Definitive evidence for the insertion of terminal alkynes into ArylS-Pt bonds: "o-halogen effect" in stoichiometric and catalytic reactions

Article abstract of DOI:10.1002/anie.200604986

(Chemical Equation Presented) An unexpected acceleration of the insertion of terminal alkynes into the S-Pt bond of complexes 1 was observed when an o-halogen substituent was present in the SAr moiety (see scheme). The reaction rates for G = o-Cl, o-Br, or o-l were much greater than for electron-donating or electron-withdrawing groups. This "o-halogen effect" was also found to be relevant to the Pd-catalyzed addition of (ArS)₂ to alkynes.

Full text of DOI:10.1002/anie.200604986

Angewandte
Chemie
DOI: 10.1002/anie.200604986
Insertion Reactions
Definitive Evidence for the Insertion of Terminal Alkynes into
À
ArylS Pt Bonds: “o-Halogen Effect” in Stoichiometric and Catalytic
Reactions**
Hitoshi Kuniyasu,* Fumikazu Yamashita, Jun Terao, and Nobuaki Kambe*
Transition-metal complexes exhibit high catalytic activities
would be an ideal complex for examining the insertion of 2
À
À
for a number of cis addition reactions of compounds of the
into the S M bond, as we expected the C Cl bond-forming
reductive elimination from the vinyl platinum species pro-
duced by insertion to be a thermodynamically unfavorable
process.Furthermore, we developed recently a general
method for the preparation of 4 by using pyridine as a
catalyst for cis-to-trans isomerization.[14] We studied the
reactions of 4 with terminal alkynes 2 and report our results
herein.
À
type ArS G (1; G is H or a functional group) to unsaturated
[1]
À
C C bonds. Among the reported substrates, terminal
alkynes 2 have proven to be the most reactive, and a wide
range of substituents G are possible, including H,[2] 9-BBN,[3]
Ar’ (from ArSC(O)Ar’),[4] CO2Me,[5] C(O)NR2,[6] SiCl3,[7]
P(O)(OPh)2,[8] and SAr.[9] In most cases, PR3-ligated Pd0 or
Pt0 catalysts have been used to produce vinyl sulfides 3 with
the ArS group bonded to the internal carbon atom and G at
the terminal position.[10]
First, we monitored the reactions of trans-[Pt(SC6H4-p-
Cl)Cl(PPh3)2] (4a, 0.01 mmol) with alkynes 2a–c (0.01 mmol)
in [D8]toluene (0.6 mL) at 1108C by 1H NMR spectroscopy by
Similar pathways have been proposed for these addition
reactions (Scheme 1).Sufficient evidence has been provided
=
using (p-MeC6H4)3P S as an internal standard [Eq.(1)].
1H NMR spectra of the reaction mixture with phenylacety-
Scheme 1. Possible routes for the addition of 1 to 2 ([M]=PR3-ligated
metal fragment). With Pd cat.: G=H, 9-BBN, CO2Me, C(O)NR2,
P(O)(OPh)2, SAr. With Pt cat.: G=H, SiCl3, Ar’.
