Bi(I)-Catalyzed Transfer-Hydrogenation with Ammonia-Borane

A catalytic transfer-hydrogenation utilizing a well-defined Bi(I) complex as catalyst and ammonia-borane as transfer agent has been developed. This transformation represents a unique example of low-valent pnictogen catalysis cycling between oxidation states I and III, and proved useful for the hydrogenation of azoarenes and the partial reduction of nitroarenes. Interestingly, the bismuthinidene catalyst performs well in the presence of low-valent transition-metal sensitive functional groups and presents orthogonal reactivity compared to analogous phosphorus-based catalysis. Mechanistic investigations suggest the intermediacy of an elusive bismuthine species, which is proposed to be responsible for the hydrogenation and the formation of hydrogen.


General Procedure for Optimization Using 1,2-Diphenyldiazene (2a) as standard substrate
A culture tube equipped with a stir bar was charged with 1,2-diphenyldiazene (2a, 36.4 mg, 0.2 mmol, 1.0 equiv.) and ammonia-borane (1.0-2.0 equiv.). A teflon cap was fitted, and the tube was evacuated and refilled with argon (3 cycles). The tube was transferred to a glove box, and catalyst (1-4 mol%) was added. The tube was removed from the glove box and subjected to a positive pressure of argon. The corresponding solvent was added (1.0 mL) and the reaction was then stirred at the desired temperature (35 to 50 °C). After the indicated time, the yield was calculated by 1 H NMR using 1,3,5trimethoxybenzene as internal standard. Table S1. Optimization of reaction conditions for the transfer hydrogenation of 2a.
a 1 H NMR yield using 1,3,5-trimethoxybenzene as internal standard.; b starting material recovery 41%; c starting material recovery 14%; d starting material recovery 11%; e starting material recovery 42%; f starting material recovery 5%; g starting material recovery 12%; h starting material recovery 53%. A culture tube equipped with a stir bar was charged with ammonia-borane (6.2 mg, 0.2 mmol, 1.0 equiv.). A teflon cap was fitted, and the tube was evacuated and refilled with argon (3 cycles). The tube was transferred to a glove box, and Bi(I) catalyst (1, 0.9 mg, 1 mol%) was added. The tube was removed from the glove box, and placed under a positive pressure of argon. The corresponding solvent was added (1.0 mL) together with nitrobenzene (20.6 uL, 0.2 mmol). The reaction was stirred at 35 °C. After 2h, the reaction was judged complete by TLC, the yield was calculated by 1 H NMR using 1,3,5trimethoxybenzene as internal standard.

N-([1,1'-Biphenyl]-2-yl)hydroxylamine (5h)
Following the general procedure B, 5h was prepared from 4h (99.5 mg, 0. Probably due to the aromatic system in 2j, reaction under the optimal conditions did not result in the corresponding N,N'-arylhydrazine. Indeed, no product was obtained even at higher temperatures (50 °C). Substrate containing acidic functionalities (2k) was also not successful, showing bad solubility in THF and hence low reactivity. Another possibility for its low conversion to the hydrogenated product might be the catalyst deactivation due to the acidic carboxylate groups. S14
Bismuthinidene 1 was synthesized following a reported protocol. 6

Note:
The carbon corresponding to the C−Bi bond is also not observable by 13 C NMR due to the high quadrupole moment of the 209 Bi nucleus (100%, I = 9/2, quadrupole moment -0.4 x 10 -28 m -2 ), which broaden the peaks corresponding to atoms bonded to the Bi center to such an extent that they are not observable under standard conditions. S15

Synthesis of fluorobismuthine 7
Scheme S5. Anion metathesis of 8 with AgF for the synthesis of fluorobismuthine 7.
A schlenk flask equipped with a stir bar was charged with 8 (261 mg, 0.5 mmol, 1.0 equiv.) and AgF (127 mg, 1 mmol, 2.0 equiv.) and MeOH (10 ml Note: 19 F NMR shown a low intensity broad peak, probably due to the high quadrupole moment of the 209 Bi nucleus (100%, I = 9/2, quadrupole moment -0.4 x 10 -28 m -2 ), which broaden the peaks corresponding to atoms bonded to the Bi center to such an extent that they are not observable under standard conditions. Similarly, the carbon corresponding to the C−Bi bond is also not observable by 13 C NMR spectroscopy.  Figure S1.   Table S4.   S20 was calculated by 1 H NMR using 1,3,5-trimethoxybenzene as internal standard (TMB, 1.0 equiv.), obtaining a 91% yield of 3a.

Reaction with amino-borane complexes
Further proof of the homogeneity of this reaction was obtained when the concentration of 1 was analyzed before and after the reaction, obtaining the same values (0.002 mmol in 1 mL of THF-d8, 0.002 mM; see Figure S3).   Table S5.       Protocol: A culture tube equipped with a stir bar was charged with 1,2-diphenyldiazene (2a, 36.4 mg, 0.2 mmol, 1.0 equiv.) and ammonia-borane (0.1 mmol, 0.33, 0.5 or 1.0 equiv.). A teflon cap was fitted, and the tube was evacuated and refilled with argon (3 cycles). The tube was transferred to a glove box, and Bi(I) catalyst (1, 1 mol%) was added. The tube was removed from the glove box and subjected to a positive pressure of argon. Then, THF was added (1.0 mL) together with water (0.1, 0.33, 0.5 or 1.0 equiv.) and the reaction was then stirred at 35 °C. After the indicated time, the yield was calculated by 1 H NMR using 1,3,5-trimethoxybenzene as internal standard.

Identification of ammonia-borane byproducts
We have analyzed the fate of the ammonia-borane reagent by NMR spectroscopy. When the reaction has reached completion (>95% conversion of 2e to 3e, simplified 1 H NMR signals), a highly insoluble material was observed. Despite its insolubility, it could be detected by 1

S46
Additionally, the 1 H-NMR also provided information about this byproduct observed. As shown in Figure S9A, a complex signal at around 5.7 ppm was detected, which correspond to the region of B(OH)x. This has been confirmed by a reference sample of boric acid ( Figure S9B). These results suggest that the polymeric byproduct structure obtained contains free OH groups attached to the B atom. Insoluble white precipitate