Functionalisation reactions of 2,5-diphenyl-1,3,4-oxadiazoles bearing a terminal ethynyl or butadiynyl substituent: X-ray crystal structures of the products

Article abstract of DOI:10.1039/b315694j

2-(4-feri-Butylphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole 1 and the butadiyne analogue 2 reacted with triethyl orthoformate in the presence of zinc iodide to give the acetal derivatives 3 and 4 which were hydrolysed with Amberlyst-15 in acetone-water to afford the alkynylaldehyde derivatives 5 and 6 in high yields. The reaction of 4,5-bis(methoxycarbonyl)-2-tributylphosphonium- 1,3-dithiole tetrafluoroborate salt 7 with 5 (nBuLi, THF) gave the Wittig product 2-(3-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}prop-2- ynylidene)-1,3-dithiole-4,5dimethyl dicarboxylate 11 (33% yield) whereas other attempted Wittig and Horner-Wadsworth-Emmons reactions led to the unexpected loss of the aldehyde group from compounds 5 and 6 to give 1 and 2, respectively. The X-ray crystal structures of compounds 3, 4, 5 and 11 are reported: the π-systems of all four molecules adopt predominantly planar conformations. A comparison of bond lengths in the structures of 5 and 11 reveals extended π-conjugation in the latter.

Full text of DOI:10.1039/b315694j

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Functionalisation reactions of 2,5-diphenyl-1,3,4-oxadiazoles
bearing a terminal ethynyl or butadiynyl substituent:
X-ray crystal structures of the products
David Kreher, Andrei S. Batsanov, Changsheng Wang and Martin R. Bryce*
Department of Chemistry, University of Durham, Durham, UK DH1 3LE.
E-mail: m.r.bryce@durham.ac.uk
Received 2nd December 2003, Accepted 23rd January 2004
First published as an Advance Article on the web 23rd February 2004
2-(4-tert-Butylphenyl)-5-(4-ethynylphenyl)-1,3,4-oxadiazole 1 and the butadiyne analogue 2 reacted with triethyl
orthoformate in the presence of zinc iodide to give the acetal derivatives 3 and 4 which were hydrolysed with
Amberlyst-15 in acetone-water to afford the alkynylaldehyde derivatives 5 and 6 in high yields. The reaction of
4,5-bis(methoxycarbonyl)-2-tributylphosphonium-1,3-dithiole tetrafluoroborate salt 7 with 5 (nBuLi, THF) gave
the Wittig product 2-(3-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}prop-2-ynylidene)-1,3-dithiole-4,5-
dimethyl dicarboxylate 11 (33% yield) whereas other attempted Wittig and Horner–Wadsworth–Emmons reactions
led to the unexpected loss of the aldehyde group from compounds 5 and 6 to give 1 and 2, respectively. The X-ray
crystal structures of compounds 3, 4, 5 and 11 are reported: the π-systems of all four molecules adopt predominantly
planar conformations. A comparison of bond lengths in the structures of 5 and 11 reveals extended π-conjugation in
the latter.
recovered in high yields in all cases. However, functionalisation
Introduction
of both 1 and 2 proceeded smoothly using Gorgues’ protocol
2,5-Diaryl-1,3,4-oxadiazole derivatives have enjoyed wide-
spread use in organic chemistry due to their high photolumin-
(triethyl orthoformate in the presence of zinc iodide)14 to afford
3 and 4 in 85% and 77% yields, respectively (Scheme 1). Com-
pounds 3 and 4 were surprisingly resistant to hydrolysis under
classical acidic conditions. However, reaction of 3 with pure
formic acid in chloroform at <10 ЊC gave the corresponding
aldehyde 5 (57% yield). Under these conditions compound 4
decomposed and 6 could not be isolated. However, the method
escence quantum yield and their good thermal and chemical
stabilities.1 These properties, combined with the electron-
deficient nature of the oxadiazole ring, have led to their
application as electron-transporting/hole-blocking (ECHB)
materials in multilayer and blended organic light emitting
devices (OLEDs).2 Low molecular weight diaryloxadiazoles,3
of choice for this hydrolysis was treatment of 3 or 4 with the ion
star-shaped oligomers4 and polymeric derivatives (with the
exchange resin Amberlyst-15 in acetone–water,15 which gave
oxadiazole units as pendant groups or in the main chain)5 have
aldehydes 5 and 6 in >95% yield.
