Cobalt dinitrosoalkane complexes in the C-H functionalization of olefins

Article abstract of DOI:10.1021/ja800738d

The use of cobalt dinitrosoalkane complexes in the C-H functionalization of alkenes has been demonstrated. Reaction of a series of alkenes with Me₄CpCo(CO)₂ in the presence of NO generatesintermediate cobalt dinitrosoalkane complexes that can be deprotonated α to the nitrosyl group and added to various Michael acceptors. The resultant products can then undergo retrocycloaddition reactions in the presence of the original alkene to regenerate the starting cobalt dinitrosoalkane complex and release the functionalized alkene. Copyright

Full text of DOI:10.1021/ja800738d

Published on Web 03/05/2008
Cobalt Dinitrosoalkane Complexes in the C-H Functionalization of Olefins
Jennifer M. Schomaker, W. Christopher Boyd, Ian C. Stewart, F. Dean Toste,* and
Robert G. Bergman*
Department of Chemistry, UniVersity of California, Berkeley, California 94720
Received January 29, 2008; E-mail: rbergman@berkeley.edu
Scheme 1. Cobalt Dinitrosoalkane Complexes
Metal-mediated organic reactions can be divided into two broad
categories: those proceeding via complexation of a substrate to a
metal center and those involving the direct interaction of a substrate
with ligands bound to the metal center.1 Examples of the latter
include oxidations by metal oxo compounds such as the FedO
center in cytochrome P450, the Sharpless dihydroxylation of olefins
by OsO4, the reduction of carbonyl compounds by the Noyori
reaction, and the hydrosilylation of carbonyl compounds using a
(PPh3)3Re(O)2I catalyst developed by Toste.2-5 Shvo has also
reported ligand-metal bifunctional hydrogenation catalysts that
were later studied extensively by Casey and others.6
Scheme 2. Functionalization of Olefins Using Cobalt
Dinitrosoalkane Complexes
The majority of ligand-attack reactions involve transfer of oxygen
and hydrogen to the organic substrate; direct interaction with
nitrogen ligands is less frequently encountered. A particularly
intriguing example of a nitrogen ligand-based organometallic
reaction was described by Brunner in the reaction of norbornene
with a cobalt nitrosyl dimer (Scheme 1, eq 1).7 Bergman and Becker
later demonstrated that cobalt dinitrosyl complexes form direct
adducts with a wide range of substituted alkenes, and the overall
transformation can be used to generate 1,2-diamines (Scheme 1,
eq 2).8 In this communication, we report that the ligand-based
reactivity of these cobalt dinitrosyl complexes can be used to form
a new C-C bond between unfunctionalized alkenes and a variety
of Michael acceptors.
The proposed mechanism for the use of cobalt dinitrosyl
complexes in ligand-based activation of unfunctionalized alkenes
toward conjugate addition is shown in Scheme 2. We envisioned
the first step to involve the reaction of an olefin with CpCo(CO)2
in the presence of NO to give cobalt complex 1, which was
previously established to proceed via the intermediacy of CpCo-
(NO)2.8 Hopefully due to the increased acidity of the protons R to
the nitrosyl groups, deprotonation of 1 in the presence of base
should occur to generate 2. The nucleophilic intermediate 2 adds
to a Michael acceptor to give the functionalized complex 3. In the
final step, an unusual (but precedented) retrocycloaddition reaction
in the presence of excess olefin regenerates the original complex
1, releasing the functionalized alkene product 4.8
mercial solution of TBAF in THF with the less electron-rich Cp
group gave moderate yields of the desired conjugate addition
products. It was necessary to utilize anhydrous TASF when the
cobalt complexes containing the more electron-rich Cp* ligand
(Table 2) was treated with Michael acceptors in the presence of a
fluoride source.
Having established the viability of forming the anion of the cobalt
dinitrosoalkane complex, we turned our efforts toward using
unsilylated precursors. Initial attempts to deprotonate the nor-
bornene-derived cobalt complex 7a using a variety of bases in the
presence of a Michael acceptor gave poor yields and decomposition
of 7a. Among the bases screened were Et3N, DABCO, DBU, and
nBuLi as well as hydroxides, hydrides, carbonates, and hexameth-
yldisilazides. KHMDS was the only base found to give the desired
conjugate addition product 12, albeit with a low yield of 21%.
Further extensive screening of conditions for the Michael addition
yielded LHMDS as the optimum base and a mixture of 5:1 THF/
HMPA as the best solvent. In the hope that additives might promote
the reaction, a series of Lewis acids was also evaluated. The use
of Sc(OTf)3 as the Lewis acid resulted in an increased yield of the
desired product 12 from an initial 39-82% (Table 3).11
To begin our studies, a variety of stable, isolable cobalt
dinitrosoalkane complexes were synthesized by directly treating
cobalt dicarbonyl complex 5 with an excess of alkene in the
presence of NO (Table 1).8
We initially investigated the possibility of forming the anion of
the cobalt dinitrosoalkane complex using the silyl complexes 8 and
9 (Table 2). The use of a fluoride source to desilylate a C-Si bond
is a common, very mild method for generating a carbon-based
anion.9 In addition, alkenes in the form of aryl/alkenyl organosi-
