Mechanistic investigation of the hydration reaction of [Cr(η/ 6-silabenzene)(CO)3]: Hydrido(silacyclohexadienyl)chromium complex as an intermediate

Article abstract of DOI:10.1246/cl.2008.720

A key intermediate for the hydration reaction of [Cr(η⁶- silabenzene)(CO)₃], hydrido(η⁵-silacyclohexadienyl) chromium complex, was successfully synthesized by the reaction of silabenzene complex with 2mol equiv of H₂O. X-ray diffraction study revealed that the hydroxy group on the central silicon atom is located at the endo position to the metal moiety. The treatment of the endo adduct with excess of D ₂O resulted in the stereoselective formation of 1-silacyclohexa-2,4- diene bearing a deuterium atom at the methylene position. Copyright

Full text of DOI:10.1246/cl.2008.720

720
Chemistry Letters Vol.37, No.7 (2008)
Mechanistic Investigation of the Hydration Reaction of [Cr(ꢀ6-silabenzene)(CO)3]:
Hydrido(silacyclohexadienyl)chromium Complex as an Intermediate
Yusuke Tanabe, Yoshiyuki Mizuhata, and Norihiro Tokitohꢀ
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received April 11, 2008; CL-080377; E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp)
A key intermediate for the hydration reaction of [Cr(ꢀ6-
try around the central silicon atom was confirmed by the NOE
experiments and X-ray crystallographic analysis (Figure 1a).8
Both studies indicated that the OH group was oriented at the en-
do position toward the Cr(CO)3 unit. In the 1H NMR spectrum of
5 in C6D6, the signals assignable to the protons of the silacyclo-
hexadienyl ring were observed at 5.30 (t, H3), 4.97 (dd, H2 and
H4), and 2.15 (dd, H1 and H5) ppm, which were similar to those
reported for silacyclohexadienyl complexes.9 The characteristic
signal assignable to that of Cr–H was observed at ꢁ9:04 ppm (t),
which was reasonable for the chemical shift of a terminal hy-
dride bound to a chromium atom.10 Interestingly, H1 and H5
protons and Cr–H are coupling with each other (J ¼ 9:0 Hz),
indicating that the hydrido(silacyclohexadienyl) complex 5
could be regarded as an agostic complex (eq 4).11 In the
13C NMR spectrum of 5, the signal of the carbons in the carbonyl
ligands was observed as only one signal at 231.3 ppm, suggest-
ing the existence of rapid rotation of Cr(CO)3 unit. In IR
spectrum of 5, the frequencies of the carbonyl stretchings
(1991, 1910, and 1889 cmꢁ1) are smaller than those observed
for the related complex [Cr(ꢀ5-cyclopentadienyl)(CO)3(H)]10
(2018, 1947, and 1938 cmꢁ1). This result indicated that the
electron-donating ability of the ꢀ5-silacyclohexadienyl ligand
was higher than that of the ꢀ5-cyclopentadienyl ligand.
silabenzene)(CO)3], hydrido(ꢀ5-silacyclohexadienyl)chromium
complex, was successfully synthesized by the reaction of sila-
benzene complex with 2 mol equiv of H2O. X-ray diffraction
study revealed that the hydroxy group on the central silicon atom
is located at the endo position to the metal moiety. The treatment
of the endo adduct with excess of D2O resulted in the stereose-
lective formation of 1-silacyclohexa-2,4-diene bearing a deuteri-
um atom at the methylene position.
The chemistry of chromium tricarbonyl complexes having
an arene ligand has been extensively studied, and their unique
reactivities have been revealed.1 For example, the reactions of
[Cr(ꢀ6-benzene)(CO)3] with alkyllithiums as a nucleophile are
known to produce exo adducts, Li[Cr(ꢀ5-cyclohexadienyl)-
(CO)3], stereoselectively.2 Further reactions of the anionic
complexes with electrophiles resulted in the stereoselective
formations of the corresponding cyclohexa-1,3-dienes.3 On the
other hand, we have already reported the syntheses of various
metallaaromatic compounds containing a heavier group 14 ele-
ment by taking advantage of an efficient steric protection group,
2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (denoted as Tbt) and
their applications as an arene ligand for group 6 transition metal
complexes.4 We have synthesized a stable silabenzene complex,
[Cr(ꢀ6-silabenzene)(CO)3] 1, and found that the reaction of 1
with excess of water gave silanol 3 regioselectively (eq 1).4h
This result was in sharp contrast with that for the similar reaction
of free silabenzene 2,5 which resulted in the competitive 1,2- and
1,4-addition of water (eq 2). Herein, we report the detailed
mechanistic studies on the hydration and related reactions of
the silabenzene complex 1, which revealed that hydrido(silacy-
clohexadienyl) complex 5 is a key intermediate for the reaction.
