Synthesis of N-allyl-2,3-dihydrobenzoxazol-2-ylidene complexes of chromium(0) and tungsten(0)

Article abstract of DOI:10.1016/S0020-1693(99)00032-8

The reaction of 2-trimethylsiloxyphenyl isocyanide (1) with [M(CO)₅(THF)] (M=Cr, W) gives the complexes [M(2-Me₃SiO-C₆H₄NC)(CO)₅] (2) (2a, M=Cr; 2b, M=W). Hydrolysis of the Si-O bond leads to the formation of an equilibrium between complexes 3 (3a, M=Cr; 3b, M=W) with a 2-hydroxyphenyl isocyanide and complexes 4 (4a, M=Cr; 4b, M=W) with a 2,3-dihydrobenzoxazol-2-ylidene ligand. Deprotonation of mixtures 3/4 followed by alkylation with allyl bromide yields exclusively N-allyl-2,3-dihydrobenzoxazol-2-ylidene complexes (5) (5a, M=Cr; 5b, M=W). Thermally induced intramolecular substitution of one cis-carbonyl ligand in (N-allyl-2,3-dihydrobenzoxazol-2-ylidene)-pentacarbonyltungsten (5b) results in the formation of the alkene-carbene complex 6 incorporating an η²:η¹-(N-allyl-2,3-dihydrobenzoxazol-2-ylidene) ligand. The molecular structure of 5b is reported.

Full text of DOI:10.1016/S0020-1693(99)00032-8

Inorganica Chimica Acta 288 (1999) 47–52
Synthesis of N-allyl-2,3-dihydrobenzoxazol-2-ylidene complexes of
chromium(0) and tungsten(0)ꢀ
Matthias Tamm, F. Ekkehardt Hahn *
Anorganisch-Chemisches Instiut, Westfa¨lische Wilhelms-Uni6ersita¨t Mu¨nster, Wilhelm-Klemm-Straße 8, D-48149 Mu¨nster, Germany
Received 3 September 1998; accepted 7 January 1999
Abstract
The reaction of 2-trimethylsiloxyphenyl isocyanide (1) with [M(CO)5(THF)] (M=Cr, W) gives the complexes [M(2-Me3SiO–
C6H4NC)(CO)5] (2) (2a, M=Cr; 2b, M=W). Hydrolysis of the Si–O bond leads to the formation of an equilibrium between
complexes 3 (3a, M=Cr; 3b, M=W) with a 2-hydroxyphenyl isocyanide and complexes 4 (4a, M=Cr; 4b, M=W) with a
2,3-dihydrobenzoxazol-2-ylidene ligand. Deprotonation of mixtures 3/4 followed by alkylation with allyl bromide yields exclu-
sively N-allyl-2,3-dihydrobenzoxazol-2-ylidene complexes (5) (5a, M=Cr; 5b, M=W). Thermally induced intramolecular
substitution of one cis-carbonyl ligand in (N-allyl-2,3-dihydrobenzoxazol-2-ylidene)-pentacarbonyltungsten (5b) results in the
formation of the alkene–carbene complex 6 incorporating an h2:h1-(N-allyl-2,3-dihydrobenzoxazol-2-ylidene) ligand. The
molecular structure of 5b is reported. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Chromium complexes; Tungsten complexes; Carbene complexes; Alkene complexes
The position of the equilibrium depends on the ‘elec-
tron-richness’ of the metal fragment MLx. Strong (d“
p*) backbonding from relatively electron-rich metal
fragments (e.g. Re(I)) can completely suppress in-
tramolecular cyclization of the hydroxyphenyl iso-
cyanide ligand [3,4b,4d]. On the other hand, complete
ylidene formation is observed upon 2-hydroxyphenyl
isocyanide coordination to relatively electron-poor
metal fragments due to weaker backbonding (e.g.
