Epoxide ring opening catalysed by imidochromium complexes

Article abstract of DOI:10.1039/a700380c

tert-Butylimidochromium complexes catalysed ring opening of epoxides by SiMe₃(N₃) to give azidohydrins in good yields. The proposed mechanism for the chromium-mediated epoxide azidolysis involves the intermediacy of a chromium-azide species which delivers nucleophilic azide to the epoxide substrate. The azido complexes [{Cr(NBu^t)₂Cl}₂(μ-N₃) ₂] 1 and [Cr(Bu^tN=NN=NSiMe₃)(N₃)Cl₂] 2 have been isolated in the reactions of SiMe₃(N₃) with [Cr(NBu^t)₂Cl₂] and [Cr(NBu^t)Cl₃(dme)] (dme = 1,2-dimethoxyethane), respectively. The structure of complex 1 has been established by X-ray crystallography. The mean Cr-N (imide) and Cr-N (azide) distances are 1.605 and 2.061 A, respectively. Reaction of [Cr(NBu^t)Cl₃(thf)₂] (thf = tetrahydrofuran) with (S,S)-diop {4,5-[bis(diphenylphosphino)methyl]-2,2-dimethyl-1,3-dioxolane} gave the dimeric complex [{Cr(NBu^t)Cl₃}₂{μ(S,S)-diop}], which has been characterised by X-ray crystallography. The Cr-N and Cr-P distances are 1.593(9) and 2.449(4) A, respectively.

Full text of DOI:10.1039/a700380c

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Epoxide ring opening catalysed by imidochromium complexes‡
Wa-Hung Leung,*,†,a Man-Ching Wu,a Joyce L. C. Chim,a Man-Tat Yu,a Hong-wei Hou,a
Lam-Lung Yeung,*,a Wing-Tak Wongb and Yu Wangc
a
Department of Chemistry, The Hong Kong University of Science and Technology,
Clear Water Bay, Kowloon, Hong Kong
b Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong
c Department of Chemistry, National Taiwan University, Taipei, Taiwan
tert-Butylimidochromium complexes catalysed ring opening of epoxides by SiMe3(N3) to give azidohydrins in
good yields. The proposed mechanism for the chromium-mediated epoxide azidolysis involves the intermediacy of
a chromium–azide species which delivers nucleophilic azide to the epoxide substrate. The azido complexes
[{Cr(NBut) Cl} (µ-N ) ] 1 and [Cr(ButN᎐NN᎐NSiMe )(N )Cl ] 2 have been isolated in the reactions of SiMe (N )
᎐
᎐
2
2
3
2
3
3
2
3
3
with [Cr(NBut)2Cl2] and [Cr(NBut)Cl3(dme)] (dme = 1,2-dimethoxyethane), respectively. The structure of complex
1 has been established by X-ray crystallography. The mean Cr᎐N (imide) and Cr᎐N (azide) distances are 1.605 and
2.061 Å, respectively. Reaction of [Cr(NBut)Cl3(thf)2] (thf = tetrahydrofuran) with (S,S )-diop {4,5-[bis(diphenyl-
phosphino)methyl]-2,2-dimethyl-1,3-dioxolane} gave the dimeric complex [{Cr(NBut)Cl3}2{µ-(S,S )-diop}], which
has been characterised by X-ray crystallography. The Cr᎐N and Cr᎐P distances are 1.593(9) and 2.449(4) Å,
respectively.
Epoxides are an important class of building blocks for complex
organic compounds.1 With the advent of asymmetric epoxid-
ation of allylic alcohols2 and alkenes,3 optically pure epoxides
have been made easily available as valuable starting materials
for stereospecific syntheses. Of practical importance is the syn-
thesis of azidohydrins via ring opening of epoxides with
SiMe3(N3), which is known to be catalysed by organometallic
Lewis acids such as V, Ti,4 Zr 5 and Yb 6 (Scheme 1).
HO
R
N3
O
(1) SiMe3(N3), Lewis acid
(2) H +
R′
R
R′
Scheme 1
mass spectra on a Finnigan TSQ 7000 spectrometer. Elemental
analyses were performed by Medac Ltd., Brunel University,
UK.
