Sulfur-silicon bond activation catalysed by Cl/Br ions: Waste-free synthesis of unsymmetrical thioethers by replacing fluoride catalysis and fluorinated substrates in SNAr reactions

Article abstract of DOI:10.1039/c4gc00535j

In contrast to conventional activation of Nu-SiR₃ reagents by F ion attributed to the strong affinity of Si to F, S-Si activation can now be achieved using Cl/Br ions of TBAX as catalysts via formation of weaker X-Si bonds and Me₃Si-X. This led to a waste-free synthesis of unsymmetrical thioethers via F-free S_NAr reactions of activated (hetero)aryl halides and RS-SiMe₃, with recovery of the useful Me₃Si-X reagent in high yields. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00535j

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Sulfur–silicon bond activation catalysed by Cl/Br
ions: waste-free synthesis of unsymmetrical
thioethers by replacing fluoride catalysis and
fluorinated substrates in SNAr reactions†
Cite this: Green Chem., 2014, 16,
3444
Received 26th March 2014,
Accepted 19th May 2014
Xiaojuan Jia,‡a Lei Yu,‡a,b,c Jianping Liu,a Qing Xu,*a,c Marcel Sickert,b Lianhui Chena
DOI: 10.1039/c4gc00535j
www.rsc.org/greenchem
and Mark Lautens*b
In contrast to conventional activation of Nu–SiR3 reagents by
F ion attributed to the strong affinity of Si to F, S–Si activation can
now be achieved using Cl/Br ions of TBAX as catalysts via for-
mation of weaker X–Si bonds and Me3Si–X. This led to a waste-
free synthesis of unsymmetrical thioethers via F-free SNAr reac-
tions of activated (hetero)aryl halides and RS–SiMe3, with recovery
of the useful Me3Si–X reagent in high yields.
Trialkylsilyl-protected nucleophiles (Nu–SiR3, mostly R = Me)
bearing element–silicon bonds (E–Si, E = C, N, O, S, etc.) have
long been known as more effective alternatives (Scheme 1)
than the corresponding proton nucleophiles (Nu–H), because
deprotonation of certain weakly acidic Nu–H requires strong
bases that may be incompatible with the reaction conditions
and/or certain functional groups.1,2 Due to the strong affinity
of silicon to fluorine or oxygen, and because Si–F and Si–O
bonds are among the strongest single bonds with the highest
dissociation energies (Scheme 1A),2g new Si–F and Si–O bonds
can be formed to facilitate the formation of the reactive penta-
coordinate or hexacoordinate silicon intermediates intended
for further synthetic transformations, such as the addition and
cross-coupling reactions (Scheme 1B).1,2 Therefore, fluoride
salts such as TBAF (tetrabutylammonium fluoride), KF, or CsF,
polar oxide sources such as alkali metal alkoxides or phen-
oxides, as well as other neutral Lewis bases such as amines
and phosphines and their derivatives have been emerging as
Scheme 1 Backgrounds for E–Si bond activation: (A) average dis-
sociation energies (kJ mol−1) of E–Si bonds;2g (B) simplified mechanisms
for addition and cross-coupling reactions of Nu–SiMe3 reagents.1,2
effective activators of Nu–SiR3 reagents in either catalytic or
stoichiometric amounts, depending on the nature of the reac-
tions.1,2 In contrast, possibly due to the much lower dis-
sociation energies of the weaker X–Si bonds (Scheme 1A),2g the
more accessible halide ions Cl−, Br−, or I− have not been con-
sidered and used as potential activators of Nu–SiR3 reagents.
Owing to the relatively lower availability and higher cost of the
fluoride activators/catalysts and fluorinated substrates, and
because E–Si activation still remains an important issue in the
field,1,2 there is a need to develop new E–Si activation
methods, especially by using the more available and cheaper
Cl or Br ions and the corresponding chlorinated or brominated
substrates. Herein we report that the S–Si bond of organylthio-
trimethylsilanes (RS–SiMe3) can now be effectively activated by
Cl, Br, or I ions derived from TBAX salts (X ≠ F), which led to
efficient TBAX-catalysed aromatic nucleophilic substitution
aZhejiang Key Laboratory of Carbon Materials, College of Chemistry and Materials
Engineering, Wenzhou University, Wenzhou, Zhejiang 325035, China.
