The in situ generation of alk-1-ynyllead triacetates from terminal acetylenes by zinc-lead exchange and crystal structure of 2,4,7,9,13-pentamethyl-9-phenylethynyl-7,10-ethenospiro[5.5]undeca-1,4-diene-3, 8-dione

Article abstract of DOI:10.1039/a608582b

Methods involving zinc-lead exchange for the one-pot conversion of terminal acetylenes into alk-1-ynyllead(IV) triacetates have been developed, and examples of the in situ C-alkynylation of a number of carbon nucleophiles are reported. An attempt to extend the reaction to phenols by treating 2,4,6-trimethylphenol with phenylethynyllead triacetate led to formation of the spiro dienone 16, the structure of which was determined by X-ray diffraction.

Full text of DOI:10.1039/a608582b

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The in situ generation of alk-1-ynyllead triacetates from terminal
acetylenes by zinc–lead exchange and crystal structure of 2,4,7,9,13-
pentamethyl-9-phenylethynyl-7,10-ethenospiro[5.5]undeca-1,4-diene-
3,8-dione
Christopher J. Parkinson, Trevor W. Hambley and John T. Pinhey*
School of Chemistry, University of Sydney, Sydney, 2006, Australia
M ethods involving zinc–lead exchange for the one-pot conversion of terminal acetylenes into alk-1-
ynyllead(IV) triacetates have been developed, and examples of the in situ C-alkynylation of a number of
carbon nucleophiles are reported. An attempt to extend the reaction to phenols by treating 2,4,6-
trimethylphenol with phenylethynyllead triacetate led to formation of the spiro dienone 16, the structure
of which was determined by X-ray diffraction.
In an earlier paper we reported a new general method for the C-
alkynylation of soft carbon nucleophiles such as β-dicarbonyl
compounds and sodium nitronates.1 The procedure involved
the treatment of the nucleophile with an alk-1-ynyllead triacet-
ate, unstable compounds which may be produced in solution by
the reaction of either an alk-1-ynyl(trimethyl)stannane or a
di(alk-1-ynyl)mercury compound with lead tetraacetate (LTA)
(Scheme 1).
which we had noted previously,1 had occurred [eqn. (1)]. This
would suggest that in the case of the β-keto ethyl ester 1
alkynylation is slower than for the corresponding benzyl ester.
In previous work it has been noted by ourselves4 and by
Ikegami and co-workers5 that the reactivity of β-keto esters
towards organolead triacetates is dependent on the nature of
the ester group; methyl esters and benzyl esters were found to be
more reactive than ethyl esters.
At about the same time as the communication of Ikegami, we
had found that alkynylzinc chlorides reacted with LTA to give a
solution which had the capacity to carry out the electrophilic
alkynylation of β-keto esters. This reaction, which has been
developed into a useful one-pot method for the C-alkynylation
of soft carbon nucleophiles, is outlined for the synthesis of the
keto ester 3 (see Scheme 3). The most efficient and readily
Scheme 1
The development of a one-pot procedure for converting an
acetylene into an alkynyllead triacetate in situ was recognised
by us as a potentially useful extension of the method, especially
because of the moisture sensitivity of alk-1-ynyl(trimethyl)-
stannanes. Such a method was later reported by Ikegami,2 who
found that, if a tetrahydrofuran solution of a lithium acetylide
was added at low temperature to LTA in methylene dichloride,
the resulting mixture reacted at room temperature with a range
of β-keto benzyl esters to give good yields of the corresponding
α-alkynylated keto esters. We have attempted to use this pro-
cedure to produce the α-hex-1-ynyl β-keto ester 11; however, we
found that reaction of ethyl 2-oxocyclopentanecarboxylate 1
under the conditions reported, resulted in formation of the
dimer 2 in approximately 72% yield and recovery of about 9%
of the keto ester 1 (see Scheme 2). None of the hexynyl deriv-
Scheme 3 Reagents and conditions: i, BuLi, THF, Ϫ78 ЊC; ii, ZnCl2;
iii, LTA, CHCl3, Ϫ10 ЊC then RT, 0.25 min; iv, compound 1
reproduced procedure, which afforded a 73% yield of the
phenylethynyl derivative 3, involved the treatment of phenyl-
acetylene (1.2 mol equiv.) in tetrahydrofuran with butyllithium
at Ϫ78 ЊC. Zinc chloride (1.2 mol equiv.) was added to the solu-
tion which was then stirred at Ϫ10 ЊC for 20 min. A solution of
LTA (1.1 mol equiv.) in chloroform was then added at Ϫ10 ЊC
to the solution which was then stirred for 15 s. After this the
substrate 1 (1.0 mol equiv.) in chloroform was added to the
mixture which was then stirred at Ϫ10 ЊC for 1 h and a further
2 h at room temperature.
