Synthesis and crystal structure of bis[(4S,5S)-4,5-dihydro-4,5-diphenyl-2-(2′-oxidophenyl-χO)oxazole- χN]copper(II) and its application in the asymmetric Baeyer-Villiger reaction

Article abstract of DOI:10.1016/S0022-328X(00)00687-2

Two new oxazolines (3 and 4) and their copper complexes (5 and 6) were synthesized. The four compounds were characterized by elemental analysis and ¹H-NMR, IR, MS spectrometry. The molecular structure of compound 6 was determined by single-crystal X-ray diffraction analysis. The four coordinate atoms [N(1), N(2), O(2), O(4)] and the central metal atom (Cu) form a saddle configuration. Compounds 5 and 6 were found to be moderately effective in the catalytic asymmetric Baeyer-Villiger reaction of 2-phenylcyclohexanone.

Full text of DOI:10.1016/S0022-328X(00)00687-2

www.elsevier.nl/locate/jorganchem
Journal of Organometallic Chemistry 619 (2001) 204–208
Synthesis and crystal structure of bis-
[(4S,5S)-4,5-dihydro-4,5-diphenyl-2-(2%-oxidophenyl-xO)oxazole-xN]
copper(II) and its application in the asymmetric Baeyer–Villiger
reaction
Yungui Peng b, Xiaoming Feng* a,b,1, Kaibai Yu b, Zhi Li b, Yaozhong Jiang* b,2,
Chi-Hung Yeung c
a The Faculty of Chemistry, Sichuan Uni6ersity, Chengdu 610064, People’s Republic of China
b Chengdu Institute of Organic Chemistry, Chinese Academy of Sciences, Chengdu 610041, People’s Republic of China
c Open Laboratory of Chirotechnology and Department of Applied Biology and Chemical Technology, The Hong Kong Polytechnic Uni6ersity,
Hong Kong, People’s Republic of China
Received 18 April 2000; received in revised form 21 July 2000; accepted 23 August 2000
Abstract
Two new oxazolines (3 and 4) and their copper complexes (5 and 6) were synthesized. The four compounds were characterized
by elemental analysis and 1H-NMR, IR, MS spectrometry. The molecular structure of compound 6 was determined by
single-crystal X-ray diffraction analysis. The four coordinate atoms [N(1), N(2), O(2), O(4)] and the central metal atom (Cu) form
a saddle configuration. Compounds 5 and 6 were found to be moderately effective in the catalytic asymmetric Baeyer–Villiger
reaction of 2-phenylcyclohexanone. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Asymmetric catalysis; Crystal structure; Chiral oxazoline; Baeyer–Villiger reaction
1. Introduction
their efficiency in asymmetric catalytic reactions, we
tried to use compounds 5 and 6 in the asymmetric
Baeyer–Villiger reaction. Herein, we report the synthe-
sis of complexes 5 and 6 and the X-ray structure of 6.
The results of the asymmetric Baeyer–Villiger reaction
catalyzed by 5 and 6 are also reported.
Optically active oxazolines as a class of efficient
auxiliaries have been extensively used in catalytic asym-
metric synthesis [1,2]. They can transfer the chirality
from the heterocycle to newly formed bonds, thereby
generating new centers of chirality with high enantiose-
lectivity. These characteristics have attracted large ef-
fort to the design and synthesis of new kinds of chiral
oxazoline ligands [3–8]. Bolm has developed an effec-
tive method to synthesize oxazoline from amino alcohol
[3,4]. Towards this direction, we designed and synthe-
sized novel oxazolines 3 and 4. Complexes 5 and 6 were
also obtained by the reaction of 3 and 4 with copper
acetate (Schemes 1 and 2). As part of our study to test
2. Results and discussion
2.1. Synthesis
Chiral amino alcohols 1 and 2 were prepared on a
large scale [9] and were used as starting materials for
the synthesis of chiral oxazoline ligands. Compounds 3
and 4 were synthesized by the reaction of 1 and 2 with
2-hydroxy-3-tert-butyl benzonitrile in chlorobenzene in
the presence of zinc chloride (Scheme 1) [3,4]. Com-
pounds 5 and 6 were prepared by mixing the oxazolines
3 and 4 with copper acetate in absolute ethanol.
1 *Corresponding author. Fax: +86-28-522 3978; e-mail:
ulas@cioc.ac.cn.
