Hydrogen Rearrangement and Exchange

In 1985 soon after the discovery of H2 complexes the intramolecular site exchange of H atoms between H2 and hydride ligands was found by the Crabtree and Morris research groups.42 The H NMR signals of the cis H2 and hydride in [Ir(H2)H(bq)(PPh3)2] + coalesce at 240 because of exchange, and even the hydride trans to H2 in [Fe(H2)H(dppe)2]+ exchanges positions with the H atoms of i/2-H2. 

For the simple Hj/hydride situation, two general types of exchange mechanisms can be envisaged. The first is dissociative and involves homolysis of the H-H bond to produce a fluxional trihydride intermediate that facilitates intramolecular exchange of H atoms between cither adjacent or distal H2 and H ligands. Equations 

The R mechanism is important because polyhydrides can readily interchange cis hydrides by transient formation of H2-like ligands where dHH is shortened to 

Section 6.3.4). The observations indicate that exchange couplings can operate between such hydrogens if they are involved in R-type exchange.52 

Many new examples include more sophisticated systems with four or more H-donor ligands that display very complex dynamic process. The complexes encompass early to late metd species such as Cr(CO)4(H2)  

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The enzymes in the so-called "isomerase reactions contain 5 -deoxy-adenosylcorrinoids. Labeling experiments have been used to identify the C, H, and O atoms vv hich have moved in the course of the rearrangement and to show that during the reaction the hydrogen atoms of the substrate exchange with the hydrogen atoms of the C-5 atom coordinated to the cobalt ion, but not with the solvent. There is also some spectroscopic evidence that the Co—C bond is broken during the reaction. 

Transition metals catalyze a very wide variety of hydrocarbon reactions ranging from hydrogenation of olefins and exchange of paraffins with deuterium at lower temperatures to skeletal rearrangement, cyclization, hydrogenolysis, cracking, and carbiding under more severe conditions. Because of this flexibility in... 

Fluorine is usually so reactive that its use results in numerous side reactions, such as halogen-fluorine exchange, replacement of hydrogen, rearrangements or dimerization of radical intermediates. The addition of fluorine may be controlled, if fluorine is diluted with an inert gas and the temperature is lowered, or by using the Jet Fluorination technique, which was developed by Bigelow and co-workers (see also Vol. ElOa, pl59ff). 

Studies of the photochemical formation of benzyl carbanions have been reported for two systems this year. Ultra-violet light irradiation of (420) in aqueous acetonitrile resulted in formation of (421) exchange of the benzylic hydrogens of (420) was also observed when heavy water was used in the solvent. Compound (421) was shown to arise by two pathways a di-7r-methane rearrangement, and a 1,7-hydrogen shift followed by electrocyclic closure. The deuterium exchange reaction is presumed to occur due to... 

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