C-H activation of ethers and alkanes by germylene-aryl halide complexes

Article abstract of DOI:10.1021/ja0357285

Regioselective, room-temperature C-H activation of alkanes and ethers by stable germylenes and aryl halides is reported. Germylenes, Ge[CH(SiMe₃)₂]₂ and Ge[N(SiMe₃)₂]₂, and aryl halides, Ph

Full text of DOI:10.1021/ja0357285

Published on Web 07/02/2003
C-H Activation of Ethers and Alkanes by Germylene-Aryl Halide Complexes
Karla A. Miller,† Jeffrey M. Bartolin,† Rory M. O′Neill,† Ryan D. Sweeder,† Thomas M. Owens,†
Jeff W. Kampf,† Mark M. Banaszak Holl,*,† and Norman J. Wells‡
UniVersity of Michigan, Department of Chemistry, Ann Arbor, Michigan 48109, and Baldwin-Wallace College,
Department of Chemistry, Berea, Ohio 44017
Received April 21, 2003; E-mail: mbanasza@umich.edu
Selective intermolecular C-H activations and insertions have
been the focus of much research both in academia and industry.1-3
Main group radical-based C-H activations of alkanes have focused
on halogenation, sometimes under phase-transfer conditions.4,5
Although these reactions are selective for tertiary and secondary
C-H bonds, most of the tertiary activation products are unstable
to the reaction conditions and are observed only in trace amounts.
We have recently discovered a novel C-H activation of ethers
and alkanes using Ge[CH(SiMe3)2]2 (1a) or Ge[N(SiMe3)2]2 6 (1b)
and PhX (X ) I, Br, Cl) (eq 1).
Figure 1. (a) ORTEP of 3a. Selected bond lengths (Å) and angles (deg):
Ge-I, 2.5946(4); Ge-C15, 1.9866(13); C1-Ge-C8, 107.10(5); C8-Ge-
C15, 117.51(5); C8-Ge-I, 106.52(4); C15-Ge-I, 102.40(4). (b) Proposed
intermediate.
For ethers such as tetrahydrofuran (THF) (R-C-H 92 (kcal/mol),
â-C-H 97.6),7,8 and Et2O (R-C-H 89, â-C-H 112)9 the weakest
Scheme 2. A Proposed Mechanism for C-H Activation Invoking
Free Phenyl Radical
C-H bonds are selectively activated. Activation of the C-H bonds
in 1,4-dioxane (96),7 cyclohexane (97),7 cyclopentane (93),10 and
butane (97, 98)11 have also been observed (Scheme 1). Benzene,
Scheme 1. Activation of Alkanes and Ethers by GeR2 and PhI
reactants (0.2 M), the OA product, 7a, is formed in up to 40%
yield, and 7b, in 82% yield. At low concentration of PhI (0.02 M)
and 1a (<0.03 mM) the C-H activation product is produced almost
exclusively, with less than 1% of 7a formed (as determined by 1H
NMR spectroscopy). Related oxidative addition products have been
reported for Sn and Si analogues. Lappert has investigated the
reaction chemistry of the analogous stannylene Sn[N(SiMe3)2]2
toward alkyl and aryl halides and reported that the reaction of
Sn[N(SiMe3)2]2 with PhBr in THF gives a 9:1 ratio of [(Me3Si)2-
N]2Sn(Ph)Br:[(Me3Si)2N]2SnBr2.12 Similarly, West reported that
reaction of a stable silylene with PhBr in hexanes results in oxidative
addition and disilane formation.13 In neither case were the analogous
CH-activation products reported.
A proposed mechanism is presented in Scheme 2. Abstraction
of a hydrogen radical by free phenyl radical is not supported by
key experiments reported herein. Instead, we propose that the
hydrogen radical abstraction occurs at the ipso-carbon of species 8
as the Ge-X bond and an incipient phenyl radical form and the
C-X bond breaks. This is consistent with a recent theoretical
analysis of the related West silylene chemistry.14
derived from the phenyl halide, is produced along with the C-H
activation product. Labeling studies with deuterated solvents and
PhI or 2-bromotoluene, show monodeuterio arene by GC/MS. No
trace of radical coupling products was observed by 1H NMR
spectroscopy or GC/MS for the reactions shown in Scheme 1.
The CH-activation products are synthesized by slow, room-
temperature addition of 1a or 1b (0.02 M) dissolved in the alkane
or ether of interest to an equimolar solution of PhI dissolved in the
same alkane or ether. For 1a, the optimal rate of addition can be
judged by noting that the yellow-orange germylene color quickly
fades to give a colorless solution before the next drop of germylene
is added. The products in Scheme 1 have been characterized by 1H
and 13C NMR spectroscopies, elemental analysis, and in the case
of 3a, X-ray crystallography (Figure 1a).
Oxidative addition (OA) of aryl halide to germylene is a
concentration-dependent side reaction. At high concentration of
† University of Michigan.
‡ Baldwin-Wallace College.
9
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J. AM. CHEM. SOC. 2003, 125, 8986-8987
10.1021/ja0357285 CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
The regioselectivity of this reaction has been studied using 1a
and PhI. Reaction with butane at -30 °C results in a 1°:2° ratio of
0.08: 1 and reaction with methylcyclopentane at 20 °C gives a
2°:3° ratio of 1: 7.1. These ratios indicate a radical-like selectivity
related to that observed for free phenyl radical generated by
phenylazotriphenylmethane (PAT) at 60 °C (0.11-0.13:1.01; and
1:4.8 respectively).15
A difference in reactivity is observed when the halide is varied.