2 *Corresponding author.
0022-328X/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S0022-328X(00)00687-2

Y. Peng et al. / Journal of Organometallic Chemistry 619 (2001) 204–208
205
2.2. Crystal structure of 6
An air-stable single crystal of 6 suitable for X-ray
diffraction analysis was obtained by crystallization
from dichloromethane. The molecular structure of 6
and the adopted numbering scheme are shown in Fig.
1. The stereoplot of the packing of 6 is shown in Fig. 2.
The crystallographic data are given in Table 1. Selected
bond lengths and bond angles are given in Table 2.
Complex 6 crystallizes in orthorhombic space group
P212121 with one independent molecule per asymmetric
unit (Z=4). Analysis of its structure indicates that the
four atoms of the dihydrooxazole N-atoms and pheno-
lato O-atoms in each molecule are located in an almost
square planar arrangement with a great distortion to-
wards a tetrahedron configuration. This is similar to the
configuration of bis[(4S)-4,5-dihydro-4-isopropyl-2-(2%-
oxidophenyl-xO)oxazole-xN]copper(II) [4]. In complex
6, the deviation of the Cu atom from the N1–N2–O2–
Scheme 1.
Scheme 2.
,
O4 plane is −0.0364 A, which is smaller than the
corresponding deviation in bis[(4S)-4,5-dihydro-4-iso-
propyl - 2 - (2% - oxidophenyl - xO)oxazole - xN]copper(II)
,
(−0.146 A) [4]. The angles between O(4)–Cu–N(1)
and O(2)–Cu–N(2) (93.03 and 92.47°) are slightly
larger than comparable angles in bis[(4S)-4,5-dihydro-4-
isopropyl - 2 - (2% - oxidophenyl - xO)oxazole - xN]copper-
(II) (89.9 and 89.6°) [4]. All these indicate that
the degree of distortion toward tetrahedron configura-
tion in complex 6 is greater than in bis[(4S)-4,5-dihy-
dro - 4 - isopropyl - 2 - (2% - oxidophenyl - xO)oxa-
zole-xN]copper(II) [4]. This is ascribed to the steric
hindrance among the bulky tert-butyl, the oxazoline
ring and the substitute phenyl at C4. In complex 6, the
dihedral angle of the N(2)–Cu–O(4) plane with the
N(1)–Cu–O(4) plane is 27.5°, which indicates the great
deviation from planarity of the five atoms [N(1), N(2),
Cu, O(2), O(4)]. Accordingly, it is reasonable to con-
clude that the five atoms form a saddle configuration.
This saddle configuration is also different to the near-
planar geometry of the chiral Schiff base copper com-
plex [(1R,2R) - (−) - N,N% - bis(salicylidene) - trans - 1,2-
cyclohexanediiminato]copper(II) [11], which has the
same coordinating atoms in similar chelations. In com-
plex 6, the dihedral angles between O(3)–C(40)–C(41)–
C(42) and O(1)–C(15)–C(16)–C(17) are 14.6°and 2°,
respectively, which indicate the small deviation from
planarity between the 2-oxidophenyl and 4,5-dihydro-
oxazole ring. The Cu–O distances of 1.909 and 1.9257
Fig. 1. ORTEP plot of the molecular structure and numbering scheme
of 6. Hydrogen atoms are omitted for clarity.
,
A are slightly longer than those reported for bis[(4S)-
4,5 - dihydro - 4 - isopropyl - 2 - (2% - oxidophenyl - xO)-
,
oxazole-xN]copper(II) (1.889 and 1.911 A) [4]. On the
,
other hand, the Cu–N distances (1.920 and 1.932 A)
are slightly shorter than the reported ones(1.941 and
Fig. 2. ORTEP stereoplot of the packing of 6.
,
1.950 A) [4]. The two phenyl rings, which locate in the

206
Y. Peng et al. / Journal of Organometallic Chemistry 619 (2001) 204–208
Table 1
Crystallographic data for 6
plane with the C(26)–C(27)–C(28)–C(29)–C(30)–
C(31) phenyl plane and the C(34)–C(35)–C(36)–
C(37)–C(38)–C(39) phenyl plane are 106.2°and 96.0°,
respectively. The dihedral angle of the two phenyl
rings is 40.1°. In the other oxazoline unit, the dihedral
angles of the N(1)–C(7)–C(8)–O(1)–C(15) oxazoline
plane with the C(9)–C(10)–C(11)–C(12)–C(13)–C(14)
phenyl plane and the C(1)–C(2)–C(3)–C(4)–C(5)–
C(6) phenyl plane are 76.3°and 90.4°, respectively. The
dihedral angle of the two phenyl rings is 68.7°.
