Reactions of hydrido(hydrosilylene)tungsten complexes with α,β-unsaturated carbonyl compounds: Selective formation of (η3-siloxyallyl)tungsten complexes

Article abstract of DOI:10.1021/ja074629e

Hydrido(hydrosilylene)tungsten complexes (C₅Me₄R¹)(CO)₂(H)W=Si(H)[C(SiMe₃)₃] [R¹ = Me (1a), R¹ = Et (1b)] reacted with α,β-unsaturated carbonyl compounds R^2-C(O)CH=CHR³ (R² = Me, R³ = H or Ph; R² = OMe, R³ = H) at room temperature to give exo-anti (η³-siloxyallyl)tungsten complexes (C₅Me₄R¹)(CO)₂W[η³-R³HCCHCR²OSiH₂{C(SiMe₃)₃}] almost quantitatively and regioselectively. Possible reaction mechanisms involving a [2+4] cycloaddition process are proposed. Copyright

Full text of DOI:10.1021/ja074629e

Published on Web 08/23/2007
Reactions of Hydrido(hydrosilylene)tungsten Complexes with r,â-Unsaturated
Carbonyl Compounds: Selective Formation of (η3-Siloxyallyl)tungsten
Complexes
Takahito Watanabe, Hisako Hashimoto,* and Hiromi Tobita*
Department of Chemistry, Graduate School of Science, Tohoku UniVersity, Aoba-ku, Sendai 980-8578, Japan
Received July 9, 2007; E-mail: tobita@mail.tains.tohoku.ac.jp
Chart 1
In contrast to that of transition-metal carbene complexes, the
development of transition-metal silylene complexes has been slow.
Earlier studies on such silylene complexes have concentrated on
their syntheses, structures, and fundamental reactions with simple
nucleophiles.1 Recently, because of improved synthetic methodolo-
gies, a new stage in the development of silylene complexes has
opened.2-5 For example, a cationic silylene-ruthenium complex
has been found to react with enolizable ketones to afford silyl enol
ethers.2 Some silylene complexes, through their M-Si double
carbonyl compounds, the reaction of 1a with cyclohex-2-en-1-one,
bonds, can undergo [2+2] cycloaddition reactions with isocyanates.3
a cyclic derivative, resulted in a complicated mixture of unidentified
products.
Examples of stoichiometric and catalytic hydrosilylations of alkenes
and carbonyl compounds using silylene complexes have also been
demonstrated.4 However, [2+4] cycloaddition reactions involving
silylene complexes have yet to be reported.
All η3-siloxyallyl products were fully characterized using H,
13C, and 29Si NMR spectroscopy and elemental analysis.6 Existence
of the η3-siloxyallyl ligands was indicated by the 1H NMR spectra.
1
1
In the case of 2a, the three H signals in the vicinity of 2 ppm
Recently, we have found the stoichiometric hydrosilylation
reactions of acetone5a and nitriles5b with neutral hydrido(hydro-
silylene)tungsten complexes, Cp′(CO)2(H)WdSi(H)[C(SiMe3)3]
(1a, Cp′ ) Cp*; 1b, Cp′ ) η5-C5Me4Et). Herein, we report on the
new and unique reactions of 1 with R,â-unsaturated carbonyl
compounds to afford η3-siloxyallyl complexes. These reactions are
highly regioselective, nearly quantitative, and, presumably, proceed
via a [2+4] cycloaddition reaction as the main process.
Treatment of hydrido(hydrosilylene)tungsten complex 1a with
methyl vinyl ketone at room temperature resulted in the immediate
and quantitative formation of η3-siloxyallyl complex Cp*(CO)2W-
[η3-H2CCHCMeOSiH2{C(SiMe3)3}] (2a) as the sole product (eq
1).6,7 Similarly, the η5-C5Me4Et analogue, 2b, was obtained by the
reaction of 1b with methyl vinyl ketone.6 Transformations of such
silylene complexes into the corresponding η3-allyl complexes have
yet to be reported.
were assigned to the central methine proton (δ ) 2.08 ppm) and
to the terminal methylene protons (δ ) 1.47 and 2.40 ppm), and
were confirmed using 13C{1H}-1H COSY NMR experiments.6 The
two doublet signals, with AB pattern, at δ ) 4.93 and 5.02 ppm
(2JHH ) 16.5 Hz) were assigned to the diastereotopic Si-H protons
on the siloxy group. The 29Si NMR spectrum of 2a exhibited signals
for the siloxy (δ ) -19.9) and the silyl (δ ) -0.7) groups.
