Hydrodecyanation of Secondary Alkyl Nitriles and Malononitriles to Alkanes using DiMeImd-BH₃

Abstract

The decyanation of secondary aliphatic nitriles and the 2-fold decyanation of malononitriles leading to alkanes in the presence of 1,3dimethylimidazol-2-ylidene borane (diMeImd-BH 3 ) are reported. These reactions proceed via a radical mechanism that involves the addition of a borane radical to the nitrile to form an iminyl radical, followed by cleavage of a carbon−carbon bond. Theoretical calculations suggest that the β-cleavage of these iminyl radicals, which affords NHC-BH 2 CN and the corresponding alkyl radicals, is the rate-determining step in this reaction.

DFT Investigation of Hydrogen Atom Abstraction from NHC-Boranes by Methyl, Ethyl and Cyanomethyl Radicals—Composition and Correlation Analysis of Kinetic Barriers

“…This behavior is well supported by the calculated results, as the free energy barriers for the HAA reactions of NHC-BH 3 with • CH 2 CN, Me • and Et • (13.8, 14.7 and 18.9 kcal mol −1 , respectively) increase to 16.0–21.0, 17.4–20.6 and 21.0–23.8 kcal mol −1 , respectively, after substitution ( Table S6 ). These results are also in good accordance with available experimental data [ 17 , 25 , 28 , 29 ]. For instance, addition–elimination reactions between NHC-boryl radicals and cyano compounds give NHC-boryl monocyanides but further H-abstraction–addition–elimination reactions to provide NHC-boryl dicyanides are generally difficult.…”

“…The investigated HAA reactions are shown in Scheme 1 . The selected NHC-boranes, which act as H donors, include frequently used diMe-Imd-BH 3 ( 1 ) [ 5 , 16 , 19 , 21 , 25 , 28 ], those with –F, –CN, and/or –Ph substituents [ 5 , 6 , 25 , 28 , 29 ] and two special NHC-ligated boryl monocyanides, namely, tetraMe-Imd-BH 2 CN ( 6 ) and diiPr-Imd-BH 2 CN ( 8 ) [ 28 ]. The selected attacking radicals (abstractors) are methyl (Me • ), ethyl (Et • ) and cyanomethyl ( • CH 2 CN).…”

“…NHC-boranes have been used as substrates, reagents and coinitiators in various types of organic reactions, including the radical borylative cyclization of N -allylcyanamides [ 25 , 26 ], the reduction of xanthates [ 20 ] and the hydroxymethylation of organoiodides [ 27 ]. In the typical examples of reductive decyanation of several organocyanides [ 17 , 18 , 28 , 29 ], the addition of the NHC- • BH 2 radical to R-CN provides NHC-BH 2 C(R)=N • as an adduct intermediate. The subsequent elimination of R • gives the NHC-boryl monocyanide NHC-BH 2 CN.…”

“…Substituting the B-site H of an NHC-borane with an electron-withdrawing group weakens the nucleophilicities of the H atom being abstracted and the B atom of the resulting NHC-boryl radical, which theoretically leads to weaker polarity matching with the electrophilic abstracting radical and a higher HAA barrier. Thus, electron-withdrawing groups on B likely have a detrimental effect on the HAA reactions of NHC-boranes, which may be the predominant reason for B-site disubstituted NHC-boranes being obtained in poor yields [ 17 , 25 , 28 , 29 ]. However, such conjecture requires further confirmation.…”

Recent Developments in Hydrodecyanation and Decyanative Functionalization Reactions

“…Lately, Kawamoto and co‐workers reported hydrodecyanation of secondary alkyl nitriles using 1,3‐dimethylimidazol‐2‐ylidene borane 36 (diMeImd‐BH 3 ) and t BuOO t Bu combinations (Scheme 15). [31] Although secondary alkyl nitriles were tolerated but primary alkyl nitriles and tertiary alkyl nitriles afforded trace amount of the corresponding decyanated products. This reaction proceeds via a radical mechanism involving addition of a borane radical 37 to the nitrile to form an iminyl radical 38 , followed by β‐fragmentation to give the corresponding NHC‐cyanoborane and an alkyl radical 34 .…”