Formula
Mn
Color
Space group
Crystal system
Unit cell dimensions
C50H48CuN2O4
804.44
Dark-blue
P212121
Orthorhombic
,
a (A)
12.737(2)
14.050(2)
23.896(3)
4276.3(11)
,
b (A)
,
c (A)
3
,
V (A )
Z
4
2.3. Catalysis
v (mm−1
)
0.557
Crystal size (mm)
Temperature (K)
0.58×0.40×0.36
296(2)
0.71073
1.68–26.00
The Baeyer–Villiger reactions can be used in the
synthesis of many natural products, pharmaceuticals
and monomers for polymerization. During the past two
decades, attempts to study the asymmetric Baeyer–Vil-
liger reactions catalyzed by metal complexes have met
with only moderate success [2,12,13]. Only some special
substrates can be transformed to the corresponding
lactones with high enantiomeric excess [2b,2c]. Most
substrates gave low to moderate enantioselectivity when
they were transformed into lactones [2a,12,13]. In the
asymmetric Baeyer–Villiger reaction of 2-phenylcyclo-
hexanone, the best result (yield 47%, 69% enantiomeric
excess) was obtained by Bolm with bis[(4S)-4-tert-butyl-
2-(3%-tert-butyl-5%-nitro-2%-oxidophenyl-xO)-4,5-di-
hydrooxazole-xN]copper(II) as the catalyst [2a]. In or-
der to enlarge the scope of this reaction, we tested the
use of complexes 5 and 6 in the asymmetric Baeyer–
Villiger reactions using 2-phenylcyclohexanone as a test
substrate (Scheme 3). The results are shown in Table 3.
The results shown in Table 3 indicated that complex
6 was superior to complex 5 (entries 1, 11). A series of
solvents were tested (entries 2–6) and toluene was
found to give the best result (entry 4). Water was found
to be beneficial to the enantioselectivity (entries 1, 2).
The best solvent system was found to be a mixture of
toluene with water (2.5 ml–0.1 ml) (entries 10, 13). We
also investigated the cooxidants and found that the
most effective cooxidant for higher reactivity and enan-
tioselectivity was 2-ethylbutylaldehyde (entries 2, 7–9).
The effects of temperature and amount of catalyst were
also tested. The experiments showed that the reaction
proceeded very slowly when the temperature was lower
than 20°C and the reaction was suppressed when the
catalyst level was over 10 mol%. The best result (entry
,
Wavelength (A)
q range (°)
Scan type
ꢀ
No. of data collected
No. of unique data
hkl range
Rmerge
Flack
Observability criterion n, I\n|(I)
No. of data in refinement
No. of refined parameters
Goodness-of-fit on F2
Final R
6018
5546
−1:15, −1:17, −1:29
0.0119
−0.014(11) [10]
2
5546
521
0.846
0.0373
0.0485
wR
Table 2
,
Selected bond distances (A) and bond angles (°) of 6
Cu–O(2)
Cu–O(4)
1.909(2)
1.9257(19)
1.356(4)
1.298(3)
1.453(4)
1.289(4)
1.284(4)
Cu–N(2)
Cu–N(1)
1.920(3)
1.932(3)
1.463(4)
1.343(4)
1.307(3)
1.472(4)
1.470(4)
O(1)–C(15)
O(2)–C(21)
O(3)–C(33)
N(1)–C(15)
N(2)–C(40)
O(1)–C(8)
O(3)–C(40)
O(4)–C(46)
N(1)–C(7)
N(2)–C(32)
O(2)–Cu–N(2)
N(2)–Cu–O(4)
N(2)–Cu–N(1)
C(15)–O(1)–C(8)
C(40)–O(3)–C(33) 107.3(3)
C(15)–N(1)–C(7) 109.8(3)
C(7)–N(1)–Cu
C(40)–N(2)–Cu
N(1)–C(7)–C(6)
O(1)–C(8)–C(7)
N(1)–C(15)–O(1) 115.2(3)
O(1)–C(15)–C(16) 117.5(3)
O(2)–C(21)–C(16) 123.2(3)