As we found previously in the case of Sn–Pb and Hg–Pb
exchange reactions,1 the time allowed for Zn–Pb exchange is
most important for the success of the reaction. This, as indi-
cated above, is due to disproportionation of the alkynyllead
triacetate which is outlined in eqn. (1). As can be seen from
Table 1, the optimum yield of compound 3 was obtained after a
Zn–Pb exchange time of 15 s; any increase in this time resulted
in formation of more of the dimer 4, and with a delay of 15 min
Scheme 2 Reagents and conditions: i, Mixture from reaction of
᎐
C4H9C᎐CLi and Pb(OAc)4 at Ϫ20 ЊC then 15 min at RT; ii, Pb(OAc)4
᎐
ative 11 could be detected in the reaction mixture. Compound 2
is readily formed by treatment of the β-keto ester 1 with LTA,3
which would indicate that the relatively fast disproportionation
of the alkynyllead triacetate to the tetraalkynyllead and LTA,
J. Chem. Soc., Perkin Trans. 1, 1997
1465

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Table 1 Effect of Zn–Pb exchange time on the ratio of the acetylene
derivative 3 to dimer 4 in the reaction of Scheme 3 conducted at
Ϫ10 ЊC
Zn–Pb exchange
time (min)
Molar ratio of (3):(4)
Total yield (%)
0.25
0.5
2.0
9.5:1
8.1:1
4.3:1
2:1
88
83
75
78
15.0
before addition of the substrate 1, the product contained 33%
of compound 4.
The formation of the dimer 4 in the above reaction (Scheme
3) is noteworthy, since the C᎐O linked dimer 2 is the product of
LTA oxidation of the keto ester 1.3 It appeared likely that the
presence of zinc chloride caused the change in the mode of
oxidative dimerisation, and this was examined by treating the
keto ester 1 with LTA (0.5 mol equiv.) and zinc chloride in
chloroform. The C᎐C linked dimer 4 was produced in 67% yield
as a single diastereoisomer, suggesting that the product arises in
a radical coupling of two molecules complexed to a zinc ion
through oxygen.
In exploring the generality of the above one-pot alkynylation
method, the β-diketone 5 and the sodium nitronate 6 were
found to react readily under the conditions indicated in Scheme
3 to give the phenylethynyl derivatives 7 and 9 in yields of 59
and 58%, respectively. An initial attempt to extend the method,
by replacing phenylacetylene with oct-1-yne in Scheme 3,
resulted in production of some of the expected keto ester 12;
however, there was significant dimer 4 formation. This problem
was readily overcome by introducing a catalytic amount of
mercury() acetate (0.1 mol equiv.) with the LTA and maintain-
ing all other conditions as in Scheme 3. The modified method
resulted in a 52% yield of the keto ester 12 and it would appear
to be potentially useful for the introduction of a variety of
alkynyl groups, since the nucleophiles 5 and 6 behaved similarly
to the β-keto ester 1, affording the octynyl derivatives 8 (52%)
and 10 (41%), respectively, in synthetically useful yields.
Although yields in five of the six alkynylations were below
those which we reported for alkynyllead triacetates generated
for alkynyl(trimethyl)stannanes,1 the method offers greater effi-
ciency in the use of the acetylene, and its simplicity is a con-
siderable advantage. The need to employ a catalytic amount
of mercury() acetate for the octynylation reactions requires
some comment. We had found in previous work 6 that aryl-
boronic acids react with LTA to give diaryllead diacetates,
whereas the same reaction conducted with a catalytic amount
of mercury() acetate produced the aryllead triacetate in high
yield. Thus, we reasoned that the octynylzinc reagent may be
reacting in a similar way with LTA to yield octynyllead tri-
acetate and dioctynyllead diacetate, but only the former lead
compound with LTA–Hg(OAc)2. Thus, in the absence of
Hg(OAc)2 unchanged LTA would be available to produce the
dimer 4.
