Insertion of carbon monoxide into an aldehyde CO double bond induced by an (η3-α-silabenzyl)carbonylmolybdenum complex

Article abstract of DOI:10.1039/c4ra02308k

Unusual silylcarbonylation of aldehyde through CO insertion into a CO double bond induced by a metal-silicon complex has been discovered: treatment of an (η³-α-silabenzyl)carbonylmolybdenum complex with alkylaldehydes afforded isolable O-silylated Mo-C-O three-membered-ring complexes, and one of them was subsequently converted to an η³- oxaallyl complex at room temperature through CO insertion into the metallacycle C-O bond and incorporation of a CO molecule. the Partner Organisations 2014.

Full text of DOI:10.1039/c4ra02308k

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Insertion of carbon monoxide into an aldehyde
C]O double bond induced by an (h3-a-silabenzyl)-
carbonylmolybdenum complex†
Cite this: RSC Adv., 2014, 4, 19068
Received 17th March 2014
Accepted 10th April 2014
*
Yuto Kanno, Takashi Komuro and Hiromi Tobita
DOI: 10.1039/c4ra02308k
www.rsc.org/advances
Unusual silylcarbonylation of aldehyde through CO insertion into a Si(p-Tol)3} (1; Cp* ¼ h5-C5Me5, p-Tol ¼ p-C6H4Me) bearing a
C]O double bond induced by a metal–silicon complex has been unique h3-coordination of silicon and two aromatic carbons to
discovered: treatment of an (h3-a-silabenzyl)carbonylmolybdenum molybdenum.4 Since the coordinated arene moiety of 1 is labile,
complex with alkylaldehydes afforded isolable O-silylated Mo–C–O complex 1 serves as a synthetic equivalent of a reactive, coor-
three-membered-ring complexes, and one of them was subsequently dinatively unsaturated silyl complex Cp*Mo(CO)2{Si(p-Tol)3} (10)
converted to an h3-oxaallyl complex at room temperature through CO (Scheme 2).4
insertion into the metallacycle C–O bond and incorporation of a CO
This time, we examined the reactions of 1 with alkylalde-
hydes. Complex 1 reacted with RCHO (R ¼ i-Pr and t-Bu) quickly
at room temperature in toluene to give Mo–C–O three-
membered-ring complexes Cp*Mo(CO)2{h2(C,O)–CH(R)OSi(p-
Tol)3} (2: R ¼ i-Pr and 3: R ¼ t-Bu) in 83% and 76% yields,
respectively (Scheme 3). Complex 2 was obtained as a mixture of
two diastereomers 2a (major) and 2b (minor) in a 5 : 1 molar
ratioꢁ (determined by 1H NMR spectroscopy in CD2Cl2 at
ꢀ33 C),5 whereas complex 3 was obtained as a single diaste-
reomer. Although a few O-silylated M–C–O three-membered-
ring complexes have previously been reported,6 2 and 3 are the
rst examples of this type of complex synthesised by reactions
of metal–silicon complexes with carbonyl compounds.
molecule.
Carbon monoxide is a simple and widely-used C1 building
block for production of organic compounds such as hydro-
formylation of unsaturated organic molecules catalysed by
transition-metal complexes.1 As a catalytic transformation
reaction involving incorporation of not only a CO molecule but
also a silyl group into organic molecules, Murai et al. have
developed silylformylation of aldehydes catalysed by
a
Co2(CO)8–PPh3 mixture where aldehydes react with tertiary
silane and CO to give a-(siloxy)aldehydes (Scheme 1).2 Wright
et al. then found that some rhodium complexes are also cata-
lytically active for the silylformylation.3 A reasonable mecha-
nism for these catalytic reactions involves CO insertion into an
M–C(OSiR3) (M ¼ Co or Rh) bond of an a-(siloxy)alkyl complex,
which is formed by migratory insertion of an aldehyde C]O
double bond into an M–SiR3 bond of a silyl-complex interme-
diate.2,3 In this study, we discovered a new stoichiometric
silylcarbonylation reaction of aldehyde induced by a metal–
silicon complex, which proceeds through insertion of a CO
molecule into an aldehyde C]O double bond (Scheme 1).
