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Azonia Derivatives of Arenes Synthesis and Properties of 10c-Azoniafluoranthene.

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8
J = 14.47, 11.01 Hz, lH,olefinic),6.35(dd, J = 15.22, 10.55Hz, lH,olefinic),
6.19 (dd, J = 14.46Hz. 10.41 Hz, IH, olefinic), 6.03 (t. J = 10.90Hz. l H ,
olefinic),5.72 (dd. J = 15.05. 7.05 Hz, l H , olefinic), 5.42(m,2H, olefinic), 5.39
(m, lH,oIefinic),4.11 (m, lH,CH-OH), 3.68(m, lH,CH-OH), 3.64(s, 3H,
COOCH,), 2.91 (t, J = 7.12 Hz, 2H, his-allylic), 2.33 (t. J = 6.70 Hz, 2H,
CH,COOCH,), 2.15 (s, 2H, OH), 2.03 (4, J = 7.02 Hz, 2H, allyhc), 1.23 (m,
10H. CH,), 0.86 (t. J = 6.47 Hz, 3H, CH,).
Hydrogenution u f 4 10 give S4: To a 25-mL round-bottom flask equipped with
a magnetic stirrer and filled with argon was placed a solution of 4 (40 mg,
0.069mmol) in distilled methanol (2mL) and 10% PdjC (Xmg, 20% by
weight). The argon was replaced with hydrogen and the reaction mixture was
vigorously stirred under hydrogen for 18 h while monitored by TLC. The hydrogen was removed and the catalyst was filtered off through a celite pad. The
solution was concentrated and subjected to flash column chromatography (silica, 10% petroleum ether in ether), furnishing saturated 4 as a white waxy solid
(19.8 mg, XO%). 4: R, = 0.47 (silica,ether); [a:" + 0.55"(c = 0.55, CHCI,). MS
calculated for C,,H,,O,, 35X.131; found, 376.343 ( M + NH,): IR (CHCI,):
C,,,,,, = 3420,3020.2930.2858,1730.1467.1440,1218,755,670cm~'. 'H NMR
(CDCI,, 500 MHz): 6 = 3.65 fs, 3H, COOCH,), 3.57 (m, 2H. CH-OH), 2.35 (t.
J = 7.36Hz,2H,CH,COOCH,),l.84(s,
lH,OH),1.58(s,1H,OH),1.47-1.42
(m, 5H, CH,), 1.23 (s. 25H. CH,), 0.86 (t, J = 6.77 Hz, 3H, CH,).
9
Ih
SiMe,
SiMe,
I1
10
12
2
OSitBuMe,
Received: December 8, 1988;
revised: February 6, 1989 [Z3081 IE]
German version: Angew. Chem. 101 (1989) 621
OSitBuMe,
I
O\ LCOOMe
IA
COOMe
OSitBuMe,
OSitBuMe,
13
3
I
s4
Scheme 3. Total synthesis of 4 and S4. a) TosNHNH, (2.0 eq.)/NaOAc
(3.0eq.). THF:H,O ( l : l ) , reflux, 4h. 54%. b) HCEC-SiMe, (1.0eq.).
Pd(PPh,), (0.04eq.)/CuI (0.16eq)/Et2NH, 0°C. lh, 90%. c) PPh, (1.3 eq.)/
CBr,(1.3eq.),CH2C1,, -3O"C,2.5h,Yl%.d) PPh3(1.2eq.),CH,CN,36h,
reflux, 92%. e) KN(SiMe,), (1.05eq.), THF, -78-0"C, 1 h, then -78"C,
hexanal (1.2 eq.), 2 h, 82% (ci.s:truns ca. 4:l). 0 Chromatography (SiO,, petroleum ether). g) KCN (7.0eq.)/AgN03(4.0eq.). THF:EtOH:H,O (1 :1 :l),
O'C, 2 h, 79%. h) 1,3,2-Benzodioxaborole ("catecholborane") (5.0 eq.),
benzene, 75 "C. 28 h, 55%. i) CrC1, (6.0 eq.)/CH,I (2.0 eq.), THF, 0 "C, 8 h.
