Catalytic and stoichiometrically directed synthesis of less accessible bromothiophenes and bromobithiophenes. Trapping and characterization of catalytic intermediates of trans-PdBr(C4H4-nBrn-1S-C)(PPh 3)2</

Article abstract of DOI:10.1021/om9800142

A facile one-pot procedure for synthesizing isomerically pure bromothiophenes and bromobithiophenes was achieved by palladium-catalyzed hydrodebromination. The compounds synthesized include 2,3,4-tribromothiophene (2), 3,4-dibromothiophene (3), 2,3-dibrom

Full text of DOI:10.1021/om9800142

3988
Organometallics 1998, 17, 3988-3995
Ca ta lytic a n d Stoich iom etr ica lly Dir ected Syn th esis of
Less Accessible Br om oth iop h en es a n d
Br om obith iop h en es. Tr a p p in g a n d Ch a r a cter iza tion of
Ca ta lytic In ter m ed ia tes of
tr a n s-P d Br (C4H4-n Br n -1S-C)(P P h 3)2 (n ) 1-4),
tr a n s-P d Br (C8H4Br S2-C)(P P h 3)2, a n d
tr a n s,tr a n s-P d 2Br 2(µ-C8H6-n Br n -2S2-C,C′)(P P h 3)4 (n ) 2, 4)
Yang Xie,† Bo-Mu Wu,‡ Feng Xue,‡ Siu-Choon Ng,*,† Thomas C. W. Mak,*,‡ and
T. S. Andy Hor*,†
Department of Chemistry, Faculty of Science, National University of Singapore, Kent Ridge,
Singapore 119260, and Department of Chemistry, The Chinese University of Hong Kong,
Shatin, New Territories, Hong Kong
Received J anuary 13, 1998
A facile one-pot procedure for synthesizing isomerically pure bromothiophenes and
bromobithiophenes was achieved by palladium-catalyzed hydrodebromination. The com-
pounds synthesized include 2,3,4-tribromothiophene (2), 3,4-dibromothiophene (3), 2,3-
dibromothiophene (7), 3-bromothiophene (4), 3,3′,5-tribromo-2,2′-bithiophene (10), 3,3′-
dibromo-2,2′-bithiophene (11), and 3-bromo-2,2′-bithiophene (12). The regioselectivity is
catalytically dictated by the metalation site, whereas the extent of debromination is
conveniently controlled by the stoichiometry and substrate quantity. Under favorable
conditions, complete debromination takes place, giving rise to thiophene and bithiophene.
The catalytic intermediates trans-PdBr(3,4-C4HBr2S-C)(PPh3)2 (16), trans,trans-Pd2Br2(µ-
C8H2Br2S2-C,C′)(PPh3)4 (17), trans,trans-Pd2Br2(µ-C8H4S2-C,C′)(PPh3)4 (18), trans-PdBr(C8H4-
BrS2-C)(PPh3)2 (19), trans-PdBr(C4H3S-C)(PPh3)2 (20), and trans-PdBr(3,4,5-C4Br3S-C)(PPh3)2
(21) were isolated. Palladium insertion invariably occurs at the C(R)-Br bonds of the
thiophene or bithiophene rings. Single-crystal X-ray crystallography analyses of 20, 16,
and 21 gave three mononuclear square-planar Pd(II) thienyl structures with phosphines in
trans orientations.
In tr od u ction
the electronic factors favor the formation of 2-bro-
mothiophene (22),6 2,5-dibromothiophene,7 2,3,5-tribro-
mothiophene (6),8 tetrabromothiophene (1),9 5-bromo-
2,2′-bithiophene (15), 5,5′-dibromo-2,2′-bithiophene (14),
or 3,3′,5,5′-tetrabromo-2,2′-bromothiophene (9).10 For
the syntheses of other isomers, alternative methods
must be used. Common methodologies include the use
of thienyllithium derivatives for the synthesis of 3-bro-
mothiophene (4; from 6), 2,3-dibromothiophene (7; from
6),11 3,4-dibromothiophene (3; from 1), and 2,3,4-tribro-
Halothiophenes and halobithiophenes are widely used
as substrates for a variety of C-C coupling reactions.1
Especially common are the brominated derivatives,
which serve as important intermediates in the syntheses
of glassy materials,2 conducting polymers,3 and biologi-
cally active compounds.4 These bromothiophenes and
bromobithiophenes are usually prepared from direct
bromination of thiophene or 2,2′-bithiophene with Br2.5
However, this usually gives a mixture of products, and
(4) Kim, B. M.; Guare, J . P.; Vacca, J . P.; Michelson, S. R.; Darke,
P. L.; Zugay, J . A.; Emini, E. A.; Schlief, W.; Lin, J . H.; Chen, I. W.;
Vastag, K.; Anderson, P. S.; Huff, J . R. Bioorg. Med. Chem. Lett. 1995,
5, 185. Gronowitz, S.; Zhang, Y.; Hornfeldt, A. Acta Chem. Scand. 1992,
46, 654. Prugh, J . D.; Hartman, G. D.; Mallorga, P. J .; McKeever, B.
M.; Michelson, S. R.; Murcko, M. A.; Schwan, H.; Smith, R. L.; Sondey,
J . M.; Springer, J . P.; Sugrue, M. F. J . Med. Chem. 1991, 34, 1805.
D’Auria, M.; De Mico, A.; D’Onofrio, F.; Piancatelli, G. J . Chem. Soc.,
Perkin Trans. 1987, 1, 1777.
(5) Mozingo, R.; Harris, S. A.; Wolf, D. E.; Hoffhine, C. E.; Easton,
N. R.; Folkers, K. J . Am. Chem. Soc. 1945, 67, 2092.
(6) Tamaru, Y.; Yamada, Y.; Yoshida, Z. Tetrahedron 1979, 35, 329.
(7) Markovitz, M. Dissertation, New York University, 1963; Diss.
Abstr. 1964, 24, 3100.
† National University of Singapore.
‡ The Chinese University of Hong Kong.
(1) Dauria, M. J . Photochem. Photobiol. A 1995, 91, 187. Dagostini,
A.; Dauria, M. J . Chem. Soc., Perkin Trans. 1994, 1245. Zhang, Y. H.;
Hornfeldt, A. B.; Gronowitz, S. J . Heterocycl. Chem. 1995, 32, 435.
(2) Whitaker, C. M.; McMahon, R. J . J . Phys. Chem. 1996, 100, 1081.
(3) Zhao, Z. S.; Pickup, P. G. J . Electroanal. Chem. 1996, 404, 55.
Ng, S. C.; Chan, H. S. O.; Miao, P. J . Mater. Sci. Lett. 1997, 16, 1170.
Chan, H. S. O.; Ng, S. C.; Seow, R. S. H.; Moderscheim, M. J . G. J .
Mater. Chem. 1992, 2, 1135. Krische, B.; Hellberg, J .; Lilja, C. J . Chem.