Virtually all 1a is consumed instantly upon addition to a PhI/THF
solution. When PhBr is used, the reaction takes 3 days to go to
completion. In the case of PhCl, the reaction is 33% complete after
12 days at room temperature, or complete after 2 days at reflux.
This suggests that cleavage of the C-X bond is involved in the
rate-limiting step.
dilution conditions minimize the likelihood of reaction between R′•
and GeR2, thus avoiding steps 3-7 and the manifold of reactions
required to form OA product 7. Species 8 is consistent with the
different isotope effects observed for 1a and 1b, the differing isotope
effect observed for 1a and PAT, and the differing substrate reactivity
observed. Steps 3-7 are similar to the mechanism proposed by
Lappert for the reaction of stannylene chemistry and aryl halides.11
Single-electron transfer (SET) is not uncommon in germanium
chemistry16,17 and plays an important role in main group C-H
activations as well. To test whether the reactions reported herein
proceed by SET from PhI to Ge, forming a radical germyl anion,
the radical anion of 1a was generated using sodium metal in THF
according to the method of Gaspar et al.18,19 No trace of a C-H
1
activation product was observed by H NMR spectroscopy. Ad-
Differing, detailed reactivity is observed for germylenes 1a and
1b, and is attributed to their different Lewis acidities. Although
both activate alkanes and ethers, 1b reacts more slowly than 1a.
Parallel activations of methylcyclopentane run under identical
conditions show that all of 1a has been consumed, while up to
ditionally, during the reaction of 1a with PhI, UV-vis spectroscopy
showed only loss of the germylene peak at 420 nm as the reaction
progressed; the characteristic green color of the germyl radical anion
(λmax ) 666 nm) was not observed. Thus, the reactions described
in this communication likely do not involve radical anion species.
In summary, a new reaction for regioselective activation of C-H
bonds using a germylene/aryl halide reagent has been discovered.
The reaction works for both straight-chain and cyclic alkanes and
a variety of ethers. In contrast to previously reported main group
C-H activations that result in a carbon-halogen bond, these C-H
activated products have a C-Ge bond. High yields of C-H
activation products can be obtained through the use of high-dilution
techniques.
1
35% of 1b is still observed via H NMR spectroscopy. Reactions
run with 1b produce more 7. For example, when the synthesis of
2b is undertaken employing the optimized conditions for 2a, 7b is
produced in 13% yield, whereas the yield for 7a is <1%. When
aromatic substrates, such as toluene, ethyl benzene, or cumene are
used, the products from benzylic C-H activation are observed when
1a is employed. However, reaction of aromatic substrates with 1b
results only in slow formation of 7b.
Competition experiments were performed at 60 °C using a 50:
50 mol:mol solution of cumene:THF-d8. When 1a and PhI are added
to the solution, a 4.7 ( 0.4 ratio of C-H activation products is
observed. When PAT is used to generate phenyl radical, the ratio
of cumene/THF activation is 3.4 ( 0.3, suggesting that a species
other than free phenyl radical is involved in the C-H activation
initiated by 1a and PhI. If this competition is performed using 1b
and PhI, only 7b and the C-H activation product derived from
THF-d8 are observed; no trace cumene activation is observed. Free
phenyl radical, generated by PAT, readily abstracts H radical from
aromatic substrates such as cumene and toluene.15 Hence, these
results in which C-H activation of THF by 1b and PhI occurs in
the presence of cumene are inconsistent with formation of free
phenyl radical. Had free phenyl radical formed, one would have
expected to observe a ∼3.4:1 ratio of cumene/THF-d8 C-H
activation.
Competition experiments at 60 °C using a 1:1 ratio of THF:
THF-d8 with germylene 1a and PhI gives a kH/kD of 5.0 ( 0.2 as
determined by GC/MS; for 1b the ratio is 4.1 ( 0.2; for phenyl
radical generated by PAT the ratio is 4.2 ( 0.2. The different kH/
kD values for 1a and 1b argue against a common intermediate,
including free phenyl radical, being formed in reactions employing
1a or 1b. The different kH/kD values for 1a and phenyl radical
generated from PAT also argue against free phenyl radical being
generated in the C-H activation. Although the kH/kD values for 1b
and PAT are not statistically different, the cumene/THF competition
experiment above argues against free phenyl radical playing a role
in the case of 1b.
Acknowledgment. The Research Corporation and the Petroleum
Research Fund are thanked for support of this research. E.N.G.
Marsh is thanked for helpful discussions.
Supporting Information Available: Experimental details (PDF).
An X-ray crystallographic file in CIF format for compound 3a. This
material is available free of charge via the Internet at http://pubs.acs.org.
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