ACS Catalysis
Letter
Scheme 4. Competition Reactions
and Xantphos (L10) catalyst system under CO pressure, the
effects of additives and then solvents were checked (Table 1).
Both linear and branched thioesters can be formed in our
testing with total conversion of styrene. Interestingly, in the
testing of acid additives, 84% yield of 4 together with 12% yield
of 3 can be achieved when a combined acid additive was
used.10 The ratio of the two acid additives were checked as
well, and the selectivity dropped dramatically when 10 mol %
of 5-chloro-2-hydroxybenzoic acid (5-Cl-SA) was used instead
of 20 mol %. In the testing of solvents, excellent conversion of
styrene can be obtained but with decreased selectivity (Table
1, entries 7−10). However, only a trace amount of thioester
was formed when DMSO was used as the solvent, and
thioether (none-carbonylation product) can be detected as
well (Table 1, entry 11). In our attempts to decrease the
reaction temperature, no conversion of substrate could be
detected, which might be due to the poison effect of thiol to
the palladium catalyst. The selectivity and yield were reduced
as well when we performed the reaction under lower CO
pressure. However, a 70% yield of linear product 4 can still be
formed with 1 mol % of palladium catalyst (Table 1, entry 12).
Subsequently, various phosphine ligands were tested by
using B(OH)3/5-Cl-SA as the acidic additive (Table 2). A very
low conversion of styrene was obtained when basic and
electron-rich phosphine ligands were applied, such as PCy3 and
BuPAd2 (Table 2, L1−L3). Here, the reaction between basic
ligand and acid might be the reason for the loss of activity.
Significantly improved yields of branched thioester 3 can be
achieved when tris(aryl)phosphine ligands were tested (Table
2, L4−L9). The best yield of 3 with 93% can be produced
when tris(4-methoxyphenyl)phosphine was used as the ligand
(Table 2, L8). Remarkably, the selectivity between 4 and 3 can
be reversed when bidentate phosphine ligands were tested, and
4 became the main product in those cases (Table 2, L10−
L17). Toward the linear selectivity, a 94% yield of 4 was
produced with DPEphos as the ligand (Table 2, L16).
desired linear thioesters will be eliminated together with the
regeneration of LPdII−H complex for the next catalytic cycle.
With the optimal conditions in our hand, the scope and
limitation of this transformation was carried out immediately
(Scheme 3). Using 1-octanthiol as the reaction partner, various
styrenes were reacted at the first step. In general, moderate to
excellent yields of linear thioesters were produced (Scheme 3,
4−15). Both electron-donating and electron-withdrawing
groups can be well tolerated. Besides halogen groups, even
the nitro group is also compatible here, which is relatively
easily reduced under carbon monoxide atmosphere in the
presence of a metal catalyst (Scheme 3, 12). Bpin-substituted
styrene can also be transformed to give the desired linear
thioester in 35% yield (Scheme 3, 15). This reaction can also
be performed on 1 mmol scale without loss of efficiency and
selectivity (Scheme 3, 4).
However, when β-methylstyrol was tested as an example of
internal aromatic substituted alkene, low or no thioester could
be formed, which is mainly due to the steric effect raised by the
ligand applied (Scheme 3, 16). Notably, the added trans-β-
methylstyrol stays nonreacted, and the conversion of cis-β-
methylstyrol was less than 20%. Then three examples of
allylarenes were tested under our standard conditions, and
moderate to good yields of the desired linear products can be
isolated without any problem (Scheme 3, 17−19). Aliphatic
alkenes as an interesting class of olefins were tested in this
system without exception (Scheme 3, 20−32). Several
examples of highly functionalized aliphatic alkenes were
transformed into the corresponding linear thioesters in
moderate to good yields. However, in the case of tested α-
methyl substituted alkene 31, no target thioester can be
obtained, although some thioether was detected. In the testing
of thiols, in addition to alkyl thiols, thiophenols can be applied
as well. Moderate to excellent yields of the desired thioesters
can be produced with styrene or 3,3-dimethyl-1-butene as the
reaction partner (Scheme 3, 33−43). Ethylene can be used as
starting material as well, 98% yield of the target product can be
formed (Scheme 3, 44). Finally, some examples of bioactive
molecule substituted styrene derivatives were tested. In our
tested cases, the final products can be isolated without any
problem (Scheme 3, 45−47).
A possible reaction mechanism is proposed on the basis of
our results and previous reports (Scheme 2).1−6,10 The
reaction starts with the generation of the LPdII−H complex
A from the palladium precursor, ligand, and acid additive. After
addition with alkenes, alkylpalladium complex B will be
formed, which will be transformed into acylpalladium complex
C after the coordination and insertion of CO. Finally, the
Finally, competition reactions between different substrates
were carried out under our standard conditions as well
(Scheme 4). With styrene as the substrate, primary and
3617
ACS Catal. 2021, 11, 3614−3619
Ai, Han-Jun
