that time involves the formation of vinyl radicals. More recently, Yao and co‐workers35, 36 reported the alkenylation of trialkylboranes with β‐nitrostyrene, but this nonchain reaction requires the use of a threefold excess of the trialkylborane. Therefore, we decided to investigate the reaction of B ‐alkylcatecholboranes with alkenyl sulfones.…”