Importantly, in all cases, the formation of a single isomer was
observed among four possible isomers, the syn and anti geometrical
isomers of the siloxy group in the η3-allyl ligand and their exo and
endo rotomers, as shown in Chart 1.8
Structural analysis using X-ray crystallography was employed
to determine the stereochemistries of η3-siloxyallyl complexes 2b,
3b, and 4b.9 The results indicated that the three complexes have
identical stereochemistries6 (ORTEP drawings of 2b and 3b are
shown in Figure 1), and possess the exo-anti configuration as
illustrated in Chart 1. For all three complexes, the C-C bond
between the three allyl carbons [1.405(11)-1.444(10) Å] are shorter
than the C-C single bond of ethane (1.54 Å) but longer than the
C-C double bond of ethene (1.34 Å), thus supporting their η3-
allyl coordination mode. In all cases, the W-C(5) bond is longer
than the W-C(3) and W-C(4) bonds, which can be attributed to
the steric repulsion between the carbonyl ligand and the oxygen
atom of the siloxy group.
All three η3-siloxyallyl complexes are suggested to take the exo-
anti configuration also in solution, because steric repulsion between
the large siloxy and Cp* groups is likely to prevent the formation
of other isomers according to the X-ray structures. Similarities
between the 1H NMR spectra of the Cp* and the η5-C5Me4Et
analogues (Table S1, in the Supporting Information) further indicate
that 2a, 3a, and 4a possess the same exo-anti configuration.6
Interestingly, the anti selectivity of our methodology is opposite to
the syn selectivity for the formation of structurally related Tp-
(CO)2Mo[η3-H2CCHCMeOSitBuMe2] [Tp ) tris-1-(pyrazolyl)-
borate],10 which was obtained using a significantly different route
Our synthetic scheme was subsequently applied to other R,â-
unsaturated carbonyl compounds. The reaction of 1a with ben-
zylideneacetone was complete within 10 min to afford Cp*(CO)2W-
[η3-PhHCCHCMeOSiH2{C(SiMe3)3}] (3a) in 81% isolated yield.
The reaction of 1a with methyl acrylate required more time (24 h)
to afford Cp*(CO)2W[η3-H2CCHC(OMe)OSiH2{C(SiMe3)3}] (4a)
in 82% yield (eq 2). The η5-C5Me4Et analogues 3b and 4b were
similarly obtained.6 In contrast to the linear R,â-unsaturated
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C O M M U N I C A T I O N S
unsuccessful formation of the η3-siloxyallyl complex using 1a and
cyclohex-2-en-1-one, which does not allow the s-cis conformation
required for the [2+4] cycloaddition. Although direct observation
of A has yet to be achieved, there is evidence that indicates the
interaction between the oxygen atom of a ketone with the silylene
silicon atom of 1a. Treatment of 1a with acetone at 250 K resulted
in the formation of Cp*(CO)2(H)2WSi(H)[OC(Me)dCH2][C(SiMe3)3]
(5),12 via coordination of the oxygen atom of acetone to the silylene
silicon, followed by the R-H migration to W, although 5 is
transformed into the hydrosilylation product upon warming to room
temperature.5a This would support that A is a key intermediate of
route 1 (Scheme 1).13
Figure 1. ORTEP drawings of 2b (a) and 3b (b) showing 50% thermal
probability ellipsoids. Hydrogen atoms are omitted for clarity. Selected bond
length (Å) and angles (deg). 2b: W-C(1), 1.966(7); W-C(2), 1.941(7);
W-C(3), 2.298(8); W-C(4), 2.223(7); W-C(5), 2.412(6); C(3)-C(4),
1.427(10); C(4)-C(5), 1.444(10); C(5)-O(3), 1.414(8); O(3)-Si(1), 1.650-
(6). 3b: W-C(1), 1.951(5); W-C(2), 1.941(5); W-C(3), 2.343(4);
W-C(4), 2.215(4); W-C(5), 2.376(4); C(3)-C(4), 1.418(6); C(4)-C(5),
1.434(6); C(5)-O(3), 1.421(5); O(3)-Si(1), 1.661(3).
Acknowledgment. This work was supported by the Ministry
of Education, Culture, Sports, Science and Technology of Japan
[Grants-in-Aid for Scientific Research Nos. 18350027, 18064003,
16750044, 19550056, and 19029003].
Supporting Information Available: Experimental procedures and
characterization data; X-ray crystallographic data (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
Scheme 1. Possible Reaction Mechanisms
References
(1) (a) Tilley, T. D. In The Chemistry of Organic Silicon Compounds; Patai,
S., Rappoport, Z., Eds.; John Wiley & Sons: New York, 1989; Chapter
24. (b) Eisen, M. S. In The Chemistry of Organic Silicon Compounds
Vol. 2; Rappoport, Z., Apeloig, Y., Eds.; John Wiley & Sons: New York,
1998; Chapter 35. (c) Ogino, H. Chem. Rec. 2002, 2, 291. (d) Okazaki,
M.; Tobita, H.; Ogino, H. J. Chem. Soc., Dalton Trans. 2003, 493. (e)
Braunstein, P.; Knorr, M. J. Organomet. Chem. 1995, 500, 21. (f)
Braunstein, P.; Knorr, M.; Stern, C. Coord. Chem. ReV. 1998, 178-180,
903.
(2) Zhang, C.; Grumbine, S. D.; Tilley, T. D. Polyhedron 1991, 10, 1173.
(3) Mitchell, G. P.; Tilley, T. D. J. Am. Chem. Soc. 1997, 119, 11236.