An attempt to extend the method to include the alkynylation
of phenols was not successful, and we attribute this to their
lower reactivity towards organolead triacetates as found by us
in earlier work.7–9 When 2,4,6-trimethylphenol (mesitol) 13 was
added to the mixture produced by use of the above conditions
for Zn–Pb exchange with phenylacetylene, none of the 6-
phenylethynyl dienone 14 (or its dimer)8 was detected. The
product of Wessely acetoxylation, compound 15, was present in
11% yield (see Scheme 4); however, the major product of the
reaction was the novel spirodienone 16, the structure of which
was determined by X-ray diffraction (see below). The pathway
to compound 16 no doubt involves a Diels–Alder condensation
between the 6-phenylethynylcyclohexadienone 14 and the mesi-
tol oxidation product 17 (see Scheme 5). This latter compound
could arise by elimination of HX from either the 6-acetoxy-
Scheme 4 Reagents and conditions: i, BuLi, THF, Ϫ78 ЊC; ii, ZnCl2;
iii, LTA, CHCl3, Ϫ10 ЊC then RT, 0.25 min; iv, compound 13
Scheme 5
cyclohexadienone 15 or the corresponding chloride as shown.
The fact that compound 16 is obtained as a single diastereo-
isomer is readily explicable in terms of the least hindered
approach of the dienophile in the condensation.
A single-crystal X-ray analysis (see Experimental section) of
the spiro compound 16 showed that the structure consists of
neutral molecules packed with no contacts shorter than those
expected on the basis of the van der Waals radii as shown in
Fig. 1. There is a triple bond between C(19) and C(20) and
double bonds between C(4) and C(5), C(9) and C(10), C(12)
and C(13), C(2) and O(1), and between C(11) and O(2). The
phenylacetylene and cyclohexa-2,5-dienone moieties are both
planar to within 0.05 Å. Long bonds between C(3) and C(8)
[1.61(1) Å] and between C(7) and C(8) [1.57(1) Å] reflect the
strain about the quaternary atom C(8).
Experimental
Mps were determined on a Kofler hot-stage and are uncor-
rected. Column chromatography was carried out on Merck
Kieselgel 60 (230–240 mesh). IR spectra were recorded on a
Digilab FTS-80 spectrometer, and NMR spectra were deter-
1466
J. Chem. Soc., Perkin Trans. 1, 1997

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allowed to warm to Ϫ10 ЊC, at which temperature it was stirred
for 30 min; it was then diluted with chloroform (10 cm3)
at Ϫ10 ЊC. Lead tetraacetate (2.03 g, 4.58 mmol) and mercuric
acetate (0.15 g, 0.46 mmol) in chloroform (20 cm3) were added
to the solution after which it was stirred at Ϫ10 ЊC for 15 s. The
substrate (4.17 mmol) in chloroform (3 cm3) (for 1 and 5) or
tetrahydrofuran (8 cm3) (for 6) was added to the solution
which was then stirred at Ϫ10 ЊC for 1 h, followed by 2 h at
room temperature. The mixture was finally poured into diethyl
ether (200 ml), filtered through Celite, and evaporated to give
the crude product.
The following compounds were obtained by the above
method.
(i) Ethyl 1-(oct-1-ynyl)-2-oxocyclopentanecarboxylate 12.
Separated by flash chromatography in ethyl acetate–light pet-
1
roleum (7:95) as a colourless oil (0.57 g, 52%). The H NMR
and IR specroscopic data were identical with those obtained
previously.1
(ii)
2-Acetyl-2-(oct-1-ynyl)-3,4-dihydronaphthalen-1(2H )-
one 8. Obtained by flash chromatography in ethyl acetate–light
petroleum (1:19) as an oil (0.64 g, 52%). The 1H NMR and IR
spectroscopic data were in accord with literature values.1
(iii) 2-Methyl-2-nitrodec-3-yne 10. Separated by flash chro-
matography in ethyl acetate–light petroleum (3:97) as an oil
Fig. 1 X-Ray molecular structure of spirocyclohexa-2,5-dienone 16
(with atomic numbering used in the crystallographic data)
mined with SiMe4 as internal standard on Bruker AMX-400
and AC-200B spectrometers. J Values are given in Hz. Micro-
analyses were performed by the microanalytical unit of the
School of Chemistry, University of New South Wales, and
mass spectra were recorded on an AEI model MS902 double
focusing instrument.