70%. j) 2 (1.4eq.). Pd(PPh,), (0.25 eq.)/TlOH (4.4eq.). THF:hexane:H,O,
25°C. lh, 55%. k) H,-Pd/C, MeOH, 25°C. 5h. 80%. 1) nBu,NF (2.2eq.),
T H E 25'C. 3 h. m) CH,N,, Et,O, O'C, 0.5 h, 85%.
sodium chloride solution (15 mL). The organic phase was dried (MgSO,) and
filtered through a celite pad. concentrated, and subjected to flash column chromatography (silica, 3% ether in petroleum ether), furnishing 14 as a colorless
oil (82 mg, 5 5 % ) . 14: R, = 0.31 (silica, 3% ether in petroleum ether); [a];'
+2.93" ( c = 0.48, CHCI,). MS' calculated for C,,H,,O,Si,, 578.418; found,
579.426 ( M + 1). IR (neat): CmaX = 3010, 2940, 2860, 1755, 1470. 1440, 1358.
1267.1170.1112,1000.840,780cm~'.UV(CH,OH):i,., = 205+262(sh),274,
285 (sh) nm. ' H NMR (CDCI,, 500 MHz): 6 = 6.45 (t, J = 12.59 Hz, lH,
olefinic), 6.16 (m, 2H, olefinic), 6.01 (t, J = 10.88 Hz, lH, olefinic). 5.61 (dd,
J = 15.38, 7.49 Hz, lH, olefinic), 5.39 (m, 3H. olefinic), 3.98 (t. J = 4.32 Hz,
IH, CH-OSi), 3.67 (s, 3H, COOCH,), 3.56 (4, J = 3.05 Hz, 1H. CHOSi), 2.92
( t , J = 7.30 Hz, 2H. his-allylic), 2.25 (t, J = 7.52 Hz. 2H, CH,COOCH,), 2.05
(4, J = 6.92 Hz, 2H, allylic). 1.55 (m, 2H, CH,), 1.28 (m. XH, CH,), 0.88 (m,
21H, 2xSi ~ Bu,CH,), 0.04 (s, 12H, SiMe,).
De.si/.vlution9/14 to give 4: Compound 14 (76 mg, 0.1 3 mmol) was azeotropically dried with benzene and dissolved in dry T HF (3.0 mL). The magnetically
stirred solution was treated with tetra-n-butylammoniuni fluoride (I M solution
in THF. 300 gL,0.300 mmol) at 25 "C. Stirring was continued for 3 h at RT
while the reaction was monitored by TLC. The solution was then diluted with
ether (100 mL) and washed with pH 6 phosphate buffer (1 mL). The organic
layer was separated and washed with saturated sodium chloride solution
(2 mL), dried (MgSO,), and concentrated. Ether (25 mL) was added followed
by cooling to 0 "C and diazomethane treatment to give, after concentration, the
crude methyl ester, which was subjected to flash column chromatography (silica, ether), furnishing pure 4 as a colorless oil (38.6 mg, 85%). 4: R, = 0.52
(silica. ether); [a]:" +0.91" (c = 0.44, CHCI,). MS: calculated for C,,H,,O,,
350.246: found, 333.243 (M-OH). IR (neat): I;,,, = 3385, 3000, 2910, 2825.
1728, 1425, 1240, 1156, 1072, 988, 905, 722cm-'. UV (CH,OH): L,,, = 320.
283 (sh), 274. 261 (sh) 205 nm. 'HNMR (CDCI,, 500 MHz): 6 = 6.52 (dd.
588
c> VCH ~/er/ug.s~e.~L~llschu~
mbH, D-6940 Weinhrim, 1989
[I] Isolation: a) J. Haeggstrom, J. Meijer, D. Radmark, J. B i d . Chem. 261
(1986) 6332; b) P. Borgeat, B. Samuelsson. ibid. 254 (1979) 7865; c) J.
Haeggstrom. A. Wetterhoim. M. Hamberg, J. Meijer, R. Zipkin, 0. Radmark, Biuchim. Biophys. Actu 958 (1988). 469. Synthesis: d) J. Adams, B. J.
Fitzsimmons, Y Girard, J. F. Evans, J. Rokach, J. Am. Chem. Soc. 107
(1985) 464.
[2] C. N. Serhan, M. Hamberg, B. Samuelsson, Proc. Null. Acad. Sci. U S A 81
(1984) 5335.
[3] J. Uenishi, J.-M. Beau, R. W. Armstrong, Y Kishi, J. A m . Chem. SOC.109
(1987) 4756. and references cited therein.