lene (2a) indicated clean formation of the vinyl platinum
complex 5a on the basis of a signal at d = 7.62 ppm (t, 3JP, H
=
4.0 Hz) assigned to a vinyl hydrogen atom.[15] The yield
reached 83% after 6 h.Compound 5a was isolated by
recrystallization in 87% yield from a reaction carried out on
a preparative scale, and its structure was determined by X-ray
crystallographic analysis.The double bond in 5a has the
Z configuration with the ArS group at the internal position
and Pt at the terminal position (Figure 1).[16]
The configuration and substitution pattern of 5a are in
agreement with structure 5 in Scheme 1 and provide the first
solid evidence for the insertion of a terminal alkyne into the
bond between a PPh3-ligated Group 10 metal and a sulfur
atom.[17,18] Similar insertions were confirmed for the reactions
of 1-octyne (2b) and propargyl alcohol (2c): The correspond-
ing vinyl platinum complexes 5b (only trans) and 5c (only
trans) were obtained in 87 and 69% yield after 16 and 6 h,
respectively.The observation that no alkyne-exchange reac-
for step a, the oxidative addition of 1 to the low-valent metal
center M.[2,4,5,11] However, information about step b (the
À
À
insertion of 2 into an S M or a G M bond) and step c
À
À
(reductive elimination to form a C G or a C S bond) is very
limited, probably because reductive elimination is faster than
insertion.[12] Accordingly, to elucidate step b, the reaction
system must be designed suitably to prevent reductive
elimination.[13] We predicted that [Pt(SAr)Cl(PPh3)2] (4)
[*]Dr. H. Kuniyasu, F. Yamashita, Dr. J. Terao, Prof. N. Kambe
Department of Applied Chemistry
Graduate School of Engineering, Osaka University
Suita, Osaka 565-0871 (Japan)
Fax: (+81)6-6879-7390
E-mail: kuni@chem.eng.osaka-u.ac.jp
kambe@chem.eng.osaka-u.ac.jp
=
tion took place upon the treatment of trans-[Pt{(Z)-CH C-
(SC6H4-o-Cl)Ph}Cl(PPh3)2] with 1-octyne (2b) even after 6 h
at 708C suggested that the insertion step is an irreversible
process.In agreement with previous findings that internal
[**]F.Y. thanks the Center of Excellence (21COE) program “Creation of
Integrated EcoChemistry” of Osaka University.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
À
alkynes are generally inert in addition reactions of ArS G (1)
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À
Figure 2. Hammett plot of the rates of insertion of 2a into the S Pt
bond of 4a–e.
Figure 1. Molecular structure of trans-5a (thermal ellipsoids set at the
50% probability level). CH2Cl2 solvent and Ph groups of PPh3 are
omitted for clarity.
approximately 19 times faster than that of the complex with a
PhS group (Table 1, entries 3 and 9).Whereas similar
facilitation of the insertion step was also observed with an
1
to 2 with PR3-ligated Pd or Pt catalysts, no insertion took
place with 4-octyne.
o-Br substituent (t = = 0.26 h; the insertion proceeds approx-
2
imately 20 times faster than with PhS; Table 1, entry 10) and
The effects of substituents in the ArS moiety of the
platinum complex 4 were then examined in the presence of an
excess amount of 2a (0.17m).The consumption rate of the
starting complex 4 at 708C obeyed pseudo-first-order kinetics.
an o-I substituent (t1/2 = 0.19 h; the insertion proceeds
approximately 28 times faster than with PhS; entry 11), the
insertion was suppressed significantly with an o-F substituent
1
(t = = 7.3 h; 1.4 times slower than with PhS; entry 12). Thus,
2
1
The half-life (t = ) of 4 for each reaction is shown in Table 1.
we could conclude that the low-energy lone pairs of electrons
2
1
The values range from t = = 2.8 h for the platinum complex
on F do not facilitate the insertion step.On the other hand, no
2
large difference was observed between the substituents p-
1
1
Table 1: The effect of substituents in the ArS moiety.[a]
2
2
OMe (t = = 2.8 h; Table 1, entry 5) and o-OMe (t = = 1.5 h;
entry 13): We postulate that steric retardation and a certain
degree of electronic facilitation by the o-OMe group cancel
each other out.