CAUTION: high-valent chromium complexes are potentially
carcinogenic; avoid contact with skin and inhalation.
While organoimido complexes (M᎐NR) have been success-
᎐
fully used as well defined promoters for alkene metathesis and
ring-opening metathesis polymerisation of cyclic alkenes,7 there
are few reports on the applications of these complexes in
organic transformations. Recently we found that imido-
chromium complexes catalyse regioselective ring opening of
epoxides8 and N-toluene-p-sulfonylaziridines,9 demonstrating
that these complexes have potential applications in organic syn-
theses. The salient features of this new class of Lewis acids
include high solubilities in organic solvents, little tendency to
oligomerise, high tolerance to heteroatom functional groups,
and their well defined organometallic chemistry. The study of
chromium-mediated epoxide ring opening is of significance
because Jacobsen and co-workers10–12 recently discovered that
chiral chromium complexes are unique catalysts for enantio-
selective ring opening of meso-epoxides10 and kinetic resolution
of racemic epoxides.11 Herein we describe the isolation of
azido(imido)chromium complexes, which are believed to be the
active species of chromium-mediated epoxide ring opening
and the preparation of a chiral imidochromium catalyst for
enantioselective epoxide ring opening.
Materials
The compounds [Cr(NBut)2Cl2],13 [Cr(NBut)Cl3(thf)2] (thf =
tetrahydrofuran), [Cr(NBut)Cl3(dme)] (dme = 1,2-dimethoxy-
ethane),14 and [{Ti(NBut)Cl2(NH2But)2}3]15 were prepared as
described elsewhere. The epoxide substrates and (S,S )-diop
{4,5-[bis(diphenylphosphino)methyl]-2,2-dimethyl-1,3-dioxol-
ane} were obtained from Aldrich and used as received.
Typical procedure for ring opening of styrene oxide
To a stirred solution of styrene oxide (1 mmol) and [Cr(NBut)-
Cl3(dme)] (0.05 mmol) in dry CH2Cl2 (15 cm3) at room temp-
erature was added SiMe3(N3) (3 mmol). After stirring over-
night, the reaction mixture was treated with dilute H2SO4
(1 mol dmϪ3, 10 cm3). The organic layer was collected and dried
with anhydrous MgSO4. Evaporation of the volatiles gave an
oil, which was loaded onto a column of silica gel. The azido-
phenylethanol products were eluted with 15% Et2O–hexane,
isolated as an inseparable mixture. The ratio of the products
was determined by 1H NMR spectroscopy.
Experimental
All manipulations were carried out under nitrogen using stand-
ard Schlenk techniques. Solvents were purified, distilled and
degassed prior to use. Proton NMR spectra were recorded on a
JEOL EX 400 or Bruker ALX 300 spectrometer, chemical shifts
(δ, in ppm) being referenced to SiMe4. Infrared spectra were
obtained on a Perkin-Elmer 16PC FT-IR spectrophotometer,
Spectroscopic data for the azidohydrins and chlorohydrin. 2-
Azido-2-phenylethanol. 1H NMR (CDCl3): δ 1.90 (t, J = 6.5,
1 H, OH), 3.76 (t, J = 6.5, 2 H, CH2OH), 4.68 [t, J = 6.4
Hz, CH(N3)] and 7.32–7.41 (m, 5 H, aromatic). IR (cmϪ1): 3386
(br) [ν(OH)] and 2102 [ν(N3)]. Chemical ionisation (CI) mass
spectrum: m/z 163 (Mϩ).
2-Chloro-2-phenylethanol. 1H NMR (CDCl3): δ 2.06 (dd,
J = 6.4, 7.7, 1 H, OH), 3.94 (dt, J = 1.8, 6.8, 2 H, CH2OH), 5.00
† E-Mail: chleung@usthk.ust.hk
‡ Non-SI unit employed: µB = 9.27 × 10Ϫ24 J TϪ1
.