141 For other examples of borane-vinyl iodide coupling, see: a) W R. Roush,
R. Riva, J. Org. Chem. 53 (1988) 710; b) F. Haviv, J. D. Ratajczyk, R. W.
DeNet. Y C. Martin, R. D. Dyer. G . W. Carter, J. Med. Chem. 30 (1987)
254; c) G. Cassani, P. Massardo, P. Piccardi, Tarahedron Let/. 24 (1983)
2513; d) N. Miyaura. A. Suzuki. J. Orgunomel. Chem. 213 (1981) 653.
IS] K. C. Nicolaou, C. A. Veale. S. E. Wehber, H. Katerinopoulos, J. Am.
Chem. Soc. 107 (1985) 7515, and references cited therein.
[6] N. Miyaura, H. Suginome, A. Suznki. Tetrahedron 39 (1983) 3271.
[7] T. Kazuhiko, K. Nitta, K. Utimoto, J. Am. Chem. SOC.108 (1986) 7408.
181 Small amounts of the &-lactonecorresponding to 4 were also formed in this
reaction. Conversion of this lactone to 4 was easily accomplished by exposure to Et,N in methanol.
191 All new compounds exhibited satisfactory spectroscopic and analytical and/
or exact mass data. Yields refer to chromatographically and spectroscopically homogeneous materials.
Azonia Derivatives of Arenes: Synthesis
and Properties of l0c-Azoniafluoranthene **
By Marc Fourmigut?,* Kamal Boubekeur, Patrick Batail,
and Klaus Bechgaard
Radical cation salts of arenes such as naphthalene and
fluoranthene exhibit an extremely narrow solid-state EPR
line (10-20 mG, compared to 1.35 G for DPPH"'). This is
believed to be due to the strongly one-dimensional nature of
the conducting electrons and related to the metallic behavior
arising from a strict 2: I stoichiometry of the mixed-valent
(aryl),X salts.'', 31 Promising applications of these compounds as magnetic field probes are restricted, however, because of their lack of
[*I
[**I
Dr. M. Fourmigui, K. Bonbekeur, Dr. P. Batail
Laboratoire de Physique des Solides associt au CNRS,
Universite de Paris-Sud
F-91405 Orsay (France)
Prof. Dr. K . Bechgaard
H. C. 0rsted Institutet
DK-2100 K~benhavn(Denmark)
This work was supported by the University of Copenhagen, CNRS, and
ANVAR (Paris).
0570-0833189/0505-0588 $0Z.S0/0
Anger. Chem. Int. Ed. Engl. 28 (1989) No. 5
..
~
..
Heteroatom substitution at appropriate positions in the
carbon framework of the arene molecule is expected to enhance the stability of the corresponding radical species. We
therefore investigated the closed-shell cation 1 e, which is
isoelectronic with fluoranthene. We report here the synthesis
and characterization of le and demonstrate that its reduction affords the neutral radical lo, which is isoelectronic
with the fluoranthene radical anion and was characterized by
EPR and cyclic voltammetry experiments.
concomitant with the intramolecular quaternization, leads
directly to the salt 5. Reaction of 5 with the anhydrous glyoxa1 equivalent 619]in the presence of an equimolar amount of
triethylamine allows the double condensation and simultaneous elimination to give the fully aromatized bromide salt
l e [Eq. (b)].
5
The cation le is especially attractive for the following
reasons (Fig. 1): (1) the crisscross mode of overlap of 2: 1
fluoranthenium salts, which actually minimizes the intradimer van der Waals interactions, should be preserved in lo in
the solid state and in (1),X salt^;[^,^^ (2) there is essentially
no odd-electron density in the SOMO of l o at the nitrogen
atom. Therefore, the hyperfine interaction with the nitrogen
atom ( S = 1) is minimal and consequently no EPR line broadening should occur in the solid because of the replacement
of C by N.
Fig. 1. Left: The overlap between adjacent molecules in the fluoranthenium
salts [ S ] . Right: The electronic density in the fluordnthene LUMO. Radii are
proportional to the coefficients of the LUMO [16].
