Anti-Bredt N-heterocyclic carbene: an efficient ligand for the gold(i)-catalyzed hydroamination of terminal alkynes with parent hydrazine

Abstract

An anti-Bredt N-heterocyclic carbene gold(I) chloride complex was synthesized by taking advantage of the reversible insertion of the free carbene into the NH bond of hexamethyldisilazane. This precatalyst promotes the parent hydrazine hydroamination of terminal alkynes at room temperature.

Room‐Temperature Hydrohydrazination of Terminal Alkynes Catalyzed by Saturated Abnormal N‐Heterocyclic Carbene–Gold(I) Complexes

“…Bertrand and co‐workers have reported a cyclic alkyl amino carbenes (CAAC)–gold(I) complex (Figure 1, I ) and an anti‐Bredt NHC–gold(I) complex (Figure 1, II ) for the catalyzed reaction between alkynes and anhydrous hydrazine 19. The former ( I ) was able to catalyze the addition of hydrazine to a few terminal and internal alkynes as well as diynes at high temperatures (100 °C),19a the latter ( II ) catalyzed the hydrazination of terminal alkylalkynes at room temperature, although terminal arylalkynes or internal alkynes still required high temperatures 19b. The anti‐Bredt NHC complex ( II ) is more similar to CAACs than to classical NHCs in terms of its electronic properties, because one nitrogen adjacent to the carbene atom is placed in a strained bridgehead position, thus being restricted from donating its lone pair.…”

“…Gratifyingly, appreciable reactivity was observed when tetrakis‐(3,5‐bis(trifluoromethyl)phenyl)borate (BAr F 4 ) salts were employed for the halide abstraction from gold(I) complex 4 a (entries 5–6). Especially effective was the potassium salt (entry 6), which allowed 95 % conversion of phenylacetylene 1 a and 92 % yield of the corresponding hydrazone (note that this transformation is not catalyzed by Bertrand’s anti‐Bredt‐NHC complex19b at room temperature, although 87 % yield was obtained at 90 °C).…”

Mesoionic Carbene‐Gold(I) Catalyzed Bis‐Hydrohydrazination of Alkynes with Parent Hydrazine

“…[20] The carbene-Aub ond length [1.991(6) ] is comparable to those of the (CAAC)AuCl [ 1.973(4) ] [18] and (pyrNHC)AuCl [1.983(3) ]. [10] Initial catalytic studies were conducted at 100 8C, using 1mol %o fastoichiometricm ixture of (L)AuCl( 7 and 8)w ith Figure 1. CAAC, pyrNHCand saNHC ligands previously used in gold-catalyzed hydrohydrazination, and BAC 1 and MIC 2 considered in this study (Dipp:2,6-diisopropylphenyl).…”

Experimental and Computational Studies of Anti‐Bredt Amidinium Salts

“…Even though potassium hexamethyldisilazide deprotonates 1 a at −78 °C, the resulting hexamethyldisilazane and carbene 2 a reacts together when warming the reaction mixture to room temperature to form adduct 6 a 9a. Interestingly, this reaction is reversible, because according to NMR spectroscopy data, dissolving crystals of 6 a in benzene gives back a small amount of 2 a and hexamethyldisilylazane 9b…”