Highly Chemoselective Hydroboration of Alkynes and Nitriles Catalyzed by Group 4 Metal Amidophosphine–Borane Complexes

Abstract

We report a series of titanium and zirconium complexes supported by dianionic amidophosphine−borane ligands, synthesized by amine elimination and salt metathesis reactions. The Ti IV complex [{Ph 2 P(BH 3 )N} 2 C 6 H 4 Ti(NMe 2 ) 2 ] (1) was obtained by the reaction between tetrakis-(dimethylamido)titanium(IV) and the protic aminophosphine−borane ligand [{Ph 2 P(BH 3 )NH} 2 C 6 H 4 ] (LH2) at ambient temperature. Both the heteroleptic zirconium complexes[η 5 -(C 5 H 5 ) 2 Zr{Ph 2 P-(BH 3 )N} 2 C 6 H 4 ] (2) and [[{Ph 2 P(BH 3 )N} 2 C 6 H 4 ]ZrCl 2 ] (3)and the homoleptic zirconium complex [[{Ph 2 P(BH 3 )-N} 2 C 6 H 4 ] 2 Zr] (4) were obtained in good yield by the salt metathesis reaction of either zirconocene dichloride [η 5 -(C 5 H 5 ) 2 ZrCl 2 ] or zirconium tetrachloride with the dilithium salt of the ligand [{Ph 2 P(BH 3 )NLi} 2 C 6 H 4 ] ( LLi2), which was prepared in situ. The molecular structures of the complexes 1, 2, and 4 in their solid states were confirmed by single-crystal X-ray diffraction analysis. Of these complexes, only titanium complex 1 acts as an effective catalyst for the facile hydroboration of terminal alkynes, yielding exclusive E-isomers. The hydroboration of organic nitriles yielded diborylamines with a broad substrate scope, including broad functional group compatibility. The mechanism of hydroboration occurs through the formation of titanium hydride as an active species.

Overview of Regioselective and Stereoselective Catalytic Hydroboration of Alkynes

“…The 3d transition metals are earth‐abundant and demonstrate less toxic behavior, which makes them one of the prior choices for catalysis. In this regard, various first‐row transition metals are also used as efficient catalysts in syn ‐selective hydroboration, for instance, Ti, [48,49] Mn, [50] Fe, [51–58] Co, [59–61] Ni, [62] Cu, [63–77] and Zn [78] …”

Catalytic Nitrile Hydroboration: A Route to N,N‐Diborylamines and Uses Thereof

“…Slightly improved yields of N,N‐diborylamines and higher reaction rates have been reported by Panda et al. in 2020 for hydroboration of nitriles with HBPin, catalyzed by Ti(IV) amide 5 (Scheme ) . Similarly with 4 ‐catalyzed transformations (Scheme ), the reactions were performed at 60 °C under solvent‐free conditions.…”

“…[19,20] During the last decade, since the discovery of Mo-catalyzed dihydroboration of nitriles, [19,20] a number of research publications concerning other catalytic systems have appeared in the literature, with the vast majority of developments being accomplished in the area of transition metal catalysis. The newly developed methods mostly rely on Ru [22,29,30] and Co [21,23,31,32] catalysts, with a handful of examples using Ti, [33,34] Mn, [35] Fe [36] and Ni. [37] With regards to f-block elements, only two reports on using Er, Y, Dy, Gd [38] and Th [39] systems in dihydroboration of nitriles have been disclosed.…”

“…Slightly improved yields of N,N-diborylamines and higher reaction rates have been reported by Panda et al in 2020 for hydroboration of nitriles with HBPin, catalyzed by Ti(IV) amide 5 (Scheme 4). [34] Similarly with 4-catalyzed transformations (Scheme 3), the reactions were performed at 60°C under solvent-free conditions. Compared to 4-catalyzed hydroboration, the catalyst loading was reduced from 10 to 3 mol% and excellent conversions of nitriles were achieved within less than 10 hours.…”

Sodium as High‐efficient Catalyst in Hydroboration of Unsaturated Compounds

“…[7] The hydroboration catalyzed by well-defined transition metal catalysts is particularly prevalent. Numerous transition metal complexes involving Mo, [8] Fe, [9] Co, [10] Ru, [11] Th, [12] Hf [13] , Ti, [14] Zn, [15] and Mn [16] have exhibited outstanding catalytic activity, and a variety of valuable organoboranes are formed. Besides, the Lewis acid/Lewis acid-base pairs synthesized by main group catalysts based on Mg [17] and Al [3b,18] have achieved great success in the hydroboration of nitriles with high efficiency.…”