Tbt
OR
1
1
2
2
Si
Si
ROX
Si Tbt
CO
OR
Tbt
(2 equiv)
3
3
(3)
Cr0
5
5
4
4
C6H6, r.t.
CrII
CrII
OC
OC
OC
OC
OC
X
CO
X
CO
CO
1
5 (82%, R = X = H)
-
-
5-d2 (69%, R = X = D)
6 (80%, R = Me, X = H) 7 (R = Me, X = H)
H1
Tbt
H
Tbt
OH
OH
H5
Si
Si
Tbt
Si
(4)
H
Tbt OH
Si
Tbt OH
Si
OH
CO
CO
Si Tbt
0
Cr
0
CrII
H2O (excess)
C6H6, r.t.
Cr
OC
OC
H
(1)
CO
OC
OC
Cr
CO
CO
CO
OC
CO
The reaction of 1 with 2 mol equiv of D2O gave deuteride-
4
(-)
1
3
(72%)
(silacyclohexadienyl)chromium complex 5-d2 in 69% yield.
1H and 2H NMR studies showed the product contained 81%
deuterium at the OH group and Cr–H moiety, suggesting those
H atoms were surely derived from water. The treatment of 1 with
2 mol equiv of methanol resulted in the formation of the endo
and exo adducts, 6 and 7, respectively, in the ratio of 10:1.12
The structure of endo adduct 6 was determined by 1H, 13C,
and 29Si NMR spectra, elemental analysis, and X-ray diffraction
study (Figure 1b).8
In general, nucleophilic attack to ꢀ6-benzene chromium tri-
carbonyl complexes are known to afford the corresponding exo
adducts due to the steric hindrance of Cr(CO)3 unit.2 Therefore,
the fact that the endo adduct was obtained as a major product
Tbt
Si
H2O (excess)
THF, r.t.
(2)
3
4
(30%)
(54%)
2
First of all, the reaction of 1 with 2 mol equiv of H2O was
performed. As shown in eq 3, the reaction afforded the hydrido-
(silacyclohexadienyl) complex 5 in 82% yield stereoselective-
ly.6,7 This is the first chromium complex bearing a silacyclohex-
adienyl ligand. The complex 5 was characterized based on the
1
results of H, 13C, and 29Si NMR and IR spectroscopic studies
together with elemental analysis. In addition, the stereochemis-
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.7 (2008)
721
Tbt Reductive
Tbt
OH
H
Elimination
Si
Si
OH
D2O
(excess)
Tbt OH
Si
H
H
CrII
Cr 0
H
– [CrL (CO) ]
OC
OC
H
3
3
OC
OC
L
CO
CO
5
3
H/D
scrambling
D2O
(excess)
L = D2O
Reductive
Tbt
Tbt
OD
Elimination
D2O
Si
Si
H
OD
Tbt OH
Si
(excess)
H
D
D
CrII
.
– [CrL (CO) ]
3 3
OC
OC
Cr 0
Figure 1. ORTEP drawings of 5 (a) and [6 0.5C6H6] (b) (30%
probability). The hydrogen atoms and benzene molecule were
omitted for clarity.
