Platinum-catalyzed reaction of alkynes with ArI (Ar = aryl and thienyl) and Ar′SM (M = Na, K, and Sn(Bu-n)3): Three- vs two-component cross-coupling reaction

Article abstract of DOI:10.1021/om050340z

The Pt-catalyzed reactions of terminal alkynes HCCR (2) with ArI (6) (Ar = aryl and thienyl) and Ar′SM (M = Li, Na, K, and Sn(n-Bu₃)) (7) have been examined. Among them, the combined use of 2- and 3-iodothiophene with PhSK resulted in the formation of (Z)-Ar(H)C=C(SAr′)(R) (5) in moderate to good yields. When aryl iodide was employed as a coupling partner, thioether ArSAr′ (4), a direct cross-coupling product between 6 and 7, was competitively produced together with 5. The ratios of formation of 5/4 were significantly dependent on the substituent in Ar of 6 and Ar′ of 7 as well as species of 2. The mechanistic study indicated that 4 was produced by the alkyne-participated reaction of 6 with Pt(Ar)(SAr′)(PPh₃) ₂ (1).

Full text of DOI:10.1021/om050340z

566
Organometallics 2006, 25, 566-570
Articles
Platinum-Catalyzed Reaction of Alkynes with ArI (Ar ) aryl and
thienyl) and Ar′SM (M ) Na, K, and Sn(Bu-n)3): Three- vs
Two-Component Cross-Coupling Reaction
Hitoshi Kuniyasu,* Fumikazu Yamashita, Takayoshi Hirai, Jia-Hai Ye,
Shin-ichi Fujiwara, and Nobuaki Kambe
Department of Molecular Chemistry & Frontier Research Center, Graduate School of Engineering,
Osaka UniVersity, Suita, Osaka 565-0871, Japan
ReceiVed April 30, 2005
The Pt-catalyzed reactions of terminal alkynes HCCR (2) with ArI (6) (Ar ) aryl and thienyl) and
Ar′SM (M ) Li, Na, K, and Sn(n-Bu3)) (7) have been examined. Among them, the combined use of 2-
and 3-iodothiophene with PhSK resulted in the formation of (Z)-Ar(H)CdC(SAr′)(R) (5) in moderate to
good yields. When aryl iodide was employed as a coupling partner, thioether ArSAr′ (4), a direct cross-
coupling product between 6 and 7, was competitively produced together with 5. The ratios of formation
of 5/4 were significantly dependent on the substituent in Ar of 6 and Ar′ of 7 as well as species of 2. The
mechanistic study indicated that 4 was produced by the alkyne-participated reaction of 6 with Pt(Ar)-
(SAr′)(PPh3)2 (1).
Scheme 1. Strategy for Carbothiolation of Alkyne (2)
Introduction
Transition metal-catalyzed cross-coupling reactions have
served as powerful strategies to build up desired chemical
compounds. Along with numerous C-C bond-forming reac-
tions,1 C-S bond formations have also been developed.2,3 On
the other hand, we have recently reported on a very simple
prototype for achievement of a three-component coupling
reaction that enabled the regio- and stereoselective introduction
of carbon and sulfur functionalities into terminal alkynes. The
basic concept is shown in Scheme 1.4 The generation of
platinum complex 1 having a C-Pt-S moiety resistant to C-S
bond-forming reductive elimination patterned after the Ni- or
Pd-catalyzed syntheses of thioethers 4 sets up the reaction.2 Then
cis-insertion of HCCR (2) into the Pt-S bond of 1 with Pt bound
at the terminal position affords the vinyl platinum complex 3,
possessing a C-Pt-C fragment, and finally C-C bond-forming
reductive elimination of vinyl sulfide 5 completes one catalytic
cycle to regenerate the Pt(0) complex. According to this working
hypothesis, the Pt-catalyzed arylthiolation with aryl thioester,5a
Scheme 2. Methodology of Generation of 1
thienylthiolation with thienyl thioester,5b pyridylthiolation with
iodopyridine and ArSK,5c and furylthiolation with furyl thioester5d
of 2 have been realized gratifyingly.
