Pt(0)-catalyzed alkynylation of aryl iodides with lithium alkynyltriisopropoxy borates

Article abstract of DOI:10.1055/s-2004-832819

An efficient cross coupling reaction of various lithium alkynyltriiospropoxyborates with a wide array of aryl iodides was catalyzed by Pt(PPh₃)₄-CuI in DMF as a solvent. These cross coupling reactions are general and permit the new sp-sp² carbon-carbon bond formation.

Full text of DOI:10.1055/s-2004-832819

LETTER
2091
Pt(0)-Catalyzed Alkynylation of Aryl Iodides with Lithium Alkynyl-
triisopropoxy Borates
P
t(0)-Catalyzed A
h
lkynylation
a
of
A
ryl Iod
n
ides g Ho Oh,* V. Raghava Reddy
Department of Chemistry, Hanyang University, Sungdong-Gu, Seoul 133-791, Korea
Fax +82(2)22990732; E-mail: changho@hanyang.ac.kr
Received 1 July 2004
reactivity, presumably due to the diffused d-orbital. We
imagined that the use of less reactive platinum compounds
could provide an interesting alternative for selective
cross-coupling reactions of alkynylborates with aryl io-
dides in the presence of labile organic bromides. To the
best of our knowledge, there was no precedent on cross-
coupling of alkynylborates with aryl iodides or any other
electrophiles by using platinium catalysts. On other hand,
platinum-catalyzed Suzuki coupling reaction,15 Heck
reaction,16 hydrosilylation of terminal alkynes,17 intramo-
lecular C-H activation,18 and olefin amination 19 reactions
are known.
Abstract: An efficient cross coupling reaction of various lithium
alkynyltriiospropoxyborates with a wide array of aryl iodides was
catalyzed by Pt(PPh3)4–CuI in DMF as a solvent. These cross cou-
pling reactions are general and permit the new sp-sp2 carbon-carbon
bond formation.
Key words: alkynylation, lithium alkynylborate, Pt-catalyst,
Suzuki–Miyaura coupling
Transition metal-catalyzed coupling reactions of metal
acetylides are very efficient and reliable methods for con-
structing new carbon-carbon bonds.1 In particular, the
Sonogoshira reaction has been used extensively over the
past three decades for the construction of C(sp2)-C(sp)
bonds.2 The reaction is a straightforward way to achieve
arylalkynes, enynes, and other unsaturated compounds,
many of which possess a diverse range of potential appli-
cations on such as molecular electronic devices,3 den-
drimers,4 benzaannulenes,5 foldamers or polymers,6
pharmaceuticals,7 and natural product synthesis.8 There-
fore there have been extensive efforts to find better cata-
lysts which provide high turnover number, high functional
group compatibility, and increased scope of arylalkyne
formation.
On the basis of these promising potentialities, our efforts
have been made in developing new synthetic methodolo-
gies using organoboron compounds,20 herein we wish to
report a new Pt(0)-catalyzed cross-coupling of alkynylbo-
rates with aryl iodides with an exquisite chemoselectivity.
To uncover a suitable Pt-catalyst and to search the optimal
conditions, we tried to perform the reaction of phenylacet-
yleneborate (1a) with p-iodoanisole (2a). Among PtCl2,
PtCl4, and Pt(PPh3)4, Pt(PPh3)4 was shown to be the most
effective catalyst. We considered as Cu(I) salt as a poten-
tial co-catalyst because of ability to catalyze Sonogashira
reaction. To our great delight the use of Pt(PPh3)4–CuI
system was effective in view of the yield of the product
3aa. Then, we carried out this reaction in several solvents
such as toluene, CH2Cl2, MeCN, DME, and DMF. The re-
action was found to be very sensitive to DMF as solvent,
where significant enhancement of the catalytic activity in
DMF was observed. The other solvents CH2Cl2, MeCN,
and DME were totally ineffective while toluene afforded
3aa only in trace amounts. Gratifyingly, the use of
Pt(PPh3)4 (2.5 mol%) with CuI (5 mol%) in DMF at
120 °C resulted in exclusive formation of 3aa in 82%
yield.
Of the many organometalloids being used in cross-cou-
pling reactions, the organoboron reagents are the most in-
triguing since they are stable in air, inexpensive, and
environmentally benign.9 Utility of alkynylborates in met-
al-catalyzed reactions have become the subject of intense
interest due to their enhanced reactivity. Successful trans-
formations include Suzuki–Miyaura cross coupling,10 al-
lylic additions to allylic carbonate,11 conjugate addition to
enones,12 Mannich reaction,13 and homocoupling reac-
tions.14 Although we and others have applied the use of
these reagents in palladium-catalyzed Suzuki–Miyaura
coupling reactions, limitations remain: the conditions of-
ten require stoichiometric or excess amount of base, and
two equivalents of borate salts are indispensable for
smooth transformations. In spite of the synthetic value of
organoborates, extensive research is being performed
only in the fields of Pd- and Ni-catalyzed reactions whilst
the use of Pt catalysts remains mainly unnoticed. Com-
pared to Ni or Pd compounds, Pt compounds have low
Having established the present methodology, we studied
this cross-coupling reaction with various aryl iodides
(Equation 1 and Table 1). Notably, most of the reactions
gave good results. No significant electronic effects were
observed for either para- or meta-substituted aryl iodides.
However, the presence of a free hydroxyl group in aryl
iodides resulted in some homocoupled products (entries 8
and 9). Even sterically hindered aryl iodides worked well:
2-iodotoluene (2k) and 2,6-dimethyliodobenzene (2l)
showed to have virtually the same reactivity toward phe-
nylacetyleneborate under the present conditions (entries
11 and 12), implying that steric effect is not a sole gover-
SYNLETT 2004, No. 12, pp 2091–2094
0
1
.1
0
.2
0
0
4
Advanced online publication: 21.09.2004
DOI: 10.1055/s-2004-832819; Art ID: U19004ST
© Georg Thieme Verlag Stuttgart · New York