Soc., Chem. Commun. 1987, 1476. Tanguy, J .; Pron, A.; Zargoska, M.;
Kulszewics-Bajer, I. Synth. Met. 1991, 45, 81. Zargoska, M.; Kulsze-
wics-Bajer, I.; Pron, A.; Firlej, I.; Bernier, P.; Galtier, M. Synth. Met.
1991, 45, 385. Roncali, J . Chem. Rev. 1992, 92, 711. Chan, H. S. O.;
Ng, S. C. Prog. Polym. Sci., in press. Chan, H. S. O.; Ng, S. C.; Huang,
H. H.; Prog. Pac. Polym. Sci., Proc. Pac. Polym. Conf. 1994, 3, 237.
Chan, H. S. O.; Ng, S. C.; Huang, H. H.; Seow, R. S. H. J . Chem. Res.,
Synop. 1996, 232; J . Chem. Res., Miniprint 1996, 1285.
(8) Gronowitz, S.; Raznikiewicz, T. Org. Synth. 1964, 44, 9. Gronow-
itz, S. Acta Chem. Scand. 1959, 13, 1045.
(9) Gronowitz, S.; Moses, P.; Hakansson, R. Ark. Kemi 1960, 16, 267.
(10) Khor, E.; Ng, S. C.; Li, H. C.; Chai, S. Heterocycles 1991, 32,
1805.
S0276-7333(98)00014-4 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 08/08/1998

Bromothiophenes and Bromobithiophenes
Ta ble 1. Ca ta lytic Debr om in a tion of Br om oth iop h en esa
product yield (%)
Organometallics, Vol. 17, No. 18, 1998 3989
entry no.
substrate
NaBH4:substrate
time (h)
conversn (%)
2
6
7
8
3
4
5
1
2
1
1
1
1
2
6
6
7
8
3
1
2
3
4
6
7
8
2.5:1
5:1
2.5:1
2.5:1
2:1
1.5:1
3.3:1
1.5:1
1.5:1
1.5:1
10:1
7.8:1
4.5:1
2:1
6
8
6
6
6
6
7
6
5
100
100
91
93
0
72
70
0
0
0
0
0
0
0
0
0
0
0
0
0
0
7
0
4
8
6
8
0
95
10
8
90
0
trace
trace
trace
2
trace
16
17
16
16
∼100
∼100
∼100
∼100
∼100
∼100
∼100
∼100
3b
4c
5
85
100
100
100
100
100
100
100
100
100
100
100
100
100
100
6
7
8
9
10
11
12
13
14
15
16
17
18
92
2
6
1
trace
0
81
82
83
83
7
22
18
15
8
14
10
8
7:1
4:1
4.5:1
1.5:1
22
3
a
Conditions: substrate, 1 mmol; catalyst, Pd(PPh3)4, 1 mol equiv of substrate; solvent, CH3CN, 10 mL; 70 °C. The conversion and
product distributions were analyzed by GC. First recycle of catalyst used in entry 1. c Second recycle of catalyst used in entry 1.
b
mothiophene (2; from 1),12 Grignard reagents,13,14 zinc
reductive reactions8 (for example, of 6, 1, and 9, giving
4, 3, and 11, respectively), and rearrangement of 2,5-
dibromothiophene by NaNHC6H5 in liquid NH3, giving
3.15 These approaches are generally handicapped by the
required use of expensive and/or air-sensitive reagents
and two-step or multistep synthetic routes. The syn-
(1)
thetic utilities of these materials demand a simple
preparative method which can give products in good
yields and isomerically pure forms. In a recent com-
munication, we reported a catalytic approach to the
debromination of 2,3,5-tribromothiophene.16 In this
paper, we extended this strategy to other polybro-
mothiophenes and polybromobithiophenes, whereby a
facile one-pot synthesis for the less accessible bromi-
nated thiophene and bithiophene isomers is accom-
plished. The catalytic mechanism will also be discussed.
20-25% lower (entries 3 and 4). Similar work on 2
gives 3 (entry 5), which illustrates that 2 is a probable
precursor to its lower brominated analogues. Similarly,
2,3,5-tribromothiophene (6) almost exclusively gives 7
(entry 6) or 4 (entry 7), depending on the NaBH4
concentration (eq 2). The three dibromothiophene
Resu lts a n d Discu ssion
Our previous results established the efficiency of Pd-
(PPh3)4 as a catalyst when NaBH4 is the reducing
agent.16 In this work, a series of bromothiophenes are
examined for their debromination behaviors under
catalytic conditions (Table 1). Debromination of tetra-
bromothiophene (1; eq 1) at a NaBH4:substrate molar
ratio of 2.5:1 in the presence of 1% of Pd(PPh3)4 catalyst
at 70 °C results in a total (100%) conversion of the
substrate, giving predominantly (93%) 2,3,4-tribro-
mothiophene (2; Table 1, entry 1). Increasing the
NaBH4:substrate ratio to 5.0:1 remarkably gives almost
exclusively (95%) 3,4-dibromothiophene (3; entry 2).
When the catalyst (of entry 1) is recovered and recycled,
it is still active, although the yields of 2 are generally
(2)
isomers, viz. 3, 7, and 8, inevitably give 4 as the
isomerically pure monobromothiophene product (>80%
yields) (entries 8-10) (eqs 1-3). Complete debromina-
(3)
(11) Lawesson, S.-O. Ark. Kemi 1957, 11, 317, 325, 373.
(12) Decroix, B.; Morel, J .; Paulmier, C.; Paastour, P. Bull. Soc.
Chim. Fr. 1972, 1848.
(13) Troyanowsky, C. Bull. Soc. Chim. Fr. 1955, 424. Gronowitz, S.
Ark. Kemi 1954, 7, 267.
(14) Nemec, M.; Vopatrna, P.; J anda, M.; Srogl, J .; Stibor, I.
Synthesis 1972, 545.
(15) Reinecke, M. G.; Adickes, H. W.; Pyun, C. J . Org. Chem. 1971,
36, 2690.
(16) Xie, Y.; Ng, S. C.; Hor, T. S. A.; Chan, H. S. O. J . Chem. Res.,
Synop. 1996, 3, 150.
tion of 3, 6, 7, and 8 giving 4 and thiophene, 5, as a
side-product is also evident (entries 7-10) (eqs 1-3).
In the presence of a 2-3-fold excess of NaBH4, all
bromothiophenes, 1-8, regardless of the bromine con-

3990 Organometallics, Vol. 17, No. 18, 1998
Xie et al.
Ta ble 2. Ca ta lytic Debr om in a tion of
Br om obith iop h en esa
gives chiefly 12 (80%) (entry 4). It is evident that this
synthesis can be targeted at a specific product by
controlling the extent of debromination through the
measured use of the reductant. The same approach can
be used to convert 5,5′-dibromo-2,2′-bithiophene (14) to
5-bromo-2,2′-bithiophene (15; 81%) (eq 5) (Table 2, entry
product yield (%)
entry sub-
NaBH4: time conversn
no. strate substrate (h)
(%)
10 11 12 15 13
1
2
3
4
5
6
7
9
9
9
9
14
9
1.5:1
2.0:1
3.5:1
5.0:1
1.5:1
8:1
4
4
5
6
3
100
100
100
100
100
100
100
80
45
20 trace
52
0
0
0
0
0
0
6
19
3
trace 88 12
14 80
0
0
81
12
10
∼100
14
4:1
∼100
a
Conditions: substrate, 1 mmol; catalyst, Pd(PPh3)4, 1 mol
equiv of substrate; solvent, CH3CN, 10 mL; 70 °C. The conversion
and product distributions were analyzed by GC using a HP5890
Series II Plus gas chromatograph with an HP-1 capillary column.