J. Chem. Soc., Dalton Trans., 1997, Pages 3525–3529
3525

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(t, J = 6.6 Hz, 1 H, CHCl) and 7.33–7.43 (m, 5 H, aromatic). CI
mass spectrum: m/z 156 (Mϩ).
Table 1 Catalytic ring opening of styrene oxide by SiMe3(N3)
2-Azido-1-phenylethanol. 1H NMR (CDCl3): δ 2.38 (s br, 1 H,
OH), 3.44 (dd, J = 4.2, 12.6, 1 H, CH2N3), 3.50 (dd, J = 7.8, 12.6
Hz, 1 H, CH2N3), 4.90–4.93 (m, 1 H, CHOH) and 7.32–7.39 (m,
5 H, aromatic).
O
(1) SiMe3(N3), catalyst N3
OH
HO
Ph
N3 Cl
+
OH
+
(2) H +
2
1
Ph
Ph
Ph
Preparation of [{Cr(NBut)2Cl}2(ì-N3)2] 1
I
II
III
The compound SiMe3(N3) (0.44 cm3, 3.31 mmol) was added to
a solution of [Cr(NBut)2Cl2] (0.22 g, 0.83 mmol) in hexane (10
cm3). The reaction mixture was refluxed overnight. The solvent
was removed in vacuo and the residue recrystallised from
toluene. Dark red crystals were obtained after cooling the
solution to Ϫ10 ЊC overnight {yield 25% with respect to [Cr-
% Yield
Catalyst a
Solvent
I
II
III Reaction time
[Cr(NBut)2Cl2]
CH2Cl2 90
CH2Cl2 90
CH2Cl2 63 30
thf
MeCN
CH2Cl2
0
0
7
0
3
2
0
0
0
9
12 h
10 h
5 h
12 h
24 h
5 d
[{Cr(NBut)2Cl}2(N3)2]
[Cr(NBut)Cl3(dme)]
42 10
1
(NBut)2Cl2] used}. H NMR (CDCl3): δ 1.64 (s, 36 H, But).
4
0
0
0
0
0
[CrL(NBut)Cl2]b
IR (cmϪ1): 1198 ν(Cr᎐NBut) and 2098 ν(N ) (Found: C, 35.2;
᎐
3
[Mo(NBut)2Cl2(dme)]
[{Ti(NBut)Cl2(NH2But)2}3]
CH2Cl2 45
CH2Cl2 45
2 d
20 h
H, 6.8; N, 26.8. Calc. for C16H36Cl2Cr2N10: C, 35.4; H, 6.6;
N, 25.8%).
a
Catalyst loading was 10 mol % except for the reactions with [Cr(N-
But)Cl3(dme)], in which 5 mol % catalyst was used. b L = (η-C5H5)Co-
{PO(OEt)2}3.
Preparation of [Cr(ButN᎐N᎐N᎐NSiMe )(N )Cl ] 2
᎐
᎐
3
3
2
The compound SiMe3(N3) (0.50 cm3, 3.77 mmol) was added to
a solution of [Cr(NBut)Cl3(dme)] (0.30 g, 0.94 mmol) in CH2Cl2
(10 cm3). The reaction mixture was stirred at room temperature
overnight. The solvent was removed in vacuo and the residue
recrystallised from CH2Cl2–diethyl ether to give a dark yellow
solid (yield 75%). IR (cmϪ1): 2110 ν(N3). CI mass spectrum: m/z
264 (Mϩ Ϫ N2 Ϫ isobutene). µeff(Evans method,16 CHCl3) =
3.52 µB (Found: C, 23.4; H, 5.2; N, 25.4. Calc. for
C10H27Cl2CrN8Si2: C, 23.9; H, 5.1; N, 27.9%).
diffractometer at 25 ЊC. Intensity data were corrected for
Lorentz-polarization effects. The structure was solved by direct
methods (SIR 9218) and refined by full-matrix least-squares
analysis with hydrogen atoms placed in calculated positions.
CCDC reference number 186/650.