We are investigating the synthesis and reactivity of an (h3-a-
silabenzyl)carbonylmolybdenum complex Cp*Mo(CO)2{h3(Si,C,C)-
Recrystallization of a mixture of 2a and 2b from hexane at
ꢀ35 ꢁC afforded an orange single crystal involving only 2a. The
X-ray crystal structure analysis of 2a revealed the existence of a
Mo–C–O three-membered ring on which the i-Pr group is anti to
the Cp* ligand (Fig. 1).‡ The Mo(1)–C(3) and Mo(1)–O(1)
Department of Chemistry, Graduate School of Science, Tohoku University, Sendai
980-8578, Japan. E-mail: tobita@m.tohoku.ac.jp; Fax: +81 22 795 6543; Tel: +81 22
795 6539
† Electronic supplementary information (ESI) available: Experimental details on
synthesis and characterisation of 2–4. CCDC 990556 and 990557. For ESI and
crystallographic data in CIF or other electronic format see DOI:
10.1039/c4ra02308k
Scheme 1 Illustrations of catalytic silylformylation of aldehydes (top)2,3
and silylcarbonylation of aldehyde via CO insertion into a C]O double
bond developed in this work (bottom).
19068 | RSC Adv., 2014, 4, 19068–19071
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
those of the corresponding carbons for metallacycles A [d 75.6
(R0 ¼ H) and 77.6 (R0 ¼ Me)].8b This implies that the metallacycle
carbons in 2 and 3 have some carbene character as drawn in
Scheme 4 by a resonance between two canonical forms, i.e.,
alkyl(silyl ether) form B and carbene(siloxy) form C.
To determine the stereochemistry of diastereomers 2a and
2b, the 1H–1H NOESY spectrum of the mixture was measured at
Scheme 2 Complex 1 as a synthetic equivalent of coordinatively
unsaturated complex 10.
ꢁ
ꢀ33 C. The major isomer 2a shows a pair of NOE cross peaks
between the 1H signals of the metallacycle C–H (d 3.16) and Cp*
(d 1.65), indicating that these protons are spatially close to each
other. Thus, the i-Pr group on the metallacycle carbon in 2a is
anti to the Cp* ligand, and this arrangement agrees with that of
the molecule in the crystal (Fig. 1). The NOESY spectrum of 3 at
ꢁ
ꢀ33 C exhibits a pair of strong cross peaks between the met-
allacycle C–H (d 3.62) and the Cp* (d 1.60) signals together with
weak cross peaks between the t-Bu (d 0.71) and the Cp* signals.
This implies that the geometric arrangement of substituents on
the metallacycle of 3 is the same as that of 2a.
Complexes 2 and 3 are thermally unstable both in solution
and in the solid state at room temperature. Although products
of the thermal conversion of 2 were not characterised, the main
product of that of 3 was isolated and structurally determined.
When the reaction of 1 with 2,2-dimethylpropanal was per-
formed at room temperature for a longer period (5 h), the colour
of the solution initially changed from dark purple of 1 to orange
of 3, and then gradually changed to dark-red of 4. A siloxy-
substituted h3-oxaallyl complex Cp*Mo(CO)2[h3(O,C,C)-OC
{OSi(p-Tol)3}CH(t-Bu)] (4) was isolated as reddish purple crystals
from the reaction mixture in 40% yield (Scheme 3).10 The yield
of 4 was improved to 75% NMR yield when the same reaction
was carried out under CO atmosphere (1 atm) in C6D6 (see ESI†
for details). This result as well as the structure of 4 clearly
indicates that one molecule of CO is incorporated to 3 either
from another molecule of 3 or by the reaction with free CO to
form 4. The NMR-tube reaction of isolated 3 in C6D6 at room
temperature also afforded complex 4 in 41% NMR yield, which
conrms that 3 is an intermediate for the formation of 4
(see ESI†).
