Synthesis and reactivity of functionalized arylcopper compounds by transmetalation of organosilanes

Article abstract of DOI:10.1021/ja8070804

Reactive organometallics are part and parcel of synthetic chemistry. Organosilanes potentially represent a cheap, robust, and environmentally benign precursor to reactive organometallics, but the nature of the very stable C-Si bond has generally prevented their use as precursors to more reactive organometallics. We present investigations into a copper fluoride complex which activates organosilanes in anhydrous media under mild conditions, effecting transmetalation to produce stable and isolable organocopper species containing sensitive functional groups including carbonyl groups, aryl bromides, benzylic chlorides, and alkyl ketones. This discovery allows us to better understand the fundamental reactivity of presumed intermediates in copper-catalyzed reactions and to develop new catalytic bond-forming processes of organosilane reagents. Copyright

Full text of DOI:10.1021/ja8070804

Published on Web 11/14/2008
Synthesis and Reactivity of Functionalized Arylcopper Compounds by
Transmetalation of Organosilanes
Jessica R. Herron and Zachary T. Ball*
Department of Chemistry, Rice UniVersity, Houston, Texas 77005
Received September 15, 2008; E-mail: zb1@rice.edu
Table 1. Synthesis of Functionalized Organocopper Compounds
The generation of functionalized organometallic reagents is of
great value for the synthesis of complex target structures. In one
approach to this problem, we have initiated a program to develop
and study functional-group tolerant methods for the generation of
organocopper species. Copper reagents are generally produced from
organomagnesium or other highly reactive organometallics, by
selective metalation of organohalides,1 or through direct metalation
with strongly basic reagents.2 Important efforts have been made to
develop selective cupration strategies, including from stable orga-
noboron3 and organostannane4 precursors. However, there are
significant potential benefits to the use of organosilanes as
potentially cheap, stable, and environmentally benign direct precur-
sors to functionalized organocopper reagents.
The use of fluoride to activate organosilanes is well established,
but this approach to the synthesis of reactive organometallics, and
to organocopper reagents in particular, is limited by the poor
availability of soluble, anhydrous fluoride sources,5 the instability
of copper(I) fluoride toward disproportionation,6 and the relatively
inert C-Si bond. The intermediacy of organocopper species has
been inferred in reactions of highly strained silanes7 and of those
activated by intramolecular alkoxides.8 A few reports demonstrating
copper catalysts for organosilane dimerization,9 for coupling of
alkynylsilanes,10 or for nucleophilic addition of organosilanes have
appeared,11 primarily with more easily transferable allylsilane
nucleophiles.12,13 The intermediacy of organocopper species can
be inferred in these reactions on the basis of kinetics, NMR, and
enantioselection observations,11,14 but it can be difficult to distin-
guish reactivity of an organocopper intermediate from that from a
fluoride-activated silicate.15 To our knowledge, direct observation
or characterization of silicon-to-copper transmetalation has not been
observed with simple sp2- or sp3-organosilanes.16
To surmount the difficulties associated with copper(I) fluoride
complexes, we employ a N-heterocyclic carbene ligand for copper17,18
to stabilize the copper(I) fluoride and to afford solubility in
anhydrous, aprotic solvents. Specifically, we employ the fluoride
complex (IPr)CuF (IPr ) 1,3-bis(2′,6′-diisopropylphenyl)imidazol-
2-ylidene), available upon treatment of (IPr)Cu(Ot-Bu) with
Et3N·3HF in anhydrous benzene, followed by filtration of the
product.19 Uniquely, (IPr)CuF is a monomeric, anhydrous copper(I)
fluoride complex soluble in ethereal solvents.
Treatment of (IPr)CuF in THF-d8 with phenyltriethoxysilane
induced clean transmetalation within 1 h at rt, affording the product
(IPr)CuPh (Table 1, entry 1). Gratifyingly, the product organome-
tallic could be isolated in analytically pure form by precipitation
upon dilution with pentane and cooling. We set out to explore the
generality and functional group tolerance of our approach to
organocopper reagents. Although we have focused on triethoxysi-
lanes, (dialkyl)monoalkoxysilyl groups are much more stable to
silica chromatography and to hydrolysis generally, and so we were
pleased to find that phenyldimethylethoxysilane (1b) undergoes
efficient transmetalation as well (entry 2), with a rate only slightly
a All reactions were performed at room temperature in THF unless
otherwise indicated. Yields indicate isolated yields of analytically pure
material. b Reaction performed in ether. c Yields in parenthesis are NMR
yields.
slower than the trialkoxysilane 1a. Despite the bulk of the IPr ligand,
ortho-substituted arylcopper reagents are readily formed (entries
3-4), and both electron-rich and electron-poor arylsilanes are
tolerated (entries 5-6).
The reaction of allyltriethoxysilane with (IPr)CuF is quite fast,
proceeding to completion in less than 12 min by 1H NMR analysis
of the mixture in THF-d8 (entry 7). The product (IPr)Cu(allyl) (2g)
can be isolated in a manner similar to the arylcopper compounds
above. Only two resonances corresponding to the allyl unit are seen
1
in the H NMR (δ 6.42, quintet, J ) 11.1 Hz, 1H); 3.08, d, J )
11.1 Hz, 4H), indicating fast exchange between η1 and η3
coordination modes.20 Cooling a THF-d8 solution of the product
allylcopper 2g resulted in significant broadening of the resonances
corresponding to the allyl moiety, but no static structure was
observed upon cooling to -90 °C.
Having established a general preparative method for polyfunc-
tionalized aromatic copper compounds, we next examined the
reactivity of arylcopper compounds 2. Pure phenylcopper 2a reacts
cleanly with allyl bromide, affording allylbenzene (quant yield by
NMR). Other organocopper reagents, including bromide 1j have
been generated in quantitative yield (NMR) by treating 1j with 1.2
9
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2008 American Chemical Society

C O M M U N I C A T I O N S
metallic precursors.18,23 species. (NHC)copper species serve as
increasingly valuable catalysts for reactions assumed to involve
organocopper intermediates, and we believe that the method
described here will allow a more thorough investigation of the
mechanism and selectivity questions in these processes. Finally,
copper(I) fluoride complexes have seen limited use in synthesis
owing to the lack of soluble, anhydrous complexes available without
the presence of protic solvent molecules, and the work here
demonstrates that new, well defined, anhydrous copper(I) fluoride
complexes enable new reactivity and catalysis.
Scheme 1. Reactivity of Organocopper Species
Acknowledgment. We thank Joseph P. Sadighi and David S.
Laitar for input and discussion and Adam Colson and Jake A. Smith
for experimental assistance. The authors acknowledge financial
support provided by Rice University and the Robert A. Welch
Foundation research grant C-1680.
Supporting Information Available: Complete experimental details
and spectral data for all new compounds. This material is available
free of charge via the Internet at http://pubs.acs.org.
References
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