loxanes and boronic esters have been utilized in Pd-, Ru-, or Rh-
catalyzed additions to R,â-unsaturated carbonyl compounds.10
Indeed, treatment of the cobalt dinitrosoalkanes 8 and 9 with Bu4-
NF (TBAF) or (NMe2)3SSiMe3F2 (TASF) in the presence of a
Michael acceptor gave the desired products. The use of a com-
The anion of cobalt dinitrosoalkane 7a was added to a variety
of conjugated acceptors using LHMDS as the base, Sc(OTf)3 as
the Lewis acid, and 5:1 THF/HMPA as the solvent to determine
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J. AM. CHEM. SOC. 2008, 130, 3777-3779
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C O M M U N I C A T I O N S
Table 1. Synthesis of Cobalt Dinitrosoalkane Complexes
Table 4. Conjugate Addition Reactions of 7a
Table 2. Reaction of Silylated Cobalt Dinitrosoalkane Complexes
with Michael Acceptors in the Presence of a Fluoride Source
*Based on recovered starting material.
using 2-cyclohexen-1-one, A, or phenyl vinyl sulfone, B, as the
electrophile (Table 5). Cobalt complexes derived from strained ring
systems, such as those containing the [2.2.1]bicycloheptane frame-
work (7a-c, Table 5), reacted smoothly to give good yields of the
desired products. The new complexes were monosubstituted using
A as the electrophile (entries 1, 3, and 5) and disubstituted using
the less sterically hindered B as the Michael acceptor (entries 2
and 4). Substrate 7d did not react with A but gave 50% yield of
the disubstituted 15d using B as the electrophile, possibly due to
steric effects. The complex formed from endo dicyclopentadiene
(7e) did not undergo Michael addition, but the exo complex 7f did,
presumably due to decreased steric interactions. Substrate 7g was
unstable under the basic reaction conditions and did not give useful
yields of the product 14g, even when the reaction temperature was
reduced to -78 °C. Cobaltacycle 7h, formed from highly strained
cyclobutene, also gave good yields of conjugate addition products
albeit as a 2:1 mixture of diastereomers (entries 11 and 12). Finally,
cobalt dinitrosoalkane complexes containing a variety of five-
membered rings (7i-l) gave products that were obtained in
moderate yields but had to be handled quickly due to their
propensity to decompose in solution (entries 13-17).
*Reactions using TBAF were performed in THF; reactions using TASF
were carried out in 1,2-dimethoxyethane.
Table 3. Evaluation of Lewis Acids
To complete the overall transformation illustrated in Scheme 1,
it was necessary to show that treatment of products with an excess
of starting olefin could displace the functionalized alkene and
regenerate the starting complex.8 For example, treatment of 12 with
an excess of norbornene at 90 °C gave a nearly quantitative yield
1
of the functionalized alkene 17 by H NMR and regenerated the
starting cobalt complex 7a (Table 6, entry 1). The complex 14b
formed from highly strained norbornadiene required elevated
temperatures for retrocycloaddition (entry 2), but additional sub-
stitution R to the nitrosyl group attenuated the temperature required
for releasing the product alkene 19 (entry 3), perhaps due to steric
effects. The cobalt complex 16 required much lower temperatures
for the retrocycloaddition reaction with cyclopentene (entry 4).
the electrophile scope. The majority of the electrophiles gave the
product as one major diastereomer (Table 4).
To determine the range of alkenes that would participate in this
reaction, the cobalt dinitrosoalkanes from Table 1 were screened
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C O M M U N I C A T I O N S
Table 5. Conjugate Addition Reactions of Cobalt Dinitrosoalkanes
In conclusion, we have demonstrated the use of the ligand-based
reactivity of a series of cobalt dinitrosyl complexes for the C-H
functionalization of alkenes. Although the alkene scope is still
somewhat limited, and we have not yet achieved catalytic turnover,
the formation of intermediate cobalt dinitrosoalkane complexes
allows for the utilization of masked alkenes as nucleophiles in
conjugate addition reactions. The resulting new cobaltacycles can
then undergo retrocycloaddition reactions in the presence of the
original alkene to regenerate the starting cobalt dinitrosoalkane
complex and release the functionalized alkenes. Future studies are
directed toward expansion of the scope of the reaction, rendering
the reaction asymmetric, and efforts to carry it out under catalytic
conditions.
Acknowledgment. R.G.B. acknowledges NSF grant CHE-
0345488, and F.D.T. acknowledges support from the Director,
Office of Science of the U.S. Department of Energy under Contract
No. DE-AC02-05CH11231. J.M.S. thanks the NIH for a NRSA
postdoctoral fellowship. W.C.B. and I.C.S. acknowledge financial
support from NSF predoctoral fellowships. The authors thank
Vincent S. Chan, Lee Bishop, Melanie Chiu, Stuart Smith, and
Pablo Mauleon for useful discussions.
Supporting Information Available: Experimental procedures and
spectral information for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
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*Yields determined by 1H NMR using mesitylene as an internal standard.
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