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14 Activation of Carbon-Hydrogen Bonds in Alkanes and Other Organic Molecules Using Organotransition Metal Complexes Robert G. Bergman Department of Chemistry, University of California, Berkeley, and Materials and Chemical Sciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720
The scope, selectivity, and mechanism of reactions in which low -valent iridium and rhodium complexes undergo intermolecular C-H oxidative addition reactions were investigated. Progress was made on the development of stoichiometric methods for converting the metal complexes formed by C-H activation into functionalized organic molecules. In an effort to locate an inert solvent for the reaction, C-H activation reactions were carried out in liquefied noble gases such as liquid xenon and liquid krypton. This approach allowed us to directly examine the C-H activation reactivity of gaseous and solid substrates. It also provided a means for carrying outflashkinetic studies designed to generate transient coordinatively unsaturated metal complexes and directly measure the rates of their reactions with C-H bonds.
A . L K A N E S ARE A M O N G T H E MOST C H E M I C A L L Y INERT Organic molecules.
The potential for using alkanes as feedstocks i n chemical synthesis has stimulated a search for metal complexes capable of undergoing the C - H oxidative addition process shown i n eq 1, so that alkane chemistry more selective than that available using free radical reagents might be developed (1-3). M + R-H R - M - H 0065-2393/92/0230-0211$06.00/0 © 1992 American Chemical Society
(1)
212
H O M O G E N E O U S TRANSITION M E T A L C A T A L Y Z E D REACTIONS
Intramolecular C - H oxidative addition to metal centers has been known for some time. However, despite many efforts the corresponding intermolecular C - H oxidative addition process illustrated in eq 1 was not observed directly until 1982. At that time our group (4, 5) and Graham's (6) indepen dently found that irradiation of complexes such as C p * ( L ) I r H
[1, C p * is
2
( T | - C M e ) , L is PMe )] and C p * I r ( C O ) causes successful insertion of the
Homogeneous Transition Metal Catalyzed Reactions Downloaded from pubs.acs.org by UNIV OF CALIFORNIA SAN DIEGO on 01/28/16. For personal use only.
5
5
5
3
2
C p * I r L fragment into C - H bonds in alkanes. This procedure leads to stable alkyliridium hydride complexes Cp*(L)Ir(R)(H) (2). This reaction is exceed ingly general. So far no organic liquid in which C p * ( L ) I r H has been irra 2
diated has failed to react with the intermediate generated in the reaction. Some of the solvents that have been observed to give intermolecular C - H insertion products upon irradiation of 1 are illustrated in Scheme I.
HC
hv/-H + R-H
3
R-H = Q ,
Δ
2
Cp* \ Ir. / > L
. Me C, CHaiCHafeCH* CH CN, 4
3
R
ζ}
Scheme I.
Characteristics of Oxidative Addition Reactions Much has been learned about the scope and mechanism of this reaction (7-9). Following our initial studies with iridium, alkane C - H oxidative ad dition was observed at several other second- and third-row transition metal centers (where M - H and M - R bonds are expected to be relatively strong) such as rhodium, rhenium, platinum, and osmium, and even at one firstrow metal, iron (10-14). The selectivity of the iridium and rhodium C - H oxidative addition re action was investigated by carrying out competition studies with various hydrocarbon substrates. The results are summarized in Scheme II. Although the absolute magnitudes of the selectivities are different for Ir and Rh, their trends are parallel. This observation indicates that C - H oxidative additions proceed by similar mechanisms at these two metal centers, but that the Rh reactions are less exothermic than their Ir counterparts.
The selectivity
experiments also suggest that steric effects and C - H acidities, rather than
14.
BERGMAN
Activation of Carbon-Hydrogen Bonds in Alkanes
ki[Ri-Hl Μθ^Γ \
hv -50°
Me M" / ΡΜθ
213
Cp*(L)M
