Blue light-induced diazo cross-coupling: synthesis of allyldiazo compounds through reshuffling of functionalities

In this paper, we describe a new type of cross-coupling between simple diazo and vinyldiazo compounds that gives access to unusual allyldiazo products. Blue light discriminates two diazo compounds towards free carbene formation, triggering sequential cyclopropenation, (3+2) cycloaddition and ring opening rearrangement processes. This strategy involves an overall reshuffle of diazo functionality and olefinic carbons of vinyldiazo compounds with an extrusion of nitrogen. Mechanistic studies including a 15N-labelling experiment demonstrate that the diazo functionality of allyldiazo products derives from simple diazo compounds, while vinyldiazo reagents are selectively decomposed via energy transfer with thioxanthone photocatalyst. The obtained allyldiazo compounds can be efficiently converted into synthetically useful structures such as 1,3-dienes, gem-difluoro-1,4-diene, hydrazine, dihydropyrazole, pyridazine, and bicyclobutane.

Borane-Catalyzed Coupling of Diazooxindoles and Difluoroenoxysilanes to Tetrasubstituted Monofluoroalkenes

A highly stereoselective coupling reaction of diazooxindoles with difluoroenoxysilanes catalyzed by Lewis acidic boranes has been developed. The reaction proceeded at ambient temperature under transition metal-free conditions with wide functional group tolerance. By using this simple procedure, a series of tetrasubstituted monofluoroalkenes can be accessed in good yield with high selectivity.

Kinetics of N2 Release from Diazo Compounds: A Combined Machine Learning-Density Functional Theory Study

Diazo compounds are commonly employed as carbene precursors in carbene transfer reactions during a variety of functionalization procedures. Release of N2 gas from diazo compounds may lead to carbene formation, and the ease of this process is highly dependent on the characteristics of the substituents located in the vicinity of the diazo moiety. A quantum mechanical density functional theory assisted by machine learning was used to investigate the relationship between the chemical features of diazo compounds and the activation energy required for N2 elimination. Our results suggest that diazo molecules, possessing a higher positive partial charge on the carbene carbon and more negative charge on the terminal nitrogen, encounter a lower energy barrier. A more positive C charge decreases the π-donor ability of the carbene lone pair to the π* orbital of N2, while the more negative N charge is a result of a weak interaction between N2 lone pair and vacant p orbital of the carbene. The findings of this study can pave the way for molecular engineering for the purpose of carbene generation, which serves as a crucial intermediate for many chemical transformations in synthetic chemistry.

B(C6F5)3-Catalyzed Diastereoselective and Divergent Reactions of Vinyldiazo Esters with Nitrones: Synthesis of Highly Functionalized Diazo Compounds

Herein we report a mild, transition-metal-free, highly diastereoselective Lewis acid catalyzed methodology toward the synthesis of isoxazolidine-based diazo compounds from the reaction between vinyldiazo esters and nitrones. Interestingly, the isoxazolidine products were identified to have contrasting diastereoselectivity to previously reported metal-catalyzed reactions. Furthermore, the same catalyst can be used with enol diazo esters, prompting the formation of Mukaiyama-Mannich products. These diazo products can then be further functionalized to afford benzo[b]azepine and pyrrolidinone derivatives.

Lewis Acid Assisted Brønsted Acid Catalysed Decarbonylation of Isocyanates: A Combined DFT and Experimental Study

An efficient and mild reaction protocol for the decarbonylation of isocyanates has been developed using catalytic amounts of Lewis acidic boranes. The electronic nature (electron withdrawing, electron neutral, and electron donating) and the position of the substituents (ortho/meta/para) bound to isocyanate controls the chain length and composition of the products formed in the reaction. Detailed DFT studies were undertaken to account for the formation of the mono/di-carboxamidation products and benzoxazolone compounds.

Borane-catalyzed arylation of aryldiazoacetates with N,N-dialkylanilines

A selective arylation of donor-acceptor diazo compounds with aniline derivatives catalyzed by Lewis acidic boranes is developed. This simple reaction protocol provides an efficient method for the synthesis of diarylacetates under metal-free conditions.

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6 F5 )3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N2 release. Recently, tris(pentafluorophenyl)borane [B(C6 F5 )3 ] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C6 F5 )3 is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.

Triarylborane Catalyzed Carbene Transfer Reactions Using Diazo Precursors

Reactive carbenes generated from diazo compounds are key intermediates for a range of organic reactions to afford synthetically useful organic compounds. The majority of these reactions have been carried out using transition metal catalysts. However, the formation of carbene intermediates using main group elements has not been widely investigated for synthetic purposes. Recent studies have demonstrated that triarylboranes can be used for the in situ generation of reactive carbene intermediates in both stoichiometric and catalytic reactions. These new reactivities of triarylboranes have gained significant attention in synthetic chemistry particularly in catalytic studies. The range of organic compounds that have been synthesized through these reactions are important as pharmaceuticals or agrochemicals. In this perspective, we highlight the recent progress and ongoing challenges of carbene transfer reactions generated from their corresponding diazo precursors using triarylboranes as catalysts. We also highlight the stoichiometric use of triarylboranes in which the boranes not only activate the diazo functionality to afford a carbene intermediate but also actively participate in the reactions as a reagent. The different mechanisms for activation and carbene transfer are described along with the mechanistic and computational studies that have aided the elucidation of these reaction pathways. Potential opportunities for the use of boranes as a catalyst toward different carbene transfer reactions and their future prospects are discussed.