(5)
tent, undergo complete debromination to give near-
quantitative yields of thiophene (5) after 24 h (entries
11-17). This demonstrates the high catalytic efficiency
of the present system toward hydrodebromination.
On the basis of these pilot results, large-scale prepa-
rations of 2,3,4-tribromothiophene (2), 2,3-dibromothio-
phene (7), 3,4-dibromothiophene (3), and 3-bromothio-
phene (4) have been achieved in yields of 82%, 83%,
86%, and 81% respectively. These catalytic yields are
at least comparable with those literature yields of 2
(85%11 or 70%,12 both by n-C4H9Li from 1), 3 (75%14 by
Mg, 68%8 by Zn, or 75%11 by n-C4H9Li on 1 or 80%15 by
NaNHC6H4 and NH3 on 2,5-dibromothiophene), 7
(<12%17 by C2H5MgBr on 6 or 97%18 by Br2 and CCl4
on 3-bromothiophene (4)), and 4 (71%8 by Zn or 84%11
by n-C4H9Li on 6). Our “one-pot” method avoids the use
of expensive and/or air-sensitive reagents. It is notable
that 2,3-dibromothiophene (7) can be obtained in good
yield (92%, entry 6), whereas its preparation in a
Grignard reaction is problematic and gives yields <12%.17
Although 7 has been synthesized in good yield (97%)
from the bromination of 3-bromothiophene (4), this
method is not straightforward since 4 needs to be
prepared carefully from another precursor 6.8,18
5). Complete debromination of 9 and 14, giving
bithiophene (13), is achieved when NaBH4 is in excess
(entries 6 and 7). These results prompted us to carry
out preparative-scale synthesis of 10-12 (yields 75%,
85%, and 76%, respectively). The yields of 11 and 12
are comparable with those from other methods.10 Com-
pound 12 is a useful precursor for a series of 4-substi-
tuted 2,2′-bithiophenes.19
A distinct advantage of this catalytic approach is
witnessed in the total conversion of the bromothiophenes
or bromobithiophenes tested. The high regioselectivity
of the products offers a synthetic advantage to this
strategy. These results suggest that the conversion
from higher to lower brominated thiophenes or bithio-
phenes is a sequential process driven by the catalyst
and directed by the amount of the reductant used.
Under catalytic influence, the R-position is most sus-
ceptible to substitution. The preferential formation of
7 (from 6) over its isomers (8 or 3) has been described
in our earlier communication.16 Exclusive formation of
4 from both 6 and 7 without other monobromothiophene
isomers is a manifestation of this R-directing effect.
Degradation of 1-8 to thiophene (entries 11-17 in Table
1) and of 9 and 10 to 2,2′-bithiophene (entries 6 and 7
in Table 2) demonstrates a technological application of
this system in the hydrodebromination of polybro-
mothiophenes and polybromobithiophenes.
This synthetic method can be extended to higher
bromobithiophenes. For example, debromination of 9
giving 10 (eq 4) in 80% yield occurs at a NaBH4:
substrate molar ratio of 1.5:1 with 11 as a minor product
(Table 2, entry 1). Increasing the molar ratio to 3.5:1
These catalytic reactions proceed through repetitive
and sequential oxidative addition of the Pd(0) active
catalyst with the C-Br bonds of the thiophene. The
C-Br bond adjacent to the thiophenic sulfur is the most
reactive and holds the key to the substitution at the
R-position20 (Scheme 1). This mechanism is supported
by the trapping of the key intermediate trans-PdBr(3,4-
C4HBr2S-C)(PPh3)2 (16), isolated from a stoichiometric
reaction of 6 with Pd(PPh3)4. The single 31P resonance
(22.9 ppm) is consistent with a model containing trans-
oriented phosphines. The high-field shift of the sole
proton of 6 (i.e. the proton at the 4 (or â-)-position) (6.90
ppm) upon complex formation (5.56 ppm) is an indica-
tion of metalation at the adjacent carbon: viz., at either
the 3- or 5-position in 6 (numbering scheme illustrated
(4)
(17) Gronowitz, S. Ark. Kemi 1958, 12, 115.
(18) Gronowitz, S.; Dahlgren, K. Ark. Kemi 1963, 21, 201.
(19) Rasmussen, S. C.; Pichens, J . C.; Hutchison, J . E. J . Heterocycl.
Chem. 1997, 34, 285.
(20) Xie, Y.; Ng, S. C.; Wu, B., Xue, F., Mak, T. C. W.; Hor, T. S. A.
J . Organomet. Chem. 1997, 531, 175.
gives 11 as the dominant product (88%) and 12 as a
byproduct (entry 3). Further increase to a 5:1 ratio

Bromothiophenes and Bromobithiophenes
Organometallics, Vol. 17, No. 18, 1998 3991
Ta ble 3. Cr ysta llogr a p h ic Da ta a n d Str u ctu r e Refin em en t Deta ils for tr a n s-P d Br (C4H3S-C)(P P h 3)2 (20),
tr a n s-P d Br (3,4-C4HBr 2S-C)(P P h 3)2 (16), a n d tr a n s-P d Br (3,4,5-C4Br 3S-C)(P P h 3)2 (21)
20
16
21
formula
mol wt
C
794.0
40H33BrP2PdS
C40H31Br3P2PdS
951.7
C40H30Br4P2PdS
1030.7
color and habit
cryst size (mm3)
cryst syst
space group
T, K
yellow prism
0.30 × 0.30 × 0.40
orthorhombic
Pbcn (No. 60)
294
yellow prism
0.30 × 0.24 × 0.18
monoclinic
C2/c (No. 15)
294
yellow prism
0.20 × 0.20 × 0.30
monoclinic
P21/n
294
a, Å
b, Å
c, Å
19.354(1)
10.760(2)
16.451(1)
90
1600
3426(2)
4
25.681(1)
13.546(1)
11.619(1)
113.87(1)
1872
3696(2)
4
3.939
1.710
11.361(2)
13.049(3)
26.670(5)
102.00(3)
2008
3867(2)
4
4.777
â, deg
F(000)
V, Å3
Z
abs coeff, mm-1
1.891
1.539
F
calcd, g cm-3
index ranges
1.770
-24 e h e 24, -13 e k e 13, 0 e 1 e 20 0 e h e 32, -17 e k e 17, -14 e 1 e 13 0 e h e 13, 0 e k e 15, -31 e 1 e 30
no. of rflns collected 11 410
6356
7181
no. of indep rflns
final R Indices
(obsd data)
3606 (Rint ) 2.36%)
R ) 6.14%, wR ) 5.93%
3696 (Rint ) 6.77%)
R ) 5.29%, wR ) 13.45%
6815 (Rint ) 2.69%)
R ) 3.75%, wR ) 4.00%
goodness of fit
1.49
1.12
1.12
largest diff peak and 0.75 and -0.87
0.69 and -0.88
0.42 and -0.52
hole (e Å-3
)
Sch em e 1. P r op osed Ca ta lytic Mech a n ism for th e
Debr om in a tion of 2,3,5-Tr ibr om oth iop h en e
F igu r e 1. ORTEP plot of the molecular structure of trans-
PdBr-σ-3,4-C4HBr2S-C)(PPh3)2 (16) (30% probability el-
lipsoids).