Results and Discussion
Ring opening of cyclohexene oxide by [Cr(NBut)Cl3(dme)] with
(S,S )-diop
Catalytic azidolysis of styrene oxide
tert-Butylimidochromium complexes [Cr(NBut)2Cl2] and
[Cr(NBut)Cl3(dme)] catalyse ring opening of styrene oxide with
SiMe3(N3) to give the vicinal azidohydrins in good yields and
the results are summarised in Table 1. Typically reaction of
styrene oxide with SiMe3(N3) in the presence of 10 mol %
of [Cr(NBut)Cl3(dme)] afforded a 1:2 mixture of azidohydrins
I and II in 93% yield.
A mixture of cyclohexene oxide (0.61 mmol), [Cr(NBut)-
Cl3(dme)] (10 mmol), (S,S )-diop (10 mmol) and SiMe3(N3)
(1.83 mmol) was stirred in CH2Cl2 (5 cm3) at room temperature
for 3 d. The reaction mixture was worked up as described earlier
and the products were identified as racemic mixtures of 2-
azidocyclohexanols (30%) and 2-chlorocyclohexanol (20%) by
GLC.
Depending on the purity of the catalyst and experimental
conditions, some chlorohydrin III was also detected. It is
apparently derived from the chloride of the catalyst because
none was detected when the azidochromium complex [{Cr-
(NBut)Cl(N3)}2(µ-N3)2] (see later) was employed as the catalyst.
The corresponding reactions with [Cr(NBut)2Cl2] and [Mo-
(NBut)2Cl2]19 were slower but selective, yielding the 2-azido-
alcohol I exclusively. The preference for the formation of I by
the ring opening of styrene oxide may be explained in terms of
the stability of the benzylic carbocation intermediate, which
is susceptible to nucleophilic attack by azide. The activity of
imidometal complexes in epoxide ring opening was found to
decrease in the order CrV > TiIV > CrVI > MoVI, which reflects
the trend in Lewis acidity of the metal centre. The lower Lewis
acidity of [Cr(NBut)2Cl2] compared with that of [Cr(NBut)-
Cl3(dme)] may be reasoned by the fact that the former complex,
although in a higher oxidation state, is stabilised by two strong-
ly π-donating imides and is therefore a weaker Lewis acid. No
catalytic reaction was observed when co-ordinatively saturated
[(η-C5H5)Co{PO(OEt)2}3Cr(NBut)Cl2]20 was used as catalyst,
suggesting that a vacant site on chromium is essential for the
catalytic ring opening. However, there is no appreciable differ-
ence in catalytic activity between [Cr(NBut)Cl3(dme)] and the
more substitutionally labile [Cr(NBut)Cl3(thf)2], indicating
that dissociation of the ether ligands in the imidochromium()
complexes is not a rate-limiting step in the chromium-catalysed
azidolysis reaction. The catalytic epoxide ring opening is rather
insensitive to the choice of solvent, although the yield for
CH2Cl2 was found to be slightly higher than that for thf. This is
in contrast to the Cr(salen) [H2salen = N,NЈ-bis(salicylidene)-
ethane-1,2-diamine] system,10 in which epoxide ring opening
Preparation of [{Cr(NBut)Cl3}2{ì-(S,S )-diop}] 4
The compound (S,S )-diop (0.27 g, 0.54 mmol) was added to a
solution of [Cr(NBut)Cl3(thf)2] (0.20 g, 0.54 mmol) in CH2Cl2
(10 cm3). The reaction mixture was stirred at room temperature
overnight. The solvent was removed in vacuo and the residue
washed with toluene. Recrystallisation from CH2Cl2–Et2O
afforded green prisms, suitable for X-ray diffraction study
{Yield 33% with respect to [Cr(NBut)Cl3(thf)2] used} (Found:
C, 55.7; H, 5.8; N, 2.5. Calc. for C53H66Cl6Cr2N2O2P2: C, 55.3;
H, 5.7; N, 2.6%).