In attempts to extend further the π-electron conjugated
system, we explored Wittig and Horner–Wadsworth–Emmons
reactions of 5 and 6 with a range of 2-trialkylphosphonium-
been studied in this context.
In recent years there has been a renaissance in the chemistry
of new alkyne and diyne systems. Their syntheses have been
greatly facilitated by developments in organometallic coupling
1,3-dithiole salts 7–9 and the related phosphonate ester deriv-
methodology, notably the Sonogashira reaction,6 and their sp
atives 10, under standard conditions16 (nBuLi or LDA in
carbon frameworks provide interesting rigid molecular archi-
tectures, the structures of which are simplified compared to
THF). Numerous reactions were tried [specifically: all the salts
7–9 with 5; and 7, 8 and 9 (R = Ph and n-Bu, R1 = Me, SC5H11)
alkene analogues due to the lack of E/Z isomerism. The extent
and 10 (R1 = Me, SC5H11 and CH2OAc) with 6]. With one
of conjugation through sp hybridised carbon frameworks con-
exception (see below), the only compounds isolated after
tinues to be widely debated among experimentalists7 and theor-
chromatographic purification were recovered starting materials
5 or 6 (45–65% yields) and the terminal alkynes 1 or 2 (25–45%
eticians,7,8 and the potential of ethynyl derivatives of arenes and
heteroarenes to function as “molecular wires” is a hot topic.9
yields). The mechanism by which the aldehyde group is lost
For example, ethynyl and butadiynyl derivatives of por-
under these conditions is at present not known. The exception
phyrins,10 tetrathiafulvalenes11 and organometallic complexes12
have been synthesised as building blocks for studies in this field.
In this paper we combine these two contemporary themes
was the isolation of compound 11 (33% yield) from the reaction
of salt 7 with 5 (nBuLi, THF). Compound 11 is an air-stable,
yellow–orange crystalline solid (λmax 384 nm in CH2Cl2). The
(i.e. diaryl-1,3,4-oxadiazole and alkyne/diyne chemistry) and
cyclic voltammogram of 11 displays the typical quasi-reversible
describe reactions involving the terminal ethynyl and buta-
one-electron oxidation wave of a 1,3-dithiol-2-ylidene unit17 at
diynyl units of the new 2,5-diphenyl-1,3,4-oxadiazole deriv-
atives 1 and 2, along with X-ray crystal structures of four of the
products obtained.
E ox ϩ0.95 V (in CH2Cl2) or ϩ0.84 V (in MeCN) vs Ag/AgCl.
It is known that complexation of alkyne or butadiyne units
to Co2(CO)6 clusters masks the reactivity of the triple bond(s).18
We sought, therefore, to enhance the reactivity of the aldehyde
group of 5 and 6 in Wittig reactions by using the derived cobalt
carbonyl complexes 12 and 13 which were readily obtained in
Results and discussion
Compounds 1 and 2 have recently been synthesised in our
laboratory.13 Initial attempts at functionalisation of the ter-
minal sp carbon atoms in 1 and 2 by deprotonation (with NaH,
DBU, LDA or t-BuLi) and reaction with 4-ethoxybenzaldehyde
or DMF) gave no substituted product. Starting material was
high yields. Complex 12 was obtained by reaction of dicobalt
octacarbonyl with 5; compound 13 was obtained via the tetra-
cobalt complex 14 of the acetal 4. However, no reaction was
observed between 12 and reagents 7 and 8, under the conditions
described above: starting material 12 was recovered in high
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 8 5 8 – 8 6 2
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
858

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Scheme 1 Reagents and conditions: i, triethyl orthoformate, ZnI2, (and THF for 2), heat; ii, Amberlyst-15, acetone–water, 20 ЊC; or (for 3) formic
acid, CHCl3, <10 ЊC; iii, reagent 7, n-BuLi, THF, Ϫ78 ЊC to 20 ЊC; iv, reagents 8–10, n-BuLi or LDA, THF, Ϫ78 ЊC to 20 ЊC or reflux; v, Co2(CO)8,
THF, 20 ЊC; vi, reagents 7 or 8, n-BuLi, THF, Ϫ78 ЊC to 20 ЊC; vii, trimethylamine oxide, THF, 20 ЊC.