We next sought to elucidate the mechanism underlying
the observed “o-halogen effect”.Intramolecular coordination
by o-halogen substituents in ligands of the type ArS has been
documented previously.[20,21] Thus, we carried out a phos-
phine-ligand-exchange reaction to determine whether the o-
halogen substituent accelerates the liberation of PPh3
[Eq.(2); Tol = tolyl].The treatment of trans-[Pt(SC6H4-p-
Cl)Cl(PPh3)2] (4a) with trans-[Pt(SC6H4-p-Cl)Cl{P(p-Tol)3}2]
(4a’) at 258C gave trans-[Pt(SC6H4-p-Cl)Cl(PPh3){P(p-Tol)3}]
1
1
Entry
4
X
t = [h]Entry
4
X
t = [h]
2
2
1
2
3
4
4b
4a
4c
4d
4e
4d
p-CF3
p-Cl
H
p-Me
p-OMe
p-Me
12.2
6.1
5.3
4.0
2.8
7
8
9
10
11
12
13
4 f
4g
4h
4i
4j
4k
4l
o-Me
o-iPr
o-Cl
o-Br
o-I
10.1
13.3
0.28
0.26
0.19
7.3
5
6[b]
16.4
o-F
o-OMe
1.5
[a]Reaction conditions: 4 (0.01 mmol), 2a (0.17m), C6D6, 708C. [b]PPh 3
(0.03 mmol) was added.
1
with a p-OMe-substituted aromatic ring to t = = 12.2 h with a
2
p-CF3 substituent (Table 1, entries 1–5).The Hammett plot
shows a good linear free-energy relationship with simple
s values that correlate with the acidity of the equivalently
substituted benzoic acids (Figure 2).Its negative slope ( 1 =
À0.7) indicates that electron-donating groups (EDG) slightly
facilitate the insertion.[19] In the presence of additional PPh3
1
(3 equiv), the reaction was retarded significantly (t = = 16.4 h;
2
Table 1, entry 6).The rate of insertion was influenced even
1
more drastically by ortho substituents.The t = value of 10.1 h
(4a’’) in 23% yield after 1 h.A similar ligand exchange with
complexes 4h and 4h’, which contain an o-Cl substituent,
took place at a slightly lower reaction rate (12% yield of 4h’’
after 1 h).These results may rule out the possibility that the
dissociation of one phosphine ligand triggered by the
coordination of o-X (X = Cl, Br, or I) to Pt (7 in Scheme 2)
2
observed for 4 with an o-Me substituent (Table 1, entry 7) and
the t = value of 13.3 h with an o-iPr group (entry 8) suggest
that the steric hindrance caused by the substituent in the ortho
1
2
1
position retards the insertion.Intriguingly, a t = value of 0.28 h
2
was observed for o-Cl-substituted 4h: The reaction was
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Chemie
The experiments described above suggest the following
reaction pathway (Scheme 3): Complex A (Y= Cl, SAr)
reacts via complex B, which is formed by the liberation of
Scheme 2. “o-Halogen effect”: Possible roles of the ortho substituent
(X=Cl, Br, I).
À
promotes the insertion of an alkyne into the S Pt bond.
Another possibility is that the o-halogen coordinates to the
vacant site generated by the migration of ArS to the alkyne (8
in Scheme 2).[22] It is also possible that one of the electron lone
Scheme 3. A plausible reaction route for the insertion of an alkyne into
À
the S Pt bond of trans-[Pt(SAr)Y(PPh3)2](Y =Cl, SAr).
À
pairs on X interacts with the S Pt s* orbital and thus weakens
À
the S Pt bond (9 in Scheme 2).In fact, X-ray crystallographic
analysis of trans-[Pt(SC6H4-o-Br)Cl(PPh3)2] (4i) showed that
PPh3, with alkyne 2 to afford the alkyne complex C.The
À
the Br S distance of 3.2 Š is within the sum of the van der
Waals radii (3.6 Š) of the two atoms (Figure 3).
migration of ArS to the coordinated alkyne then gives
complex D.Coordination of PPh at the vacant site results
3
in the formation of E as the kinetic product,[27] which
isomerizes to the thermodynamically more stable trans
isomer F.
Finally, the effect of an ortho substituent was examined in
the Pd-catalyzed dithiolation of 1-octyne (2b) with a disulfide
12 of the type (ArS)2 (G = SAr in Scheme 1).[9a] The treat-
ment of 2b with 12a or 12b at 608C for 5 h produced the
adducts 13a and 13b, respectively (Table 2, entries 1 and 2).