1
the H spectrum shows two species (δ -9.73 ppm, dd,
2
2
2J PH ) 82.5 Hz, J PH ) 3.3 Hz; δ -11.86 ppm, t, J PH
)
8.7 Hz) attributable to the cis and trans hydrido thienyl
complexes. Similar palladium hydrido complexes have
been reported.21 The hydrido resonances are not sus-
tainable. GC/MS assay of the resultant mixture con-
firms 7 as the predominant product.
in Scheme 1). The metalated carbon is deshielded
(148.1 ppm) compared to other carbon resonances
(110.6-132.2 ppm), although the exact position of
palladation in the thienyl moiety cannot be unambigu-
ously assigned on the basis of these spectra.
The substrate 6 is an ideal model for crystallographic
analysis because it contains the maximum regioselec-
tivity information. Although R-substitution (i.e. at the
2- and 5-positions) is expected, the â-position cannot be
ignored, as it also offers some steric and electronic
(activation from neighboring bromine) advantages. Be-
A single-crystal X-ray analysis was carried out to
confirm the solid-state structure of 16 (Figure 1, Table
3). It reveals a trans Pd(II) square-planar thienyl
complex arising from oxidative addition of 6 to Pd(0).
This structure provides unequivocal evidence that met-
alation occurs preferentially at the carbon between the
sulfur atom and the 4 (or â-)-carbon. Subsequent
hydrodebromination of 16 followed by proton transfer
would account for the formation of 7 (Scheme 1).
Reaction of 16 with NaBH4 in CD3CN in an NMR tube
at 60 °C gives a weak and short-lived singlet at -11.06
ppm attributed to the fragile Pd-H moiety. In CDCl3,
(21) Reger, D. L.; Garza, D. G. Organometallics 1993, 12, 554. Siedle,
A. R.; Newmark, R. A.; Gleason, W. B. Inorg. Chem. 1991, 30, 2005.
Di Bugno, C.; Pasquali, M.; Leoni, P.; Sabatino, P.; Braga, D. Inorg.
Chem. 1989, 28, 1390. Clark, H. C.; Goel, A. B.; Goel, S. Inorg. Chem.
1979, 18, 2803. Saito, T.; Munakata, H.; Imoto, H. Inorg. Synth. 1977,
17, 83. Green, M. L. H.; Munakata, H. J . Chem. Soc., Dalton Trans.
1974, 269.

3992 Organometallics, Vol. 17, No. 18, 1998
Xie et al.
Ta ble 4. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for tr a n s-P d Br (C4H3S-C)(P P h 3)2 (20),
tr a n s-P d Br (3,4-C4HBr 2S-C)(P P h 3)2 (16), a n d
tr a n s-P d Br (3,4,5-C4Br 3S-C)(P P h 3)2 (21)
tween the two R-bromine atoms, the one at the 2-posi-
tion is electronically favored because of possible acti-
vation from both neighboring atoms, viz. Br and S; the
5-bromine has an advantage from a steric viewpoint.
These differences are subtle, but they decide the meta-
lation site and hence determine the regioselectivity of
the reaction.
(a) trans-PdBr(C4H3S-C)(PPh3)2 (20)
Bond Lengths
Pd(1)-C(19)
Pd(1)-P(1)
S(1)-C(19)
C(19)-C(20)
P(1)-C(6)
1.993(4)
2.351(1)
1.710(3)
1.363(5)
1.826(4)
1.826(4)
Pd(1)-Br(1)
C(21)-C(21a)
S(1)-C(21a)
C(20)-C(21)
P(1)-C(18)
2.526(1)
1.361(1)
1.709(3)
1.430(4)
1.838(5)
Despite the presence of sterically demanding ligands,
16 maintains a near-ideal square-planar geometry with
virtually linear disposition of the trans ligands ( P(1)-
Pd(1)-P(1a) ) 179.5(1)° and Br(1)-Pd(1)-C(19) )
180.0°) (Figure 1, Table 4). To minimize repulsion
between the thienyl and phenyl planes so as to support
a strong Pd-thienyl interaction (Pd(1)-C(19) ) 1.998-
(3) Å), the thienyl plane is tilted at a dihedral angle of
109.6° to the Pd(II) coordination plane. There is no
indication of η1(S) coordination or any η2- or η4-metal
interaction using the π-bonds on the thienyl ring, even
though such coordination modes are commonly found
in other metals.22 The strong trans influence of the
thienyl group gives a significantly long Pd-Br bond
(2.5089(8) Å) compared to many other PdII-Br bonds,
e.g. in trans-[PdBr2(PPh3)2] (2.425(1) Å)23 and trans-
PdBr2[2-(2′-thienyl)pyridine)]2 (2.431(1) Å).24 The weak-
ness of this Pd-Br bond also supports the ready
hydroreduction of 16, which is a crucial step governing
the efficiency of the catalyst. To confirm that the
regioselective attack is independent of the degree of
bromination of the thiophene ring, we have also deter-
mined the X-ray crystallographic structures of trans-
PdBr(C4H3S-C)(PPh3)2 (20) (Figure 2) and trans-PdBr-
(3,4,5-C4Br3S-C)(PPh3)2 (21) (Figure 3), which are
obtained similarly from 22 and 1, respectively. Both
reveal similar σ-thienyl structures with metalation at
the C atom next to the thiophene sulfur and two trans-
directing phosphines. The slight and progressive weak-
ening of the Pd-C bonds from 20 (1.993(4) Å) to 16
(1.998(3) Å) and to 21 (2.004(8) Å) is consistent with
the weaker donicity of the thienyl ligand as its size or
bromo content increases. The corresponding strength-
ening of the Pd-P bonds also emphasizes the impor-
tance of electronic over steric effects. Very notably, the
weaker σ-effect of the (higher) bromothienyl derivative
weakens its trans influence. This is translated to a
progressive strengthening of the Pd-Br bonds from 20
(2.526(1) Å) to 16 (2.5089(8) Å) and to 21 (2.483(1) Å).
The weakness of the Pd-Br bonds (in 20 and 16) is
consistent with the ease of debromination of 22 and 6,
respectively. It takes a shorter time and/or less NaBH4
to complete the degradation of 22 and 6 compared to
that of 1 (Table 1, entries 1, 6, and 18).