The geometry of the diethoxy-propyne and -pentadiyne
moieties in 3 and 4 are unexceptional19 and can be compared to
20
᎐
᎐
᎐
(Ph P)(C H )NiC᎐CCH(OEt)
and {(MeO) P} (OC) IFeC᎐
᎐
3
5
5
2
3 2 2
CCH(OMe)2.21 In 5, which to our knowledge is the first struc-
turally characterised alkynylaldehyde, the C(9)᎐O(2) bond is
᎐
twisted out of the ring B plane by 25.7Њ and shows no special
conjugation with the triple bond, the C(8)–C(9) bond [1.445(3)
Å] being marginally longer than the standard C(sp1)–C(sp2)
bond (1.431 Å).19 (Unfortunately X-ray quality crystals of 6
could not be obtained). On the contrary, in 11 the dithiole ring
is flat and coplanar within 0.7Њ to the benzene ring B, the C(8)–
C(9) bond is shortened to 1.419(2) Å, indicating some delocal-
isation. Similar bond distances have been observed in buta-
diynyl derivative of tetrathiafulvalene,11 although the precision
of the structure was low. As in the latter, in 11 the methoxy-
carbonyl substituents at C(17) and C(18) have different
orientations, inclined to the dithiole ring by 4.4Њ and 78.2Њ.
Molecules of 3 and 11 form continuous stacks in the crystal
structures. Adjacent molecules in each stack are inversion-
related, hence they overlap in a head-to-tail fashion and their
planes are strictly parallel. The mean interplanar separations in
3 alternate between 3.42 and 3.44 Å, in 11 between 3.49 and
3.56 Å. Molecules 4 and 5 are stacked into centrosymmetric
(head-to-tail) dimers with interplanar separations of 3.34 (4)
and 3.36 Å (5). However, these dimers do not form infinite
stacks, but pack in a herringbone fashion, contacting at a
dihedral angle of 34.4Њ (4) and 49.7Њ (5).
yield. Compound 13 behaved differently: the aldehyde group
was lost (as above with compounds 5 and 6) and compound 15
was obtained in 30–45% yield. Again, no Wittig product was
observed. Compound 15 was also obtained directly from 2
in 72% yield. Decomplexation of 12–15 to regenerate the
corresponding ethynyl or butadiynyl system occurred cleanly
(44–66% yields) under standard conditions,18b viz. trimethyl-
amine oxide in THF.
X-Ray crystal structures of compounds 3, 4, 5 and 11
All four molecules (Fig. 1; Table 1) adopt predominantly planar
conformations, with the exception of both OEt groups in 3
and 4, the keto-oxygen in 5, one of the CO2Me groups in 11,
and the tert-butyl atoms C(4), C(5) and C(6) in each molecule,
all non-hydrogen atoms lie in one plane with the average
deviations of 0.08 (3, 4), 0.03 (5), 0.06 (11) and the maximum
deviations of 0.25 (3), 0.24 (4), 0.08 (5) and 0.12 Å (11).