Re(V)) [2c,2d,4d]. Furthermore, it could be shown that
even electronically stabilized isocyanide complexes of
the type A can be converted to ylidene derivatives by
deprotonation and subsequent N-alkylation. In depro-
tonated complexes of type A the phenolate oxygen
atom is a strong nucleophile causing complete cycliza-
tion. N-Alkylation of the resulting monoanionic ben-
zoxazol-2-yl complex results in the formation of
complexes with N-alkyl-2,3-dihydrobenzoxazol-2-yli-
dene ligands and prevents reopening of the N,O-
heterocycle.
1. Introduction
The use of 2-trimethylsiloxyphenyl isocyanide (1) [1]
as a suitable starting material for the synthesis of
N,O-heterocarbene complexes has been demonstrated
[2,3]. A more detailed examination of this type of
reaction revealed that coordination of 1 to a transition
metal fragment followed by hydrolysis of the Si–O
bond results in a chemical equilibrium between com-
plexes with a 2-hydroxyphenyl isocyanide (A) or a
2,3-dihydrobenzoxazol-2-ylidene ligand (B) (Scheme 1)
[4].
Scheme 1.
In this contribution we like to report on the synthesis
and characterization of the N-allylated complexes (N-
allyl-2,3-dihydrobenzoxazol-2-ylidene)pentacarbonyl-
chromium (5a) and -tungsten (5b). These compounds
ꢀ This paper is dedicated to Professor Dr B. Krebs, on the occasion
of his 60th birthday.
* Corresponding author. Tel.: +49-251-8333 110/111; fax: +49-
251-8333 3108.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00032-8

48
M. Tamm, F.E. Hahn / Inorganica Chimica Acta 288 (1999) 47–52
are potentially useful for the synthesis of carbene–
alkene complexes [5,6], which have been proposed as
intermediates in several catalytic processes such as
olefin metathesis, cyclopropanation of alkenes and
heterogenous Ziegler–Natta polymerization of alke-
nes.
resonances allowed us to determine the ratio of com-
plexes 3/4 showing that the chromium complexes 3a/
4a form
a 29:71 mixture, whereas the tungsten
derivatives 3b/4b form a 15:85 mixture [4b]. As the
position of the equilibrium for both complexes resides
more strongly on the right, resulting in an excess of
the carbene component, all 13C NMR resonances can
be unambiguously assigned to complexes 3 or 4, re-
spectively (Table 1).
2. Results and discussion
The CO carbon resonances of 4a and 4b are ob-
served at a lower field than those of 3a and 3b, indi-
cating that the carbene ligand in 4 is a weaker
p-acceptor than the isocyanide ligand in 3 [7]. Conse-
quently, the 13C NMR spectra of complexes 4 exhibit
larger Dl differences between the trans- and cis-CO
resonances than those of complexes 3, as the trans-
carbonyl ligand directly competes with the ylidene or
isocyanide ligand for electron-density from the metal
center.
The treatment of 2-trimethylsiloxyphenyl isocyanide
(1) with [M(CO)5(THF)] gives complexes [Cr(1)(CO)5]
(2a) and [W(1)(CO)5] (2b) as colorless viscous oils in
good yields (Scheme 2). Hydrolysis of the Si–O bond
is best achieved by stirring complexes 2 in methanolic
solution with a catalytic amount of KF for 2 days
[2,4]. After chromatography on Al2O3 (4% H2O) mix-
tures of the desilylated complexes 3 and 4 are isolated
as off-white solids.
The isocyanide complexes 3 can be easily identified
in these mixtures by the strong IR absorptions due to
the CꢀN stretching mode (2132 cm−1 for 3a; 2141
cm−1 for 3b), while the carbene compounds 4 show a
characteristic IR absorption for the N–H bond (3436
After treatment of mixtures 3/4 with KOtBu in
DMF followed by addition of allyl bromide the sta-
ble N-allylated carbene complexes 5a and 5b are ex-
clusively isolated, which cannot rearrange to the
‘open’ isocyanide form (Scheme 2). No 2-allyl-
oxyphenyl isocyanide derivatives can be isolated from
the reaction mixture. In deprotonated 3, the nega-
tively charged phenolate oxygen atom is a strong nu-
cleophile which rapidly attacks the isocyanide carbon
atom under formation of anionic benzoxazol-2-yl
complexes, which are subsequently allylated at the ni-
trogen atom. Naturally, the spectroscopic data of the
carbene derivatives 4 and 5 are very similar.