Herein we wish to report on details about carbothiolation
using ArX (Ar ) aryl and thienyl; X ) Cl, Br, I, and OTf) (6)
and Ar′SM (M ) Li, Na, K, and Sn(Bu-n)3) (7) as the sources
of formation of platinum intermediate 1 (M′ ) Pt) via oxidative
addition of 6 to Pt(0) to yield the Pt(II) complex Ar-Pt-X (8)
and subsequent transmetalation with 7 (Scheme 2).6
* To whom correspondence should be addressed. E-mail:
kuni@chem.eng.osaka-u.ac.jp.
(1) Metal-catalyzed Cross-coupling Reactions; Diederich, F., Stang, P.
J., Eds.; Wiley-VCH: New York, 1998.
(2) (a) Kosugi, M.; Shimizu, T.; Migita, T. Chem. Lett. 1978, 13. (b)
Murahashi, S.; Yamamura, M.; Yanagisawa, K.; Mita, N.; Kondo, K. J.
Org. Chem. 1979, 44, 2408. (c) Migita, T.; Shimizu, T.; Asami, Y.; Shiobara,
J.; Kato, Y.; Kosugi, M. Bull. Chem. Soc. Jpn. 1980, 53, 1385. (d) Kosugi,
M.; Ogata, T.; Terada, M.; Sano, H.; Migita, T. Bull. Chem. Soc. Jpn. 1985,
58, 3657. (e) Carpita, A.; Rossi, R.; Scamuzzi, B. Tetrahedron Lett. 1989,
30, 2699. (f) Cristau, H. J.; Chabaud, B.; Chene, A.; Christol, H. Synthesis
1981, 892. (g) Takagi, K. Chem. Lett. 1987, 2221. (h) Deardorff, D. R.;
Linde, R. G.; Martin, A. M.; Shulman, M. J. J. Org. Chem. 1989, 54, 2759.
(i) Martinez, A. G.; Barcina, J. O.; de Fresno, C. A.; Subramanian, L. R.
Synlett. 1994, 561. (j) Rane, A. M.; Miranda, E. I.; Soderquist, J. A.
Tetrahedron Lett. 1994, 35, 3225. (k) Beletskaya, I. P. J. Organomet. Chem.
1983, 250, 551. (l) Cristau, H. J.; Chabaud, B.; Labaudiniere, R.; Christol,
H. Organometallics 1985, 4, 657. (m) Dickens, M. J.; Gilday, J. P.; Mowlem,
T. J.; Widdowson, D. A. Tetrahedron 1991, 47, 8621.
Results and Discussion
Arylthiolation or Thienylthiolation vs Cross-Coupling
Reaction (Table 1). First, the arylthiolation of 1-octyne (2a,
(5) (a) Sugoh, K.; Kuniyasu, H.; Sugae, T.; Ohtaka, A.; Takai, Y.;
Tanaka, A.; Machino, C.; Kambe, N.; Kurosawa, H. J. Am. Chem. Soc.
2001, 123, 5108. (b) Hirai, T.; Kuniyasu, H.; Kambe, N. Chem. Lett. 2004,
33, 1148. (c) Hirai, T.; Kuniyasu, H.; Kambe, N. Tetrahedron. Lett. 2005,
46, 117. (d) Hirai, T.; Kuniyasu, H.; Terao, J.; Kambe, N. Synlett 2005, 7,
1161.
(3) Kondo, T.; Mitsudo, T. Chem. ReV. 2000, 100, 3205.
(4) Kuniyasu, H.; Kurosawa, H. Chem. Eur. J. 2002, 8, 2660.
(6) An example of arylthiolation of 1-octyne using PhI and PhSNa has
already been reported in a communication (ref 5a).