N(2)–C(32)–C(33) 102.9(3)
O(3)–C(33)–C(34) 110.4(3)
N(2)–C(40)–C(41) 126.8(3)
O(4)–C(46)–C(41) 122.2(3)
92.47(12)
90.22(11)
162.94(11)
107.8(3)
O(2)–Cu–O(4)
O(2)–Cu–N(1)
O(4)–Cu–N(1)
C(21)–O(2)–Cu
C(46)–O(4)–Cu
C(15)–N(1)–Cu
158.43(9)
90.64(11)
93.03(11)
127.1(2)
125.5(2)
124.6(3)
125.6(2)
125.4(3)
111.0(3)
103.1(3)
C(40)–N(2)–C(32) 108.9(3)
C(32)–N(2)–Cu
N(1)–C(7)–C(8)
O(1)–C(8)–C(9)
125.6(2)
102.3(3)
111.1(3)
N(1)–C(15)–C(16) 127.3(3)
O(2)–C(21)–C(20) 119.5(3)
N(2)–C(32)–C(31) 113.3(3)
O(3)–C(33)–C(32) 104.2(2)
N(2)–C(40)–O(3) 116.7(3)
O(3)–C(40)–C(41) 116.4(3)
O(4)–C(46)–C(45) 120.2(3)
two sides of the oxazoline ring, take almost perpendicu-
lar orientation with the oxazoline ring. The dihedral
angles of the N(2)–C(40)–O(3)–C(33)–C(32) oxazoline
Scheme 3.

Y. Peng et al. / Journal of Organometallic Chemistry 619 (2001) 204–208
207
Table 3
Asymmetric Baeyer–Villiger reaction of 2-phenylcyclohexanone catalyzed by 6 a
Entry
Catalyst
Solvent
Cooxidant
Time (h)
Conversion (%)
Enantiomeric excess of 7 (%)
1
2
3
4
5
6
7
8
5
5
5
5
5
5
5
5
5
5
6
6
6
C6H6–H2O
C6H6 (dry)
THF (dry)
C7H8 (dry)
CH2Cl2
ClCH2CH2Cl
C6H6 (dry)
C6H6 (dry)
C6H6 (dry)
C7H8–H2O
C6H6–H2O
C7H8–H2O
C7H8–H2O
(CH3)3CCHO
(CH3)3CCHO
(CH3)3CCHO
(CH3)3CCHO
(CH3)3CCHO
(CH3)3CCHO
PhCHO
(CH3)2CHCHO
(C2H5)2CHCHO
(C2H5)2CHCHO
(CH3)3CCHO
(CH3)3CCHO
(C2H5)2CHCHO
38
35
36
36
36
36
36
37
37
48
36
38
24
24
24
3
22
2
23
11
7
14
21
14
17
9
15
26
26
25
25
7
10
37
57
62
19
21
34
9
10 b
11
12
13
a The reactions were carried out at room temperature in the dark with 2 mol% catalyst and 1.5 equivalent of cooxidant; conversions were
determined by GC; enantiomeric excesses were determined by GC on CYDEX-b 25×0.22mm chiral capillary column.
b The enantiomeric excess of the recovered 2-phenylcyclohexanone was 25%.
10) was obtained at ambient temperature with 2 mol%
catalyst; this was inferior to the literature reported
value for enantioselectivity [2a]. This was caused by the
difference in structure of the two kinds of catalyst
which were used in the reaction.
cooled under argon. After cooling to room tempera-
ture, 10 ml of chlorobenzene were added followed by 5
mmol (0.876 g) of the 2-hydroxy-3-tert-butyl benzoni-
trile and 5 mmol (1.07 g) of the amino alcohol. The
mixture was refluxed for 20 h. The solvent was removed
under reduced pressure to give an oily residue, which
was dissolved in 20 ml dichloromethane. The solution
was extracted three times with 20 ml of water and the
aqueous phase with 10 ml of dichloromethane. The
combined organic phases were dried over anhydrous
sodium sulfate and the solvent was removed in vacuum.
The residue was purified by flash chromatography
(petroleum ether–ethyl acetate 40:1) [3].
3. Conclusion
Two novel chiral oxazolines 3, 4 and their copper
complexes 5, 6 were prepared. The structure of complex
6 was analyzed by X-ray diffraction. We also estab-
lished their catalytic activity in the asymmetric Baeyer–
Villiger reaction. Further efforts will be devoted to
improve the rate and enantioselectivity of the reaction.