1
cm−1 for 4a; 3433 cm−1 for 4b). The H NMR spec-
tra in CD2Cl2 exhibit broad singlets for the OH reso-
nances at about 5.6 ppm as well as for the NH
resonances at about 10.4 ppm. Integration of these
The molecular structure of 5b was established by
X-ray diffraction analysis, and Fig. 1 shows an ORTEP
diagram. The bond distances and angles compare well
with equivalent parameters in related tungsten com-
plexes with 2,3-dihydrobenzoxazol-2-ylidene ligands
[2b]. The O–C–N angle at the carbene carbon atom
Table 1
Selected 13C NMR dataa
Compound
M–C
trans-CO
cis-CO
Dlb
2a
2b
3a
3b
4a
4b
5a
5b
173.0
153.6
216.9
196.5
218.1
197.5
222.2
202.3
221.1
201.8
214.6
194.1
215.6
194.9
217.7
197.8
216.5
197.3
2.3
2.4
2.5
2.6
4.5
4.5
4.6
4.5
174.0
c
232.0
211.6
233.8
215.5
a l (ppm), 62.90 MHz; 2 in CDCl3, 3 and 4 in d6-acetone, 5 in
CD2Cl2.
b l(trans-CO)–l(cis-CO).
c Not observed.
Scheme 2.

M. Tamm, F.E. Hahn / Inorganica Chimica Acta 288 (1999) 47–52
49
ylidene) ligand was isolated as a yellow solid in a
relatively low yield (14%, Scheme 3).
The intramolecular olefin coordination is easily de-
1
tected by H NMR spectroscopy, and the resonances
and coupling constants for the protons of the allyl
group are summarized in Table 2.
1
The H NMR spectra of 5b and 6 showing only the
allyl resonances are depicted in Fig. 2. All resonances
for the allyl protons are observed at higher field upon
coordination of the alkene moiety with the resonances
of cis- and trans-H3 being most strongly shifted by ca.
2 ppm. In 6, two resonances are observed for the
diastereotopic allylic protons H1 with a 2J coupling
constant of 12.5 Hz. Thereby, the resonance of proton
H2 changes from a ddt in 5b to a dddd in 6.
The use of N-allyl-2,3-dihydrobenzoxazol-2-ylidene
complexes for the synthesis of alkene–carbene com-
plexes could be demonstrated. This has prompted us to
synthesize related 2,3-dihydro-1H-benzimidazol-2-yli-
dene complexes, which can be doubly allylated at the
nitrogen atoms [10]. Currently, the suitability of these
ligands for the synthesis of carbene-bis(alkene) com-
plexes or even homoleptic bis(carbene)–tetrakis(alkene)
complexes is being investigated.
˚
Fig. 1. ORTEP drawing of 5b. Selected bond lengths (A) and angles
(°): W–C1 2.041(6), W–C2 2.030(5), W–C3 1.993(5), W–C4
2.051(5), W–C5 2.048(5), W–C6 2.198(5), O–Ccarb 1.120(6)–
1.142(6), O6–C6 1.368(5), O6–C7 1.374(5), N1–C6 1.349(6), N1–
C12 1.403(5), N1–C13 1.457(5), C7–C12 1.361(6), C13–C14
1.487(6), C14–C15 1.293(7); C6–O6–C7 109.5(3), C6–N1–C12
110.2(3), C6–N1–C13 125.1(4), C12–N1–C13 124.6(4), W–Ccarb–O
176.7(4)–180.0(5), W–C6–O6 118.9(3), W–C6–N1 134.9(3), O6–
C6–N1 106.2(4).