1
(0.34 g, 41%). The H NMR and IR spectroscopic data were
identical with those previously reported.10
Oxidation of ethyl 2-oxocyclopentanecarboxylate 1 by LTA in
the presence of ZnCl2
Lead tetraacetate (2.42 g, 5.46 mol) in chloroform (10 cm3) was
added to a solution of ethyl 2-oxocyclopentanecarboxylate
(1.53 g, 9.83 mmol) in chloroform (10 cm3) and tetrahydrofuran
(10 cm3) at Ϫ10 ЊC, and the mixture was stirred at Ϫ10 ЊC for 1
h, followed by 2 h at room temperature. The mixture was then
poured into diethyl ether (200 cm3), filtered through Celite and
evaporated. The crude material was fractionated by flash chro-
matography (ethyl acetate–light petroleum, 2:23) to yield the
dimer 4 as a colourless oil (1.025 g, 67%) (Found: C, 61.7; H,
7.4. C16H22O6 requires C, 61.9; H, 7.1%); δH (CDCl3) 4.29 (4 H,
q, J 7.1, 2 × CO2CH2CH3), 2.86–2.72 (2 H, dt, J 14.1, 10.4, 3-H
and 3Ј-H), 2.71–2.27 (6-H, m, 3-H, 3Ј-H, 5-H2 and 5Ј-H2), 2.26–
2.07 (4-H, m, 4-H2 and 4Ј-H2) and 1.32 (6 H, t, J 7.1,
2 × CO2CH2CH3); δC(CHCl3) 205.9 (C-2 and C-2Ј), 166.9
(2 × CO2Et), 69.4 (C-1 and C-1Ј), 62.7 (2 × CO2CH2CH3),
38.1 (C-3 and C-3Ј), 35.0 (C-5 and C-5Ј), 18.8 (2 × CH3) and
13.6 (C-4 and C-4Ј); νmax(CHCl3)/cmϪ1 1768 and 1723; m/z 164
(M Ϫ 2 × CO2Et, 13%), 162 (26), 145 (13), 135 (25), 134 (18),
132 (14), 123 (13), 122 (26), 120 (20), 117 (28), 109 (66), 108
(34), 107 (76), 99 (32), 89 (28), 82 (41), 81 (26), 73 (20) and 55
(100).
General method for the reaction of carbon nucleophiles with
phenylethynyllead triacetate generated in situ by Zn–Pb
exchange
Phenylacetylene (0.51 g, 5.0 mmol) was dissolved in dry tetra-
hydrofuran (5 cm3). Butyllithium solution (2.0 ; 2.5 cm3, 5.0
mmol) in hexane was added at Ϫ78 ЊC to the mixture which was
then stirred for 20 min under nitrogen. Zinc chloride (0.68 g, 5.0
mmol) was added to the mixture after which it was allowed to
warm to Ϫ10 ЊC; after being stirred at Ϫ10 ЊC for 30 min the
mixture was diluted with chloroform (20 cm3) at Ϫ10 ЊC. Lead
tetraacetate (2.03 g, 4.58 mol) in chloroform (20 cm3) was added
to the mixture after which it was stirred at Ϫ10 ЊC for 15 s. The
substrate (4.17 mmol) in chloroform (4 cm3) (for 1 and 5) or
THF (8 cm3) (for 6) was added to the solution which was then
stirred at Ϫ10Њ for 1 h, followed by 2 h at room temperature. The
mixture was then poured into diethyl ether (200 cm3), filtered
through Celite and evaporated to give the crude product.
The following compounds were synthesised by the above
method.