X-Ray crystallography
Crystal data and experimental details for complexes 1 and 4 are
listed in Table 2. Measurements for 1 were made on an Enraf-
Nonius CAD-4 diffractometer. X-Ray-quality crystals of
[{Cr(NBut)2Cl}2(µ-N3)2] 1 were obtained from a toluene solu-
tion at Ϫ10 ЊC. Lattice parameters for 1 were obtained from 25
reflections with 2θ 19.02–27.70Њ. All reflections were corrected
for Lorentz-polarisation effects. All data reduction and refine-
ment was performed using the NRCVAX packages.17 The
structure was solved by the Patterson method and refined on F
by full-matrix least-squares analysis. All hydrogen atoms were
refined isotropically and all other atoms anisotropically.
Hydrogen atoms on the organic ligands were calculated in ideal-
ised positions and included in the structure-factor calculation.
X-Ray-quality crystals of [{Cr(NBut)Cl3}2{µ-(S,S )-diop}] 4
were obtained by slow diffusion of ether into a CH2Cl2
solution. Measurements were made on a Rigaku AFC7R
3526
J. Chem. Soc., Dalton Trans., 1997, Pages 3525–3529

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Table 2 Crystallographic data for [{Cr(NBut)2Cl}2(µ-N3)2] 1 and
[{Cr(NBut)Cl3}2{µ-(S,S )-diop}] 4
Table 3 Selected bond lengths (Å) and angles (Њ) for complex 1
Cr ؒ ؒ ؒ CrЈ
Cr᎐N(1)
Cr᎐N(4)
N(1Ј)᎐Cr
N(2)᎐N(3)
3.368(2)
2.020(4)
1.607(4)
2.101(4)
1.119(5)
Cr᎐Cl
2.288(2)
2.101(4)
1.603(4)
1.221(5)
1
4
Cr᎐N(1Ј)
Cr᎐N(5)
N(1)᎐N(2)
Chemical formula
M
C8H18ClCrN5
271.71
C53H66Cl6Cr2N2O2P2
1141.77
Crystal dimensions/mm 0.40 × 0.50 × 0.60 0.23 × 0.24 × 0.34
Crystal system
Space group
a/Å
b/Å
Monoclinic
P21/n
Orthorhombic
P212121
18.822(4)
27.220(4)
10.984(3)
CrЈ᎐Cr᎐Cl
121.16(5)
34.3(1)
113.6(2)
94.9(2)
70.4(1)
95.4(2)
CrЈ᎐Cr᎐N(1)
CrЈ᎐Cr᎐N(4)
Cl᎐Cr᎐N(1)
36.0(1)
115.1(2)
86.8(1)
96.8(2)
98.1(2)
123.1(2)
112.9(2)
122.0(3)
179.1(5)
CrЈ᎐Cr᎐N(1Ј)
CrЈ᎐Cr᎐N(5)
Cl᎐Cr᎐N(4)
N(1)᎐Cr᎐N(1Ј)
N(1)᎐Cr᎐N(5)
N(1Ј)᎐Cr᎐N(5) 123.4(2)
CrЈ᎐N(1)᎐CrЈ 109.6(2)
Cr᎐N(1)᎐N(2) 128.4(3)
Cr᎐N(4)᎐C(1) 162.2(4)
10.552(4)
12.429(4)
11.320(2)
97.13(3)
1473.2(7)
4
25
0.710 69
1.225
Cl᎐Cr᎐N(5)
c/Å
N(1)᎐Cr᎐N(4)
N(1Ј)᎐Cr᎐N(4)
N(4)᎐Cr᎐N(5)
Cr᎐N(1)᎐N(2)
N(1)᎐N(2)᎐N(3)
β/Њ
U/Å3
5627(1)
4
25
0.710 73
1.348
ω–2θ
45
7.68
Z
T/ЊC
λ/Å
Do/g cmϪ3
Scan type
2θmax/Њ
θ–2θ
50
7.338
µ/cmϪ1
F(000)
570
2376
Reflections measured
Observed reflections
Weighting scheme
R a
2586
8308
1611 [I > 2σ(I)]
1/σ2(Fo)
0.045
0.032
2.22
2701 [I > 3σ(I)]
1/[σ2(Fo) ϩ (0.006Fo /4)]
2
0.052
0.047
2.25
¹
RЈb
Goodness of fit c
a
R = (Σ|Fo| Ϫ |Fc|)/|Fo|. b RЈ = [(Σw2|Fo| Ϫ |Fc|)2/Σw2|Fo|2)]². c Goodness of
¹
fit = [Σw(|Fc| Ϫ |Fo|)2/(Nobs Ϫ Nparam)]².
was found to be faster in thf and ether. The catalytic reaction is
very slow in acetonitrile possibly because the imidochromium
complexes have a strong affinity for N-donor ligands and bind
to acetonitrile very tightly.
