Synthesis and structures of gold and copper carbene intermediates in catalytic amination of alkynes

Abstract

Metal carbenes are often proposed as reactive intermediates in the late transition metal-catalysed transformations of alkynes. Owing to their high reactivity, however, isolation and structural characterization of metal carbene intermediates in these transformations has remained unknown. Herein, we report the isolation of two acyclic gold and copper carbene intermediates in either Au(I)- or Cu(I)-catalysed cyclization of N-alkynyl formamidines through five-exo-dig cyclization. X-ray diffraction, ¹³C NMR spectra data and computational analyses provide evidence for the formation of a gold carbene intermediate with a carbocation-like electronic character. Using the intrinsic bond orbital (IBO) approach, we also evaluate the π-stabilizing effects of organic substituents at the carbene carbon atom in the gold carbene intermediate. Another rare six-membered copper carbene complex is also obtained through 6-endo-dig cyclization. These metal carbenes have proven reactive toward oxidation. The metal-promoted cyclization of N-alkynyl formamidine provides a facile approach to synthesize metal carbene species.

Figure 1. Selected metal carbene species involved in late transition metal-mediated transformations.

(a) Late transition metal carbenes generated by decomposition of the diazo precursors. (b) Proposed late transition metal carbenes in catalytic cycloisomerization of alkynes. (c) Gold and copper carbene intermediates isolated in gold- and copper-catalysed cyclization of N-alkynyl formamidines.

Figure 2. Cu-mediated cyclization of N-alkynyl formamidine.

(a) Cu-promoted amidiniumation of N-alkynyl formamidine 1. (b) Cu-catalysed cyclization oxidation of N-alkynyl formamidine 1 and transformation of copper intermediate 2 into compound 6.

Figure 3. Molecular structure of 6 and 7.

(a) Molecular structure of 6 with 20% probability. H atoms in aryl rings have been omitted for clarity. (b) Molecular structure of 7·CH₂Cl₂ with 20% probability. The counterion (NTf₂⁻) and CH₂Cl₂ in 7 and H atoms in aryl rings have been omitted for clarity.

Figure 4. Formation of compound 6 and copper carbene 7 and related mesoionic compound G.

(a) IPr*CuNTf₂-catalysed cyclization oxidation of formamidine 1 and related copper carbene 7. (b) Copper carbene complex 7 bearing mesoionic imidazolium-4-olate ring and related mesoionic 1,2,3-triazolium-4-olate G.

Figure 5. Preparation of gold carbene 8 and related heteroatom-stabilized gold carbenes 9–13.

(a) Au-catalysed cyclization oxidation of N-alkynyl formamidine 1 and gold carbene intermediate 8. (b) Heteroatom-stabilized gold carbenes 9–13.

Figure 6. Molecular structure of 8 with 20% probability.

The counterion (OTf⁻) and H atoms in aryl rings have been omitted for clarity.

Figure 7. Known gold carbene complexes 14 and 15 and comparison of them with gold carbene 8.

(a) Previously reported gold carbene complexes 14 (top) and 15 (bottom) prepared through gold-mediated cyclization of allenes. (b) Comparison of related bond lengths and characteristic NMR spectroscopic data of complex 8 with those of reference complexes 15 and 15a; see ref. (c) Comparison of Mayer bond order of complex 8 with those of reference complexes 14 and 15; see refs and .

Figure 8. C¹-stabilizing IBOs of gold complex 8.

Numbers in parentheses indicate the fraction of electrons of the doubly occupied orbital assigned to the individual atoms. (a) Coordinative bond between the lone pair electrons of C¹ and Au. (b) Delocalized π bond between C¹ and phenyl ring. (c) d-π backbond between Au and C¹. (d) Delocalized π bond between C¹ and imidazolium-4-olate ring.

Figure 9. Cu-activated amidiniumation of formamidine 17 having a terminal alkyne in the presence of a base.

CuBr-mediated cyclization of formamidine 17 in the presence of N(iPr)₂Et as a base afforded a 6-membered vinylcopper species 18, which could be transformed into a divinylcopper species 19. Vinylcopper 18 is prone to undergo carbene oxidation to give a zwitterionic oxo-adduct 20, suggesting that 18 is more like a copper carbene species 18′.

Figure 10. Molecular structure of 19 with 20% probability.

H atoms in aryl rings have been omitted for clarity.