E-mail: qing-xu@wzu.edu.cn; Fax: (+)-86-577-86689302; Tel: (+)-86-138-57745327
bDavenport Chemistry Laboratories, Department of Chemistry, University of Toronto,
80 St. George Street, Toronto, ON M5S 3H6, Canada.
E-mail: mlautens@chem.utoronto.ca; Fax: (+1)-416-946-8185;
Tel: (+1)-416-978-6083
cSchool of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou,
Jiangsu 225002, China
†Electronic supplementary information (ESI) available: Detailed condition
screening table, experimental details, and 1H and 13C NMR spectra of the pro-
ducts and Me3SiCl. See DOI: 10.1039/c4gc00535j
‡Both authors contributed equally to this work.
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(SNAr) reactions of activated (hetero)aryl halides and RS–SiMe3. successful with 3a (run 2), which is not unusual, and therefore
This new method provides a transition-metal (TM)-, base-, and a stoichiometric amount of TBAF was required to obtain a
fluorine-free protocol for a waste-free synthesis of the versatile satisfactory result under the same conditions (run 3). Interest-
unsymmetrical heteroaryl and diaryl thioethers from the ingly, we were surprised to find that TBAF began to show cata-
more available and much cheaper fluorine-free catalysts and lytic activity at slightly higher temperatures (80–100 °C)10 to
substrates, with recovery of the useful Me3Si–X reagent in high give a high yield of 6aa at 100 °C (run 4). This is also greatly
yields and on a large scale (Scheme 1).
different from the SNAr reactions of EWG-activated aryl halides
Thioethers are known as important building blocks of (X ≠ F) that required excess amounts of TBAF7f (vide infra).
natural products, biological, pharmaceutical, and agrichemical This result may also imply that, after being initiated by F−, the
compounds,3 key precursors to the corresponding sulfoxides reaction could also be catalysed by Cl− released from 3a via
and sulfones,4 good ligands in TM catalysis,5 and even activation of 1a by formation of the Cl–Si bond. More control
effective organocatalysts in synthesis.6 Therefore, there is a reactions with 2-bromo- and 2-iodopyridines (4a and 5a) also
great interest in thioethers in our laboratories based on our showed that although the dissociation energies of Br–Si and
continuous studies on organochalcogen compounds.7 As to I–Si bonds are lower than that of S–Si (Scheme 1A), the in situ
heteroaryl thioethers, in comparison with TM-catalysed8 and generated Br− and I− could similarly activate 1a and catalyse
conventional SNAr reactions9 of the toxic and sensitive thiols the reaction under the same conditions, albeit giving lower
with heteroaryl halides that require strong basic conditions yields of 6aa (runs 5 and 6). Similar to the above observations,
and the corresponding sodium/potassium thiolates or excess other TBAX salts (X = Cl, Br, I) also showed excellent catalytic
amounts of strong bases, alternative methods using RS–SiMe3 activities in the reaction to give good to high yields of 6aa
reagents should have potential advantages such as milder and (runs 7–9), implying that Cl−, Br−, and I−, either derived from
base-free neutral conditions, higher tolerance of reactive func- TBAX or released from 2-halopyridines 3a–5a, are all effective
tional groups, and accordingly broader scope of the substrates activators of the S–Si bond and efficient catalysts for the reac-
and applications in synthesis.
