Zinc Hydride Catalyzed Chemoselective Hydroboration of Isocyanates: Amide Bond Formation and C=O Bond Cleavage

Gallium Hydride-Catalyzed Selective Hydroboration of Unsaturated Organic Substrates

The first examples of tetrasubstituted conjugated bis-guanidinate (CBG) supported monomeric and thermally stable gallium dihalides [LGaX2], (X=Cl (Ga-Cl), I (Ga-I)) and dihydride (Ga-H) [LGaH2] (where L={(ArHN)(ArN)-C=N-C=(NAr)(NHAr)}; Ar=2,6-Et2-C6H3) compounds are reported. The reaction of in situ generated LLi with 1.0 equiv. GaX3 (X=Cl, I) afforded compounds Ga-Cl and Ga-I. The reaction between Ga-Cl and Li[HBEt3] in benzene yielded the dihydride compound Ga-H. All reported compounds (Ga-Cl, Ga-I, and Ga-H) were characterized by NMR, HRMS, and single-crystal X-ray diffraction studies. Ga-H was probed for the hydroboration of carbodiimides (CDI), isocyanates, and isothiocyanates with HBpin. Compound Ga-H was also found effective for the catalytic hydroboration of imines, nitriles, alkynes, esters, and formates, affording the corresponding products in quantitative yields. Stoichiometric reactions with a CDI were performed to establish the catalytic cycle.

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

In this work, the molecular aluminium dihydride complex bearing an N, N'-chelated conjugated bis-guanidinate (CBG) ligand is used as a catalyst for reducing a wide range of aryl and alkyl esters with good tolerance of alkene (C=C), alkyne (C≡C), halides (Cl, Br, I and F), nitrile (C≡N), and nitro (NO₂ ) functionalities. Further, we investigated the catalytic application of aluminium dihydride in the C-O bond cleavage of alkyl and aryl epoxides into corresponding branched Markovnikov ring-opening products. In addition, the chemoselective intermolecular reduction of esters over other reducible functional groups, such as amides and alkenes, has been established. Intermediates are isolated and characterized by NMR and HRMS studies, which confirm the probable catalytic cycles for the hydroboration of esters and epoxides.

Applications of catalysis in hydroboration of imines, nitriles, and carbodiimides

The catalytic hydroboration of imines, nitriles, and carbodiimides is a powerful method of preparing amines which are key synthetic intermediates in the synthesis of many value-added products. Imine hydroboration has perennially featured in notable reports while nitrile and carbodiimide hydroboration have gained attention recently. Initial developments in catalytic hydroboration of imines and nitriles employed precious metals and typically required harsh reaction conditions. More recent advances have shifted toward the use of base metal and main group element catalysis and milder reaction conditions. In this survey, we review metal and nonmetal catalyzed hydroboration of these unsaturated organic molecules and group them into three distinct categories: precious metals, base metals, and main group catalysts. The TON and TOF of imine hydroboration catalysts are reported and summarized with a brief overview of recent advances in the field. Mechanistic and kinetic studies of some of these protocols are also presented.

Catalyzed reaction of isocyanates (RNCO) with water

The reactions between substituted isocyanates (RNCO) and other small molecules (e.g. water, alcohols, and amines) are of significant industrial importance, particularly for the development of novel polyurethanes and other useful polymers. We present very high-level ab initio computations on the HNCO + H₂O reaction, with results targeting the CCSDT(Q)/CBS//CCSD(T)/cc-pVQZ level of theory. Our results affirm that hydrolysis can occur across both the N[double bond, length as m-dash]C and C[double bond, length as m-dash]O bonds of HNCO via concerted mechanisms to form carbamate or imidic acid with ΔH_0K barrier heights of 38.5 and 47.5 kcal mol^-1. A total of 24 substituted RNCO + H₂O reactions were studied. Geometries obtained with a composite method and refined with CCSD(T)/CBS single point energies determine that substituted RNCO species have a significant influence on these barrier heights, with an extreme case like fluorine lowering both barriers by close to 15 kcal mol^-1 and most common alkyl substituents lowering both by approximately 3 kcal mol^-1. Natural Bond Orbital (NBO) analysis provides evidence that the predicted barrier heights are strongly associated with the occupation of the in-plane C-O* orbital of the RNCO reactant. Key autocatalytic mechanisms are considered in the presence of excess water and RNCO species. Additional waters (one or two) are predicted to lower both barriers significantly at the CCSD(T)/aug-cc-pV(T+d)Z level of theory with strongly electron withdrawing RNCO substituents also increasing these effects, similar to the uncatalyzed case. The 298 K Gibbs energies are only marginally lowered by a second catalyst water molecule, indicating that the decreasing ΔH_0K barriers are offset by loss of translational entropy with more than one catalyst water. Two-step 2RNCO + H₂O mechanisms are characterized for the formation of carbamate and imidic acid. The second step of these two pathways exhibits the largest barrier and presents no clear pattern with respect to substituent choice. Our results indicate that an additional RNCO molecule might catalyze imidic acid formation but have less influence on the efficiency of carbamate formation. We expect that these results lay a firm foundation for the experimental study of substituted isocyanates and their relationship to the energetic pathways of related systems.