Significantly lower conversions were observed for the reac-
tions with 12c, 12d, and 12e (Table 2, entries 3–5).
Table 2: Pd-catalyzed addition of (ArS)2 (12) to 1-octyne (2b).[a]
Figure 3. Molecular structure of 4i (thermal ellipsoids set at the 50%
Entry
12
Ar
13
Yield [%][b]
À
À
probability level). Selected bond lengths [Š]: Pt S 2.299(2), Pt Cl
1
2
3
4
5
12a
12b
12c
12d
12e
o-ClC6H4
o-BrC6H4
p-ClC6H4
p-BrC6H4
o-MeC6H4
13a
13b
13c
13d
13e
68 (60)[c]
À
À
À
À
2.342(2), Pt P1 2.332(2), Pt S 2.319(2), S C1 1.73(1), Br C2 1.82(2),
80 (75)[c]
À
Br S 3.202(2).
2
13
18
The “o-halogen effect” was next studied with the trans and
cis dithiolate complexes [Pt(SC6H4-o-Cl)2(PPh3)2] (10).[23] The
treatment of trans-10 with 10 equivalents of phenylacetylene
(2a) at 258C furnished the vinyl platinum complex 11 in 40%
yield (only cis) after 4 h and 95% yield (cis/trans 85:15) after
55 h [Eq.(3)]. [24,25] In stark contrast, 11 was not formed at all
when cis-10 was treated for 4 h under the same reaction
conditions.The higher reactivity of trans-10 relative to that of
cis-10 toward the insertion of an alkyne may be attributed to
the stronger trans effect of PPh3 relative to that of SAr.[26]
[a]Reaction conditions: 12 (0.5 mmol), 2b (0.6 mmol), [Pd(dba)2]
(0.015 mmol), PPh3 (0.033 mmol), C6H6 (0.5 mL), 608C, 5 h. [b]The
yield was determined by 1H NMR spectroscopy. [c]The yield of the
isolated product is given in parentheses. dba=dibenzylideneacetone.
In conclusion, the present study demonstrates that the
platinum complexes 4 (M = Pt, G = Cl) and 10 (M = Pt, G =
À
SAr) are useful for examining the insertion of 2 into the S M
bond of PPh3-ligated complexes (M: Group 10 metal).Details
À
of the mechanism of the insertion of alkynes into the S Pt
bond and the application of the observed “o-halogen effect”
to other catalytic reaction systems are currently under
extensive investigation.[6b]
Experimental Section
Typical procedure: The complex trans-4a (9.0 mg, 0.0010 mmol),
=
S P(p-Tol)3 (1.7 mg, 0.0051 mmol (as an internal standard), and C6D6
Angew. Chem. Int. Ed. 2007, 46, 5929 –5933
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www.angewandte.org
5931

Communications
(0.6 mL) were placed in a pyrex NMR tube under a nitrogen
[8] L.-B. Han, M. Tanaka, Chem. Lett. 1999, 863.
atmosphere.After the relative sensitivity of the signals of trans-4a
[9] a) H.Kuniyasu, A.Ogawa, S.Miyazaki, I.Ryu, N.Kambe, N.
Sonoda, J. Am. Chem. Soc. 1991, 113, 9796; b) Y.Gareau, A.
Orellana, Synlett 1997, 803.
1
and S = P(p-Tol)3 had been checked by recording H and 31P NMR
spectra of this mixture, phenylacetylene (2a; 10.2 mg, 0.1 mmol) was
added to the NMR tube under a nitrogen atmosphere.The reaction
[10] In the following discussion, cis and trans refer to the relationship
of the two PPh3 groups at a Pt center; E and Z refer to the
configuration of the double bond of a vinyl substituent.