Benzene rings A and B are inclined to the oxadiazole ring by
13.0 and 0.9Њ in 3, 7.2 and 7.8Њ in 4 (the dihedral angle between
rings A and B is only 3.3Њ), 3.2 and 2.1Њ in 5, and 6.8 and
3.0Њ in 11, respectively.
Conclusions
We have explored functionalisation of the terminal carbon
atoms of the ethynyl and butadiynyl derivatives 1 and 2.
Appropriate reaction conditions have been defined for the effi-
cient two-step conversion of 1 and 2, via acetal derivatives 3 and
4, into the corresponding alkynylaldehyde derivatives 5 and 6.
The Wittig reaction product 11 has been obtained from
5. Cobalt carbonyl complexes 12–15 are reported, and their
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 8 5 8 – 8 6 2
859

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2-(4-tert-Butylphenyl)-5-[4-(5,5-diethoxy-1,3-pentadiyn-1-yl)-
phenyl]-1,3,4-oxadiazole 4. Following the method used to
prepare 3, compound 213 (1.0 g, 3.1 mmol), ZnI2 (105 mg,
0.31 mmol), THF (10 cm3) and triethyl orthoformate (30 cm3)
were stirred under Ar for 3 h (oil-bath temperature 110 ЊC).
Chromatography (silica, DCM–diethyl ether 92 : 8, v/v) and
recrystallisation from ethanol–ethyl acetate gave 4 as white
needles (1.0 g, 77%) mp: 141–143 ЊC. δH (CDCl3): 1.28 (t,
J = 7.2 Hz, 6H), 1.38 (s, 9H), 3.66 (m, 2H), 3.78 (m, 2H), 5.41 (s,
1H), 7.56 (d, J = 8.0 Hz, 2H), 7.65 (d, J = 8.0 Hz, 2H), 8.06 (d,
J = 8.0 Hz, 2H), 8.11 (d, J = 8.0 Hz, 2H); δC (CDCl3): 15.3, 31.3,
35.4, 61.6, 69.7, 75.7, 78.3, 79.1, 91.8, 121.1, 124.7, 124.8, 126.4,
127.0, 127.1, 133.5, 155.9, 163.9, 165.2; MS (EI): m/z (%): 428
(Mϩ, 88), 383 (98), 355 (100). UV/Vis (DCM): λmax 324 nm.
Anal for C27H28N2O3 (428.21): calcd C, 75.68; H, 6.59; N, 6.54.
Found: C, 75.66; H, 6.57; N, 6.57%.
{3-[5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-
propynal 5. Method (a). Compound 3 (1.0 g, 2.5 mmol) was
dissolved in acetone (100 cm3). Amberlyst-15 resin (1.0 g) and
water (1 cm3) were added and the mixture was stirred vigorously
for 36 h at 20 ЊC. The precipitate was removed by suction
filtration and washed with DCM. The filtrate was evaporated
in vacuo to yield a residue which was chromatographed (silica,
DCM–diethyl ether 94 : 6, v/v) and recrystallised from hexane–
ethyl acetate to yield 5 as white needles (0.80 g, 97%) mp: 152–
155 ЊC (decomp.). δH (CDCl3): 1.38 (s, 9H), 7.57 (d, J = 8.8 Hz,
2H), 7.78 (d, J = 8.8 Hz, 2H), 8.08 (d, J = 8.8 Hz, 2H), 8.20 (d,
J = 8.8 Hz, 2H), 9.47 (s, 1H); δC (CDCl3): 31.3, 35.6, 90.0, 93.4,
120.9, 122.7, 126.4, 127.1, 127.2, 134.0, 156.0, 163.6, 165.4,
176.7; MS (EI): m/z (%): 330 (Mϩ, 67), 315 (100). UV/Vis
(DCM): λmax 316 nm. Anal for C21H18N2O2 (330.14): calcd C,
76.34; H, 5.49; N, 8.48. Found: C, 76.32; H, 5.47; N, 8.50%.