(i) Ethyl 2-oxo-1-(phenylethynyl)cyclopentanecarboxylate 3
(0.78 g, 73%). Separated by HPLC (Whatman Partisil 10 M20)
in ethyl acetate–light petroleum (1:49) as a colourless oil. It had
1H NMR and IR spectroscopic data in accord with literature
values.1
Reaction of 2,4,6-trimethylphenol (mesitol) 13 with
phenylethynyllead triacetate produced by Zn–Pb exchange
Butyllithium in hexane (2.0 ; 3.5 cm3, 7.06 mmol) was added
to phenylacetylene (0.72 g, 7.06 mmol) in dry THF (7.0 cm3) at
Ϫ78 ЊC, and the mixture was stirred for 20 min under N2. Zinc
chloride (0.96 g, 7.06 mmol) was added to the solution which
was then allowed to warm to Ϫ10 ЊC, at which temperature it
was stirred for 30 min. Chloroform (25 cm3) was added to
the mixture followed by LTA (2.84 g, 6.42 mmol) in chloroform
(25 cm3); the mixture was then stirred at Ϫ10 ЊC for 15 s. Mesi-
tol 13 (0.80 g, 5.88 mmol) in chloroform (6 cm3) and pyridine
(1.01 g, 12.8 mol) were then added to the mixture after which it
was stirred at Ϫ10 ЊC for 1 h followed by 3 h at room temper-
ature. After this the mixture was poured into diethyl ether (200
cm3), filtered through Celite, and the filtrate washed in turn with
dilute sulfuric acid (1.0 M; 2 × 100 cm3), water (2 × 100 cm3)
and saturated brine (100 cm3) and then dried (Na2SO4) and
evaporated. The residue was chromatographed on a column
of silica gel in ethyl acetate–light petroleum (1:19) to yield
(ii) 2-Acetyl-2-(phenylethynyl)-3,4-dihydronaphthalen-1(2H )-
one 7. Separated as an oil (0.71 g, 59%) by flash chromato-
1
graphy in ethyl acetate–light petroleum (1:19). The H NMR
and IR spectra were identical with those obtained previously.1
(iii) 3-Methyl-3-nitro-1-phenylbut-1-yne 9. Obtained as an oil
(0.46 g, 58%) by flash chromatography in ethyl acetate–light
1
petroleum (1:19). The H NMR and IR spectra correspond
with those obtained previously.1
General method for the reaction of carbon nucleophiles with oct-
1-ynyllead triacetate in situ by Zn–Pb exchange
Oct-1-yne (0.55 g, 5.0 mmol) was dissolved in dry tetrahydro-
furan (5 cm3). A solution of butyllithium (2.2 ; 2.25 ml, 5.0
mmol) in hexane was added at Ϫ78 ЊC to the mixture which
was then stirred for 20 min under nitrogen. Zinc chloride
(0.68 g, 5.0 mmol) was added to the solution after which it was
J. Chem. Soc., Perkin Trans. 1, 1997
1467

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2,4,7,9,13-pentamethyl-9-phenylethynyl-7,10-ethenospiro[5.5]-
undeca-1,4-diene-3,8-dione 16 (0.89 g, 41%) as pale yellow crys-
tals (from ethyl acetate–light petroleum), mp 132–133 ЊC
(Found: C, 84.4; H, 7.2. C26H26O2 requires C, 84.3; H, 7.1%);
δH (CDCl3) 0.98 (3 H, s, 7-Me), 1.55 (3 H, s, 9-Me), 1.80 (1 H,
dd, J 14.2, 3.0, 11-H), 1.85 (3-H, d, J 1.5, 4-Me), 1.89 (3 H, J
1.0, 2-Me), 2.00 (3-H, d, J 1.7, 13-Me), 2.70 (1 H, dd, J 14.2,
2.5, 11-H), 2.83 (1 H, m, 10-H), 5.47 (1 H, dq, J 1.7, 1.2, 12-H),
6.46 (1 H, dq, J 3.3, 1.0, 1-H), 6.82 (1 H, dq, J 3.3, 1.5, 5-H),
7.28–7.35 (3 H, m, phenyl-H) and 7.39–7.43 (2 H, m, phenyl-
H); δC(CDCl3) 13.6 (7-Me), 16.1 (9-Me), 16.4 (9-Me or 13-Me),
21.8 (4-Me), 25.3 (2-Me), 33.6 (C-11), 44.4 (C-6), 48.0 (C-9),
shifts < 0.07σ) with R † 0.052, Rw 0.051 and w = 2.19/
2
[σ2(Fo) ϩ 0.000 11Fo ]. Maximum excursions in a final differ-
ence map were ϩ0.2 e ÅϪ3 and Ϫ0.2 e ÅϪ3. Scattering factors
and anomalous dispersion terms used were those supplied in
SHELX-76.13 All calculations were carried out using SHELX-
7613 and plots were drawn using ORTEP.14 The atom number-
ing scheme is given in Fig. 1. Atomic coordinates, bond lengths
and bond angles, and thermal parameters have been deposited
at the Cambridge Crystallographic Data Centre. See Instruc-
tions for Authors (1997), J. Chem. Soc., Perkin Trans. 1, 1997,
Issue 1. Any request to the CCDC for this material should
quote the full literature citation and the reference number
207/99.