2092
C. H. Oh, V. R. Reddy
LETTER
nor in this reaction. It is noteworthy that this Pt-catalyzed It is noteworthy that 4-bromoiodobenzene (2j) underwent
alkynylation can be performed in ambient atmosphere chemoselective coupling producing the corresponding
without scarifying yield (entries 3 and 4). A wide variety bromoarylalkyne derivative (entry 10). As anticipated, the
of aryl iodides were found to undergo facile cross-cou- selective coupling with aryl iodide over bromide afford-
plings by phenylacetyleneborate (1a). The cross-coupling ing 3aj is caused by the fast oxidative addition into the
reaction with 1-iodonaphthalene (2m) gave good yield, C(sp2)-I bond over the C(sp2)-Br bond. The electronic
without significant loss of yield, heterocyclic iodides 2n properties of the ligand are certainly of importance for this
and 2o also underwent smooth cross-coupling thereby af- reaction; thus even platinum(0) complexes are not suffi-
fording 3an and 3ao in acceptable yields of 73% and 71%, ciently electron rich to undergo fast oxidative addition
respectively (entries 13–15). No coupling product was ob- into aryl bromides.21 Encouraged by these results, we be-
tained when bromobenzene or p-bromoanisole were used. gan to evaluate the general applicability as well as relative
reactivity of various alkynylborates (Equation 2).
_
To this end we have prepared a series of alkynylborates
from the commercially available triisopropoxyboarate
B(OiPr)3
Li+
+
Ar-I
1a
with alkynyllithiums, which were prepared in situ from
Pt(PPh3)4 (2.5 mol%) CuI (5 mol%)
DMF, 120 °C
Ar
Table 2 Platinum-Catalyzed Cross-Coupling of Various Alkynyl-
borates (1b–p) with Iodobenzene (2b).
Equation 1
Table 1 Platinum-Catalyzed Cross-Coupling of Various Aryl Io-
dides (2a–o) with Phenylalkynylborate 1a.
Entry
1
Substrate
Time (h) Product
Yield (%)
72
12
12
14
3bb
3cb
3db
Z- =
I
CH3O- (2a)
H- (2b)
I
I
O2N
CH3
H3C
CH3
NO2- (2c)
1b
CH3CO- (2d)
CH3O2C- (2e)
p-CH3- (2f)
m-CH3- (2g)
HO- (2h)
2
3
76
80
2l
2k
Me
Z
1c
I
I
I
HOCH2- (2i)
p-Br- (2j)
H3CO
S
N
2n
1d
tert-C4H9
2m
Entry
1
2o
4
5
15
13
12
12
10
12
12
15
12
3eb
3fb
3gb
3hb
3ib
71
73
61
76
62
74
71
75
69
Substrate
2a
Time (h)
12
Product
3aa
3ab
3ac
3ad
3ae
3af
Yield (%)a
1e
n-C4H9
82
1f
Me3Si
6
2
2b
2c
12
78
1g
TBSOCH2
3
12
86 (82)b
85 (80)b
84
7
4
2d
2e
12
1h
THPOCH2
8
5
12
1i
9
3jb
3kb
3lb
TBSO(CH2)2
6
2f
12
78
1j
7
2g
12
3ag
3ah
3ai
75
10
11
12
TBSO(CH2)3
8
2h
2i
16
23 (58)c
35 (41)c
73
1k
BnO(CH2)4
9
18
1l
3mb
OTBS
10
11
12
13
14
15
2j
15
3aj
2k
2l
14
3ak
3al
61
1m
14
59
13
12
3nb
64
OTBS
2m
2n
2o
17
3am
3an
3ao
74
1n
1-Np
14
73
14
15
15
12
3ob
3pb
62
71
NCH3
17
71
1o
1p
a Isolated yields.
b Reaction performed in presence of air.
c Homocoupled product (diphenyl-1,3-butadiyne) was observed.
Synlett 2004, No. 12, 2091–2094 © Thieme Stuttgart · New York