Method (b). A solution of 3 (1.0 g, 2.5 mmol) in a mixture of
chloroform (60 cm3) and pure formic acid (30 cm3) was stirred
under Ar for 1 h at 10 ЊC and then stored for 24 h at 6–8 ЊC.
Water was added and the organic phase was washed with water
again several times (3 × 100 cm3), dried (MgSO4) and concen-
trated in vacuo. The residue was purified by chromatography
(silica, DCM–diethyl ether 95 : 5, v/v) to yield 5 (0.47 g, 57%)
identical with the sample above.
Fig. 1 X-Ray structures of 3, 4, 5 and 11, showing 50% atomic
displacement ellipsoids.
Table 1 Bond distances (Å)
3
4
5
11
C(24)–C(7)
C(7)–C(8)
C(8)–C(9)
C(9)–C(10)
C(10)–C(27)
C(9)–O(2)
C(9)–O(3)
1.441(2)
1.193(2)
1.478(2)
—
—
1.408(2)
1.406(2)
1.432(2)
1.200(2)
1.375(2)
1.201(2)
1.476(2)
1.393(2)a
1.420(3)a
1.430(2)
1.206(3)
1.445(3)
—
—
1.192(3)
—
1.433(2)
1.204(2)
1.419(2)
1.357(2)
1.357(2)
—
—
a Bonds C(27)–O(2) and C(27)–O(3).
decomplexation reactions occur smoothly to regenerate the
corresponding alkyne or diyne systems. X-Ray crystal struc-
ture analyses reveal that the π-systems of compounds 3, 4, 5
and 11 adopt predominantly planar conformations. Further
uses of the novel alkynes 1 and 2 as building blocks for the
synthesis of extended π-electron systems for advanced materials
applications are underway in our laboratory.13b
5-{4-[5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-
penta-2,4-diynal 6. By analogy with the preparation of 5, com-
pound 9 (0.6 g, 1.4 mmol), acetone (100 cm3), Amberlyst-15
resin (1 g) and water (1 cm3) followed by chromatography
(silica, DCM–diethyl ether 92 : 8, v/v) gave 6 as a pale yellow
solid (0.5 g, 95%) mp: 127–130 ЊC (decomp.). δH (CDCl3): 1.38
(s, 9H), 7.66 (d, J = 8.4 Hz, 2H), 7.72 (d, J = 8.4 Hz, 2H), 8.07
(d, J = 8.4 Hz, 2H), 8.16 (d, J = 8.4 Hz, 2H), 9.31 (s, 1H);
δC (CDCl3): 31.3, 35.4, 83.8, 84.5, 87.6, 120.9, 123.2, 126.4,
127.1, 127.2, 132.8, 133.4, 155.9, 156.0, 163.8, 165.4, 175.8; MS
(EI): m/z (%): 354 (Mϩ, 26), 190 (98), 105 (100). UV/Vis
(DCM): λmax 333 nm. Anal for C23H18N2O (354.14): calcd C,
77.95; H, 5.12; N, 7.90. Found: C, 78.00; H, 5.14; N, 7.89%.
Experimental
General
The details are the same as those reported recently.3b
2-(4-tert-Butylphenyl)-5-[4-(3,3-diethoxypropyn-1-yl)phenyl]-
1,3,4-oxadiazole 3. Compound 113 (1.0 g, 3.3 mmol) and ZnI2
(100 mg, 0.33 mmol) were dissolved in triethyl orthoformate (30
cm3). The mixture was stirred under Ar for 5 h (oil-bath tem-
perature 140 ЊC) then cooled to 20 ЊC. The solvent was removed
in vacuo and the residue was chromatographed (silica, DCM–
diethyl ether 94 : 6, v/v) and recrystallised from ethanol–ethyl
acetate to obtain 3 as white needles (1.2 g, 85%) mp: 82–84 ЊC.