Scheme 3 Synthesis of Mo–C–O three-membered-ring complexes
2, 3 and conversion of 3 to h3-oxaallyl complex 4. Conditions: (i)
toluene, r.t., 1 min (ii) toluene, r.t., 5 h.
Fig. 1 Crystal structure of 2a. One of the two independent molecules
2a-A and 2a-B in the asymmetric unit, i.e., molecule 2a-A, is depicted.
Selected interatomic distances (A˚) and angles (ꢁ): Mo(1)–O(1) 2.175(4),
Mo(1)–C(3) 2.167(6), O(1)–C(3) 1.434(7), O(1)–Mo(1)–C(3) 38.56(19).7
Single crystal X-ray analysis revealed that 4 adopts a four-
legged piano-stool geometry composed of h5-C5Me5, two
carbonyl and h3-oxaallyl ligands (Fig. 2).‡ The Mo(1)–O(3),
Mo(1)–C(3) and Mo(1)–C(4) distances (2.186(4), 2.396(5) and
7
˚
2.343(6) A, respectively) for the oxaallyl ligand clearly indicate
3
distances [2.167(6) and 2.175(4) A, respectively] support the its h -coordination.11 The O(3)–C(3) and C(3)–C(4) distances
7
˚
h2-coordination of the siloxymethyl ligand.8
(1.284(7) and 1.403(9) A, respectively) are also close to the
7
˚
Each of the 1H NMR spectra of 2a, 2b and 3 in C6D6 at room corresponding distances (1.302(4) and 1.435(5) A, respectively)
temperature shows a doublet (for 2a,b) or a singlet (for 3) signal for an (h3-oxaallyl)tungsten complex CpW(CO)2{h3(O,C,C)-
assignable to the methine proton on the metallacycle carbon at OC(NEt2)CH2}.11a
d 3.49, 4.12 and 3.94, respectively. These chemical shis are
˚
close to those of the corresponding signals for related O-alky-
lated Mo–C–O three-membered-ring complexes (h5-C5H4R0)
Mo(CO)2{h2(C,O)-cyclo-CH(CH2)3O} (A) [d 4.22 (R0 ¼ H) and 4.05
(R0 ¼ Me)].8b In the 13C{1H} NMR spectra of 2a and 3 in CD2Cl2 at
ꢁ
ꢀ33 C, a signal assignable to the metallacycle carbon appears
at d 87.5 (2a) and 93.8 (3). These signals are shied downeld in
comparison with that of the methine carbon of the a-siloxyalkyl
Scheme
4 Two possible canonical forms of Mo–C–O three-
ligand in CpMo(CO)2(PPh3){CH(OSiHPh2)(CH3)} (d 69.24)9 and membered-ring complexes 2 (R ¼ i-Pr) and 3 (R ¼ t-Bu).
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 19068–19071 | 19069

View Article Online
RSC Advances
Communication
et al. in the reduction of thioamides with hydrosilanes catalysed
by an iron complex CpFe(CO)2Me.12 The carbene ligand then
couples with a CO ligand in E to produce an h2-ketene complex
F.13 Finally, migration of the siloxy ligand in F to the carbonyl
carbon of the ketene ligand (forming G) followed by incorpo-
ration of CO as a ligand gives complex 4. By this reaction
mechanism, a CO molecule is inserted into an aldehyde C]O
double bond.
In summary, stoichiometric silylcarbonylation of aldehyde
induced by (h3-a-silabenzyl)carbonylmolybdenum complex 1
gave h3-oxaallyl complex 4 via an O-silylated Mo–C–O three-
membered-ring intermediate 3. This result implies that metal–
silicon complex 1 is potentially useful as a reagent transforming
carbonyl compounds through cleavage of their C]O double
bonds.