P(1)-C(12)
Bond Angles
C(19)-Pd(1)-P(1)
S(1)-C(19)-Pd(1)
C(19)-S(1)-C(21a)
88.8(1) P(1)-Pd(1)-P(1a)
120.7(1) C(20)-C(19)-Pd(1)
93.2(2) S(1)-C(19)-C(20)
177.5(1)
136.2(2)
103.1(3)
C(19)-C(20)-C(21) 125.5(5) C(20)-C(21)-C(21a) 100.6(3)
S(1)-C(21a)-C(21) 117.5(1) C(6)-P(1)-Pd(1)
C(12)-P(1)-Pd(1) 110.7(1) C(18)-P(1)-Pd(1)
113.3(2)
120.5(1)
(b) trans-PdBr(3,4-C4HBr2S-C)(PPh3)2 (16)
Bond Lengths
Pd(1)-C(19)
Pd(1)-P(1)
S(1)-C(21a)
C(21)-C(21a)
P(1)-C(7)
1.998(3)
2.340(1)
1.710(5)
1.362(6)
1.828(5)
Pd(1)-Br(1)
S(1)-C(19)
C(21)-Br(2)
P(1)-C(1)
2.5089(8)
1.710(5)
1.854(5)
1.825(5)
1.830(5)
P(1)-C(13)
Bond Angles
C(19)-Pd(1)-P(1)
S(1)-C(19)-C(20)
S(1)-C(19)-Pd(1)
89.8(1) P(1)-Pd(1)-P(1a)
107.6(3) C(20)-C(19)-Pd(1)
119.0(1) C(19)-S(1)-C(21a)
179.5(1)
133.4(2)
90.5(2)
C(19)-C(20)-C(21) 120.9(3) C(20)-C(21)-C(21a) 102.5(1)
S(1)-C(21a)-C(21)
S(1)-C(21a)-Br(2a) 116.0(3) C(1)-P(1)-Pd(1)
C(7)-P(1)-Pd(1) 116.3(1) C(13)-P(1)-Pd(1)
118.4(3) Br(2)-C(21)-C(21a) 125.5(5)
109.4(4)
116.1(1)
(c) trans-PdBr(3,4,5-C4Br3S-C)(PPh3)2 (21)
Bond Lengths
Pd(1)-C(1)
Pd(1)-P(1)
C(1)-C(2)
S(1)-C(4)
C(3)-C(4)
C(3)-Br(3)
P(1)-C(5)
P(1)-C(17)
P(2)-C(29)
2.004(8)
2.329(2)
1.357(10)
1.719(9)
1.360(10)
1.865(7)
1.833(8)
1.812(7)
1.822(8)
Pd(1)-Br(1)
Pd(1)-P(2)
S(1)-C(1)
C(2)-C(3)
C(2)-Br(2)
C(4)-Br(4)
P(1)-C(11)
P(2)-C(23)
P(2)-C(35)
2.483(1)
2.339(2)
1.730(7)
1.425(11)
1.871(7)
1.868(8)
1.804(7)
1.813(7)
1.816(9)
Bond Angles
90.1(2)
89.0(1)
176.7(2)
121.3(4)
92.7(4)
115.0(6)
111.5(6)
122.6(5)
124.5(6)
127.5(6)
105.4(2)
115.0(3)
115.7(3)
C(1)-Pd(1)-P(1)
Br(1)-Pd(1)-P(1)
Br(1)-Pd(1)-C(1)
S(1)-C(1)-Pd(1)
C(1)-S(1)-C(4)
C(1)-C(2)-C(3)
S(1)-C(4)-C(3)
C(3)-C(2)-Br(2)
C(4)-C(3)-Br(3)
C(3)-C(4)-Br(4)
C(11)-P(1)-Pd(1)
C(23)-P(2)-Pd(1)
C(35)-P(2)-Pd(1)
C(1)-Pd(1)-P(2)
Br(1)-Pd(1)-P(2)
P(1)-Pd(1)-P(2)
C(2)-C(1)-Pd(1)
S(1)-C(1)-C(2)
C(2)-C(3)-C(4)
C(1)-C(2)-Br(2)
C(2)-C(3)-Br(3)
S(1)-C(4)-Br(4)
C(5)-P(1)-Pd(1)
C(17)-P(1)-Pd(1)
C(29)-P(2)-Pd(1)
91.7(2)
89.5(1)
174.6(1)
129.3(5)
109.3(6)
111.5(7)
122.4(6)
123.9(5)
121.0(4)
120.0(3)
116.7(2)
111.8(3)
While the unsubstituted thienyl ring (in 20) and the
bromide thienyl rings (in 16) are essentially planar
(mean deviation 0.008 and 0.031 Å, respectively), the
tribromothienyl ring (in 21) forces a slight but noticeable
distortion on the metal coordination plane (mean devia-
tion 0.069 Å, with the P atoms on one side and the Br-
(1) and C(19) atoms on the other), as well as some
angular distortions from linearity in P(1)-Pd(1)-P(2)
(174.6(1)°) and C(1)-Pd(1)-Br(1) (176.7(2)°). The
metalation imparts little effect on the strength of the
thienyl C-Br bonds (e.g. mean C-Br ) 1.854(5) Å in
16, and mean C-Br ) 1.868 Å in 21, compared to C-Br
) 1.865 Å in 2-bromothiophene25). This is consistent
with the proposal that activation of the C-Br bond
takes place via complex formation.
(22) A typical example of η1(S) coordination is found in ReCp*(CO)2-
(C4H4S), which reacts with Fe2(CO)9 to give a η1:η4-bridging complex.
See: (a) Choi, M.-G.; Angelici, R. J . J . Am. Chem. Soc. 1989, 111, 8753.
η2 complexes are also known. See: (b) Cordone, R.; Harman, W. B.;
Taube, H. J . Am. Chem. Soc. 1989, 111, 5969. (c) Deeming, A. J .; Arse,
A. J .; de Sanctis, Y.; Day, M. W.; Hardcastle, K. I. Organometallics
1989, 8, 1408. (d) Arse, A. J .; Deeming, A. J .; de Sanctis, Y.; Machada,
R.; Rivas, C. J . Chem. Soc., Chem. Commun. 1990, 1568.
(23) Xie, Y.; Ng, S.-C.; Mak, T. C. W.; Hor, T. S. A. Unpublished
results.
(24) Giordano, T. J .; Butler, W. M.; Rasmussen, P. G. Inorg. Chem.
1978, 17, 1917.

Bromothiophenes and Bromobithiophenes
Organometallics, Vol. 17, No. 18, 1998 3993
(22.5-22.8 ppm) fall into a narrow band, which is
consistent with what is observed for the trans-oriented
phosphines found in trans,trans-Pd2X2(µ-4,4′-biphenyl)-
(PPh3)4.26 Palladation invariably occurs at the carbon
at the 2 (or R-)-position, i.e. adjacent to the sulfur. This
1
regioselectivity is indicated in the H NMR spectra, in
which the resonances of the protons at the 5 (or â-)-
position adjacent to the metalated site are inevitably
shielded with respect to those of the parent bithiophenes.