10.1021/om050340z CCC: $33.50 © 2006 American Chemical Society
Publication on Web 12/30/2005

Pt-Catalyzed Reaction of Alkynes
Organometallics, Vol. 25, No. 3, 2006 567
Table 1. Carbothiolations of 2 with ArX (6) and Ar′YM′ (7)
a 2 (1.2 mmol), 6 (1.0 mmol), 7 (1.1 mmol), Pt(PPh3)4 (0.05 mmol), and toluene (0.5-1.0 mL) under reflux for 24 h. b Isolated yield. c CH2dC(SPh)(C6H13-
n) was produced in 62% yield. d 2.5 mmol of 2d in a sealed tube at 110 °C. e CH2dC(SPh)(C6H13-n) was produced in 59% yield. f Pd(PPh3)4. g Without 2a.
h 7b (2.2 mmol) and 2e (2.4 mmol).
1.2 mmol) was examined using PhX (6, 1.0 mmol) and PhSM
(7, 1.1 mmol) in the presence of Pt(PPh3)4 (0.05 mmol) under
vigorous toluene reflux for 24 h. Both reactions using PhSNa
(7a) with PhCl (6a) and with PhBr (6b) did not produce
arylthiolation products (entries 1 and 2), while the reactions
using PhI (6c) and PhOTf (6d) gave the anticipated (Z)-Ph-
(H)CdC(SPh)(C6H13-n) (5a) in 83% and 40% yields, respec-
tively (entries 3 and 4).2a,c
The reaction of 6c with PhSK (7b) also provided 5a in 87%
yields (entry 5); however, the reaction using PhSLi (7c)
produced only 3% of 5a and the hydrothiolation product CH2d
C(SPh)(C6H13-n)7 was instead obtained in 62% yield (entry 6).
The reaction of 6c with PhSSn(Bu-n)3 (7d) yielded 5a in 66%
yield (entry 7).2d Interestingly, contrary to the cases of arylthio-
lation using thioesters,5a PhSPh (4a), a direct cross-coupling
product between 6c and 7, can also be furnished under these
reaction systems.8 For example, 4a was obtained in 3% and
25% yields when 6c and 7a or 6c and 7d were employed as
coupling components (entries 3 and 7). The amount of 4 was
significantly influenced by the electronic nature of Ar in 6 and
Ar′ in 7. The introduction of an electron-withdrawing group in
Ar of ArI slightly increased the yield of 4 (entry 9; Ar )
p-CF3C6H4, 6f, 15% of p-CF3C6H4SPh (4c)) compared to the
case of Ph (entry 3; 6c, 3% of 4a) or Ar with an electron-
donating group (entry 8; Ar ) p-MeC6H4, 6e, 4% of p-MeC6H4-
SPh (4b)). On the other hand, the introduction of electron-
donating groups in Ar′ of Ar′SNa increased the formation of 4
(entry 10; Ar′ ) p-MeC6H4, 7e, 25% of 4b, entry 11; Ar′ )
p-MeOC6H4, 7f, 42% of 4d, and entry 13; Ar′ ) p-MeOC6H4,
7f, 35% of 4e), while an electron-withdrawing group suppressed
the generation of 4 (entry 12; Ar′ ) p-CF3C6H4, 7g, 0% of 4c).
The ratios of 5 and 4 also varied with the species of alkyne (2).
(7) Kuniyasu, H.; Ogawa, A.; Sato, K.; Ryu, I.; Kambe, N.; Sonoda, N.
J. Am. Chem. Soc. 1992, 114, 5902.
(8) Thioesters were also produced as byproducts under the Pt-catalyzed
pyridylthiolation of alkynes by iodopyridine and ArSK. See ref 5c.

568 Organometallics, Vol. 25, No. 3, 2006
Kuniyasu et al.
Scheme 3. Possible Reaction Routes to the Formation of 4
First, the Pt-catalyzed cross-coupling reaction between
p-MeC6H4I (6e) and p-MeOC6H4SNa (7f) was attempted;
however, only a trace amount of 4e (<1%) was detected after
20 h at 110 °C (eq 1).
