Abstract
Many transition-metal complexes MLn decompose diazo compounds N2=CR1R2 generating metal-carbenes LnM=CR1R2 which transfer the carbene group to other substrates, constituting an important tool in organic synthesis. All previous reports have shown that the CR1R2 fragment at the metal-carbene remains intact from the parent diazo compound. Herein we report the detection and isolation of a monosubstituted copper carbene where the CR1R2 ligand has undergone a modification from the initial diazo reagent. When TpMsCu(THF) (TpMs = hydrotris(3-mesityl)pyrazolylborate ligand) was reacted with N,N-diethyl diazoacetamide [N2=C(H)(CONEt2)], the stable copper carbene TpMsCu=C(H)(NEt2) was isolated, resulting from a decarbonylation process, with carbon monoxide being trapped as TpMsCu(CO). The simultaneous observation of products derived from the intramolecular carbene insertion reaction into C–H bonds demonstrates that the expected TpMsCu=C(H)(CONEt2) complex is also formed. Experimental data, DFT calculations, and microkinetic models allow us to propose that the latter undergoes CO loss en route to the former.
Highlights
More than a century after Buchner postulated the existence of carbene CR2 groups during the thermal decomposition of ethyl diazoacetate,[1] the catalytic transfer of such moiety from diazo compounds yet constitutes an area of continuous growth.[2,3] Such process consists of the metal-induced decomposition of the diazo reagent in a process in which molecular N2 is extruded and a metallocarbene intermediate MC is formed (Scheme 1a).[4]
All metals from groups 8 to 11 are known to catalyze the carbene transfer from diazo compounds (Scheme 1b), and at least one detected/isolated example of a metalcarbene intermediate formed from a diazo compound has been reported for each of them.[8−18] In all cases, the CR1R2 moiety in the initial diazo compound appears unmodified in the subsequent metal-carbene intermediate (Scheme 1b), from where it is further transferred to the nucleophile
One of the most popular diazo reagents are diazocarbonyl compounds,[4] with a CO group directly bonded to the diazo functionality: the acceptor nature of the −COR group favors the transfer of the carbene group toward the nucleophile
Summary
More than a century after Buchner postulated the existence of carbene CR2 groups during the thermal decomposition of ethyl diazoacetate,[1] the catalytic transfer of such moiety from diazo compounds yet constitutes an area of continuous growth.[2,3] Such process consists of the metal-induced decomposition of the diazo reagent in a process in which molecular N2 is extruded and a metallocarbene intermediate MC is formed (Scheme 1a).[4] This species is electrophilic[5] in nature and reacts with available nucleophiles transferring the carbene group, liberating the metal to continue the catalytic cycle. One of the most popular diazo reagents are diazocarbonyl compounds,[4] with a CO group directly bonded to the diazo functionality: the acceptor nature of the −COR group favors the transfer of the carbene group toward the nucleophile
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