tion. The results showed that TBAC and TBAB were more
In previous studies, we have described the synthesis of effective; they afforded 6aa in the highest and close yields of
diaryl thioethers via TBAF-catalysed and -mediated SNAr reac- 92% and 94%, respectively (runs 7 and 8). Thus, the cheapest,
tions of RS–SiMe3 with electron-withdrawing group (EWG)-acti- least hygroscopic, and easiest to handle TBAB was selected as
vated aryl halides and nitroarenes.7e,f In the present study, we the precatalyst of the reaction and we found its loading could
initially intended to extend the TBAF-catalysed method to be reduced to only 10 mol% without affecting the product
heteroaryl thioether synthesis via similar reactions of hetero- yield (run 10). During the optimization of the reaction con-
aryl fluorides. Thus, as expected, TBAF readily catalysed the ditions, we also screened more catalysts, solvents, and other
reaction of PhS–SiMe3 (1a) and 2-fluoropyridine (2a) at 60 °C conditions, but TBAX was still found to be the most effective
in acetonitrile, giving a good yield of the target thioether 6aa precatalyst and acetonitrile the best solvent.10
(Table 1, run 1). Since 2-chloropyridine (3a) is a much cheaper
The optimized conditions were then applied to various
substrate than 2a, we incidentally investigated the reac- heteroaryl halides and RS–SiMe3 to extend the scope of the
tions of other 2-halopyridines (2a–5a). TBAF catalysis was not method (Table 2). Like the model reaction (run 1), electron-
rich 2-chloropyridines 3b–f, including those bearing the reac-
tive amino group, all reacted efficiently with 1a to give good to
high yields of the target thioethers (runs 2–6). Possibly due to
deactivation of the 2-chloro moiety by the electron-donating
p-amino group, 3f was less reactive and required 1.1 equiv. of
TBAB to facilitate the reaction (run 6). As to dichloropyridines
3g–j, those with unreactive chloro groups (3g, 3i) gave good
yields of the target products under the standard conditions
(runs 7 and 10); those with reactive chloro groups (3h, 3j) gave
the mono-substituted products in good yields and selectivities
with formation of di-substituted products in minor amounts
(runs 8 and 11). By using 2.2 equiv. of 1a and 30 mol% of
TBAB, 3h and 3j reacted efficiently with 1a to give di-substi-
tuted products in high selectivities and good yields (runs 9
and 12). Similarly, electron-deficient 2-chloropyridines 3k–m
bearing the reactive nitrile and nitro groups11 also gave the
target products in good to high yields under similar conditions
(runs 13–15). The carboxyl group was tolerated in the reac-
tions, but a stoichiometric amount of TBAB was needed to give
good results (runs 16 and 17). Moreover, other heteroaryl
halides like 4-iodopyridine 5p, 2-chloroquinoline 3q, 2-chloro-
Table 1 Condition screening for S–Si bond activation in the reaction of
PhS–SiMe3 and 2-halopyridinesa,10
Run
X (2–5)
TBAX (mol%)
T, t
6aa%b
1
2
3
4
5
6
7
8
9
10
F
TBAF (30)
TBAF (30)
TBAF (120)
TBAF (30)
TBAF (30)
TBAF (30)
TBAC (30)
TBAB (30)
TBAI (30)
TBAB (10)
60 °C, 24 h
60 °C, 24 h
60 °C, 24 h
100 °C, 24 h
100 °C, 24 h
100 °C, 24 h
100 °C, 48 h
100 °C, 48 h
100 °C, 48 h
100 °C, 48 h
78
30
92
87
71
76
92
94
84
96
Cl
Cl
Cl
Br
I
Cl
Cl
Cl
Cl
a The mixture of 1a (0.55 mmol, 1.1 equiv.), 2a–5a (0.5 mmol), and TBAX
in solvent (0.5 mL) was sealed under N2 in a 10 mL Schlenk tube and
then heated and monitored by TLC. b Isolated yields based on 2a–5a.
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Table 2 Substrate extension for heteroaryl thioether synthesisa
pyrimidine 3r, 4-chloro-2-phenylquinazoline 3s, and 2-chloro-
benzothiazole 3t (runs 18–23) and other RS–SiMe3 (runs
24–26)12 all reacted efficiently under the standard TBAB-cata-
lysed conditions to give generally good to high yields of the
products. Consistent with the results in Table 1, the reaction
of 5p confirmed that I− is both a good activator and a good
catalyst for the reaction (runs 18 and 19). In this case, TBAC
might be a better precatalyst because the TBAC-catalysed reac-
tion of 5p gave a higher yield of 6ap (91%, run 19) than the
one used TBAB (86%, run 18).
