Intermediates, Isolation and Mechanistic Insights into Zinc Hydride-Catalyzed 1,2-Regioselective Hydrofunctionalization of N-Heteroarenes

Synthesis and utility of N-boryl and N-silyl enamines derived from the hydroboration and hydrosilylation of N-heteroarenes and N-conjugated compounds

Catalytic hydroboration and hydrosilylation have emerged as promising strategies for the reduction of unsaturated hydrocarbons and carbonyl compounds, as well as for the dearomatization of N-heteroarenes. Various catalysts have been employed in these processes to achieve the formation of reduced products via distinct reaction pathways and intermediates. Among these intermediates, N-silyl enamines and N-boryl enamines, which are derived from hydrosilylation and hydroboration, are commonly underestimated in this reduction process. Because these versatile intermediates have recently been utilized in situ as nucleophilic reagents or dipolarophiles for the synthesis of diverse molecules, an expeditious review of the synthesis and utilization of N-silyl and N-boryl enamines is crucial. In this review, we comprehensively discuss a wide range of hydrosilylation and hydroboration catalysts used for the synthesis of N-silyl and N-boryl enamines. These catalysts include main-group metals (e.g., Mg and Zn), transition metals (e.g., Rh, Ru, and Ir), earth-abundant metals (e.g., Fe, Co, and Ni), and non-metal catalysts (including P, B, and organocatalysts). Furthermore, we highlight recent research efforts that have leveraged these versatile intermediates for the synthesis of intriguing molecules, offering insights into future directions for these invaluable building blocks.

Zinc hydride catalyzed hydroboration of esters

The conjugated bis-guanidinate (CBG)-supported zinc hydride {LZnH}2; L = {(ArHN)(ArN)-CN-C(NAr)(NHAr); Ar = 2,6-Et2-C6H3} (I) is utilized as a catalyst for the hydroboration of esters with pinacolborane (HBpin) under mild reaction conditions. Various aryl and alkyl substrates containing electron-donating, withdrawing, and cyclic groups of esters are effectively converted into alkoxy boronate esters as products upon hydroboration. Furthermore, stoichiometric experiments have been performed to understand the plausible reaction mechanism for the hydroboration of esters. Additionally, complex (I) was used for the hydroboration of carbonate, carboxylic acid, and anhydride substrates to showcase the broad substrate scope.

2-Anilidomethylpyridine-Derived Three-Coordinate Zinc Hydride: The Journey Unveils Anilide Backbone's Reactive Nature

Low-coordinate heteroleptic zinc hydrides are catalytically important but rare and synthetically challenging. We herein report three-coordinate monomeric zinc hydride on a 2-anilidomethylpyridine framework (NNL). The synthetic success comes through systematically screening a few different routes from different precursors. During the process, the ligand's anilide backbone interestingly appears to be more reactive than Zn's terminal site to electrophilic Lewis and Brønsted acids. The proligand NNLH reacts with [Zn{N(SiMe3)2}2] and ZnEt2 to give [(NNL)ZnA] (A = N(SiMe3)2 (1), Et(2)). Both are inert to PhSiH3 and H2 but react with HBpin only through the internal Zn-Nanilide bond to give the borylated ligand NNLBpin (3). The reactions of 1 and 2 with Ph3EOH (E = C, Si) afford a series of divergent compounds like [(NNLH)Zn(OSiPh3)2] (4), [Zn3(OSiPh3)4Et2] (5), and [EtZn(OCPh3)] (6). But in all cases, it is invariably the Zn-Nanilide bond protonated by the -OH with equal or higher preference than the terminal Zn-N or Zn-C bonds. A DFT analysis rationalizes the origin of such a reactivity pattern. Realizing that an acid-free route might be the key, reacting [(NNL)Li] with ZnBr2 gives [(NNL)Zn(μ-Br)]2 (7), which on successively treating with KOSiPh3 and PhSiH3 gives the desired [(NNL)ZnH] (8) as a three-coordinate monomer with a terminal Zn-H bond. Estimating the ligand steric in 8 shows the openness in Zn's coordination sphere, a desired criterion for efficient catalysis. This and a positive influence of the pyridyl sidearm is reflected in 8's superior activity in hydroborating PhC(O)Me by HBpin in comparison to Jones' two-coordinate anilido zinc hydride.

Zinc-Catalyzed Chemoselective Reduction of Nitriles to N-Silylimines through Hydrosilylation: Insights into the Reaction Mechanism

The N,N'-chelated conjugated bis-guanidinate (CBG) supported zinc hydride (Zn-1) pre-catalyzed highly challenging chemoselective mono-hydrosilylation of a wide range of nitriles to exclusive N-silylimines and/or N,N'-silyldiimines is reported. Furthermore, the effectiveness of pre-catalyst Zn-1 is compared with another pre-catalyst analogue, i.e., ^DiethylNacNac zinc hydride (Zn-2), to know the ligand effect. We observed that pre-catalyst Zn-1 shows high efficiency and better selectivity than pre-catalyst Zn-2 for reducing nitriles to N-silylimines. Mechanistic studies indicate the insertion of the C≡N bond of nitrile into Zn-H to form the zinc vinylidenamido complexes (Zn-1' and Zn-2'). The active catalysts Zn-1' and Zn-2' are confirmed by NMR, mass spectrometry, and single-crystal X-ray diffraction analyses. A most plausible catalytic cycle has been explored depending on stoichiometric experiments, active catalysts isolation, and in situ studies. Moreover, the synthetic utility of this protocol was demonstrated.