Similarly, the metalated carbon is characterized by a
low-field shift to 142.3-148.0 ppm for 17-19, whereas
the nonmetalated R-carbons resonate at 111.4-132.1
ppm. Formation of 11 and 13 can similarly be explained
by a selective insertion of the [Pd(PPh3)2] moieties into
the C-Br bonds at the R-position of the bithiophene
sulfur in 9 and 14. Formation of 19 over 18 can be
controlled stoichiometrically. Reaction of 9 with Pd-
(PPh3)4 takes place rapidly, giving only 17 as the
isolated product.
F igu r e 2. ORTEP plot of the molecular structure of trans-
PdBr(C4H3S-C)(PPh3)2 (20) (30% probability ellipsoids).
The present results suggest that isomerically pure
bromothiophenes and bromobithiophenes can be syn-
thesized by a Pd-catalyzed reduction method, whereby
the degree of hydrodebromination can be controlled
conveniently by a measured use of the reductant. The
easy trapping of some reaction intermediates would
pave a way for us to introduce other functionalities to
the thienyl complexes. Such work could lead to a
significant methodology, on the basis of which nucleo-
philic substitutions of thiophenes and bithiophenes and
the designed synthesis of thiophene-containing oligo-
mers and polymers can be carried out. Current work
in our laboratory is directed at these initiatives.
Exp er im en ta l Section
2-Bromothiophene, 2,4-dibromothiophene, 2,3,5-tribromothio-
phene, tetrabromothiophene, and NaBH4 were commercial
products and were used without further purification. Pd-
(PPh3)4 was prepared according to published procedures.27 5,5′-
Dibromo-2,2′-bithiophene and 3,3′,5,5′-tetrabromo-2,2′-bithio-
phene were synthesized by direct bromination of 2,2′-bi-
thiophene with 2 and 4 molar equiv of bromine, respectively,
in a chloroform-acetic acid mixture.10 Reagent-grade aceto-
nitrile was distilled from CaH2 under argon. Toluene, benzene,
and diethyl ether were distilled from Na/benzophenone under
argon. NMR spectra were acquired on a Bruker ACF300
spectrometer. 1H NMR spectra had SiMe4 as the internal
reference with CDCl3 as solvent. 13C NMR spectra were
referenced to the same solvent. 31P NMR spectra were
referenced internally relative to the deuterium lock signal
using the SR command of standard Bruker software with the
standard 85% H3PO4-D2O.
F igu r e 3. ORTEP plot of the molecular structure of trans-
PdBr(3,4,5-C4Br3S-C)(PPh3)2 (21) (30% probability ellip-
soids).
To verify that the synthesis and mechanism can be
extended to the bithiophene systems, we have trapped
the debromination intermediates trans,trans-Pd2Br2(µ-
4,4′-C8H2Br2S2-C,C′)(PPh3)4 (17) from the stoichiometric
reaction of 9 with Pd(PPh3)4 and, similarly, trans,trans-
Pd2Br2(µ-C8H4S2-C,C′)(PPh3)4 (18) and trans-PdBr(C8H4-
BrS2-C)(PPh3)2 (19) from 14 and Pd(PPh3)4. Complex
Gen er a l P r oced u r e for th e Ca ta lytic Debr om in a tion
of Br om oth ioph en es an d Br om obith ioph en es. An oxygen-
free mixture of bromothiophenes or bromobithiophene (1
mmol) and Pd(PPh3)4 (0.011 g, 0.01 mmol) in CH3CN (10 mL)
was stirred for about 5 min, after which NaBH4 was added.
(Tables 1 and 2). The mixture was stirred at 70 °C under
argon for a specified period of time. At the conclusion of the
reaction, an internal standard, anthracene, was added to the
mixture, and the conversion and yield of products were
(25) Harshbarger, W. B.; Bauer, S. H. Acta Crystallogr. 1970, B26,
1010. Derissen, J . L.; Kocken, J . W. M.; Weelden, R. H. Acta Crystal-
logr. 1971, B27, 1692. Karl, R. R.; Bauer, S. H. Acta Crystallogr. 1972,
B28, 2619.
(26) Manna, J .; Kuehl, C. J .; Whiteford, J . A.; Stang, P. J . Organo-
metallics 1997, 16, 1897.
19 is an intermediate of 18. Isolation of these complexes
shows clearly the sequential proceedings of the oxidative
addition and hydrodebromination steps. The 31P shifts
(27) Coulson, D. R. Inorg. Synth. 1972, 13, 121.

3994 Organometallics, Vol. 17, No. 18, 1998
Xie et al.
1
determined by capillary GC (Hewlett-Packard 5890 Series II
Plus, 25 m × 0.32 mm HP-1 column, 80-300 °C).
benzene (20 mL). Yield: 0.85 g (90%). Mp: ∼235 °C dec. H
NMR: δ 7.59 (m, 12H, PPh3), 7.36 (t, 18H, PPh3), 5.56 (s, 1H,
H5-Pd). 31P NMR: δ 22.9. 13C{1H} NMR: δ 134.6 (t, J CP
)
3
Recycle Use of th e Ca ta lyst. At the end of the catalytic
reaction, the mixture was filtered under argon and the residual
solid was washed with CH3CN (2 mL × 2). This solid was
used as the catalyst in a recycled debromination reaction.
Gen er a l P r oced u r e for th e P r ep a r a tive Syn th esis of
Br om oth iop h en es a n d Bith iop h en es. An oxygen-free mix-
ture of bromothiophenes or bromobithiophenes and Pd(PPh3)4
(0.231 g, 0.2 mmol) in CH3CN was stirred at 70 °C under
nitrogen for about 10 min. A specified quantity of NaBH4 was
added in small portions over a period of time (t). The mixture
was continuously stirred at 70 °C for that period of time. Upon
completion of reaction (monitored by TLC), the solvent was
removed and the crude compound distilled in vacuo (for the
liquid product). Solid products were washed with water, dried,
and recrystallized from hexane.
6.3 Hz, Co-P), 130.5 (s, Cp-P), 130.3 (t, 2J CP ) 24.3 Hz, Ci-P),
4
2
128.0 (t, J CP ) 5.3 Hz, Cm-P); 148.1 (t, J CP ) 8.3 Hz, C1-
Pd), 110.6 (s, C3-Pd), 114.0 (s, C4-Pd), 132.2 (s, C5-Pd). Anal.
