Catalysis Science & Technology
Paper
alkynes, under the optimized reaction conditions, was Acknowledgements
expanded (Table 3). Catalyst 2a revealed good functional
L. C. T. and D. I. B. acknowledge the National Research
Foundation, South Africa (NRF 115642; NRF 94784; NRF
100119; NRF 111705), and Sasol Technology R&D Pty. Ltd.
(South Africa) for financial support (D. I. B.). I. F. is grateful
to the Spanish Ministerio de Economía y Competitividad and
FEDER (Grants CTQ2016-78205-P, PID2019-106184GB-I00 and
CTQ2016-81797-REDC). G. G.-B gratefully acknowledges
MICIU/AEI/FEDER “Una manera de hacer Europa” (PGC2018-
093382-B-I00 and RTI2018-098903-J-I00) for financial support.
group tolerance with no coupled bis-β-, β-vinyl sulfide
products detectable. By examination of the results gathered
in Table 3, we can confirm that the neutral dimetallic
complex 2a [Rh2Cl2(cod)2(μ-COC)] proved to be very selective
towards the branched α-vinyl sulfide product when aliphatic
alkynes and both aryl and alkyl thiols were used as substrates
in the absence of an internal base or hemilabile ligand
function (Table 3, entries 1–7). In contrast, only moderate
selectivities were obtained for aryl alkynes (Table 3, entries 8–
9), especially in the case where both the alkyne and thiol
contain an aryl group (entry 8). Finally, we evaluated the
catalytic activity of complex 2a [Rh2Cl2(cod)2(μ-COC)] in the
base-free hydrothiolation of an internal alkyne such 3-hexyne
with thiophenol under the same reaction conditions (Table 3,
entry 10). A lower conversion of 26% was observed with
preferential formation of (E)-3-phenylsulfanyl-3-hexene, in
comparison with the high yield reported for the formation of
this E-isomer.13
Notes and references
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Conclusions
The catalytic activity of a series of mono- and dimetallic
rhodium(I) and iridium(I) complexes bearing carbene-linker-
carbene (CXC) bis-triazolylidene ligands (with X = O, N)
ligands were assessed in the hydrothiolation model reaction
of the terminal 1-hexyne with thiophenol. The catalytic
studies revealed that, in general, the Rh(I) complexes
outperformed the related Ir(I) derivatives, both in terms of
activity and selectivity. Among them, the rhodium complexes
bearing the (COC) ether bridged bis-triazolylidene ligand
have shown superior activity and selectivity towards the
sought after branched α-vinyl sulfide product, in comparison
with related Rh(I) counterparts featuring the (CNC) amino
bridged derivative. Finally, the neutral dimetallic rhodium(I)
complex 2a containing the carbene–ether–carbene (COC)
ligand proved to be the best catalyst in terms of activity and
selectivity for a series of aliphatic alkynes and different
thiols, demonstrating good functional group tolerance.
Notably, the identified best-performing rhodium catalyst 2a
and selective iridium catalyst 8a employed in this study, the
absence of a basic, hemilabile coordinating moiety and the
selectivity improvement observed for cod co-ligands instead
of stronger π-acidic carbonyl ligands, point to an alternative
catalytic mechanism for these complexes, compared to the
non-oxidative route exemplified by catalysts A/B. Indeed, DFT
calculations carried out for the transformation involving the
Rh(I) catalyst 3 suggest that the process involves an initial
oxidative addition step followed by a selectivity-determining
step consisting of the alkyne insertion into the Rh–S bond.
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Conflicts of interest
There are no conflicts to declare.
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Catal. Sci. Technol.