In good accordance with this fact, heating the toluene-d8 (0.6
mL) solutions of trans-Pt(C6H4Me-p)(SC6H4OMe-p)(PPh3)2 (1a,
0.01 mmol)9 and PPh3 (0.05 mmol) added as trans-to-cis
isomerization catalyst either in the presence or in the absence
of 6e did not produce p-MeC6H4SC6H4OMe-p (4e) at all (eq
2). It should be noted that 35% of 4e was produced under the
reaction of 2a with 6e and 7f (entry 13 in Table 1).
The replacement of 2a for branched aliphatic terminal alkyne
2b increased the generation of 4 to furnish 69% of 5h and 27%
of 4a (entry 14 vs entry 5). The arylthiolation of alkynes having
-CN and -OH groups also provided the three-component
coupling products 5i and 5j in moderate yields (entries 15 and
16). When phenyl acetylene (2e) and (trimethylsilyl)acetylene
(2f) were employed, major products were ended in 4a to yield
30% of 5k with 66% of 4a and 12% of 5l with 64% of 4a,
respectively (entries 17 and 18). Moreover, the formation of 5
and 4 was both totally suppressed when more bulky (triisopro-
pylsilyl)acetylene (2g) was employed as the substrate (entry 19),
suggesting that the coordination of 2 to the Pt complex must
be prerequisite for the formation of thioether (4) as well as the
production of 5. Next, the thienylthiolations using ArX and
Ar′SM were examined. Although chloride again did not show
any activity toward carbothiolation (entry 20), the reaction
employing bromide afforded the desired product 5n in moderate
yield (entry 21). On the other hand, the thienylthiolation of 2a
employing 2-iodothiophene (6g) with PhSK (7b) more ef-
ficiently produced desired product 5n in 98% yield (entry 23).
The potassium thiolate exhibited the highest activity among
alkali metals examined (entries 22-24). It is a noteworthy fact
that palladium complex (Pd(PPh3)4), which catalyzed the direct
cross-coupling reaction between 6g and 7b to afford 4f in good
yield (entry 26) and did not catalyze the arylthiolation of alkyne
by aryl thioester,5a also possessed catalytic activity for thie-
nylthiolation, albeit in lower yield (59%) and with formation
of a mixture of stereoisomers (E/Z ) 16/84) (entry 25).5b
Thiophene derivative 5o, with a more extended conjugated
moiety, was obtained in 54% yield by employing 2,5-diiodot-
hiophene (entry 27). 3-Thienylthiolation by 3-iodothiophene (6k)
also successfully proceeded to afford 5p in 80% yield (entry
28). 2-Thienylthiolation was also successfully applied to other
terminal alkynes (2c, 2d, 2e, HCCC6H4OMe-p (2h), and HCC-
(CH2)2OTHF (2i)) to give the corresponding three-component
coupling products in moderate to good yields (entries 29-33).
Again, the formation of 4f, a direct cross-coupling product
between 6i and 7b, was fairly suppressed under these thie-
nylthiolations.
1
Then the reaction of 1a with 2a was monitored by H and
31P NMR spectroscopy (eq 3). After 10 h at 110 °C, the
formation of 5g (99%) and Pt(PPh3)2(HCCR) (11a; 36%) was
confirmed;10 however, 4e was again not detected at all. It must
also be noted that no signal of suspected vinyl platinum 3
(Scheme 1) was detected during the course of the reaction,
presumably because the C-C bond-forming reductive elimina-
tion11 from 3 was much faster than the insertion of 2 into the
Pt-S bond of 1.