The cation l @was synthesized as follows: The salt 5, with
two potentially nucleophilic sites available for a Westphal
condensation with an a-dioxo compound, was selected as
Compound 5 was
starting material for the synthesis of 1@.i61
obtained in two steps [Eq. (a)],"' starting with the known
addition of alkyl or aryl Grignard reagents to pyridine N-oxide.'*]
6
Cyclic voltammetry experiments performed in acetonitrile
(NBu,PF,, 0.1 M) demonstrate that le is reduced at
E,,, = -0.9 V versus SCE. Sweep-rate dependance of the
voltammogram indicates a quasi-reversible behavior.[lo1
This is in contrast to most of the pyridinium-like cations
(e.g., N-methylpyridinium, -quinolinium, -acridinium,
-quinolizinium, and acridizinium) in which reduction to the
neutral radical is irreversible unless the cation carries an
electron-attracting substituent at the position of highest electron density in the LUM0.L'
The neutral radical generated by reduction of le in acetonitrile solution on Na/Hg amalgam is persistent for several
hours when kept under oxygen-free conditions. The EPR
solution spectrum gives five equally spaced, rather broad
lines with an apparent hyperfine splitting (a,) of 5.17 G. This
spectrum is similar to that of the isoelectronic fluoranthene
radical anion (aH3= 5.24, aH1= 3.93, aH8= 1.2 G["]) when
line broadening occurs. Attempts to isolate the neutral radical lo in the solid state have been so far unsuccessful.
In order to characterize this new heterocyclic structure
more completely, we also carried out an X-ray crystal structure investigation of 1 . PF,. The cation is essentially planar.
The single salient structural difference with fluoranthene is a
significant contraction of the bond lengths around the nitrogen atom (Fig. 2).
c2
f
&
+I1
c2*
-12
+I1
-9
-11
-10
2
3
4
5
(a)
The use of the ortho-substituted bromobenzene 3 gives the
pyridine 4, isolated in 40% yield after a dehydration step
with acetic anhydride. Hydrolysis of 4 in concentrated HBr,
Angew. Chtm. I n l . Ed. EngI.28 (1989)
No.5
-20
-10
Fig. 2. Molecular structure of 1 . PF, [I 31. Ellipsoids are scaled to enclose 50 %
of the electronic density. Significant bond distances [A] and angles ['I: N-C1
1.378(5), N-C5 1.384(4), C1-C2 1.402(4). C2-C3 1.358(5), C3-C4 1.400(5),
C4-C5 1.358(5), CS-C6 1.452(4), C6-C6* 1.402(5), C6-C7 1.395(5), C7-C8
1.381 ( 5 ) . C8-C8* 1.396(6); C1-N-CS 123.7(2), CS-N-CY 112.6(3), N-Cl-C2
115.8(2), C2-Cl-C2* 128.3(4), CI-C2-C3 121.0(3), C2-C3-C4 121.7(3), C3C4-C5 118.4(3), C4-C5-N 119.4 (3), N-CS-C6 105.6(3), C4-C5-C6 135.0(3),
C5-C6-C6* 108.1(3). C5-C6-C7 130.8 (3). C6*-C6-C7 121.1 (3). C6-C7-C8
117.4(3), C7-CS-C8* 121.5(3). Right: Bond-length variations from fluoranthene to l a (in
A) 1151.
Q VCH Verlugrgeseilschufr inhH. 0-6940 Weinheim.1989
0570-0X33iR9j0505-0SR9 $02.5OjO
589
Experimental Procedure
4: A T H F solution of 2-methylpyridine N-oxide (75.3 g, 0.69 mol) [151 was
added dropwise to a THF solution of 3, prepared from the corresponding
ortho-substituted bromobenzene (138.7 g, 0.69 mol, Mg (16.8 g, 0.69 mol), and
EtBr(18.8 g.0.17 mo1)indryTHF.Themixturewasstirredca. 12 h. Hydrolysis
yielded a pasty white precipitate. The T H F solution was decanted and the
precipitate washed with CHCI,. The combined solutions were washed with
0.1 N NaOH followed by 0.1 N HCI and then evaporated. The neutral clear oil
was boiled with 500 mL of acetic anhydride for 3 h. After evaporation of the
anhydride, the dark residue was treated with 200 mL of 0.1 N NaOH and extracted with toluene three times. The combined solution was extracted with 2 N
HCI and the acidic solution cooled and basified with concentrated NaOH. The
supernatant oil was extracted with ether and the solution was dried with MgSO,
and evaporated. Vacuum distillation yielded a slightly fluorescent oil (60 g;
41%), b.p. 114-116"C(0.5torr). 'HNMR(CDCI,.60 MHz.TMS):b =2.55
(s, 3H, Me-C), 3.3 (3H. s, OMe). 4.55 (2H, s. CH,). 7.0-7.7 (7H, m, aryl H).