(4) (a) Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21,
4065. (b) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2002,
21, 4648. (c) Glaser, P. J.; Tilley, T. D. J. Am. Chem. Soc. 2003, 125,
13640.
(5) (a) Watanabe, T.; Hashimoto, H.; Tobita, H. Angew. Chem., Int. Ed. 2004,
43, 218. (b) Watanabe, T.; Hashimoto, H.; Tobita, H. J. Am. Chem. Soc.
2006, 128, 2176.
involving the reaction between (DMF)3Mo(CO)3 and methyl vinyl
ketone, followed by the addition of BuMeSiCl and KTp.
t
(6) See the Supporting Information for details.
Two most possible reaction mechanisms that explain the high
stereoselective formation of the anti-η3-siloxyallyl complex are
illustrated in Scheme 1. In route 1, the enone is coordinated to the
silicon atom of the silylene ligand of 1a by the carbonyl oxygen
atom to form intermediate A, which undergoes a [2+4] cyclo-
addition via a six-membered transition state to give intermediate
B. Subsequently, B undergoes a Si-H reductive elimination to form
16-electron η1-allyl intermediate C, which is then saturated via
coordination of the intramolecular C-C double bond, followed by
rearrangement to form anti-η3-siloxyallyl complex 2a. In route 2,
the 16-electron silyl complex D, which is formed from 1a by a
1,2-hydrogen migration from W to Si, reacts with the enone to
afford C, which then rearranges to 2a.
Murai et al. have previously reported on the reaction between
silyl complex (CO)4CoSiMe3 and methyl vinyl ketone to give
(CO)3Co[η3-H2CCHCMeOSiMe3] as a mixture of the syn and anti
isomers (syn, 12% yield; anti, 46% yield). For reactions with other
R,â-unsaturated carbonyl compounds, the syn/anti selectivity greatly
depends on the substituents. Because details of the mechanism
remain unclear,11 it is difficult to select route 2 (via silyl intermediate
D) as the only mechanism to describe the formation of the anti
isomer. In our opinion, route 1 is more likely, not only because of
the exclusive formation of the anti isomer, but also because of the
(7) 2a: 1H NMR (300 MHz, C6D6): δ ) 0.33 (s, 27H, SiMe), 1.47 (dd, 1H,
2
3
η3-H2CCHCMe, JHH ) 3.3 Hz, JHH ) 6.9 Hz), 1.55 (s, 15H, C5Me5),
2.08 (dd, 1H, η3-H2CCHCMe, JHH ) 9.1 Hz, JHH ) 6.9 Hz), 2.19 (s,
3
3
3H, η3-H2CCHCMe), 2.40 (dd, 1H, η3-H2CCHCMe, 2JHH ) 3.3 Hz, 3JHH
) 9.1 Hz), 4.93 (d, 1H, SiH, 2JHH ) 16.5 Hz), 5.02 (d, 1H, SiH, 2JHH
)
16.5 Hz).
(8) For the definition of exo and anti isomers of η3-allyl complexes, see:
Ariafard, A.; Lin, Z. Organometallics 2005, 24, 3800.
(9) Crystal data (150 K) for 2b: C27H52O3Si4W; fw ) 720.90; orthorhombic;
space group Pna21 (No. 33); a ) 13.8869(4) Å, b ) 8.9296(4) Å, c )
26.3746(8) Å, V ) 3270.6(2) Å3, density (calcd) 1.464 Mg/m3, Z ) 4.
Final R indices R ) 0.0374, Rw ) 0.0851 for all data, 6991 unique
reflections. 3b: C33H56O3Si4W; fw ) 796.99; monoclinic; space group
P21/c (No. 14); a ) 8.2146(4) Å, b ) 31.6002(11) Å, c ) 14.2624(3) Å,
â ) 89.5622(18)°, V ) 3702.2(2) Å3, density (calcd) 1.430 Mg/m3, Z )
4. Final R indices R ) 0.0469, Rw ) 0.1052 for all data, 8186 unique
reflections. For the data of 4b, see the Supporting Information. Crystal-
lographic information has been deposited with the Cambridge Crystal-
lographic Data Centre (CCDC No. 608679 for 2b, No. 608680 for 3b,
and No. 608681 for 4b).
(10) Ward, Y. D.; Villanueva, L. A.; Allred, G. D.; Liebeskind, L. S.
Organometallics 1996, 15, 4201.
(11) Chatani, N.; Yamasaki, Y.; Murai, S.; Sonoda, N. Tetrahedron Lett. 1983,
24, 5649.
(12) Complex 5 was not isolable but characterized by 1H, 13C, and 29Si NMR
data (250 K).6
(13) Besides the mechanisms described in Scheme 1, the possibility of the
[2+2] cycloaddition mechanism suggested by a reviewer cannot be ruled
out. But we think at present that the mechanisms described in Scheme 1
are more preferable, because the [2+2] cycloaddition mechanism cannot
fully explain the absolute anti selectivity of the products.
JA074629E
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