With respect to the C-S bond formation from Pd(II)
complexes, Hartwig et al. have disclosed that the introduction
of an electron-withdrawing group in the detaching carbon
functional group and an electron-donating group in the sulfur
functional group promoted the reaction.12 The electronic effects
observed in entries 8-13 of Table 1 apparently accord with
the electronic nature of C-S bond formation from Pd(II)
complexes. However, the above experimental results indicated
that the participation of path a of Scheme 3 for the generation
of thioether (4) was unlikely under the Pt-catalyzed reaction of
alkyne (2) with ArX (6) and Ar′SM (7).13
Then to examine the possibility of the participation of both
6 and 2 in the formation of 4, the reaction of trans-Pt(C6H4-
Mechanistic Study: Arylthiolation vs Cross-Coupling
Reaction. With respect to the formation of direct cross-coupling
product ArSAr′ (4), three reaction routes are conceivable
(Scheme 3). Path a showed the C-S bond-forming direct
reductive elimination of 4 from Pt(II) complex (1) with the
regeneration of Pt(0), which can be trapped by 6 (X ) I) to
give 8. The σ-bond metathesis between the Pt-S bond of 1
and the X-Ar bond of 6 via the four-membered intermediate 9
(path b) and reductive elimination via Pt(IV) intermediate 10
formed by the oxidative addition of 6 to 1 (path c) were also
possible. Then to get information about the reaction route to 4,
the following reactions were next examined.
(9) The authentic trans-1, which was thermodynamically more stable
than cis-1, was successfully prepared by the reaction of Pt(PPh3)4 with ArI
then Ar′SNa.
(10) The low yield of 11a may be attributable to the formation of Pt-
(PPh3)4, which cannot be detected by 31P NMR spectroscopy due to rapid
liberation of PPh3 in solution.
(11) (a) Stang, P. J.; Kowalski, M. H. J. Am. Chem. Soc. 1989, 111,
3356. (b) Merwin, R. K.; Schnabel, R. C.; Koola, J. D.; Roddick, D. M.
Organometallics 1992, 11, 2972, and references therein.
(12) (a) Mann, G.; Baranano, D.; Hartwig, J. F.; Rheingold, A. L.; Guzei,
I. A. J. Am. Chem. Soc. 1998, 120, 9205. (b) Baranano, D.; Hartwig, J. F.
J. Am. Chem. Soc. 1995, 117, 2937.

Pt-Catalyzed Reaction of Alkynes
Organometallics, Vol. 25, No. 3, 2006 569
Me-p)(SC6H4OMe-p)(PPh3)2 (1a, 0.01 mmol) with HCCC6H13-n
(2a) (0.1 mmol) was next performed in the presence of
p-MeC6H4I (6e, 0.1 mmol) (eq 4). Contrary to the cases of
reactions of 1a with 6e (eq 2) and 1a with 2a (eq 3), the
formation of 4e (36%) was confirmed after 6 h together with
5g (63% based on SC6H4OMe-p of 1a) and trans-Pt(C6H4Me-
p)(I)(PPh3)2 (8a, 100% based on Pt).
substituents (entries 10, 11, and 13 in Table 1) both can
accelerate such a polar σ-bond metathesis. However, considering
the fact that C-S bond formation of MeSMe took place from
the Pt(IV) complex generated by the oxidative addition of MeI
to [Pt(SMe)(PMe2Ph)(µ-SMe)]2,14 the route of path c cannot
be fully ruled out, because the trans C-Pt-S geometry can be
retained during the oxidative addition and following reductive
elimination sequence.
Furthermore, to get insight about the origin of the Ar moiety
of Ar-SAr′ (4), the reaction of 1a (0.01 mmol) and 2a (0.03
mmol) with p-CF3C6H4I (6f, 0.1 mmol) (eq 5) and the reaction
of trans-Pt(C6H4CF3-p)(SC6H4OMe-p)(PPh3)2 (1b, 0.01 mmol)
and 2a (0.03 mmol) with p-MeC6H4I (6e, 0.1 mmol) (eq 6) were
next examined.