Correct C,H,N elemental analysis.
A Joint Structural Characterization of Colloidal
Platinum by EXAFS and High-Resolution Electron
Microscopy
By Daniel G. Duff,Peter P. Edwards*, John Evans,
J. Trevor Gaunilett, David A . Jejyerson, Brian I;: G. Johnson,
Angus I. Kirkland, and David J. Smith
The structures and morphologies adopted by small metal
particles continue to attract widespread interest. Of particular importance in this regard are the structures of particles of
metals such as Pd, Pt, Ru, and Cu, which find extensive
application in commercial catalysis.
To date, the most successful techniques for probing the
structure of such species have been bright- and dark-field
transmission electron microscopy (TEM)". to yield infor5: A solution of 4 (60 g, 0.28 mol) in 48 % HBr (500 mL) and AcOH (500 mL)
mation about the morphology and surfaces in projection,
was refluxed for 4 h. Evaporation and recrystallization from EtOH yielded 5 as
coupled with electron diffraction data from larger particles
its bromide (47.4 g, 64%), m.p. 245-248°C. 'H NMR (CF,CO,H. 60 MHz,
TMS):d = 3.0(3H,s,Me),5.75(2H,s,CH,),7.7-8.6(7H,m,arylH).UV/VIS to yield unit-cell dimensions and orientation, and high resolution electron microscopy (HREM) to yield precise infor(ELOH): I[nm] = 252(1g&= 4.08), 318(4.02). Correct C,H.N analysis.
mation on the atomic arrangement and the nature of defect
1°: A solution of 5 (13 g, 0.05 mol), 6 (7 g, 0.05mol), and Et,N (7mL.
structures such as twin planes and stacking f a u l t ~ . [ ~How-~1
0.05 mol) in EtOH was refluxed for 1 h, then cooled, evaporated, and precipiever, when faced with the task of characterizing particles
tated by addition of Et,O. The dark crystalline product was recrystallized twice
from EtOH/AcOEt (1: 1) and then from dimethyl sulfoxide (DMSO) (yield of
from a colloidal solution, special problems arise. Although
1 . Br: 4.7 g, 33%). Addition of 70% HPF, to an ethanolic solution of 1 . Br
the homogeneity of the sol may be reliably ascertained by
resulted in the precipitation of a PFF salt, which was recrystallized from
examining more than one specimen grid and a number of
acetonitrile as transparent needles. 'H NMR ([D6]DMS0. 90 MHz, TMS):
grid squares within each, there still remains the possibility
6 = 9.5(2H,d),8.7-9.2(6H,m),S.l5(2H,pseudoq).
'3CNMR([D,]DMS0.
22.6 MHz, TMS): 6 = 139.17, 137.73, 136.43, 131.75, 129.80, 126.35, 123.04,
that what is imaged in the electron microscope may not be
120.83. UVjVIS (EtOH):
[nm] = 220(Ig&= 4.58), 240(4.15), 252(4.16),
the same as the species present in solution. Electron micros272 (4.4), 295 (3.96), 306(sh, 3.89), 343 (sh, 3.77), 358 (3.92), 377 (3.86). Correct
copy necessitates desiccation of the sample followed by elecC,H,N analysis.
tron irradiation : indeed, Thomas, Millward, and HarritnanI6l
have noted beam-induced reduction of iridium dioxide colloids, and preliminary results of our own suggest that for
Received: December 7, 1988 [Z 3077 IE]
very small particles this may happen too quickly to be obGerman version: Angew. Chem. 101 (1989) 607
served in some systems.
Hence we require an ensemble technique capable of operating on solutions to confirm the structural information provided by electron microscopy. Visible spectrophotometry
[1] DPPH = a,a'-diphenyl-P-picrylhydrazyl, a stable free radical used as an
EPR reference.
provides only limited and qualitative information con[2] V. Enkelmann, Adv. Chem. Ser. 217 (1988) 177.
cerning particle size and degree of aggregation. However,
[3] G. Sachs, E. Dormann, M. Schwcerer, Solid State Commun. 53 (1985) 73.