Calcd for C40H31Br3P2PdS: C, 50.48; H, 3.28; Br, 25.18; P, 6.51;
Pd, 11.18; S, 3.37. Found: C, 50.72; H, 3.32; Br, 24.70; P, 6.13;
Pd, 10.84; S, 3.22.
tr a n s,tr a n s-P d 2Br 2(µ-4,4′-C8H2Br 2S2-C,C′)(P P h 3)4 (17):
Pd(PPh3)4 (1.156 g, 1.0 mmol), 3,3′,5,5′-tetrabromo-2,2′-
bithiophene (9; 0.484 g, 1.0 mmol), toluene (30 mL). Yield:
0.80 g (92%). Mp: ∼220 °C dec. 1H NMR: δ 7.57 (m, 24H,
PPh3), 7.29 (t, 36H, PPh3), 5.42 (s, 2H, C5-Pd). 31P NMR: δ
22.6. 13C NMR: δ 134.6 (t, J CP ) 6.1 Hz, Co-P), 130.8 (s,
3
2
4
Cp-P), 130.3 (t, J CP ) 24.8 Hz, Ci-P), 128 (t, J CP ) 5.2 Hz,
Cm-P); 148.0 (t, 2J CP ) 8.0 Hz, C1-Pd), 111.4 (s, C3-Pd), 128.6
(s, C4-Pd), 133.0 (s, C5-Pd). Anal. Calcd for C80H62Br4P4-
Pd2S2: C, 55.10; H, 3.58; Br, 18.33; P, 7.10; Pd, 12.21; S, 3.68.
Found: C, 55.28; H, 3.71; Br, 18.01; P, 6.87; Pd, 11.21; S, 3.20.
tr a n s,tr a n s-P d 2Br 2(µ-C8H 4S2-C,C′)(P P h 3)4 (18): Pd(P-
Ph3)4 (1.156 g, 1.0 mmol), 5,5′-dibromo-2,2′-bithiophene (14;
0.162 g, 0.5 mmol), toluene (30 mL). Yield: 0.67 g (85%). Mp:
∼195 °C dec. 1H NMR: δ 7.52 (m, 24H, PPh3), 7.23 (t, 36H,
2,3,4-Tr ibr om oth iop h en e (2): CH3CN (100 mL), tetra-
bromothiophene (1; 8.0 g, 20 mmol), NaBH4 (1.9 g, 50 mmol).
t: 2 h. Yield: 5.3 g (82%). Bp: 131-132 °C /10 mmHg (lit.11
132-134 °C/10 mmHg). 1H NMR: δ 7.36 (s, 1H). MS: m/z
318/320/322/324 (M+: 60/100/100/60). Purity: in excess of 95%
(determined by GC).
2,3-Dibr om oth iop h en e (7): 2,3,5-tribromothiophene (6;
6.42 g, 20 mmol), NaBH4 (1.2 g, 30 mmol), CH3CN (100 mL).
t: 1.5 h. Yield: 4.0 g (83%). Bp: 92-94 °C/10 mmHg (lit.11
88-90 °C/9 mmHg). 1H NMR: δ 7.25 (d, 1H, J ) 5.6 Hz),
6.91 (d, 1H, J ) 5.6 Hz). MS: m/z 240/242/244 (M+: 50/100/
50). Purity: in excess of 95%.
3,4-Dibr om oth iop h en e (3): tetrabromothiophene (1; 8.0
g, 20 mmol), NaBH4 (3.8 g, 100 mmol), CH3CN (100 mL).
Adding time: 3 h. Yield: 4.2 g (86%). Bp: 92-93 °C/10
mmHg (lit.11 93-93 °C/10 mmHg). 1H NMR: δ 7.30 (s, 2H).
MS: m/z 240/242/244 (M+: 60/100/60). Purity: in excess of
96%.
3-Br om oth iop h en e (4): 2,3,5-tribromothiophene (6; 6.42
g, 20 mmol), NaBH4 (2.5 g, 6.6 mmol), CH3CN (100 mL). t: 3
h. Yield: 2.64 g (81%). Bp: 50-52 °C/20 mmHg (lit.11 44-
46 °C/11 mmHg). 1H NMR: δ 7.27 (d, 1H, J ) 5.0 Hz), 7.21
(d, 1H, J ) 1.3 Hz), 6.98 (d, 1H, J ) 3.1 Hz). MS: m/z 162/
164 (M+: 100/100). Purity: in excess of 95%.
PPh3), 5.84 (d, 2H, 3J HH ) 3.4 Hz, H4-Pd), 5.58 (d, 2H, 3J HH
)
3.4 Hz, H5-Pd); 31P NMR: δ 22.5. 13C NMR: δ 134.6 (t, J CP
3
2
) 6.1 Hz, Co-P), 131.1 (s, Cp-P), 129.8 (t, J CP ) 24.8 Hz,
4
2
Ci-P), 127.7 (t, J CP ) 5.2 Hz, Cm-P); 142.3 (t, J CP ) 8.0 Hz,
C1-Pd), 132.1 (s, C3-Pd), 124.5 (s, C4-Pd), 131.3 (s, C5-Pd).
Anal. Calcd for C80H64Br2P4Pd2S2: C, 60.58; H, 4.07; Br, 10.07;
P, 7.81; Pd, 13.42; S, 4.04. Found: C, 61.10; H, 4.24; Br, 9.93;
P, 7.52; Pd, 12.98; S, 4.00.
tr a n s-P d Br (C8H4Br S2-C)(P P h 3)2 (19): Pd(PPh3)4 (1.156 g,
1.0 mmol), 5,5′-dibromo-2,2′-bithiophene (14: 0.324 g, 1.0
mmol), toluene (30 mL). Reaction time: 2 h. Yield: 0.79 g
(83%). Mp: ∼180 °C dec. 1H NMR: δ 7.56 (m, 12H, PPh3),
3
7.27 (t, 18H, PPh3), 6.32 (d, 1H, J HH ) 5.3 Hz, H4-Pd), 5.82
3
3
(d, 1H, J HH ) 5.3 Hz, H5-Pd), 6.28 (d, 1H, J HH ) 3.8 Hz,
H4′-Pd), 6.78 (d, 1H, 3J HH ) 3.8 Hz, H5′-Pd). 31P NMR: δ 22.8.
13C NMR: δ 134.7 (t, J CP ) 6.1 Hz, Co-P), 130.9 (s, Cp-P),
3
2
4
130.1 (t, J CP ) 24.8 Hz, Ci-P), 127.9 (t, J CP ) 5.2 Hz, Cm-
P); 146.8 (t, 2J CP ) 8.0 Hz, C1-Pd), 132.1 (s, C3-Pd), 128.5 (s,
C4-Pd), 132.1 (s, C5-Pd), 108.4 (s, C1′-Pd), 131.3 (s, C3′-Pd),
3,3′-Dibr om o-2,2′-bith iop h en e (11): 3,3′,5,5′-tetrabromo-
2,2′-bithiophene (9; 9.64 g, 20 mmol), NaBH4 (2.7 g, 70 mmol),
CH3CN (120 mL). t: 2.5 h. Yield: 5.51 g (85%). Mp: 96-97
°C (lit.10 mp 95-97 °C). 1H NMR: δ 7.41 (d, 2H, J ) 5.4 Hz),
7.09 (d, 2H, J ) 5.4 Hz). MS: m/z 322/324/326 (M+: 80/100/
80).
125.4 (s, C4′-Pd),130.4 (s, C5′-Pd). Anal. Calcd for C44H34
-
Br2P2PdS2: C, 55.34; H, 3.59; Br, 16.73; P, 6.49; Pd, 11.14; S,
6.71. Found: C, 56.01; H, 3.91; Br, 16.11; P, 6.95; Pd, 11.76;
S, 5.91.
