Catalytic Homologation of Vinyltributylstannane to Allyltributylstannane by Mo(IV) Complexes in the Presence of Ethylene

Article abstract of DOI:10.1021/ja039173p

We have found that CH₂=CHSnBu₃ is converted into CH₂=CHCH₂SnBu₃ catalytically in the presence of Mo(IV) olefin complexes such as Mo(NAr)(CH₂CH₂)[biphen] (where Ar = 2,6-i-Pr₂C₆H₃ and [biphen]^2- = 3,3′-di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate). The proposed mechanism involves formation of a metalacyclopentane (MC₄) complex from ethylene and CH₂=CHSnBu₃, contraction of this MC₄ complex to a metalacyclobutane (MC₃) complex, and finally metathesis of the MC₃ complex to give CH₂=CHCH₂SnBu₃ and Mo(NAr)(CH₂)[biphen]. These new findings suggest (inter alia) that contraction of an MC₄ ring to an MC₃ ring may be a much more common mode of decomposition of metalacyclopentane rings in d⁰ complexes than previously believed. Copyright

Full text of DOI:10.1021/ja039173p

Published on Web 01/29/2004
Catalytic Homologation of Vinyltributylstannane to Allyltributylstannane by
Mo(IV) Complexes in the Presence of Ethylene
Richard R. Schrock,*,† M. Duval-Lungulescu,† W. C. Peter Tsang,† and Amir H. Hoveyda‡
Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, and
Department of Chemistry, Merkert Chemistry Center, Boston College, Chestnut Hill, Massachusetts 02467
Received October 22, 2003; E-mail: rrs@mit.edu
Recently we have been interested in imido alkylidene complexes
of (primarily) molybdenum and tungsten that contain an enantio-
merically pure biphenolate or binaphtholate ligand.1 Since ethylene
is often a product in metathesis reactions that involve terminal
olefins, we have been exploring reactions between these asymmetric
imido alkylidene complexes and ethylene. We have found that
M(IV) ethylene complexes are formed as end products and that
they are in equilibrium with metalacyclopentane complexes.2 We
have now found that Mo(IV) imido bisalkoxide olefin complexes
are catalysts for the conversion of vinyltributylstannane to allyl-
tributylstannane, a one methylene olefin homologation reaction, and
propose a mechanism that involves contraction of a metalacyclo-
pentane (MC4) ring to a metalacyclobutane (MC3) ring.
ppm (∼triplet) in the 1H NMR spectrum. Ethylene and vinyltribu-
tylstannane were slowly consumed over the next 48 h to give
allyltributylstannane in 75% yield and complexes 1a and 1b in a
2:3 ratio. Other minor products (e.g., Bu3SnCHdCHCH2SnBu3 and
Bu3SnCH2CHdCHCH2SnBu3) were also observed at the end of
the experiment. Complexes 1a and 1b are analogous to related
hexafluoro-tert-butoxide species.4
The reaction was discovered in the process of exploring potential
cross-metathesis reactions3 between vinyltributylstannane and, for
example, 1-hexene catalyzed by asymmetric molybdenum imido
alkylidene complexes, as well as Mo(NAr)(CHCMe2Ph)[OC(CF3)2-
Me]2 (Ar ) 2,6-i-Pr2C6H3).1 No cross-metathesis between vinyl-
tributylstannane and 1-hexene or homometathesis of vinyltributyl-
stannane to give Bu3SnCHdCHSnBu3 was observed, although
1-hexene was homometathesized to yield 5-decenes. Peculiarly,
however, vinyltributylstannane was consumed in these reactions
in a catalytic fashion to give largely allyltributylstannane and small
quantities of Bu3SnCHdCHCH2SnBu3 (according to comparison
