Synthesis, reactivity and catalytic application of ferrocenyl-functionalized 1,2,3-triazol-5-ylidene complexes

Abstract

Herein is presented the synthesis and characterization of ferrocenyl functionalized 1,2,3-triazolium salts, as precursors to stable mesoionic carbenes (MICs) or 1,2,3-triazol-5-ylidenes (trz). The synthesis of the N1, N3-diarylated ferrocenyl triazolium salt (1: [N1-dipp, N3-dipp, 4-Fc(trz-H)]PF6) (dipp = 2,6-diisopropylphenyl) involved the cycloaddition of N-chlorotriazenes with ethynyl ferrocene. For comparative purposes, N1-aryl, N3-alkyl functionalized triazolium salts (2: [N1-dipp, N3-ethyl, 4-Fc (trz-H)]BF4; 3: [N1-dipp, N3-ethyl, 4-Ph (trz-H)]BF4) were synthesized according to the one-pot copper catalyzed alkyne-azide cycloaddition (CuAAC) of 2,6-diisopropylaniline and the respective alkyne (2: ethynylferrocene; 3: phenylacetylene), followed by alkylation on N3. It was reasoned that the ligand scaffold of the ferrocenyl triazolium salts (1 and 2) would enhance the donating ability of the corresponding carbene when coordinated, given that ferrocene is a strong electron donor. However, on closer scrutiny of the chemical shifts (in NMR spectroscopy) of the acidic triazolium protons of salts 1–3, it was found that the arylated substituent on N3 has a more profound effect on the electronics of the precursor salt, than the substituent on C4 (ferrocene vs. phenyl).

Coordination of the triazolium salts to rhodium(I) cod chloride (cod = 1,5-cyclooctadiene) precursors followed either the free carbene route (synthesis of 4) or base mediated C-H activation (synthesis of 5 and 6) to synthesize the corresponding [Rh(trz)(cod)Cl] complexes (4–6). The electronic properties of the triazolylidenes were further investigated by means of measuring the stretching frequencies of the corresponding dicarbonyl complexes (7–9), whereby the calculated TEPs of all three complexes were found to be equal (2047 cm-1). Slight differences in donating strengths were observed in the carbene resonances, whereby the trend were followed regarding the acidity of their triazolium precursor salts. Furthermore, the Rh(I)-cod complexes, 4–6, underwent reactivity testing for the hydroformylation of 1-octene. To the best of our knowledge, no examples of rhodium(I) triazolylidenes as precursors catalysts for hydroformylation exist, and thus the Rh(I)-cod complexes, N (Rh(cod)(trz-NBoc) and O ([Rh(cod)(trz-NMe2), were also evaluated alongside complexes 4–6 for comparative reasons. All complexes were active in the hydroformylation of 1-octene and reported TOFs of 151.6–78.4 hr-1, and n/iso values of 2.43–1.16 were obtained. The addition of an oxidant to 4, to generate 4ox in situ resulted in increased activity at a cost to regioselectivity.

Lastly, the synthesis of the gold(I) chlorido complex 10 follows the hydrolysis of the gold(I) phenyl complex 12. Gold(I) chlorido triazolylidene complexes have been applied to various catalytic conversions in organic syntheses in the presence of silver salts. Indeed, complex 10 is an active precursor catalyst in the synthesis of oxazolines, with an increase in conversion with the addition of silver salt.