3–5: FGb
6: yield%c
Inspired by the above positive results, we then considered
extending the scope of the present S–Si activation method. In
previous studies, we have observed that even TBAF was unable
to catalyse the SNAr reactions of RS–SiMe3 and EWG-activated
aryl halides (X ≠ F) and thus it had to be used in more than
stoichiometric amounts.7f Believing that the same activation
protocol may also be applicable to other substrates, in spite of
the previous negative results,7f we continued to investigate the
reactions of EWG-activated aryl halides and PhS–SiMe3 to
develop a catalytic method for diaryl thioether synthesis.
Similarly to previous findings7f and differently from the
reactions of heteroaryl halides (vide supra), initial reactions of
1a and p-chloronitrobenzene 7a under the present optimized
conditions were not successful even at elevated temperatures
and using higher amounts of TBAB (Table 3, runs 1–3). Fortu-
nately, further condition screening showed that a modified
procedure using DMF as the solvent was more suitable for the
reaction (run 4) than using CH3CN. Although TBAB and TBAI
showed low catalytic activities in DMF (runs 4 and 5), TBAC
exhibited a much higher activity than TBAB and TBAI under
the same conditions (run 6) even at lower temperatures (runs 7
and 8). Thus, TBAC smoothly catalysed the reaction to com-
pletion to afford a high yield of the target diaryl thioether 8a
(run 6).
Run
1
2
3
4
3a: H
6aa: 96
6ab: 95
6ac: 93
6ad: 85
6ae: 73
6af: 63
6ag: 81
6ah: 68
3b: 4-Me
3c: 5-Me
3d: 3-NH2
3e: 4-NH2
3f: 5-NH2
3g: 3-Cl
5
6d
7
8e
3h: 4-Cl
6ah/6ah′: 74/26
6ah′: 92
6ah′/6ah: 99/1
9e, f
3h: 4-Cl
10
3i: 5-Cl
3j: 6-Cl
6ai: 74
6aj: 81
6aj/6aj′: 90/10
6aj′: 75
6aj′/6aj: 99/1
6ak: 87
6al: 72
6am: 78
6an: 96
11e
12e, f
3j: 6-Cl
13
3k: 4-CN
3l: 5-CN
3m: 5-NO2
3n: 4-CO2H
3o: 5-CO2H
14g
15e
16d
17d
6ao: 73
18
5p
3q
3r
6ap: 86
6ap: 91
19h
20
21
22
6aq: 78
6ar: 85
6as: 90
These optimized conditions were then applied to other
EWG-activated aryl halides for diaryl thioether synthesis
3s
Table 3 Condition modification for S–Si bond activation in the reaction
of PhS–SiMe3 and p-chloronitrobenzenea
23
24
3t
6at: 87
6ba: 84
3a
25
26
3a
3a
6ca: 75
6da: 90
Run
TBAX (mol%)
sol., T, t
8a%b
1
2
3
4
5
6
7
8
TBAB (10)
TBAB (10)
TBAB (30)
TBAB (30)
TBAI (30)
TBAC (30)
TBAC (30)
TBAC (30)
CH3CN, 100 °C, 24 h
CH3CN, 150 °C, 24 h
CH3CN, 150 °C, 24 h
DMF, 150 °C, 24 h
DMF, 150 °C, 24 h
DMF, 150 °C, 43 h
DMF, 120 °C, 24 h
DMF, 100 °C, 24 h
NRc
NR
NR
54
50
95
a Unless otherwise noted, the mixture of 1 (0.55 mmol, 1.1 equiv.), 3–5
(0.5 mmol), and TBAB (10 mol%) in CH3CN (0.5 mL) was sealed under
N2 in a 10 mL Schlenk tube and then heated and monitored by TLC
and/or GC-MS. b FG = substituting functional group. c Isolated yields
based on 3–5. Ratios of major/minor products determined by GC-MS.
d 1.1 equiv. TBAB, 120 °C. e 30 mol% TBAB. f 2.2 equiv. 1a. g 60 °C.
h 10 mol% of TBAC used instead of TBAB.