LETTER
Pt(0)-Catalyzed Alkynylation of Aryl Iodides
2093
I
Representative Procedure: To a mixture of lithium alkynyltriiso-
propoxy borate (1a, 0.20200 mmol) and aryl iodide (0.24200
mmol), Pt(PPh3)4 (0.00500 mmol, 2.5 mol%), CuI (0.01012 mmol,
5 mol%), DMF (2 mL) was added under argon atmosphere in a
round bottomed flask. The resulting suspension was stirred at 120
°C. After heating for the time specified, the reaction mixture was al-
lowed to cool to r.t. The product was extracted into Et2O (3 × 10
mL), the solvent was removed by rotary evaporation, and the resi-
due was purified by silica gel chromatography.
_
+
R
B(OiPr)3 Li+
1b–p
Pt(PPh3)4 (2.5 mol%) CuI (5 mol%)
DMF, 120 °C
R
3bb–pb
Equation 2
the corresponding terminal alkynes with BuLi at –78 °C
in diethyl ether.22
Acknowledgment
Most of the alkynylborates were cross-coupled with
iodobenzene under the optimized conditions in good to
excellent yields as summarized in Table 2. Arylacetyl-
eneborates bearing both an electron-deficient group (1b)
and electron-rich group (1c and 1d) worked smoothly un-
der these conditions to afford 3bb, 3cb and 3db in 72%,
76%, and 80% yields, respectively (entries 1–3). Alkyl-
substituted acetyleneborates such as tert-butyl (1e) and n-
butyl (1f) gave the even more promising results (entries 4,
5). TMS-substituted alkynylborate (1g) also underwent
effective cross-coupling to afford the product 3gb in 61%
yield (entry 6).
This work was supported by the Korea Research Foundation (krf-
2001-015-DP0334). V. Raghava Reddy is grateful for a graduate
fellowship supported by the BK21 project.
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Synlett 2004, No. 12, 2091–2094 © Thieme Stuttgart · New York

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C. H. Oh, V. R. Reddy
LETTER
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Synlett 2004, No. 12, 2091–2094 © Thieme Stuttgart · New York