Cobalt‐Catalyzed Deoxygenative Hydroboration of Nitro Compounds and Applications to One‐Pot Synthesis of Aldimines and Amides

Abstract

The commercially available and bench‐stable Co(acac)2 ligated with bis[(2‐diphenylphosphino)phenyl] ether (dpephos) was employed for selective room temperature hydroboration of nitro compounds with HBPin (TOF up to 4615 h−1), tolerating halide, hydroxy, amino, ether, ester, lactone, amide and heteroaromatic functionalities. These reactions offered a direct access to a variety of N‐borylamines RN(H)BPin, which were in situ treated with aldehydes and carboxylic acids to produce a series of aldimines and secondary carboxamides without the need for dehydrating and/or coupling reagents. Combination of these transformations in a sequential one‐pot manner allowed for direct and selective synthesis of aldimines and secondary carboxamides from readily available and inexpensive nitro compounds.

Hydroboration of isocyanates: cobalt-catalyzedvs.catalyst-free approaches

“…of HBPin to equimolar mixtures of PhNCO with alkenes (styrene, cyclohexene), 4-octyne, 32 bromobenzene, ethyl acetate, γ-butyrolactone, pyridine, thiophene, and N-substituted benzamides (Table 2, conditions B). Despite the previously reported high catalytic activity of Co(acac) 2 /dpephos in the hydroboration of nitriles 28 and nitro compounds, 29 chemoselective monohydroboration of PhNCO was also demonstrated in the presence of benzonitrile and nitrobenzene (Table 2, entries 7 and 12, conditions B). For all intermolecular hydroboration tests, high conversions of PhNCO to 3a and the complete NMR recovery of competing substrates were observed.…”

“…With regards to the mechanism of the Co(acac) 2 /dpephos-catalyzed hydroboration of isocyanates, based on our previous studies of the hydroboration of nitriles 28 and nitro compounds 29 as well as the literature reports on the hydroboration of amides, 42 we propose activation of the Co( ii ) pre-catalyst with HBPin to generate the catalytically active hydride (dpephos)Co I (H) ( A ; Scheme 6). The formation of Co(0) particles seems unlikely since the hydroboration of PhNCO to N -phenylformamide in the presence of excess Hg showed the identical catalytic activity of the system compared with the reaction in the absence of mercury.…”

“…We have recently reported the application of a bench-stable and commercially available Co(acac) 2 /dpephos catalytic system for the efficient hydroboration of nitriles 28 and nitro compounds 29 with HBPin to produce primary amines. Our studies have suggested that the reactions are initiated by HBPin to generate the Co( i ) hydride catalytic species.…”

The transition metal-catalysed hydroboration reaction

“…This allows the selective deoxygenation of sulfoxides with these functional groups (Scheme 86). In a similar fashion, the nitro group in 4-nitroanisole could be reduced to the diboryl amine using an excess of catecholborane and a rhodium( i ) catalyst, 558 to the monoboryl amine using an excess of HBpin and a cobalt( ii ) catalyst, 559 or to the amine using HBpin and chromium(0) species. 560…”

Aluminium‐Catalyzed Selective Hydroboration of Esters and Epoxides to Alcohols: C−O Bond Activation

“…Additionally, for aryl benzoates such as phenyl benzoate, 4methyl phenyl benzoate, 2-naphthyl acetate, and 1-naphthyl acetate (1 p-1 q, 1 s-1 t), we noticed 99 % yields for desired boronate ester products (2 l-2 m, 2 o-2 p) in 12 h except in 4nitrophenyl benzoate (1 r), where 80 % conversion of aryl boronic ester 2 n was found. [20] Next, we shifted our focus to reducing alkyl esters. The benzyl phenylacetate (1 u) undergoes CÀ O cleavage to afford the desired aryl borate esters 2 q and 2 a in a 99 % yield.…”