(106.2(4)°) is typical for coordinated cyclic heterocarbe-
nes [8].
3. Experimental
The carbene ligand in 5a and 5b is potentially biden-
tate, as coordination of the alkene moiety of the allyl
substituent can lead to the formation of alkene–car-
bene complexes. Several such complexes with acyclic [6]
and cyclic [9] carbene ligands have been reported, and
their role as intermediates in several catalytic processes
such as olefin metathesis, cyclopropanation of alkenes
and heterogenous Ziegler–Natta polymerization of
alkenes has been outlined extensively [5,6]. In order to
show whether intramolecular alkene coordination is
possible for complexes of the type 5, the tungsten
derivative 5b was heated in toluene under reflux for 3
days. After purification by chromatography, complex 6
containing an h2:h1-(N-allyl-2,3-dihydrobenzoxazol-2-
All operations were performed in an atmosphere of
dry argon by using Schlenk and vacuum techniques.
Scheme 3.
Table 2
Selected 1H NMR data of 5b and 6a,b,c
Compound
H1
H2
trans-H3
cis-H3
5b
5.12 (ddd)
3J=5
6.02 (ddd)
5.40 (dt)
3J=10
4J:1
5.28 (dt)
3J=17
4J:2
3J=17, 10, 5
4J:2, 1
6
4.62 (dd)
2J=12.5
3J=5
4.43 (dd)
2J=12.5
3J=5
4.88 (dddd)
3.46 (d)
3.26 (d)
3J=12, 8, 5, 3
3J=8
3J=12
a l/(ppm), 250 MHz.
b Coupling constants in Hz.
c In CDCl3.

50
M. Tamm, F.E. Hahn / Inorganica Chimica Acta 288 (1999) 47–52
Fig. 2. 1H NMR spectra (allylic resonances only) of 5b and 6 (250 MHz, CDCl3, l in ppm).
Solvents were dried by standard methods and distilled
prior to use. NMR spectra were recorded on a Bruker
AM 250 (250 MHz) instrument. IR spectra were taken
on a Perkin-Elmer 983 instrument in KBr. Elemental
analyses (C, H, N) were performed at the Freie Univer-
sita¨t Berlin on a Heraeus CHN-Rapid elemental ana-
lyzer. Mass spectra (EI, 70 eV) were recorded on a
Varian MAT 711 instrument. Ligand 1 was prepared as
described in Ref. [1].
ArH); 0.38 (s, 9H, SiCH3) ppm. 13C NMR (CDCl3,
62.90 MHz): l 196.5 (trans-CO); 194.1 (cis-CO,
1J(183W–13C)=125 Hz); 153.6 (Ar–NC); 151.8
(COSi); 130.3, 127.3, 121.7, 120.3 (Ar–C); 119.8 (C–
NC); 0.1 (SiCH3) ppm. IR (KBr): w 2148 (CN), 2062,
1980, 1963 (CO) cm−1
.
3.3. Mixture of pentacarbonyl(2-hydroxyphenyl
isocyanide)chromium(0) (3a) and pentacarbonyl-
(2,3-dihydrobenzoxazol-2-ylidene)chromium(0) (4a)
3.1. Pentacarbonyl(2-trimethylsiloxyphenyl isocyanide)-
chromium(0) (2a)
Complex 2a (from 5 g of [Cr(CO)6] and 4.35 g of 1)
was dissolved in 100 ml of methanol. After addition of
a catalytic amount of KF (50 mg) the reaction mixture
was stirred for 12 h. The solvent was evaporated in
vacuo and the residue was purified by chromatography
on Al2O3 (4% H2O). Mixture 3a/4a was eluted with
MeOH/Et2O (1:10). Evaporation of the solvents af-
forded an off-white solid, 4.16 g (60%). 1H NMR
(CD2Cl2, 250 MHz): l 10.44 (s br, NH, 4a); 7.67–6.93
(m, ArH, 3a+4a); 5.58 (s br, OH, 3a). For selected 13C
NMR data, see Table 1. Anal. Calc. for C12H5CrNO6
(M=311.17): C, 46.32; H, 1.62; N, 4.50. Found: C,
46.12; H, 1.94; N, 4.63%. IR (KBr): w 3436 (NH); 2132
(CN) cm−1. MS (EI, 70 eV): m/z (%) 311 (14) [M+].