δH (CDCl3): 1.30 (t, J = 7.2 Hz, 6H), 1.38 (s, 9H), 3.70 (m, 2H),
3.85 (m, 2H), 5.53 (s, 1H), 7.56 (d, J = 8.0 Hz, 2H), 7.64 (d, J =
8.0 Hz, 2H), 8.06 (d, J = 8.0 Hz, 2H), 8.10 (d, J = 8.0 Hz, 2H); δC
(CDCl3): 15.4, 31.4, 35.4, 61.3, 84.5, 87.4, 91.9, 121.1, 124.3,
125.4, 126.3, 126.9, 127.1, 132.8, 155.8, 164.0, 165.1; MS (EI):
m/z (%): 404 (Mϩ, 52), 359 (100). UV/Vis (DCM): λmax 305 nm.
Anal for C25H28N2O3 (404.21): calcd C, 74.23; H, 6.98; N, 6.93.
Found: C, 74.19; H, 6.95; N, 6.97%.
2-(3-{4-[5-(4-tert-Butylphenyl)-1,3,4-oxadiazol-2-yl]phenyl}-
prop-2-ynylidene)-1,3-dithiole-4,5-dimethyl dicarboxylate 11.
To a stirred solution of salt 722(1.2 g, 2.3 mmol) in dry THF
(20 cm3) under N2 at Ϫ78 ЊC, was added n-butyllithium (1.6 M
solution in hexane, 1.5 cm3, 2.4 mmol). The reaction mixture
was stirred for 30 min, then a solution of 5 (0.8 g, 2.3 mmol) in
THF (30 cm3), was added very slowly and the mixture was
left to warm to room temperature with stirring overnight. The
solvents were removed in vacuo and the residue was chromato-
graphed (silica, DCM–diethyl ether 96 : 4, v/v) and recrystal-
lised from hexane–ethyl acetate to yield 11 as yellow–orange
needles (0.40 g, 33%) mp: 201–20 ЊC. δH (CDCl3): 1.38 (s, 9H),
3.86 (s, 3H), 3.88 (s, 3H), 5.65 (s, 1H), 7.56 (d, J = 8.5 Hz,
2H), 7.58 (d, J = 8.5 Hz, 2H), 8.06 (d, J = 8.5 Hz, 2H), 8.08
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 8 5 8 – 8 6 2
860

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Table 2 Crystal data
Compound
3
4
5
11
Formula
Formula weight
T /K
Symmetry
Space group
a/Å
b/Å
c/Å
α/Њ
β/Њ
C25H28N2O3
404.49
120
Monoclinic
P21/c (# 14)
7.356(2)
19.870(5)
15.136(4)
90
91.37(1)
90
2211.7(10)
4
0.08
24719
5123
0.048
3761
0.043
0.105
C27H28N2O3
428.51
120
Monoclinic
P21/c (# 14)
14.835(5)
6.161(1)
25.549(3)
90
91.14(1)
90
2334.7(9)
4
0.08
19697
6234
0.055
4459
0.056
0.152
C21H18N2O2
330.37
120
Monoclinic
P21/c (# 14)
11.945(2)
6.183(1)
23.460(5)
90
97.94(1)
90
1716.0(5)
4
0.08
19480
3935
0.060
2788
0.053
0.161
C28H24N2O5S2
532.61
120
Triclinic
¯
P1 (# 2)
10.878(1)
11.000(1)
12.705(1)
114.87(1)
105.88(1)
97.05(1)
1276.5(2)
2
γ/Њ
V/Å3
Z
µ/mmϪ1
Refls collected
Unique refls
Rint
0.25
15831
6727
0.041
5969
0.035
0.095
Refls F 2>2σ(F 2)
R[F 2>2σ(F 2)]
wR(F 2), all data
(d, J = 8.5 Hz, 2H); δC (CDCl3): 31.4, 35.4, 53.5, 53.7, 89.8, 93.2,
99.4, 121.2, 123.4, 126.3, 126.7, 127.0, 131.7, 146.8, 155.7,
159.7, 160.0, 164.2, 165.0; MS (EI): m/z (%): 532 (Mϩ, 100).