Finally, the reaction of 2a (0.12 mmol) with 6e (0.1 mmol)
and 7f (0.1 mmol) in the presence of Pt(PPh3)4 (0.01 mmol) in
toluene-d8 (0.6 mL) was monitored by NMR spectroscopy at
105 °C (eq 7). The 31P NMR spectrum taken after 12 h showed
the formation of a mixture of 1a and 8a in 78% and 5% yields
1
based on Pt, respectively. On the other hand, its H NMR
spectrum detected the signals of 15% of 5g and 5% of 4e. Then
the yields of 5g and 4e were increased gradually (26% and 9%
after 24 h, respectively), while the yields of 1a and 8a hardly
changed, indicating that the complexes 1 and 8 existed as resting
states of the present Pt-catalyzed reaction. Judging from the
foregoing experimental data, possible routes of the present Pt-
catalyzed reaction of 2 with 6 and 7 are shown in Scheme 4.
Scheme 4. Possible Reaction Routes to 4 and 5 under the
Pt-Catalyzed Reactions of 2 with 6 and 7
The former produced p-CF3C6H4SC6H4OMe-p (4f, 46% based
on SC6H4OMe-p of 1a) and trans-Pt(C6H4Me-p)(I)(PPh3)2 (8a,
54% based on Pt) as well as 5g (27% based on SC6H4OMe-p
of 1a) and trans-Pt(C6H4CF3-p)(I)(PPh3)2 (8b, 23% based on
Pt). On the other hand, the latter produced p-MeC6H4SC6H4-
OMe-p (4e, 52% based on SC6H4OMe-p of 1a) and 8b (43%
based on Pt) as well as 5l (47% based on SC6H4OMe-p of 1a)
and 8a (39% based on Pt). These results clearly showed that
the Ar moiety of 4 was selectively derived from newly added
6. The facts that the yields of 4f and 8a as well as 5g and 8b
in eq 5 and yields of 4e and 8b as well as 5l and 8a in eq 6
were all comparable with each other did not contradict the
assumption that the formation of these compounds also cor-
related with each other.
After the generation of 1 through the oxidative addition of 6
to Pt(0) and transmetalation with 7,15 alkyne 2 would coordinate
to 1 to produce 12, whose formation can be the rate-limiting
step of this Pt-catalyzed reaction. The coordinated alkyne may
insert into the S-Pt bond to produce 3, from which C-C bond-
forming reductive elimination facilely took place to give 5 with
regeneration of Pt(0). When Ar is the aryl group, the complex
12 also competitively could react with 6 to give 8 and 4, in the
latter of which the Ar moiety was selectively incorporated from
6. This could explain why the product distribution (5/4) was
significantly influenced by the species of alkyne (2) and why
the yields of 4f and 8a, 5g and 8b, 4e and 8b, and 5l and 8a in
eqs 5 and 6 were comparable.
Foregoing data may suggest the involvement of path b of
Scheme 3 for the formation of 4, because the increase of
electrophilicity of 6 by introducing an electron-withdrawing
group (entry 9 in Table 1) and the increase of nucleophilicity
of the SAr′ moiety in 1 by introducing electron-donating
(13) The study of oxidative addition of vinyl sulfide to Pt(PPh3)(C2H4)
to yield vinyl platinum with a cis-sp2-C-Pt-S fragment supported that
the sp2-C-S bond-forming reductive elimination was also a thermodynami-
cally unfavorable process even from a cis-PPh3-ligated isomer. Kuniyasu,
H.; Ohtaka, A.; Nakazono, T.; Kinomoto, M.; Kurosawa, H. J. Am. Chem.
Soc. 2000, 122, 2375. See also ref 4.
(14) Puddephatt, R. J.; Upton, C. E. E. J. Organomet. Chem. 1975, 91,
C17.
(15) The reaction of trans-Pt(Ph)(I)(PPh3)2 (8c, 0.01 mmol) with 7a (0.1
mmol) in the presence of PPh3 (0.1 mmol) in toluene (0.6 mL) produced
trans-Pt(Ph)(SPh)(PPh3)2 (1c) and [Pt(Ph)(SPh)(PPh3)]2 (1c′) in 34% and
66% yields at 105 °C after 3 h, respectively.

570 Organometallics, Vol. 25, No. 3, 2006
Kuniyasu et al.