[Cr(CO)6] (5 g, 23 mmol) dissolved in 180 ml of THF
was irradiated in a photoreactor (high pressure mercury
vapour lamp) for 6 h. To the solution of [Cr(CO)5-
(THF)] was added 1 (4.35 g, 23 mmol) and the reaction
mixture was stirred overnight. The solvent was removed
in vacuo and the residue was used directly for subse-
1
quent reactions. H NMR (CDCl3, 250 MHz): l 7.29
(m, 2H, ArH); 6.98 (m, 2H, ArH); 0.40 (s, 9H, SiCH3)
ppm. 13C NMR (CDCl3, 62.90 MHz): l 216.9 (trans-
CO); 214.6 (cis-CO); 173.0 (Ar–NC); 151.6 (COSi);
130.0, 127.2, 121.6, 120.2 (ArC); 120.7 (C–NC); 0.1
(SiCH3) ppm.
3.2. Pentacarbonyl(2-trimethylsiloxy)phenyl isocyanide)-
tungsten(0) (2b)
3.4. Mixture of pentacarbonyl(2-hydroxyphenyl
isocyanide)tungsten(0) (3b) and pentacarbonyl-
(2,3-dihydrobenzoxazol-2-ylidene)tungsten(0) (4b)
Complex 2b was prepared analogously to 2a from
[W(CO)6] (2.56 g, 7 mmol) and 1 (1.39 g, 7 mmol).
1H NMR (CDCl3, 250 MHz): l 7.46–6.91 (m, 4H,
Complex 2b (from 2.56 g of [W(CO)5]) was used, and
the preparation and isolation followed the procedure

M. Tamm, F.E. Hahn / Inorganica Chimica Acta 288 (1999) 47–52
51
described under Section 3.3. Complexes 3b/4b were
obtained as an off-white solid, 1.67 g (52%). H NMR
crystal 0.34×0.25×0.20 mm, formula C15H9NO6W,
M=483.09 amu, monoclinic, space group P21/c, a=
1
˚
(CD2Cl2, 250 MHz): l 10.43 (s br, NH, 4b); 7.66–6.95
(m, CH, 3b+4b); 5.63 (s br, OH, 3b). For selected 13C
NMR data, see Table 1. Anal. Calc. for C12H5NO6W
(M=443.02): C, 32.53; H, 1.14; N, 3.16. Found: C,
32.84; H, 1.29; N, 3.30%. IR (KBr): w 3433 (NH); 2141
(CN) cm−1. MS (EI, 70 eV): m/z (%) 443 (48) [M+].
7.230(2), b=15.815(3), c=13.703(2) A, i=95.49(3)°,
3
˚
V=1559.6(10) A , Z=4, zexp=2.02, Dcalc=2.057 g
cm−3, Mo Ka radiation (u=0.71073 A, graphite
˚
monochromator), v(Mo Ka)=75.93 cm−1. In total
4541 symmetry independent diffraction data were mea-
sured at 20(2)°C in the 2q range of 2–60°. Structure
solution with Patterson and refinement with Fourier
methods, refinement of positional parameters of all
non-hydrogen atoms with anisotropic thermal parame-
ters. Hydrogen atoms on calculated positions (d(C–
3.5. (N-Allyl-2,3-dihydrobenzoxazol-2-ylidene)
pentacarbonylchromium(0) (5a)
˚
A solution of 3a/4a (805 mg, 2.6 mmol) in 20 ml of
DMF was treated with KOtBu (337 mg, 3.0 mmol) at
−40°C. After stirring for 3 h at room temperature allyl
bromide (363 mg, 3 mmol) was added and stirring
continued overnight. The solvent was evaporated in
vacuo and the residue was purified by chromatography
on Al2O3 (4% H2O) with CH2Cl2/hexane. Recrystalliza-
tion from dichloromethane afforded 5a as colorless
H)=0.95 A, [11]) with Beq(H)=1.3Beq(C). R=0.026,
Rw=0.032 for 3135 absorption corrected (five C-scans)
structure factors Fo2]3|(Fo2) and 208 refined parame-
ters. Neutral atomic scattering factors were used and all
scattering factors were corrected for anomalous disper-
sion [12]. All calculations were carried out with the
MOLEN program package [13].
