Triarylborane Catalyzed Carbene Transfer Reactions Using Diazo Precursors

Abstract

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.

Borane-catalyzed arylation of aryldiazoacetates withN,N-dialkylanilines

“…On the basis of the experimental results and previous reports, 9–13 a plausible mechanism is proposed in Scheme 4. B(C 6 F 5 ) 3 bound at the carbonyl group of α-aryl diazoacetate 1 to form intermediate I , which then generated to the carbene species II or its resonant enol cation III by release of N 2 gas.…”

“…8 In particular, B(C 6 F 5 ) 3 has been reported to active diazo compounds to generate carbene intermediate or its resonance intermediate, which then undergo insertion and nucleophilic substitution reactions (Scheme 1b). 9–13 In 2016, Zhang and co-workers described a highly chemo- and ortho -selective substitution reaction of phenols with α-aryl α-diazoacetates with B(C 6 F 5 ) 3 as the catalyst. 10 a B(C 6 F 5 ) 3 -catalyzed C–H insertion, cyclopropanation, ring-opening reactions, and C–C bond functionalization reactions have been reported by the groups of Melen, 10 b – e Wilkerson-Hill, 10 f Koenigs, 10 g and Prabhu.…”

Solvent‐free, B(C6F5)3‐Catalyzed S−H Insertion of Thiophenols and Thiols with α‐Diazoesters

“…The reaction is initiated with the reversible coordination between the organoborane Lewis acid B(C 6 F 5 ) 3 with the α‐diazoester 1 a at the electron‐rich carbonyl oxygen atom to give the active species 8 , which then quickly undergoes the irreversible N 2 ‐releasing process to generate the 1,4‐zwiterionic intermediate 9 . Notably, the 1,4‐zwiterionic form of 9 can be tautomerized into carbene form 10 , which has been elegantly applied to the alkene cyclopropanation reaction with C−C double bonds [7b] . However, for the S−H insertion reaction here, it is the 1,4‐zwiterionic intermediate 9 , rather than the carbene 10 , that further pushes the reaction forward.…”

Acid‐Catalyzed Carbene Transfer from Diazo Compounds: Carbocation versus Carbene as Key Intermediate

“…Brønsted acids can also be used to catalyze the CÀ H functionalization of N-heterocycles with diazo compounds. [11] According to recent experimental and DFT studies, it is generally believed [12] that the diazo C=N bond can be activated by Lewis acids to form neutral carbene-like intermediates [8,9,13] for further carbene transfer reactions (Scheme 1B). Such belief is also related to the fact that diazo compounds are well-known carbene precursors upon thermolysis or photolysis.…”