UV/Vis (DCM): λmax 384 nm. Anal for C28H24N2O5S2 (532.11):
calcd C, 63.14; H, 4.54; N, 5.26; S 12.04. Found: C, 63.11; H,
4.52; N, 5.24; S 12.06%. CV (c = 10Ϫ3 M in DCM–nBu4NPF6,
0.05 M, Pt electrode, scan rate = 100 mV sϪ1): E ox (V vs
Ag/AgCl) ϩ0.95; CV (same conditions in CH3CN): E ox
(V vs Ag/AgCl) ϩ0.84.
δC (CDCl3):15.2, 31.0, 34.9, 61.3, 69.7, 76.1, 78.6, 80.4, 92.0,
125.1, 126.2, 126.8, 127.1, 127.5, 133.2, 155.1, 163.7, 164.9,
197.0; MS (TOF-ES): m/z (%): 1000 (Mϩ, 12), 130 (100). UV/
Vis (DCM): λmax 305 nm. Anal for C39H28Co4N2O15 (999.88):
calcd C, 46.82; H, 2.82; N, 2.80. Found: C, 46.86; H, 2.88; N,
2.72%.
Tetracobalt dodecacarbonyl complex 15. Method (a).
Co2(CO)8 (1.3 g, 3 eq.) was added under Ar to a solution of 2
(0.40 g, 1.2 mmol) in dry tetrahydrofuran (50 cm3). After stir-
ring for 30 min, the solvent was removed in vacuo. The residue
was purified by chromatography (silica, DCM–diethyl ether
95 : 5, v/v) to yield 15 as a brown–black solid (0.80 g, 72%) mp:
> 350 ЊC. δH (CDCl3): 1.39 (s, br, 9H), 6.69 (s, br, 1H), 7.58
(d, br, J = 7.5 Hz, 2H), 7.73 (d, br, J = 7.5 Hz, 2H), 8.08 (d, br,
J = 7.5 Hz, 2H), 8.17 (d, br, J = 7.5 Hz, 2H); δC (CDCl3): 31.2,
35.6, 72.9, 126.3, 127.0, 127.7, 129.8, 142.3, 155.9, 199.5; MS
(TOF-ES): m/z (%): 898 (Mϩ, 100). UV/Vis (DCM): λmax 308
nm. Anal for C34H18Co4N2O13 (897.81): calcd C, 45.46; H, 2.02;
N, 3.12. Found: C, 45.51; H, 2.04; N, 3.11%.
Dicobalt hexacarbonyl complex 12. Co2(CO)8 (0.30 g, 1.5 eq)
was added under Ar to a solution of 5 (0.20 g, 0.6 mmol) in dry
tetrahydrofuran (20 cm3). After stirring for 30 min at 20 ЊC, the
solvent was removed in vacuo. The residue was purified by
chromatography (silica, DCM–diethyl ether 95 : 5, v/v) to
yield 12 as a black–violet solid (180 mg, 88%) mp: > 350 ЊC.
δH (CDCl3): 1.39 (s, 9H), 7.58 (d, br, J = 7.5 Hz, 2H), 7.78 (d, br,
J = 7.5 Hz, 2H), 8.11 (m, br, 4H), 10.57 (s, 1H); δC (CDCl3):
31.4, 35.4, 85.5, 89.7, 121.2, 124.3, 126.4, 127.1, 127.9, 130.5,
140.6, 155.8, 164.1, 165.1, 191.1, 197.7; MS (TOF-ES): m/z (%):
616 (Mϩ, 100). UV/Vis (DCM): λmax 310 nm. Anal for
C27H18Co2N2O8 (615.97): calcd C, 52.62; H, 2.94; N, 4.55.
Found: C, 52.64; H, 2.95; N, 4.52%.