D
CO
5-d2
OC
OC
L
CO
(R*,R*)-3-d1
Scheme 1.
even in the reaction with a larger molecule than water such as
methanol suggested the selectivity observed for the hydration
reaction of 1 is most likely due to the interaction between the
ROX and Cr(CO)3 units.
entific Research (No. 17GS0207), and the Global COE Program
(B09, ‘‘Integrated Material Science’’), from the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
As an alternative mechanism for the conversion of 1 to 5, the
reaction pathway including the demetallation/recombination
steps can be conceivable. However, the fact that [Cr(ꢀ6-
benzene)(CO)3] did not react with 3 in benzene clearly showed
that this is not the case but the Cr–H complex 5 is derived from 1
without dissociation of the SiC5 ring from the Cr center.13
When the isolated 5 was treated with excess of D2O, the
reaction furnished the final product, 1-silacyclohexa-2,4-diene
(Rꢀ; Rꢀ)-3-d1 stereo- and regioselectively (eq 5). This result
strongly suggests that the conversion of 1 to 3 proceeds via the
hydrido(silacyclohexadienyl) complex 5. The regioselective for-
mation of 1,2-adduct would be derived from the high contribu-
tion of the conjugated 1,3-diene intermediate as well as the equi-
librium of complex 5 (eq 4). 1H and 2H NMR studies of (Rꢀ; Rꢀ)-
3-d1 in benzene-d6 and benzene showed 90% incorporation of D
atom for the methylene position in the SiC5 ring. On the other
hand, the reaction of 5-d2 (81% deuterium incorporation at each
position) with excess of H2O afforded (Rꢀ; Rꢀ)-3-d1 with 30%
deuterium content at the methylene position. A plausible mech-
anism for the reaction of 5 with D2O was postulated as Scheme 1.
Formal reductive elimination of 5 leading to the formation of
3-d1 induced by D2O may occur with a rapid H/D scrambling
of Cr–H in 5. Actually, during the reaction of 5 with slight excess
of D2O in a sealed tube, 1H NMR monitoring showed the
conversion of unreacted starting complex 5 to 5-d2.12
References and Notes
1
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¨
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d) M. Rosillo, G. Domınguez, J. Perez-Castells, Chem. Soc. Rev.
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´
2
For the X-ray studies of exo adducts: a) M. F. Semmelhack, H. T. Hall,
Jr., R. Farina, M. Yoshifuji, G. Clark, T. Bargar, K. Hirotsu, J. Clardy,
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3
4
E. P. Kundig, A. F. Cunningham, Jr., P. Paglia, D. P. Simmons, G.
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Bernardinelli, Helv. Chim. Acta 1990, 73, 386.
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K. Inamura, W. Hoshino, N. Nakata, Y. Mizuhata, Y. Watanabe,
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For the complexes, see: g) N. Nakata, N. Takeda, N. Tokitoh, Angew.
Chem., Int. Ed. 2003, 42, 115. h) A. Shinohara, N. Takeda, T. Sasamori,
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5
a) K. Wakita, N. Tokitoh, R. Okazaki, S. Nagase, Angew. Chem., Int.
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Tbt
Si
OH
6
7
8
The reaction of 1 with 1 mol equiv of H2O in C6D6 resulted in the 50%
conversion of 1 to 5.
When the reaction of 1 with 2 mol equiv of H2O in THF was performed,
1:1 mixture of endo and exo adducts was obtained.
Crystallographic data reported in this manuscript have been deposited
with Cambridge Crystallographic Data Centre as supplementary
Tbt OH
D2O (excess)
H
D
Si
(5)
CrII
C6H6, r.t.
67%
OC
OC
H
CO
5
(R*,R*)-3-d1
.
publication nos. CCDC-687315 (5) and 687314 ([6 0.5C6H6]).
J. M. Dysard, T. D. Tilley, T. K. Woo, Organometallics 2001, 20, 1195,
In summary, we have revealed the reactivity of the silaben-
zene complex 1 toward water in detail and succeeded in the iso-
lation of the first chromium complex bearing a silacyclohexa-
dienyl ligand as an intermediate in the conversion of 1 to 3.
The present studies clearly showed the unique reactivities of
1 compared with its carbon analogues. Further study on the
reactivities of 1 toward various reagents is underway.
9
and references cited therein.
10 H. G. Alt, H. E. Engelhardt, W. Klaui, A. Muller, J. Organomet. Chem.
¨
¨
1987, 331, 317.
11 E. P. Kundig, D. Amurrio, G. Bernardinelli, R. Chowdhury, Organo-
¨
metallics 1993, 12, 4275.
12 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/index.html.
13 The reaction of [Cr(ꢀ6-benzene)(CO)3] with silacyclohexa-1,4-diene 4
did not proceed.
This work was partially supported by Grants-in-Aid for Sci-
Published on the web (Advance View) June 7, 2008; doi:10.1246/cl.2008.720