This work was supported by Grants-in-Aid for Scientic
Research (Grant numbers: 23750053, 22350024 and 25410058)
from the Japan Society for the Promotion of Science (JSPS). We
are grateful to Dr Eunsang Kwon (Tohoku University) for his
help with the X-ray analysis of 2a. We also acknowledge the
Research and Analytical Center for Giant Molecules, Tohoku
University, for spectroscopic measurements and elemental
analysis.
Fig. 2 Crystal structure of 4. One of the two independent molecules
4-A and 4-B in the asymmetric unit, i.e., molecule 4-A, is depicted.
Selected interatomic distances (A˚) and angles (ꢁ): Mo(1)–O(3) 2.186(4),
Mo(1)–C(3) 2.396(5), Mo(1)–C(4) 2.343(6), O(3)–C(3) 1.284(7), O(4)–
C(3) 1.355(7), C(3)–C(4) 1.403(9), O(3)–Mo(1)–C(4) 61.27(18).7
1
The H NMR spectrum of 4 shows a signal for the methine
proton of the oxaallyl ligand at d 2.30. This chemical shi is in
the range of those for h3-oxaallyl complexes of molybdenum
and tungsten (d 1.72–3.72).11a In the 13C{1H} NMR spectrum of
4, signals assignable to the central and terminal carbons of the
oxaallyl ligand appear at d 155.0 and 63.7, respectively.11 These
observations also support the structure of 4 bearing an h3-
oxaallyl ligand.
Notes and references
A possible mechanism for the reaction of 1 with alkylalde-
hydes leading to complexes 2, 3 and subsequent conversion of 3
to complex 4 is illustrated in Scheme 5. Dissociation of the
coordinated aryl carbons in 1 to generate 10 (see Scheme 2)
followed by h2-coordination of an aldehyde molecule to
molybdenum gives silyl complex D. The silyl ligand in D then
migrates to the oxygen of the aldehyde to yield complexes 2 and
3. In the case of 3, the metallacycle C–O bond is cleaved to
generate carbene(siloxy) complex E. A closely related cleavage of
a carbon–sulfur bond in S-silylated Fe–C–S three-membered-
ring intermediates has recently been proposed by Nakazawa
‡ Crystallographic data for 2a: C37H44O3SiMo, M ¼ 660.75, triclinic, space group
ꢀ
˚
P1 (no. 2), a ¼ 8.6566(3), b ¼ 10.9696(4), c ¼ 35.7255(11) A, a ¼ 95.4889(13), b ¼
3
ꢁ
˚
92.2032(14), g ¼ 97.1362(15) , V ¼ 3346.4(2) A , Z ¼ 4, T ¼ 90 K, m(Cu-Ka) ¼ 3.806
mmꢀ1, 31 297 reections were measured, 11 121 unique (Rint ¼ 0.0239), R1
¼
0.0740 and wR2 ¼ 0.1674 (all data). We could not obtain a sufficient amount of
reections for 2a: data completeness to 2q ¼ 139.42ꢁ is 88.0%. This is caused by
the gradual degradation of crystallinity of the sample under X-ray irradiation. 4:
C
39H46O4SiMo, M ¼ 702.79, monoclinic, space group P21 (no. 4), a ¼ 12.8069(3), b
3
ꢁ
˚
˚
¼ 13.1036(4), c ¼ 21.5568(7) A, b ¼ 90.7174(4) , V ¼ 3617.30(18) A , Z ¼ 4, T ¼ 150
K, m(Mo-Ka) ¼ 0.433 mmꢀ1, 51 709 reections were measured, 15 836 unique (Rint
¼ 0.1025), R1 ¼ 0.0853 and wR2 ¼ 0.1364 (all data).