with an authentic sample; see Supporting Information) and Bu3SnCH2-
CHdCHCH2SnBu3 (prepared by homometathesis of allyltributyl-
stannane with Mo(NAr)(CHCMe2Ph)[OC(CF3)2Me]2) and possibly
other species that contain two tins. Similar results were obtained
in reactions involving vinyltributylstannane and diallyl ether (which
is effectively an ethylene source) and in reactions involving
vinyltributylstannane and ethylene. Therefore, we proposed that the
reaction that produces allyltributylstannane involves ethylene (eq
1) and suspected that the catalyst was not an alkylidene complex.
Conversion of CH2dCHSnBu3 to CH2dCHCH2SnBu3 was found
to be catalytic when 31 equiv of CH2dCHSnBu3 were added to 1a
under 1 atm of ethylene. Complex 1b was the major molybdenum
complex observed in the first 60 h, with a mixture of 1a and 1b
being observed thereafter. Allyltributylstannane was obtained in
80% yield (3% Mo catalyst). The reaction was first-order with
respect to CH2dCHSnBu3 in the first 60 h (t1/2 ≈ 40 h) with a plot
of ln[CH2dCHSnBu3] versus time yielding an observed rate
constant (kobs) of 4.70 × 10-6 s-1 with R2 ) 0.9969.
When 2 equiv of CH2dCHSnBu3 were added to 1a prepared in
a reaction between Mo(NAr)(CHCMe2Ph)[biphen] and 2 equiv of
13C2H4, the allyltributylstannane that was formed in the first 12 h
was selectively 13CH2dCHCH2SnBu3 (δC ) 110.17 ppm with 1JCH
) 157.9 and 152.5 Hz and 3JSnC ) 44.8 Hz); the R and â carbons
(at 16.72 and 138.57 ppm, respectively) were not labeled. After 56
h, ∼15% of CH2dCH13CH2SnBu3 was observed. Therefore, it
appears that CH2dCH13CH2SnBu3 is formed in a secondary reaction
which we attribute to a 1,3 migration of the tributylstannane
fragment in 13CH2dCHCH2SnBu3; the mechanism is not known.
Therefore, we propose that the primary catalytic reaction being
observed is that shown in eq 3, i.e., each methylene from ethylene
is appended to the vinyl group (with the accompanying migration
of a proton from a â to an R carbon atom) to form an allyl group.
Recently,2c we found that addition of 2 or more equivalents of
C2H4 to Mo(NAr)(CHCMe2Ph)[biphen] (where [biphen]2- ) 3,3′-
di-tert-butyl-5,5′,6,6′-tetramethyl-1,1′-biphenyl-2,2′-diolate) yields
the ethylene adduct, Mo(NAr)(CH2dCH2)[biphen] (1a), quantita-
tively, along with CH2dCHCMe2Ph, the metathesis product from
the initially formed molybdacyclobutane complex. (The four
ethylene resonances were observed at 3.15, 2.30 (area 2), and 3.28
ppm in the proton NMR spectrum.) Addition of 2 equiv of
CH2dCHSnBu3 to this solution led to 1b (eq 2) in 80% yield in 1
h at 22 °C in equilibrium with 1a. The three olefinic proton
resonances in 1b were found at 3.82 ppm (dd), 2.88 (dd) and 3.64
We have been able to think of only two possible mechanisms
for this reaction. The first is that ethylene is literally split by two
Mo(IV) species (possibly in a dimer) to give two ModCH2 species.
Formation and rearrangement of an R-substituted molybdacyclobu-
tane complex (eq 4) would then produce the allyl tin product and
a Mo(IV) species again.
† Massachusetts Institute of Technology.
‡ Boston College.
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J. AM. CHEM. SOC. 2004, 126, 1948-1949
10.1021/ja039173p CCC: $27.50 © 2004 American Chemical Society