70
64
a The mixture of 1a (0.55 mmol, 1.1 equiv.), 7a (0.5 mmol), and TBAX
in solvent (0.5 mL) was sealed under N2 in a 10 mL Schlenk tube and
then heated and monitored by GC-MS and/or TLC. b Isolated yields of
8a based on 7a. c No reaction observed by TLC analysis.
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Table 4 Substrate extension for diaryl thioether synthesisa
normal reactions of the aryl chlorides catalysed by TBAC (runs
3, 9, 12, and 15), the reactions of aryl chlorides could also be
catalysed by TBAB or TBAI (runs 4, 5, 10, and 11), as well as
that catalytic reactions of aryl bromides and iodides could also
be achieved using TBAC, TBAB, or TBAI as the catalyst (runs
6–8, 13–14), all giving the target diaryl thioethers in good to
high yields. In these cases, the reactions of aryl chlorides or
TBAC-catalysed reactions usually gave higher product yields
than those of the aryl bromides/iodides or TBAB- or TBAI-cata-
lysed reactions, revealing that the cheaper aryl chlorides are
more reactive substrates and TBAC the more active activator
and catalyst for the reactions. These results (runs 3–14)
revealed again that, like Cl− and the preceding findings
(Tables 1 and 2), Br− or I−, either derived from TBAB and TBAI
or released from the corresponding aryl bromides and iodides,
can also effectively activate the S–Si reagent and make the reac-
tions catalytic.
Run
1
7
8: yield%b
95
92
2
3
91
71
69
4c
5d
6
90
71
62
7c
8d
8c
9
87
78
70
10c
11d
ð1Þ
12
77
13
14
15
8e
8e
70
72
71
ð2Þ
ð3Þ
According to the simplified mechanism of SNAr reactions
(Scheme 1B), all the above reactions may generate Me3SiX (X =
Cl, Br, I, mostly Cl) as the byproducts, which, unlike Me3SiF,
are among the most useful organosilicon reagents in syn-
thesis.1,2 To prove the formation of the weaker X–Si bonds and
generation of Me3SiX in the reactions, and to further examine
the possibility of the recovery of these useful organosilicon
reagents, large scale reactions of 1a and (hetero)aryl chlorides
were investigated. Thus, as shown in eqn (1), a solvent-free
mixture of 50 mmol of 1a and 3a with 10 mol% of TBAC
equipped with a simple distillation apparatus was heated at
100 to 120 °C under N2.10 The reaction quickly occurred to
afford, in only 6 h, thioether 6aa in a high 92% isolated yield
and Me3SiCl as a colourless liquid in a high recovery yield of
90%.13 The same method can also be applied to EWG-activated
aryl halides for large scale preparation of diaryl thioethers.
Thus, a 25 mmol scale reaction of 1a and 7a under the similar
conditions readily afforded 92% isolated yield of 8a and 83%
recovery yield of Me3SiCl (eqn (2)). Similarly, a 20 mmol scale
neat reaction of 2-bromopyridine 4a and 1a catalysed by TBAB
afforded a good 85% yield of 6aa along with a moderate 52%
yield of Me3SiBr (eqn (3)), showing that the same protocol may
also be applicable to recover Me3SiBr or Me3SiI. However, poss-
ibly due to a higher boiling point and a weaker Si–Br bond and
thus higher reactivity and easier decomposition of Me3SiBr,
16
47
a Unless otherwise noted, see run 6 of Table 3 for conditions. b Isolated
yields based on 7. c 30 mol% TBAB. d 30 mol% TBAI.
