Abstract
The photochemical reactions of Tp'Rh(PMe3)H2 (1) and thermal reactions of Tp'Rh(PMe3)(CH3)H (1a, Tp' = tris(3,5-dimethylpyrazolyl)borate) with substrates containing B-H, Si-H, C-F, and C-H bonds are reported. Complexes 1 and 1a are known activators of C-H bonds, including those of alkanes. Kinetic studies of reactions with HBpin and PhSiH3 show that photodissociation of H2 from 1 occurs prior to substrate attack, whereas thermal reaction of 1a proceeds by bimolecular reaction with the substrate. Complete intramolecular selectivity for B-H over C-H activation of HBpin (pin = pinacolate) leading to Tp'Rh(PMe3)(Bpin)H is observed. Similarly, the reaction with Et2SiH2 shows a strong preference for Si-H over C-H activation, generating Tp'Rh(PMe3)(SiEt2H)H. The Rh(Bpin)H and Rh(SiEt2H)H products were stable to heating in benzene in accord with DFT calculations that showed that reaction with benzene is endoergic. The intramolecular competition with PhSiH3 yields a ∼1:4 mixture of Tp'Rh(PMe3)(C6H4SiH3)H and Tp'Rh(PMe3)(SiPhH2)H, respectively. Reaction with pentafluoropyridine generates Tp'Rh(PMe3)(C5NF4)F, while reaction with 2,3,5,6-tetrafluoropyridine yields a mixture of C-H and C-F activated products. Hexafluorobenzene proves unreactive. Crystal structures are reported for B-H, Si-H, and C-F activated products, but in the latter case a bifluoride complex Tp'Rh(PMe3)(C5NF4)(FHF) was crystallized. Intermolecular competition reactions were studied by photoreaction of 1 in C6F6 with benzene and another substrate (HBpin, PhSiH3, or pentafluoropyridine) employing in situ laser photolysis in the NMR probe, resulting in a wide-ranging map of kinetic selectivities. The mechanisms of intramolecular and intermolecular selection are analyzed.
Highlights
Homogeneous transition metal complexes are known to activate a wide variety of strong bonds in organic molecules, and selectivities are usually observed when more than one type of bond is present
We show that the same species can react by C−F oxidative addition and that C−F bonds are sometimes activated in the presence of competing C−H bonds
These experiments show that the [Tp′Rh(PMe3)] system can activate a much wider variety of bonds than had been previously realized, opening up the possibilities of new applications through C−F, B−H, and Si−H bond activation via photolysis of Tp′Rh(PMe3)H2 or thermal reaction with Tp′Rh(PMe3)(CH3)H
Summary
Homogeneous transition metal complexes are known to activate a wide variety of strong bonds in organic molecules, and selectivities are usually observed when more than one type of bond is present. Typical hydrosilylation catalysts activate Si−H bonds in preference to C−H bonds.[1] the best borylation catalysts attack B−B and B−H bonds without affecting C−H bonds.[2] While these points may seem hardly worthy of mention, the issue of selectivity becomes critical when we consider C−H functionalization reactions such as the conversion of methane to methanol because suitable methane activators react more rapidly with product than with methane.[3] Carbon−fluorine bond activation is a rarer phenomenon, and the issue of competition between activation of C−F and C−H bonds within the same molecule becomes especially important. It is a rarity to discover complexes that activate the C−F bonds of pentafluorobenzene in preference to its C−H bond.[4]
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