[11] a) A.E.Keskinen, C.V.Senoff, J. Organomet. Chem. 1972, 37,
1
was then monitored by H and 31P NMR spectroscopy at 708C.The
NMR spectra showed clean formation of the platinum complex 5a,
and the consumption rate of the starting complex trans-4a obeyed
pseudo-first-order kinetics.The half-life of trans-4a was found to be
6.1 h.
201; b) R.Ugo, M.G.La, S.Cenini,
J. Chem. Soc. A 1971, 522;
c) R.Zanella, R.Ros, M.Graziani, Inorg. Chem. 1973, 12, 2736.
[12] The reactivity of alkyne 2 toward [Pd(SAr)2(PR3)n] (n = 1,2) was
described recently; however, no clear information about the
insertion step was provided: a) V.P.Ananikov, I.P.Beletskaya,
G.G. Aleksandrov, I.L. Eremenko, Organometallics 2003, 22,
1414; b) V.P.Ananikov, M.A.Kabeshov, I.P.Beletskaya, V.N.
Khrustalev, M.Y.Antipin, Organometallics 2005, 24, 1275.
[13] The use of 1,2-bis(diphenylphosphanyl)ethane (dppe) as a ligand
and dimethyl acetylenedicarboxylate (DMAD) as the alkyne
5a: The complex trans-4a (54 mg, 0.06 mmol), toluene (3 mL),
and 2a (61 mg, 0.06 mmol) were placed in a dry three-necked flask,
and the reaction mixture was stirred for 14 h at 1008C.Hexane (ca.
50 mL) was then added to the mixture, and the resulting precipitate
was collected by filtration.The complex 5a (54 mg, 87%) was
obtained by recrystallization from CH2Cl2/hexane as a pale-yellow
solid.Mp.:. 228
8C; 1H NMR (400 MHz, C6D6): d = 7.93–7.87 (m,
12H), 7.62 (t, JP, H = 4.0 Hz, 1H), 7.01–6.98 (m, 18H), 6.91–6.83 (m,
5H), 6.63 (d, J = 8.8 Hz, 2H), 6.22 ppm (d, J = 8.4 Hz, 2H); 31P NMR
(160 MHz, C6D6): d = 23.5 ppm (s, JPt,P = 3023 Hz); IR (KBr): n˜ =
3450, 3054, 1891, 1591, 1572, 1522, 1482, 1474, 1435, 1389, 1311,
1221, 1186, 1159, 1029, 1010, 999, 894, 810, 767, 743, 707, 693, 618, 576,
542, 522, 455, 430 cmÀ1; C,H analysis: calcd (%) for C51H42Cl4P2PtS: C
56.42, H 3.90; found: C 56.28, H 3.87.
À
enabled the direct observation of insertion into a S Pd bond: K.
Sugoh, H.Kuniyasu, H.Kurosawa, Chem. Lett. 2002, 106.
[14] F.Yamashita, H.Kuniyasu, J.Terao, N.Kambe,
Inorg. Chem.
2006, 45, 1399.
[15] P.J.Stang, Z.Zhong, M.H.Kowalski,
833.
Organometallics 1990, 9,
[16] CCDC-610343 (5a), CCDC-610574 (4i), CCDC-639788 (cis-10),
and CCDC-639789 (trans-10) contain the supplementary crys-
tallographic data for this paper.These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Received: December 9, 2006
Revised: April 17, 2007
Published online: July 6, 2007
À
[17] For the insertion of activated terminal alkynes into S M bonds
Keywords: alkynes · insertion · neighboring-group effects ·
platinum · S ligands
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À
À
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À
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À
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[25] The insertion of 2a into a trans-dithiolate complex without an o-
halogen substituent required harsher reaction conditions: The
formation of the Pt0 complex probably occurred via the divinyl
platinum complex.See the discussion about the possible course
of reactions of platinum dithiolates with alkynes in refer-
ence [4b].
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Angew. Chem. Int. Ed. 2007, 46, 5929 –5933
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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