3-Br om o-2,2′-bith iop h en e (12): 3,3′,5,5′-tetrabromo-2,2′-
bithiophene (9; 9.64 g, 20 mmol), NaBH4 (3.8 g, 100 mmol),
CH3CN (120 mL). t: 4 h. Yield: 3.72 g (76%). Bp: 123-125
°C/0.05 mmHg. 1H NMR: δH3′ 7.42 (dd, 1H, J ) 1.1, 3.7 Hz),
δH5′ 7.35 (dd, 1H, J ) 1.1, 5.1 Hz), δH5 7.19 (d, 1H, J ) 5.4
Hz), δH4′ 7.09 (dd, 1H, J ) 3.7, 5.1 Hz), δH4 7.02 (d, 1H, J )
5.4 Hz). MS: m/z 244/246 (M+: 100/100).
3,3′,5-Tr ibr om o-2,2′-bith ioph en e (10): 3,3′,5,5′-tetrabromo-
2,2′-bithiophene (9; 9.64 g, 20 mmol), NaBH4 (1.2 g, 30 mmol),
CH3CN (100 mL). t: 2 h. Yield: 6.07 g (75%). Mp: 85-87
°C. 1H NMR: δH5′ 7.41 (d, 1H, J ) 5.3 Hz), δH4′ 7.07 (d, 1H, J
) 5.3 Hz), δH4 7.06 (s, 1H). MS: m/z 400/402/404/406 (M+:
70/100/100/70).
R ea ct ion of tr a n s-P d Br (3,4-C4H Br 2S-C)(P P h 3)2 (16)
w ith Na BH4. Complex 16 (6 mg, 0.006 mmol) and NaBH4 (4
mg, 0.01 mmol) were added to CD3CN (0.6 mL) in an NMR
tube under an argon atmosphere. The tube was warmed to
60 °C for 1 h with shaking. 1H NMR data were then obtained
at ambient temperature: δ -11.06 ppm (s, Pd-H). Under
otherwise identical conditions in CDCl3, both the trans and
cis structures were detected. 1H NMR (ppm): cis δ -9.73 (dd,
2
2
2J PH ) 82.5 Hz, J PH ) 3.3 Hz, H-Pd); trans δ -11.86 (t, J PH
) 8.7 Hz, H-Pd) (∼ 1:3 ratio). The reaction of 16 with NaBH4
was also performed in CH3CN for 5 h at 60 °C, and the
resultant dark mixture was filtered. GC/MS analysis with the
help of standard samples revealed that 2,3-dibromothiophene,
PPh3, and OPPh3 were formed as the main products.
Gen er a l P r oced u r e for th e Syn th esis of 16-19.
A
mixture of Pd(PPh3)4 and bromothiophene or bromobithiophenes
in benzene or toluene was deoxygenated and stirred overnight
in a Schlenk flask under argon at room temperature. The
resultant yellow solution or suspension was evaporated to
dryness in vacuo. The solid residue was triturated with Et2O
and the ether solution discarded. The washing was repeated
twice and the residual product purified further by recrystal-
lization from CHCl3-hexane.
X-r a y Cr ysta llogr a p h y. Single crystals of 16 were grown
by a diffusion method with hexane layered on a sample
solution in CHCl3 at room temperature and those of 20 and
21 were grown similarly from CH3CN/C6H5CH3/CHCl3 and
C6H5CH3/CHCl3 mixtures, respectively. Crystals suitable for
X-ray diffraction were mounted on thin-walled Lindemann
glass capillaries under a nitrogen atmosphere. Intensity data
were measured on a Rigaku RAXIS IIC imaging plate for 16
and 20 and on a Rigaku AFC7 diffractometer for complex 21,
using graphite-monochromatized Mo KR radiation (λ ) 0.710 73
tr a n s-P dBr (3,4-C4HBr 2S-C)(P P h 3)2 (16). Pd(PPh3)4 (1.156
g, 1.0 mmol), 2,3,5-tribromothiophene (6; 0.390 g, 1.2 mmol),

Bromothiophenes and Bromobithiophenes
Organometallics, Vol. 17, No. 18, 1998 3995
Å). For complex 16, 36 oscillation frames were used with φ )
0-180°, ∆φ ) 5.0°, and scan rate 8 min per frame; for complex
20, 31 oscillation frames were used with φ ) 0-180°, ∆φ )
6.0°, and scan rate 8 min per frame; for complex 21, the
variable ω-scan technique was used with a scan rate of 2.00-
32.00°/min. Empirical absorption corrections were applied to
the raw intensities. The structures were solved by direct
methods and refined by full-matrix least squares. The aniso-
tropic thermal parameters of all non-hydrogen atoms were
varied, and hydrogen atoms were introduced in their idealized
positions with assigned isotropic temperature factors. In 16,
a crystallographic C2 axis passes through the Pd(1) and Br(1)
atoms, so that the 3,4-dibromo-2-thienyl group is subjected to
2-fold disorder. In the model chosen for refinement, the C2
axis is taken to pass through the C(19) atom and the midpoint
of the C(21)-C(21a) bond, and the X-ray scattering power of
the disordered thienyl group is accordingly represented by
C(19) and C(21) at full site occupancy together with S(1) and
C(20) at half site occupancy. In view of the close overlap
between the S(1) and ipso C(20a) atoms of the two possible
orientations of the thienyl ring, bond length restraints of 1.712-
(1), 1.424(2), and 1.362(1) Å, respectively, were imposed on the
carbon-sulfur and formal single and double carbon-carbon
bonds. Likewise, complex 20 has a crystallographic 2-fold axis
passing through Br(1), Pd(1), C(19), and the midpoint of the
C(21)-C(21a) bond, and refinement was handled in the same
manner. The atoms in complex 21 were all well-behaved. All
calculations were performed on a PC 486 with the SHELXTL-
PC program package28 for 20 and 21 and SHELXL-93 for 16.29
Crystal data, data collection parameters, and results of the
analyses are listed in Table 3. Selected bond lengths and bond
angles are listed in Table 4.
Ack n ow led gm en t. We thank the National Univer-
sity of Singapore (Grant Nos. RP950695 and RP960613)
and The Chinese University of Hong Kong (Hong Kong
Research Grants Council Ref. CUHK 311/94P) for
financial support and the technical staff in the Depart-
ment of Chemistry of NUS for general help. Steno-
graphic assistance from Y. P. Leong and valuable
discussions with S. Li and P. M. N. Low are appreciated.
Y.X. thanks NUS for a scholarship award.
Su p p or tin g In for m a tion Ava ila ble: Tables giving crys-
tal data and structure refinement details, atomic coordinates,
bond distances and angles, and thermal parameters for 16,
20, and 21 (18 pages). Ordering information is given on any
current masthead page.
OM9800142
(28) Sheldrick, G. M. In Computational Crystallography; Sayre, D.,
Ed.; Oxford University Press: New York, 1982; pp 506-514.
(29) Sheldrick, G. M. SHELX-93: A Program for the Refinement of
Crystal Structures; University of Go¨ttingen, Go¨ttingen, Germany,
1993.