Azido(imido)chromium complexes
Fig. 1 Perspective view of [{Cr(NBut)2Cl}2(µ-N3)2] 1
No reactions between styrene oxide and the imido complexes
were observed; nor was there any between SiMe3(N3) and
epoxide under the reaction conditions. This suggests that the
first step of the catalytic azidolysis may involve the interaction
of the imido complexes with SiMe3(N3). Indeed Jacobsen and
co-workers10 demonstrated that the structurally characterised
[Cr(salen)(N3)] species is responsible for the Cr(salen)-mediated
epoxide azidolysis. The formation of the diazide species [Cr-
(NBut)2(N3)2] from [Cr(NBut)2Cl2] and SiMe3(N3) was previ-
ously reported by Wilkinson and co-workers.21 However,
despite many attempts we were only able to isolate the dinuclear
species [{Cr(NBut)2Cl}(µ-N3)2] 1, which has been characterised
by X-ray crystallography, from the reaction of [Cr(NBut)2Cl2]
with SiMe3(N3), even under refluxing conditions [equation (1)].
R′
R′
R
R
O
O
N3
Cr
(a )
Cr
(b )
Scheme 2
a trivalent chromium species. On the basis of the analytical and
spectral data, it is tentatively formulated as a chromium()
tetrazene complex [CrIII(ButN᎐N᎐N᎐NSiMe )(N )Cl ]. It may
be noted that dipolar cycloaddition of the imido-metal moiety
with azide to give tetrazene is not without precedent. Reactions
of imido complexes of Os,22 Zr 23 and Ru 24 with organic azides
have been reported to give the respective tetrazene species.
᎐
᎐
3
3
2
2[Cr(NBut)2Cl2] ϩ 2SiMe3(N3) →
[{Cr(NBut)2Cl}2(µ-N3)2] ϩ 2SiMe3Cl (1)
Fig. 1 shows a perspective view of the molecule; selected
bond lengths and angles are given in Table 3. The two
Cr(NBut)2Cl cores are bridged by two azides with Cr᎐N
(azide)᎐Cr and N (azide)᎐Cr᎐N (azide) angles 109.6(2) and
70.4(1)Њ, respectively. The mean Cr᎐N (imido), Cr᎐N (azide)
and Cr᎐Cl distances are 1.605, 2.061 and 2.288(2) Å, respect-
ively. The short Cr᎐N (imido) bond lengths and large Cr᎐N᎐C
Proposed mechanism for the Cr-mediated epoxide ring opening
Two mechanisms have been proposed for the metal-catalysed
azidolysis of epoxides (Scheme 2): (a) Lewis-acid activation and
(b) nucleophilic delivery of azide. As in the case for Cr(salen)-
catalysed epoxide ring opening,10 we believe that the nucleo-
philic delivery mechanism is operative in the imidochromium
system. This is supported by fact that the azido complexes 1
and 2 are capable of catalysing azidolysis of styrene epoxide in
yields comparable to those for their precursors. It should be
noted that azidolysis of styrene oxide with 1 is only marginally
faster than that for [Cr(NBut)2Cl2], indicating that the delivery
of azide to epoxide rather than the formation of a Cr᎐N3
intermediate is rate limiting. These results are consistent with
the mechanism depicted in Scheme 3. Reaction of the pre-
catalyst [Cr(NBut)2Cl2] with SiMe3(N3) gives the azido(imido)-
angles (ca. 165.65Њ) are consistent with the formulation of
᎐
Cr᎐N triple bond. The IR spectrum of 1 shows N᎐N and
᎐
᎐
Cr᎐NBut stretches at 2098 and 1198 cmϪ1, respectively.