Method (b). Treatment of 13 with reagents 7 or 8 under the
conditions described above for the preparation of 11 gave com-
pound 15 in 30–45% yields.
Tetracobalt dodecacarbonyl complex 13. Compound 14
(0.40 g, 0.4 mmol) was dissolved in acetone (30 cm3). Amber-
lyst-15 resin (0.40 g) and water (1 cm3) were added. The mixture
was stirred vigorously for 36 h at 20 ЊC. Workup as described
for 12 with chromatography (silica, DCM–diethyl ether 95 : 5,
v/v) gave 13 as brown black solid (0.21 g, 56%) mp: >350 ЊC.
δH (CDCl3): 1.38 (s, 9H), 7.59 (d, br, J = 7.4 Hz, 2H), 7.75
(d, br, J = 7.4 Hz, 2H), 8.03 (m, br, 4H), 10.23 (s, 1H);
δC (CDCl3): 30.9, 35.2, 85.7, 87.4, 88.0, 89.6, 121.0, 124.2, 126.2,
127.1, 127.8, 130.4, 141.0, 155.1, 164.2, 165.5, 191.0, 197.2; MS
(TOF-ES): m/z (%): 926 (Mϩ, 18), 354 (100). UV/Vis (DCM):
λmax 301 nm. Anal for C35H18Co4N2O14 (925.81): calcd C, 45.38;
H, 1.96; N, 3.02. Found: C, 45.46, H, 2.01; N, 3.00%.
Decomplexation of 12–15: general procedure. Trimethylamine
oxide (5 equiv. for 12; 10 equiv. for 13–15) was added to a
solution of 12–15 (0.1– 0.2 mmol) in tetrahydrofuran (ca. 20
cm3). After stirring for 1 h at 20 ЊC, diethyl ether was added and
the mixture was extracted with water. The organic layer was
separated, dried (MgSO4) and concentrated in vacuo. The resi-
due was purified by chromatography to yield 5 (66% yield); 6
(44% yield); 4 (45% yield) and 2 (63% yield).
X-Ray crystallography
Single-crystal diffraction experiments (Table 2) were carried out
on a SMART 3-circle diffractometer with a 6 K (for 5) or 1 K
CCD area detector, using graphite-monochromated Mo-Kα
radiation (λ = 0.71073 Å) and Cryostream (Oxford Cryo-
systems) open-flow N2 cryostats. The structures were solved by
direct methods and refined by full-matrix least squares against
F 2 of all data, using SHELXTL software.23 Full crystallo-
graphic data, excluding structure factors, have been deposited
at the Cambridge Crystallographic Data Centre. CCDC
reference numbers 225628–225631. See http://www.rsc.org/
suppdata/ob/b3/b315694j/ for crystallographic data in .cif or
other electronic format.
Tetracobalt dodecacarbonyl complex 14. Co2(CO)8 (0.25 g,
3 eq.) was added under Ar to a solution of 4 (0.1 g, 0.2 mmol) in
dry tetrahydrofuran (30 cm3). After stirring for 1 h, the solvent
was removed in vacuo. The residue was purified by chromato-
graphy (silica, DCM–diethyl ether 95 : 5, v/v) to yield 14 as a
brown–black solid (0.15 g, 65%) mp: > 350 ЊC. δH (CDCl3):
1.19 (m, 6H), 1.26 (s, 9H), 3.55 (m, br, 2H), 3.79 (m, br, 2H),
5.40 (s, 1H), 7.30 (s, br, J = 7.5 Hz, 2H), 7.37 (s, br, J = 7.5 Hz,
2H), 7.83 (s, br, J = 7.5 Hz, 2H), 8.13 (s, br, J = 7.5 Hz, 2H);
O r g . B i o m o l . C h e m . , 2 0 0 4 , 2, 8 5 8 – 8 6 2
861

View Article Online
10 T. O. Screen, J. R. G. Thorne, R. G. Denning, D. G. Bucknall and
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