᎐
᎐
48.4 (C-10), 55.5 (C-7), 84.4 (C᎐C), 90.7 (C᎐C), 122.6 (phenyl
᎐
᎐
C-1), 124.1 (C-12), 128.3 (phenyl C-4), 128.4 (phenyl C-3 and
C-5), 131.5 (phenyl C-2 and C-6), 134.5 (C-13), 135.2 (C-2),
146.1 (C-4), 147.9 (C-1), 148.0 (C-5), 186.7 (C-3) and 208.0
(C-8); νmax(CHCl3)/cmϪ1 1727, 1686, 1633, 1491, 1449, 1378 and
1231; λmax(EtOH)/nm 254 and 243 (ε/dm3 molϪ1 cmϪ1 26 480
and 27 690); m/z (370, 1%) 236 (62), 235 (30), 221 (54), 207 (25),
193 (85), 192 (50), 191 (30), 179 (18), 178 (64), 165 (31), 135
(33), 134 (100), 115 (24), 106 (20), 105 (32), 91 (94), 77 (35), 65
(26) and 63 (20).
Acknowledgements
This work was supported by a grant from the Australian
Research Council. C. J. P. gratefully acknowledges receipt of an
Australian Postgraduate Award.
¹
2

2
²
† R = Σ(||Fo| Ϫ |Fc||)/Σ|Fo|, Rw = [Σw(|Fo| Ϫ |Fc|) /Σwo ] .
References
Crystal structure analysis of the spirocyclohexa-2,5-dienone 16
For diffractrometry a crystal was mounted on a glass fibre with
cyanoacrylate resin. Lattice parameters at 21 ЊC were deter-
mined by a least-squares fit to the setting parameters of 25
independent reflections, measured and refined on an Enraf-
Nonius CAD4F four-circle diffractometer employing graphite
monochromated Mo-Kα radiation.
Crystal data. Formula C26H26O2; M, 370.49, monoclinic,
space group P21/n, a 12.979(4), b 9.423(4), c 18.129(3) Å; β
108.32(2)Њ, V 2104.8(9) Å 3, Z 4, Dc 1.169 g cmϪ3, µ(Mo-Kα)
0.39 cmϪ1, λ(Mo-Kα) 0.7107 Å, F(000) 792 electrons.
Data collection and processing. Intensity data were collected
in the range 1 < θ < 22.5Њ using an ω–θ scan. The scan widths
and horizontal counter apertures employed were (1.00 ϩ 0.35
tan θ)Њ and (2.70 ϩ 1.05 tan θ) mm respectively. Data reduction
and application of Lorentz and polarisation corrections were
carried out using the Enraf-Nonius Structure Determination
Package.11 Of the 3060 reflections collected, 889 with
I > 2.5σ(I ) were considered observed and used in the
calculations.
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11 Enraf-Nonius Structure Determination Package, Enraf-Nonius,
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Structure analysis and refinement. The structure was solved
by direct methods using SHELXS-8612 and the solution was
extended by difference Fourier methods. The phenyl group was
included as a rigid group (C᎐C 1.395 Å) with isotropically
refined thermal parameters, hydrogen atoms were included at
calculated sites (C᎐H 0.97 Å) and all other atoms were refined
anisotropically. The use of these constraints allowed for a satis-
factory refinement despite the small number of observed reflec-
tions available from the small and weakly diffracting crystals.
Full-matrix least-squares refinement of an overall scale
factor, positional and thermal parameters converged (all
12 G. M. Sheldrick, in Crystallographic Computing 3, eds G. M.
Sheldrick, C. Kruger and R. Goddard, Oxford University Press,
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13 G. M. Sheldrick, SHELX-76, A Program for Crystal Structure
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14 C. K. Johnson, ORTEP, A Thermal Ellipsoid Plotting Program,
Oak Ridge National Laboratories, Oak Ridge, 1965.
Paper 6/08582B
Received 23rd December 1996
Accepted 29th January 1997
1468
J. Chem. Soc., Perkin Trans. 1, 1997