C O M M U N I C A T I O N S
The other possibility is that the molybdacyclopentane complex
formed from ethylene and vinyltributylstannane shown in eq 5
undergoes a “ring-contraction” to a molybdacyclobutane complex,
which then does not continue to rearrange (because a carbon-based
group is in the R position at that point), but metathesizes to yield
allyltributylstannane and a methylene complex. A methylene
complex that is generated in this fashion then reacts as shown in
eq 4 to yield more allyltributylstannane or decomposes to form
ethylene and/or a Mo(NAr)[biphen](olefin) complex such as 1a or
1b.
to the rate of loss of an olefin from the metalacyclopentane. As
mentioned above, two of the minor products formed in these
reactions have been identified as Bu3SnCHdCHCH2SnBu3 and Bu3-
SnCH2CHdCHCH2SnBu3. It is not yet known whether these
species are formed via rearrangement of molybdacyclobutane or
molybdacyclopentane complexes or whether they are formed in
metathetical reactions.
We could find no example in the literature of a homogeneous
catalytic one methylene homologation of an olefin in which ethylene
is the methylene source, although heterogeneous examples (e.g.,
conversion of ethylene to propylene) are known.9a Reactions are
also known that involve methylene sources such as diazomethane.9b-d
In some cases, only olefins are involved and mixtures of many
products are obtained.9d These new findings suggest (inter alia) that
contraction of an MC4 ring to an MC3 ring may be a more common
mode, perhaps even the dominant mode, of decomposition of
metalacyclopentane rings of d0 complexes. In addition to exploring
further mechanistic details, we are curious whether other vinyl
compounds (e.g., silicon) behave similarly, whether tungsten(IV)
complexes2b are also catalysts for such reactions, and whether
certain ordinary olefins could ever be homologated in this manner
under some conditions.
Acknowledgment. We thank the National Science Foundation
(CHE-0138495 to R.R.S.) and the National Institutes of Health
(GM-59426 to R.R.S. and A.H.H.) for supporting this research.
We prefer the second proposal for two reasons. First, MdCH2
species have long been known to decompose to yield ethylene or
ethylene complexes,5 and there is no precedent for this reaction
being reversible. Second, there is considerable precedent for the
ring-contraction mechanism in the chemistry of Cp*Cl2Ta(olefin)
and tantalacyclopentane complexes made from them by adding an
olefin6 and in the rearrangement of a rhenacyclopentane complex,
Cp*(CO)2Re(C4H8), in the presence of a phosphine to yield
methylcyclopropane.7
Perhaps the most surprising feature of this Mo ring-contraction
mechanism is that a methylene complex is generated from ethylene.
In fact, the possibility of generating alkylidene complexes from
reduced metal complexes in the manner shown in eq 5 (for ordinary
olefins) was recognized in a paper in 1979 that was concerned with
ring contraction of tantalacyclopentanes.8 Replacing the tributyltin
group in the R position with a methylene group when the MC4
ring contracts to an MC3 ring (eq 5) appears to trigger this
phenomenon. The second surprising feature is that only molybda-
cyclobutane and molybdacyclopentane species that contain a tin in
the R position, i.e., only those with a proton that is â with respect
to both tin and molybdenum, rearrange rapidly by a â hydride
migration process, one that can be viewed in an alternative manner
(eq 4 and ref 6) to a traditional “reductive elimination” step. All
other molybdacycles either do not form to a substantial degree (in
some cases for steric reasons) or do not rearrange rapidly relative
Supporting Information Available: Experimental details for the
synthesis of Bu3SnCHdCHCH2SnBu3 (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
References
(1) Schrock, R. R.; Hoveyda, A. H. Angew. Chem., Int. Ed. 2003, 42, 4592.
(2) (a) Tsang, W. C. P.; Schrock, R. R.; Hoveyda, A. H. Organometallics
2001, 20, 5658. (b) Tsang, W. C. P.; Hultzsch, K. C.; Alexander, J. B.;
Bonitatebus, P. J., Jr.; Schrock, R. R.; Hoveyda, A. H. J. Am. Chem. Soc.
2003, 125, 2652. (c) Tsang, W. C. P.; Jamieson, J. Y.; Aeilts, S. L.;
Hultzsch, K. C.; Schrock, R. R.; Hoveyda, A. H. Organometallics, in press.
(3) Chatterjee, A. K. In Handbook of Metathesis; Grubbs, R. H., Ed.; Wiley-
VCH: Weinheim, Germany, 2003; Vol. 2, p 246.
(4) Robbins, J.; Bazan, G. C.; Murdzek, J. S.; O’Regan, M. B.; Schrock, R.
R. Organometallics 1991, 10, 2902.
(5) (a) Schrock, R. R. In Reactions of Coordinated Ligands; Braterman, P.
R., Ed.; Plenum: New York, 1986. (b) Feldman, J.; Schrock, R. R. Prog.
Inorg. Chem. 1991, 39, 1. (c) Schrock, R. R. Chem. ReV. 2002, 102, 145.
(6) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1980, 102,
5610.
(7) Yang, G. K.; Bergman, R. G. Organometallics 1985, 4, 129.
(8) McLain, S. J.; Sancho, J.; Schrock, R. R. J. Am. Chem. Soc. 1979, 101,
5451.
(9) (a) Imamura, H.; Konishi, T. Lanthanide Actinide Res. 1991, 3, 387. (b)
Adnadevic, B. K.; Agabalyan, L. G. J. Serb. Chem. Soc. 1997, 62, 409.
(c) Tebbe, F. N.; Parshall, G. W.; Reddy, G. S. J. Am. Chem. Soc. 1978,
100, 3611. (d) Leconte, M. J. Mol. Catal. 1994, 86, 205. (e) Leconte, M.;
Theolier, A.; Rojas, D.; Basset, J. M. J. Am. Chem. Soc. 1984, 106, 1141.
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