On the other hand, reaction of [Cr(NBut)Cl3(dme)] with
SiMe3(N3) afforded an air-stable brown solid analysed as
[Cr(NBut)(N3)Cl2{Me3Si(N3)}] 2. The IR spectrum displays a
band at ca. 1500 cmϪ1, tentatively assigned as the N᎐N stretch,
᎐
along with the ν(N3) band at 2110 cmϪ1. The measured mag-
netic moment of ca. 3.52 µB suggests that complex 2 is probably
J. Chem. Soc., Dalton Trans., 1997, Pages 3525–3529
3527

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Fig. 2 Perspective view of [{Cr(NBut)Cl3}2{µ-(S,S )-diop}] 4
[Cr(NBut)2Cl2]
Table 4 Selected bond lengths (Å) and angles (Њ) for complex 3
SiMe3(N3)
Cr(1)᎐N(1)
Cr(1)᎐Cl(1)
Cr(1)᎐Cl(2)
Cr(1)᎐Cl(3)
Cr(1)᎐P(1)
Cr(1)᎐O(1)
1.593(9)
2.272(4)
2.287(4)
2.281(4)
2.449(4)
2.58(1)
Cr(2)᎐N(2)
Cr(2)᎐Cl(4)
Cr(2)᎐Cl(5)
Cr(2)᎐Cl(6)
Cr(2)᎐P(2)
Cr(2)᎐O(2)
1.603(9)
2.279(4)
2.274(4)
2.273(4)
2.450(4)
2.43(1)
SiMe3Cl
N3
Ph
OH
O
[{Cr(NBut)2Cl}2(N3)2]
1
Ph
Cl(ButN)2Cr
O
Cl(1)᎐Cr(1)᎐Cl(2)
Cl(1)᎐Cr(1)᎐P(1)
Cl(2)᎐Cr(1)᎐Cl(3)
Cl(2)᎐Cr(1)᎐N(1)
Cl(3)᎐Cr(1)᎐N(1)
Cl(4)᎐Cr(2)᎐Cl(5)
Cl(4)᎐Cr(2)᎐P(2)
Cl(5)᎐Cr(2)᎐Cl(6)
Cl(5)᎐Cr(2)᎐N(2)
Cl(6)᎐Cr(2)᎐N(2)
Cr(1)᎐N(1)᎐C(1)
91.8(1)
85.0(1)
93.7(1)
97.7(4)
95.8(4)
92.2(1)
165.0(1)
158.7(1)
96.3(4)
104.1(4)
165.6(9)
Cl(1)᎐Cr(1)᎐Cl(3) 162.3(2)
HN3
Cl(1)᎐Cr(1)᎐N(1)
Cl(2)᎐Cr(1)᎐P(1)
Cl(3)᎐Cr(1)᎐P(1)
P(1)᎐Cr(1)᎐N(1)
Cl(4)᎐Cr(2)᎐Cl(6)
Cl(4)᎐Cr(2)᎐N(2)
Cl(5)᎐Cr(2)᎐P(2)
Cl(6)᎐Cr(2)᎐P(2)
P(2)᎐Cr(2)᎐N(2)
Cr(2)᎐N(2)᎐C(5)
100.1(4)
166.9(1)
85.8(1)
95.4(4)
90.2(1)
101.3(4)
86.4(1)
85.8(1)
93.6(4)
169(1)
Ph
N3
3
Scheme 3
Ph2P
PPh2
O
O
(S, S )-diop
diop only yielded racemic mixtures of the azidohydrins and
chlorohydrins in 50% overall yield. The solid-state structure of
the phosphine adduct of the imidochromium() complex may
provide some insights into the inability of this complex to effect
asymmetric ring opening. The 2:1 adduct [{Cr(NBut)Cl3}2{µ-
(S,S )-diop}] 4 was obtained by reaction of [Cr(NBut)Cl3(thf)2]
with 1 equivalent of (S,S )-diop. The 1:1 phosphine adduct
[Cr(NBut)Cl3{(S,S )-diop}] was not obtained even when an
excess of phosphine was used. Fig. 2 shows a perspective view
of the molecule; selected bond lengths and angles are given in
Table 4. The structure consists of two Cr(NBut)Cl3 fragments,
which are bridged together via the phosphorus and oxygen
atoms of (S,S)-diop. The geometry around each Cr can be
described as distorted octahedral with the imide ligand oppos-
ite to the isopropylidene oxygen of (S,S )-diop. The Cr(1)᎐N(1)
and Cr(1)᎐P(1) distances of 1.593(3) and 2.449(4) Å are com-
parable to those in [Cr(NBut)Cl3(PEtPh2)2] [1.634(7) and 2.479
Å, respectively].26 The Cr᎐O distances [2.43(1) and 2.58(1) Å]
are long apparently due to the trans influence of the tert-
butylimide. In this co-ordination mode it seems that the steric