(Table 4). Thus, o-chloronitrobenzene and halobenzonitriles
(7b–d) reacted efficiently with 1a to give high yields of diaryl
thioethers 8b–d (runs 2–11). In addition, aryl halides bearing
the reactive carbonyl groups that may undergo addition reac-
tions with S–Si reagents under F-catalysed conditions2 were
tolerated and remained intact under the present conditions to
give good yields of the products (runs 12–16), showing high
chemoselectivity of the reactions. The reaction of p-chloro-
benzaldehyde 7g gave a relatively lower yield of target 8g due
to byproduct formation (run 16), which may be attributed to
the labile nature of the unprotected formyl group in compari-
son with the more stable acetyl group (runs 12–15). Further
attempts to improve 8g’s yield using TBAB or TBAI as the cata-
lyst only led to even lower yields. As shown in the table, slightly
different from chloronitrobenzenes 7a–b, in the cases of halo-
benzonitriles and acetylphenyl halides 7c–f, in addition to
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the reaction not only required higher reaction temperatures the RS–SiMe3 reagents, these reactions were successfully made
but was also less efficient, affording lower yields of the pro- as catalytic processes using TBAX as the effective precatalysts.
ducts. These results revealed that employing (hetero)aryl chlor- Therefore, due to the wide applications of Nu–SiR3 reagents in
ides as the substrates and TBAC as the catalyst should still be synthesis and the ready recoverability of the useful byproducts
the best protocol in large scale reactions. In addition, for some Me3SiX in high yields and in large scales, this new activation
simpler products such as 2-phenylthiopyridine 6aa that have method may become a useful protocol for E–Si activation, may
appropriate boiling points (160–162 °C/8 mmHg, 138–140 °C/ have a wider scope of substrates and reactions, and thus have
2.5 mmHg, or 121–122 °C/0.7 mmHg),14 they can be purified potentially broader utilities in synthesis. Further extension of
by vacuum distillation under reduced pressure. Therefore, pro- the activation method and its synthetic applications are
ducts of the large scale reactions can be obtained pure more underway.
conveniently by vacuum distillation of the reaction residues,
thus avoiding the tedious workup of large scale column
chromatography and the use of large amounts of volatile and
inflammable organic solvents.
Acknowledgements
The above results not only revealed the high recoverability We thank ZJNSF for distinguished young scholars (LR14B020002),
of the useful Me3SiX reagents, but also confirmed the SNAr NNSFC (20902070, 21202141), SRF for ROCS of SEM, ZJQJTP
mechanism of the present TBAX-catalysed reactions. As shown (QJD0902004), Xinmiao Talents Program of Zhejiang Province
in Scheme 2,1,2,7e,f,9 TBAX first generated the initiating X1 ion, (2013R424062), Postgraduate Innovation Foundation of
which then reacts with RS–SiMe3 (1) to give a polycoordinated Wenzhou University (31606036010198) for financial support.
silicon intermediate 9 by forming a new Si–X bond. This M.L. thanks the University of Toronto, Canada Council for the
process activated the S–Si bond of RS–SiMe3 reagents to Arts and NSERC for financial support.
provide a nucleophilic organylthio species 9 that equals
organylthio anion 10, which then adds to (hetero)aryl halides
(3–5, 7) to give intermediates 11 or 12, generating meanwhile
the recoverable Me3SiX reagents. The dearomatized anions
Notes and references
11 or 12 are relatively stable intermediates because the anionic
charge on the ring can be well-balanced by the electronegative
heteroatoms in the ring or the electron-withdrawing groups on
the ring.1,2,7e,f,9 Finally, by leaving a new halide anion X2
derived from substrate (hetero)aryl halides, aromatization of
11 or 12 will take place to give the target unsymmetrical
thioethers 6 or 8. The generated X2 ion will be recycled to work
as the new activator of the remaining RS–SiMe3 reagents, thus
acting as the catalyst of the reaction and completing the cata-
lytic cycle.
In summary, in contrast with conventional activation of
E–Si reagents by the F ion, S–Si bond activation of RS–SiMe3
can now be conveniently accessed by the more available Cl/Br
ions, which then leads to transition-metal-, base-, and F-free
processes for a waste-free synthesis of the versatile unsymme-
trical thioethers when employed in SNAr reactions of activated
(hetero)aryl halides and RS–SiMe3. Since the Cl/Br/I ions
released from (hetero)aryl halides can be recycled to re-activate
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Scheme 2 Proposed mechanism for TBAX-catalysed SNAr reactions of
RS–SiMe3 and activated (hetero)ArX (X ≠ F).
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Green Chemistry
Communication
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Green Chem., 2014, 16, 3444–3449 | 3449