(4S,5R)-4,5-dihydro-4,5-diphenyl-2-(2%-hydroxy-3%-
tert-butylphenyl)oxazoline (3). Yield 1.19 g (65%); yel-
low solid; m.p. 145–146°C; [h]2D5= −9.84° (c 0.244,
EtOH). IR (mull) w (cm−1) 1633 (CꢀN), 1251 (C–O–
4. Experimental
1
C), 1213 (C–OH). H-NMR (CDCl3, 300 MHz): d 1.47
(s, 9H), 5.81 (d, J=6.7 Hz, 1H), 5.98 (d, J=6.7 Hz,
1H), 6.88 (t, J=5.0 Hz, 1H), 7.02 (m, 10H), 7.47 (dd,
J=5.1 Hz, 1.2 Hz, 1H), 7.76 (dd, J=5.1 Hz, 1.2 Hz,
1H), 12.76 (s, 1H) ppm. 13C-NMR (CDCl3, 300 MHz):
d 167.9, 160.3, 138.1, 137.8, 136.5, 128.4, 128.4, 128.4,
128.3, 127.9, 127.1, 116.7, 110.9, 84.9, 74.0, 35.7, 30.1
ppm. EIMS (m/z): 371 (M+), 204, 161, 91 (100), 77, 41.
Anal. Calc. for C25H25NO2: C, 80.83; H, 6.78; N, 3.77.
Found: C, 80.96; H, 6.90; N, 3.87%.
4.1. General
Melting points were determined on a Southend
SS25PH apparatus and were uncorrected. Elemental
analyses were recorded on a Carlo Erba 1106 instru-
ment. Gas chromatographic analyses were performed
1
by SC-7 gas chromatography. H-NMR spectra were
performed on a Bruker 300 spectrometer. Optical rota-
tions were measured on a Perkin–Elmer 341 polarime-
ter. Enantiomeric excesses were determined by GC on
CYDEX-b 25×0.22 mm chiral capillary column.
(4S,5S)-4,5-dihydro-4,5-diphenyl-2-(2%-hydroxy-3%-
tert-butylphenyl)oxazoline (4). Yield 1.54 g (84%); yel-
low solid; m.p. 94–95.5°C; [h]2D5= +94.38° (c 0.766,
EtOH). IR (mull) w (cm−1): 1632 (CꢀN), 1251 (C–O–
4.2. Synthesis of 3 and 4
1
C), 1208 (C–OH). H-NMR(CDCl3, 300 MHz): d 1.46
(s, 9H), 5.28 (d, J=5.2 Hz, 1H), 5.36 (d, J=5.2 Hz,
1H), 6.46 (t, J=5.1 Hz, 1H), 7.35 (m, 10H), 7.44 (dd,
In a 50 ml three-necked flask, 27.2 mg (0.2 mmol) of
zinc chloride were melted under high vacuum and

208
Y. Peng et al. / Journal of Organometallic Chemistry 619 (2001) 204–208
J=5.2 Hz, 0.9 Hz, 1H), 7.71 (dd, J=5.2 Hz, 0.9 Hz,
1H), 12.67 (s, 1H) ppm. 13C-NMR(CDCl3, 300 MHz): d
167.0, 160.2, 141.9, 140.2, 138.0, 131.5, 129.6, 129.3,
128.6, 127.2, 127.1, 126.5, 118.6, 111.0, 88.6, 78.3, 35.7,
30.0 ppm. EIMS (m/z): 371 (M+), 204 (100), 161, 91,
77, 41. Anal. Calc. for C25H25NO2: C, 80.83; H, 6.78;
N, 3.77. Found: C, 80.92; H, 6.78; N, 3.76%.
5. Supplementary material
Crystallographic data for structural analysis have
been deposited with the Cambridge Crystallographic
Data Center, CCDC no. 142805 for compound 6.
Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (Fax: +44-1223-336033;
e-mail: deposit@ccdc.cam.ac.uk or www: http://
www.ccdc.cam.ac.uk).
4.3. Synthesis of 5 and 6
To a solution of 0.6 mmol (223 mg) of oxazoline in
10 ml absolute ethanol was added a solution of 60 mg
(0.3 mmol) of Cu(OAc)2·H2O in 10 ml absolute ethanol.