effect of the ligand is not sufficient to direct the orientation of
the substrate in the transition state to achieve enantioselective
ring opening. This may explain why no stereoselectivity was
observed for catalytic ring opening with complex 4. The
chromium complex 1 and SiMe3Cl. This is followed by the
rate-limiting nucleophilic delivery of the Cr-bound azide to the
epoxide and the formation of chromium azidoalkoxide com-
plex 3. It may be noted that a related Cr(salen) cyclopentoxide
complex has been isolated recently from the reaction of cyclo-
pentene oxide with [Cr(salen)(N3)].25 Reaction of 3 with HN3,
which is presumably formed by protonation of SiMe3(N3), gives
the azidohydrin and complex 1, thus completing the catalytic
cycle. The elegant work by Jacobsen and co-workers25 has
unambiguously confirmed that it is HN3, instead of SiMe3(N3),
that is responsible for the turnover of the alkoxide inter-
mediate.
Chiral imidochromium complex
The success of imidochromium complexes in azidolysis of
cyclohexene oxide8 prompts us to explore the possibility
of enantioselective ring opening of meso-epoxides with
chiral imidochromium complexes. To this end, ring opening of
meso-epoxides with SiMe3(N3) in the presence of [Cr(NBut)-
Cl3(thf)2] and chiral phosphines such as (S,S )-diop was
attempted.
Unfortunately the reaction of cyclohexene oxide with SiMe3-
(N3) in the presence of catalytic [Cr(NBut)Cl3(thf)2] and (S,S )-
3528
J. Chem. Soc., Dalton Trans., 1997, Pages 3525–3529

View Article Online
11 S. E. Schaus and E. N. Jacobsen, Tetrahedron Lett., 1996, 37, 7937;
preparation of monomeric chiral imidochromium complexes
with C2-symmetric chiral ligands is underway.
J. F. Larrow, S. E. Schaus and E. N. Jacobsen, J. Am. Chem. Soc.,
1996, 118, 7420.
12 E. N. Jacobsen, F. Kakiuchi, R. G. Konsler, J. F. Larrow and
M. Tokunaga, Tetrahedron Lett., 1997, 38, 773.
Acknowledgements
13 A. D. Danopoulos, W.-H. Leung, G. Wilkinson, B. Hussain-Bates
and M. B. Hursthouse, Polyhedron, 1990, 9, 2625.
14 W.-H. Leung, A. A. Danopoulos, G. Wilkinson, B. Hussain-Bates
and M. B. Hursthouse, J. Chem. Soc., Dalton Trans., 1991, 2051.
15 P. H. Sheridan, T. S. Lewkebandara, M. J. Heeg, A. L. Rheingold
and C. H. Winter, Inorg. Chem., 1994, 33, 5897.
Support from The Hong Kong University of Science and
Technology and The Hong Kong Research Grants Council is
gratefully acknowledged. We thank Dr Michael Lam for
helpful discussions.
16 D. Evans, J. Chem. Soc., 1959, 2003.
17 NRCVAX, E. J. Gabe, Y. Le Page, J.-P. Charland, F. L. Lee and
P. S. White, J. Appl. Crystallogr., 1989, 22, 384.
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Received 15th January 1997; Paper 7/00380C
J. Chem. Soc., Dalton Trans., 1997, Pages 3525–3529
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