EXAFS provides exactly the required information on the
[4] E. Dormann, G. Sachs, W. Stocklein, B. Bail, M. Schwaerer, Appl. Phys.
particle structure with limited perturbation of the system
A 3 0 (1983) 227.
[5] V. Enkelmann, B. S. Morra, C. Krohnke, G. Wegner, J. Heinze, Chem.
and, although not ideal where long-range order is concerned,
Phys. 66 (1982) 303.
is especially useful in the regime of small particle size (30-8,
[6] M. Jsrgensen, T. Bjsrnholm, K. Bechgaard, Proc. Int. Con/: Sci. Techno/.
diameter and below). It is thus ideally complementary to an
Synth. Met. (Santa Fe, NM, USA, 1988), Synih. Met. 27 (1988) 159.
electron-microscopic study.
[7] The unsubstituted analogue of 5 has been synthesized by a very different
route which could not be used for the synthesis of 5 : A. Fozard. C. K.
In this communication we therefore report a joint strucBradsher, J. Org. Chem. 32 (1967) 2966.
tural characterization of a platinum sol by EXAFS and
[8] T. Kato, H. Yamanaka, J. Org. Chem. 30 (1965) 910.
HREM. Our results confirm that the H R E M images ob[9] M. C. Venuti, Synthesis 1982, 61.
tained represent an accurate reflection of the structure of the
[lo] All electrochemical criteria for reversibility are satisfied for low sweep rates
(typically below 100 mVs-'); for details, see: A. J. Bard, L. R. Faulkner;
partides present in solution in this case.
Electrochemicut Methods, Wiley, New York 1980, p. 224.
The synthesis is an adaptation of one previously de[ l l ] P. Hanson, Adv. Heterocyc. Chem. 25 (1979) 205.
A greater residual molar ratio of polymer to
112) K . Mobius, M. Plato, Z . Nufu&rsrh. A 22 (1967) 929
[13] 1 'PF,: space group P2,/m with u = 6.280(3), b = 11.338(2),
c=9.736(1)A,fl=97.17(3)",V=688.1 ~ 3 , Z = 2 , @ , , , . d = 1 . 6 8 g c n ~ - 3 .
p = 2.589 cm-', Fooo= 176. Data were collected at room temperature
using graphite-monochromated Mo,, radiation (2 = 0.71073 A) and an
Enraf-Nonius CAD4-F diffractometer. The crystal structure was solved
by direct methods using the Enraf-Nonius SDP program package. Hydrogen atoms with geometrically calculated positions were included in scalefactor calculation but not refined. 950 reflections ( I .> 3 o(0) were used for
112 parameters, R = 0.045, R, = 0.043, GOF = 0.671. Further details of
the crystal structure investigation are available on request from the
Fachinformationszentrum Energie, Physik, Mathematik GmbH.
D-7514 Eggenstein-Leopoldshafen 2 (FRG), o n quoting the depository
number CSD-53732, the names of the authors, and the journal citation.
[14] A. C. Hazell, D. W. Jones, J. M. Sowden, Actu Crystathgr. Sect. 8 3 3
(1977) 1516.
1151 Aldrich reagent grade (98%) was used.
[16] E. Heibronner, P. A. Straub: Hiickel Molecular Orbitals, Springer, Berlin
1966.
590
0 VCH
Verlugsgesellschaft mbH. 0-6940 Weinhelm, 1989
[*] Dr. P.P. Edwards, D. G. Duff, Dr. D. A. Jefferson, Dr. B. F. G. Johnson,
A. 1. Kirkland
Department of Chemistry,
Lensfield Road, Cambridge CB2 1EW (England)
[**I
Prof. J. Evans. Dr. J. T. Gauntlett
Department of Chemistry, The University,
Southampton SO9 5NH (England)
Prof. D. J. Smith
Center for Solid State Science, Arizona State University
Tempe A240587 (USA)
The authors would like to thank the Science and Engineering Research
Council (SERC), the Ernest Oppenheimer Foundation, British Alcdn,
and the B.P. Venture Research Unit for financial support. D.A.J. also
acknowledges NSF grant DMR-8677609 for support at Arizona State
University.
0S70-0833/89jOSO5-059(1$02.50j0
Angew. Chem. Int. Ed. Engl. 28 (1989) No. 5
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