The reaction mixture was stirred at room temperature
for 16 h. A large amount of dark-blue precipitate was
formed. After filtration, the solid was collected and
washed with absolute ethanol three times to give the
pure product.
Acknowledgements
We thank the National Natural Science Foundation
of China (No.29832020) and The Hong Kong Polytech-
nic University ASD Fund for financial support of this
study.
Bis[(4S,5R)-4,5-dihydro-4,5-diphenyl-2-(2%-oxido-
phenyl-xO)oxazole-xN]copper(II) (5). Yield 0.18
g
(75%); dark blue solid. IR (mull) w (cm−1) 1612 (CꢀN),
1255 (C–O–C). Anal. Calc. for C50H48CuN2O4: C,
74.65; H, 6.01; N, 3.48. Found: C, 74.70; H, 5.94; N,
3.47%.
References
[1] A.K. Ghosh, P. Mathivanan, J. Cappiello, Tetrahedron: Asym-
metry 9 (1998) 1.
[2] (a) C. Bolm, G. Schlingloff, K. Weickhardt, Angew. Chem. Int.
Ed. Engl. 33 (1994) 1848. (b) C. Bolm, G. Schlingloff, J. Chem.
Soc. Chem. Commun. (1995) 1247. (c) C. Bolm, T.K.K. Luong,
G. Schlingloff, Synlett (1997) 1151.
Bis[(4S,5S)-4,5-dihydro-4,5-diphenyl-2-(2%-oxido-
phenyl-xO)oxazole-xN]copper(II) (6). Yield 0.197 g
(82%); dark blue solid. IR (mull) w (cm−1) 1614 (CꢀN).
Anal. Calc. for C50H48CuN2O4: C, 74.65; H, 6.01; N,
3.48. Found: C, 74.38; H, 6.03; N, 3.46%.
[3] C. Bolm, K. Weickhardt, M. Zehnder, T. Ranff, Chem. Ber. 124
(1991) 1173.
4.4. General procedure for the asymmetric
Baeyer–Villiger reaction
[4] C. Bolm, K. Weickhardt, M. Zehnder, D. Glasmacher, Helv.
Chim. Acta 74 (1991) 717.
[5] R.E. Lowenthal, A. Abiko, S. Masamune, Tetrahedron Lett. 31
(1990) 6005.
The 2-phenylcyclohexanone 87 mg (0.5 mmol) and
the catalyst 8 mg (0.01 mmol) were dissolved in water-
saturated toluene (2.5 ml) and treated with cooxidant
aldehyde (0.75 mmol) and water (0.1 ml). The mixture
was stirred at room temperature under an oxygen at-
mosphere in the dark for 24–48 h, diluted with diethyl
ether (25 ml) and washed with saturated aqueous
NaHCO3 solution (10 ml). The organic phase was dried
over anhydrous Na2SO4. After removal of the solvent,
the crude product was chromatographed on silica gel
(petroleum ether–ethyl acetate 3:1). The enantiomeric
excesses for lactone were determined by GC on
CYDEX-b 25×0.22 mm chiral capillary column.
[6] Q.L. Zhou, A. Pfaltz, Tetrahedron 50 (1994) 4467.
[7] S.G. Lee, C.W. Lin, C.E. Song, I.O. Kim, C.H. Jun, Tetrahe-
dron: Asymmetry 8 (1997) 2927.
[8] H. Yang, M.A. Khan, K.M. Nicholas, Organometallics 12
(1993) 3485.
[9] J. Weijlard, K. Pfister, E.F. Swanezy, C.A. Robinson, M. Tish-
ler, J. Am. Chem. Soc. 73 (1951) 1216.
[10] H.D. Flack, Acta Crystallogr. Sect. A 39 (1983) 876.
[11] K. Bernardo, S. Leppard, A. Robert, G. Commenges, F. Dahan,
B. Meunier, Inorg. Chem. 35 (1996) 387.
[12] (a) M. Del Todesco Frisone, F. Pinna, G. Strukul, Organometal-
lics 12 (1993) 148. (b) A. Gusso, C. Baccin, F. Pinna, G. Strukul,
Organometallics 13 (1994) 3442.
[13] M. Lopp, A. Paju, T. Kanger, T. Pehk, Tetrahedron Lett. 37
(1996) 7583.
.