1 (a) W. Keim, Pure Appl. Chem., 1986, 58, 825; (b)
H. M. Colquhoun, D. J. Thompson and M. V. Twigg, in
Carbonylation: Direct Synthesis of Carbonyl Compounds,
Plenum Press, New York, 1991.
2 (a) S. Murai, T. Kato, N. Sonoda, Y. Seki and K. Kawamoto,
Angew. Chem., Int. Ed. Engl., 1979, 18, 393; (b) Y. Seki,
S. Murai and N. Sonoda, Angew. Chem., Int. Ed. Engl., 1978,
17, 119.
3 (a) M. E. Wright and B. B. Cochran, J. Am. Chem. Soc., 1993,
115, 2059; (b) M. E. Wright and B. B. Cochran,
Organometallics, 1996, 15, 317.
4 T. Komuro, Y. Kanno and H. Tobita, Organometallics, 2013,
32, 2795.
5 It has not been claried whether the equilibrium between 2a
and 2b exists or not in solution.
6 (a) S. Ogoshi, M. Oka and H. Kurosawa, J. Am. Chem. Soc.,
2004, 126, 11802; (b) S. Ogoshi, H. Kamada and
H. Kurosawa, Tetrahedron, 2006, 62, 7583.
7 Selected bond distances and angles for one of the two
crystallographically independent molecules of 2a or 4 in
Scheme 5 A possible formation mechanism of Mo–C–O three-
membered-ring complexes 2, 3 and h3-oxaallyl complex 4.
19070 | RSC Adv., 2014, 4, 19068–19071
This journal is © The Royal Society of Chemistry 2014

View Article Online
Communication
RSC Advances
the asymmetric unit were only described in the text because
the geometric parameters of these molecules do not reveal
signicant difference (see Fig. S1 and S2 in ESI†).
G. A. Slough, R. Hayashi, J. R. Ashbaugh, S. L. Shamblin
and A. M. Aukamp, Organometallics, 1994, 13, 890 and
references cited therein.
8 (a) H. Adams, N. A. Bailey, P. Cahill, D. Rogers and 12 (a) K. Fukumoto, A. Sakai, T. Oya and H. Nakazawa, Chem.
M. J. Winter, J. Chem. Soc., Chem. Commun., 1983, 831; (b)
H. Adams, N. A. Bailey, P. Cahill, D. Rogers and
M. J. Winter, J. Chem. Soc., Dalton Trans., 1986, 2119; (c)
Commun., 2012, 48, 3809; (b) K. Fukumoto, A. Sakai,
K. Hayasaka and H. Nakazawa, Organometallics, 2013, 32,
2889.
´
´
A. Pizzano, L. Sanchez, E. Gutierrez, A. Monge and 13 Similar carbene–CO coupling reactions have been reported.
E. Carmona, Organometallics, 1995, 14, 14.
9 M. Akita, O. Mitani, M. Sayama and Y. Moro-oka,
Organometallics, 1991, 10, 1394.
10 The yield of 4 was calculated assuming that one molecule of
4 is formed from one molecule of 3.
11 (a) E. R. Burkhardt, J. J. Doney, R. G. Bergman and
C. H. Heathcock, J. Am. Chem. Soc., 1987, 109, 2022; (b)
For selected examples, see: (a) W. A. Herrmann and J. Plank,
Angew. Chem., Int. Ed. Engl., 1978, 17, 525; (b) H. Fischer,
Angew. Chem., Int. Ed. Engl., 1983, 22, 874; (c) T. W. Bodnar
and A. R. Cutler, J. Am. Chem. Soc., 1983, 105, 5926; (d)
D. B. Grotjahn, G. A. Bikzhanova, L. S. B. Collins,
T. Concolino, K.-C. Lam and A. L. Rheingold, J. Am. Chem.
Soc., 2000, 122, 5222.
This journal is © The Royal Society of Chemistry 2014
RSC Adv., 2014, 4, 19068–19071 | 19071