Organoborane compounds present a class of versatile synthetic intermediate for myriad organic transformations. The direct addition of a B–H bond across unsaturated bondnamely, hydroborationis a ...powerful tool for the preparation of organoborane derivatives. This review outlines recent advances in catalytic hydroboration of unsaturated organic compounds, specifically those involving C-X (X = N, O) bonds. We will discuss the chemical behavior of both transition metal catalysts and main group catalysts in hydroboration. Emphasis will also be placed on the reaction mechanism of these catalytic reactions. Furthermore, recent achievements in catalytic hydroboration of carbon dioxide CO2 will be highlighted.
Most of the chemical and biological processes involving the fixation and transformation of small molecules have long been exclusive for metal complexes. Meanwhile, the last decades have seen a ...significant advance in main group chemistry that mimics transition-metal complexes, among which various boron-containing systems have been successful in mediating the small molecule activation. In this review, we focus on boron-containing heterocycles enabling the activation of σ- and π-bonds in small molecules, in conjunction with the proposed mechanisms.
This review focuses on boron-containing heterocycles enabling the activation of σ- and π-bonds in small molecules.
N-Heterocyclic phosphenium triflates (NHP-OTf) 1 serve as efficient catalysts for the regio- and chemoselective hydroboration of pyridines under ambient condition with good functional group ...tolerance. Mechanistic studies indicate that a boronium salt, (Py)2·BpinOTf 4, is generated concomitant with NHP-H 5 via hydride abstraction from HBpin by 1 in the initial reaction step. Hydride reduction of the activated pyridine in (Py)2·BpinOTf 4 by NHP-H 5 affords the 1,4-hydroboration product selectively. Thus, the phosphenium species act as a hydrogen transfer reagent in the catalytic cycle.
Hydrophosphination of CO2 with 1,3,2‐Diazaphospholene (NHP‐H; 1) afforded phosphorus formate (NHP‐OCOH; 2) through the formation of a bond between the electrophilic phosphorus atom in 1 and the ...oxygen atom from CO2, along with hydride transfer to the carbon atom of CO2. Transfer of the formate from 2 to Ph2SiH2 produced Ph2Si(OCHO)2 (3) in a reaction that could be carried out in a catalytic manner by using 5 mol % of 1. These elementary reactions were applied to the metal‐free catalytic N‐formylation of amine derivatives with CO2 in one pot under ambient conditions.
Formate formation: Hydrophosphination of CO2 with 2‐H‐1,3,2‐diazaphospholene afforded phosphorus formate, from which transfer of the formate to Ph2SiH2 produced Ph2Si(OCHO)2. These elementary reactions were applied to the metal‐free catalytic N‐formylation of various amine derivatives with CO2 in a one‐pot approach at room temperature.
The stable cyclic (alkyl)(amino)alumanyl anion (CAAAl) 3 reacts with white phosphorus (P4) under ambient conditions, in which P4 is fragmented into a P1 unit to afford the bis(alumanyl)phosphide ...2, demonstrating the direct formation of a P anion from an Al anion. Structural and electronic features of the latter were fully characterized by standard spectroscopic means, X-ray diffraction analysis, and computational studies, which revealed that 2 bears highly polarized Al (δ+) and P (δ−) and relatively short Al–P bonds, indicative of the σ-donating and π-accepting nature of CAAAl groups.
Synthesis of a series of cationic coinage metal (Au: 2–4, Ag: 5, and Cu: 6) complexes of asymmetric diborenes (1 a, 1 b) is reported. X‐ray diffraction analysis reveals that the metal center ...interacts unsymmetrically with the B2 moiety and the terminal boron atom exhibits pronounced pyramidal geometry. A computational study shows that in complexes 2–6, the bonding between the B2 units and the metal center is dominated by electrostatic interactions concomitant with non‐negligible covalent contributions.
To coin a complex: A series of cationic coinage metal (Au: 2–4, Ag: 5, and Cu: 6) complexes of asymmetric diborenes (1 a, b) have been synthesized. X‐ray diffraction analysis reveals that the metal center interacts unsymmetrically with the B2 moiety and the terminal boron atom exhibits pronounced pyramidal geometry. The computational study shows that in the complexes the bonding between the B2 units and the metal centers is dominated by electrostatic interactions (see scheme).
3,4-dihydro-2H-1,2,4,3-triazaborol-3-yl-lithium 3 was synthesized and fully characterized. The 11B NMR spectrum, X-ray diffraction analysis, and computational studies revealed the ionic nature of the ...B–Li bond, and indeed 3 displays nucleophilic property which allowed preparation of a series of 1,2,4,3-triazaborol-3-yl-metal complexes (Al; 5, Au; 6, Zn; 7, Mg; 8, Sb; 9, and Bi; 10). 3 reacted with CO (1 atm) and various isonitriles under ambient condition, and mechanistic study suggests that the reactions with CO and aryl isonitriles proceed via an insertion of CO and isonitrile carbon into the B–Li bond followed by isomerization to yield transient carbene species, one of which was confirmed by trapping with S8. With PhNC, compounds 5 and 7·(thf) underwent exchange of THF molecule coordinating to the metal center with isonitrile, whereas insertion of isonitrile carbon occurred at the B–Bi bond in 10 which afforded stable bismuth (boryl)iminomethane 20.
Boranes have long been known as the archetypal Lewis acids owing to an empty p-orbital on the boron centre. Meanwhile, Lewis basic tricoordinate boranes have been developed in recent years. Here we ...report the synthesis of an annulated 1,4,2,5-diazadiborinine derivative featuring boron atoms that exhibit both Lewis acidic and basic properties. Experimental and computational studies confirmed that two boron atoms in this molecule are spectroscopically equivalent. Nevertheless, this molecule cleaves C-O, B-H, Si-H and P-H bonds heterolytically, and readily undergoes 4+2 cycloaddition reaction with non-activated unsaturated bonds such as C=O, C=C, C≡C and C≡N bonds. The result, thus, indicates that the indistinguishable boron atoms in 1,4,2,5-diazadiborinine act as both nucleophilic and electrophilic centres, demonstrating ambiphilic nature.
Activation of dihydrogen (H2) by 1,4,2,5-diazadiborinine is reported. Under the heating condition, treatment of 1,4,2,5-diazadiborinine with H2 results in the cleavage of HH bond, to afford product ...2, which was fully characterized by NMR and IR spectroscopy as well as X-ray diffraction and HRMS analysis. In the presence of borane dimethylsulfide complex (BH3·SMe2), 2 readily isomerizes to 3 under the mild reaction condition.
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