Hydrogenolysis of M-C Bonds

The kinetics of hydrogenolysis of a metal-alkyl have been monitored by HP IR spectroscopy for [MeIr(CO)2l3] , the resting state in the cycle for iridium catalysed methanol carbonylation [113]. On treatment with H2 at elevated temperatures, the v(CO) bands of [MeIr(CO)2l3] decayed and were replaced by new r(CO) bands at slightly higher frequency and a v(Ir-H) absorption, corresponding to Eq. (10). 

This represents one pathway to the formation of methane, a knovm by-product in iridium catalysed methanol carbonylation. The hydrogenolysis reaction was severely retarded by the presence of excess CO, indicating a mechanism involving initial dissociation of CO from [MeIr(CO)2l3] , prior to activation of H2. The mechanism therefore resembles that for hydrogenolysis of Rh acetyl complexes in hydroformylation. 

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The third preparative route consists of the hydrogenolysis of M-C bonds in the reaction (107)... 

The acidification of H2 may also be involved in hydrogenase action, where H2 is beheved to bind to an Fe(II) center. Isotope exchange between H2 and D2O is catalyzed by the enzyme see Nickel Enzymes Cofactors Nickel Models of Protein Active Sites Iron-Sulfur Proteins). Similar isotope exchange can also occur in H2 complexes. Oxidative addition to give a classical dihydride is also a common reaction. [W(H2)(CO)3(PCy3)2] is in equilibrium with about 20% of the dihydride in solution. This can lead to subsequent hydrogenolysis of M-C bonds as in the case of a cyclometallated phenylpyridine complex of Ir(III). ... 

Catalytic Hydrogenolysis of Carbon-Huonne Bonds it-Bond Participation Mechamsm Hudlicky, M J Fluorine Chem 44, 345-359 58 n o C) S ... 

Kinetic studies show that insertion (the enantioselection step) is very rapid, and that the rate-determining step is the hydrogenolysis of the M-C bond. Nonetheless, under H2-starving conditions, there is evidence that fi- I elimination can be competitive with hydrogenolysis. />-H elimination of the alkyl intermediate gives back the starting alkene and, through an equilibration process, it... 

The reaction can also be used to produce m-2-substituted halocyclopropanes, l-chloro-2-phenylcydopropenes undergoing efficient hydrogenation using palladium on calcium carbonate by cis-addition of hydrogen with no evidence for competing hydrogenolysis of the C—Cl bond 27). 

Alkynes show the same reaction, and again the product obtained is the trans isomer. After a suitable elimination from the metal the alkene obtained is the product of the trans-addition. Earlier we have seen that insertion into a metal-hydride bond and subsequent hydrogenolysis of the M-C bond will afford the c/s-alkene product. Thus, with the borohydride methodology and the hydrogenation route, both isomers can be prepared selectively. 

Reactions with Elements. There has been no report of the cleavage of a Ge-C bond by hydrogen, though for Si(CH3)4 and Sn(CH3)4 heterogeneous hydrogenolysis of an M-C bond has been observed. For the H-D exchange in basic solutions and the derived kinetic C-H acidity of M(CH3)n compounds, see [53]. 

Conventional hydrogenation eatalysts such as palladium and platinum also catalyze hydrogenolysis of the C—M bond. 

Pyrrolo[l,2- ][l,2]oxazines are a class of compounds with very few references regarding synthesis and reactivity. An interesting preparation has been described by intramolecular cyclization of IV-hydroxy pyrrolidines carrying a methoxyallene substituent at C-2 (242, Scheme 32). These compounds were obtained by addition of a lithiated allene to chiral cyclic nitrones 241. Cyclization occurred spontaneously after some days at relatively high dilution (0.05 M). Compounds 243 (obtained with excellent diastereoselectivity) can be submitted to further elaboration of the double bond or to hydrogenolysis of the N-O bond to form chiral pyrrolidine derivatives (Section 11.11.6.1) <2003EJ01153>. 

Treatment of the alcohol ( ) with trifluoromethylsulfonic anhydride (triflic anhydride) at -78 C afforded the ester (1 ) which could be isolated and characterized. We knew from previous experience (2J that sulfonyl esters vicinal to an isopropylidene acetal are relatively stable. The triflate T,) reacted cleanly with potassium azide and 18-crown-6 in dichloromethane at room temperature. The crystalline product [68% overall from (1 )] was not the azide ( ) but the isomeric A -triazoline ( )- Clearly the initially formed azide (18) had undergone intramolecular 1,3-cyclo-addition to the double bond of the unsaturated ester (21- ). The stereochemistry of the triazoline (1 ), determined by proton nmr spectroscopy, showed that the reaction was stereospecific. There are several known examples of this reaction ( ), including one in the carbohydrate series ( ). When the triazoline was treated with sodium ethoxide ( ) the diazoester ( ) was rapidly formed by ring-opening and was isolated in 85% yield, Hydrogenolysis of the diazo group of (M) gave the required pyrrolidine ester ( ) (90%). 

Three transmetallation reactions are known. The reaction starts by the oxidative addition of halides to transition metal complexes to form 206. (In this scheme, all ligands are omitted.) (i) The C—C bonds 208 are formed by transmetallation of 206 with 207 and reductive elimination. Mainly Pd and Ni complexes are used as efficient catalysts. Aryl aryl, aryl alkenyl, alkenyl-alkenyl bonds, and some alkenyl alkyl and aryl-alkyl bonds, are formed by the cross-coupling, (ii) Metal hydrides 209 are another partner of the transmetallation, and hydrogenolysis of halides occurs to give 210. This reaction is discussed in Section 3.8. (iii) C—M bonds 212 are formed by the reaction of dimetallic compounds 211 with 206. These reactions are summarized in Schemes 3.3-3.6. 

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