Electron Transfer with Halogen Electrophiles
A R T I C L E S
halogen reagents19-21 readily react with alkane C-H and/or
C-C bonds. The mechanistic understanding of the activation
step, however, is controversial and is based on either the
insertion of E+ into the σ-bonds via three centered-two electron
(3c-2e) species8,22,23 or the addition of E+ by means of direct
attack on the atoms.24
electrophiles with alkanes are at the borderline of inner and outer
electron transfer,45 and the hydrocarbon moieties in the transition
states or intermediates often resemble radical cation structures.
For instance, the lowest-lying transition states for the reactions
+
of methane with NO+ and BH2 very much resemble the
structure of the methane radical cation46-49 with two elongated
C-H bonds (i.e., [H2‚‚‚CH2-E]+) without incorporation of the
electrophile in the 3c-2e bonding.24,49 Methane, however, is not
a particularly good model for electrophilic aliphatic substitution
mechanistic studies because the triply degenerate HOMO does
not describe the individual C-H bonds.50
Experimental mechanistic studies in superacidic media5,7,25,26
reveal little about the activation step. Mechanistic computational
modeling is equally difficult as only intermediates27-30 rather
than transition structures can generally be located for these often
highly exothermic reactions. Only a limited number of transition
structures for the hydrogen exchange reactions for surface-
adsorbed alkanes were found to date.31-33 Some computations
are available for the reactions of alkanes with relatively stable
Selective heterolytic (polar) halogenations are characteristic
for adamantane,51 and for a number of other cage hydrocarbons
such as diamantane,52 protoadamantane,53 2,4-ethanoadaman-
tane,54 bicyclo[3.3.1]nonane,55 as well as for some aliphatic56,57
compounds. Functionalizations of cage compounds with some
electrophiles such as H3O2 ,
+ 22 NO+,23,24,34 carbocations,11,12,35,36
and positively charged halogens.37-39 However, there is no
decisive evidence for22,23 or against24 an electrophilic 3c-2e
activation mechanism, involving the attack on the C-H bonds,
that is, via transition structures with triangular C-H-E moieties.
In addition, our17,40,41 and other42-44 studies on hydrocarbon
activations with electrophile-oxidizers show that electron-
transfer (ET) processes cannot be ruled out and may actually
be dominating. As we pointed out recently,2 the reactions of
58
other oxidizing electrophiles such as 100% HNO3 and
nitrogen-containing electrophiles15,16,59 are similarly selective.
Inter alia, we will provide explanations for the following
combined experimental facts observed for polar halogenations
of, for example, adamantane: (i) the increase of the reaction
rates in the presence of Lewis acids60 and mixed halogens such
as I+Cl-,21 as well as Br+Cl-,61(ii) the high kinetic reaction
orders observed for halogen (we have found a reaction order of
about 7 for Br2 in CCl4), (iii) the observed 100% 3° C-H bond
regioselectivities,51 and (iv) the relatively high experimental H/D
kinetic isotope effect (KIE) for the adamantane bromination in
Br2 (kH/kD ) 3.9 ( 0.241).
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The present combination of experiment and theory reveals
key aspects of the C-H activation mechanisms of some
representative hydrocarbons (methane, isobutane, and adaman-
tane) with a set of model electrophiles ranging from nonoxi-
dizing (i.e., with low electron affinities) carbocations to
positively charged halogen oxidizers Haln+. We will show that
ET is a prime element for the detailed understanding of the
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