1
crystals, 745 mg (82%). H NMR (CDCl3, 250 MHz): l
7.68–7.26 (m, 4H, ArH); 6.08 (ddt, 1H, CHꢁCH2); 5.44
(dt, 1H, trans-CHH); 5.32 (dt, 1H, cis-CHꢁCHH); 5.18
(dt, 2H, NCH2) ppm. 13C NMR (CD2Cl2, 62.90 MHz):
l 233.8 (NCO); 221.1 (trans-CO); 216.5 (cis-CO);
152.9, 131.4 (ArC); 130.3 (HCꢁCH2); 125.0, 124.8
(ArC); 119.7 (HCꢁCH2); 111.0, 110.9 (ArC); 50.1
(NCH2) ppm. Anal. Calc. for C15H9CrNO6 (M=
351.24): C, 351.29; H, 2.58; N, 3.99. Found: C, 50.91;
H, 2.80; N, 4.08%. IR (KBr): w 2063, 1978, 1926, 1892
(CO) cm−1. MS (EI, 70 eV): m/z (%) 351 (11) [M+].
4. Supplementary material
Further details of the crystal structure investigation
may be obtained from the Fachinformationszentrum
Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Ger-
many on quoting the depository number CSD-59458
and the journal citation.
Acknowledgements
3.6. (N-Allyl-2,3-dihydrobenzoxazol-2-ylidene)
pentacarbonyltungsten(0) (5b)
Financial support of this work by the Fonds der
Chemischen Industrie, the Deutsche Forschungsge-
meinschaft and the BMBW (Graduiertenkolleg Syn-
these und Strukturaufkla¨rung niedermolekularer
Verbindungen) is gratefully acknowledged.
Complex 5b was prepared analogously to 5a from a
mixture of 3b/4b (368 mg, 0.8 mmol), KOtBu (104 mg,
0.9 mmol) and allyl bromide (112 mg, 0.9 mmol) in 8
ml of DMF. Recrystallization from dichloromethane
afforded 5b as yellowish crystals, 205 mg (53%). 1H
NMR (CDCl3, 250 MHz): l 7.67–7.28 (m, 4H, ArH);
6.02 (ddt, 1H, CHꢁCH2); 5.40 (dt, 1H, trans-CHH);
5.28 (dt, 1H, cis-CHꢁCHH); 5.12 (dt, 2H, NCH2) ppm.
13C NMR (CD2Cl2, 62.90 MHz): l 215.5 (NCO); 201.8
(trans-CO); 197.3 (cis-CO, 1J(183W–13C)=125 Hz);
153.3, 131.1 (ArC); 130.7 (HCꢁCH2); 125.8, 125.6
(ArC); 119.8 (HCꢁCH2); 111.9, 111.6 (ArC); 51.7
(NCH2) ppm. Anal. Calc. for C15H9NO6W (M=
483.09): C, 37.29; H, 1.88; N, 2.90. Found: C, 37.35; H,
2.24; N, 3.14%. IR (KBr): w 2069, 1974, 1925, 1896
(CO) cm−1. MS (EI, 70 eV): m/z (%) 483 (51) [M+].
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