Transformation of 5 into a Double Arylation Product. The
utility of the present transformation was attested by converting
the SAr group into an Ar group.2,3,16 The treatment of 5d (0.5
mmol) with PhMgBr (1.0 mmol) in the presence of Pd2(dab)3‚
CHCl3 (5 mol %) and PBu3-n (20 mol %) under THF reflux
for 8 h resulted in the formation of the desired (Z)-(Ph)(H)Cd
C(Ph)(C6H13-n) (13) in 69% yield (eq 8), enabling the introduc-
tion of two Ar groups into alkyne 2 by the sequential
Pt-catalyzed carbothiolation and Pd-catalyzed cross-coupling
reaction.
Experimental Section
General Procedures. 1H and 13C NMR spectra in CDCl3 solution
and 31P NMR spectra in toluene-d8 and benzene-d6 solution were
recorded with a JEOL JNM-Alice-400 (400 MHz). Chemical shifts
in the H NMR and 13C NMR spectra were recorded relative to
1
Me4Si and CDCl3, respectively. Chemical shifts in the 31P NMR
spectra were recorded relative to 85% H3PO4(aq) as an external
standard. Infrared spectra were obtained with a Perkin-Elmer FT-
IR (Model 1600). GC mass (EI) analyses were run using a Saturn
GCMS-2000 spectrometer. Elemental analyses were performed in
the Instrumental Analysis Center of the Faculty of Engineering,
Osaka University. Preparative TLC was carried out using Wakogel
B-5F silica gel using hexane and Et2O as eluent. Toluene was
distilled just before use. Aryl iodides, iodothiophene, PhSSn(Bu-
n)3, and alkynes were commercially available.
General Procedure of Pt-Catalyzed Arylthiolation of Alkynes
2 (entry 3 in Table 1). Into a 3 mL flask equipped with a reflux
condenser were added Pt(PPh3)4 (62.2 mg, 0.05 mmol), PhSNa,
(7a) (145.4 mg, 1.1 mmol), toluene (0.5 mL), PhI (6c) (204.0 mg,
1.0 mmol), and 1-octyne (2a) (132.2 mg, 1.2 mmol). The reaction
mixture was vigorously refluxed for 24 h under a nitrogen
atmosphere, the resulting mixture was filtrated through Celite, and
the resultant filtrate was evaporated. The products (5a) were isolated
by PTLC (eluent: hexane and ether) (83% yield). Other reactions
in Table 1 were performed similarly. In the cases of entries 8-13,
1.0 mL of toluene was used as the solvent.
Attempted Reductive Elimination of 4e from 1a (eq 2). Into
a dry Pyrex NMR tube were added 1a (9.5 mg, 0.01 mmol), PPh3
(13.1 mg, 0.05 mmol), and toluene-d8 (0.6 mL) under N2 atmo-
sphere. Then the reaction at 110 °C was monitored by NMR
spectroscopy. However, no formation of 4e was observed even after
20 h. Other mechanistic studies shown in eqs 3-7 were performed
similarly.
Conclusion
This study demonstrated that Pt-catalyzed carbothiolation of
terminal alkyne with ArX and Ar′SM can be successfully
realized by using 2- and 3-iodothiophene and Ar′SK. When
aryl-I was employed as a coupling partner, the carbothiolation
can significantly compete with C-S bond formation, where the
participation of both alkyne and ArI was indicated. It must be
emphasized that catalytic C-S bond-forming cross-coupling
may take place through other than conventional direct C-S
bond-forming reductive elimination from the M′(II) complex,
suggesting that an alternative pathway must always be taken
into account, when the mechanisms of a number of transition
metal-catalyzed cross-coupling reactions of R-X with R′M are
considered.1
Supporting Information Available: Experimental procedures
and spectral data of the products and authentic 1. This material is
available free of charge via the Internet at http://pubs.acs.org.
(16) Itami, K.; Mineno, M.; Muraoka, N.; Yoshida, J. J. Am. Chem. Soc.
2004, 126, 11778.
OM050340Z