Dehydrocyclization-hydrogenolysis mechanism

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

Having characterized the three hydrogenolysis mechanisms by their precursor species dicarbenes (Scheme 34), 7c-adsorbed olefins (Scheme-36), and metallocyclobutanes (Schemes 38 and 39), the knowledge of the overall mechanism of cyclic type isomerization requires the identification of the precursor species in 1-5 dehydrocyclization, the reverse reaction of hydrogenolysis of cyclopentanes. 

The first approach to the cyclic mechanism of isomerization was the finding that the interconversion of n-hexane and methylpentanes takes place under the conditions where the nonselective mechanism of hydrogenolysis (Mechanism A) is the only one operating that is, on 0.2% Pt/AljOj (32). The identical product distributions in isomerization of hexanes and hydrogenolysis of methylcyclopentane suggested that both reactions involve a common intermediate with a methylcyclopentane structure. It was then proposed that the species responsible for dehydrocyclization of hexanes are a,j8, -triadsorbed species involving a single metal atom (55) (Scheme 40). 

As previously mentioned, Davis (8) has shown that in model dehydrocyclization reactions with a dual function catalyst and an n-octane feedstock, isomerization of the hydrocarbon to 2-and 3-methylheptane is faster than the dehydrocyclization reaction. Although competitive isomerization of an alkane feedstock is commonly observed in model studies using monofunctional (Pt) catalysts, some of the alkanes produced can be rationalized as products of the hydrogenolysis of substituted cyclopentanes, which in turn can be formed on platinum surfaces via free radical-like mechanisms. However, the 2- and 3-methylheptane isomers (out of a total of 18 possible C8Hi8 isomers) observed with dual function catalysts are those expected from the rearrangement of n-octane via carbocation intermediates. Such acid-catalyzed isomerizations are widely acknowledged to occur via a protonated cyclopropane structure (25, 28), in this case one derived from the 2-octyl cation, which can then be the precursor... 

Diphenylmethane catalytic hydrogenolysis kinetics, 29 241-243 reduction mechanism, 29 267 cyclization, 30 65 dehydrocyclization, 28 318 [(Diphenylphosphino)alkyl]phosphonates, 42 479... 

Indeed the carbene-alkyl insertion mechanism in Scheme 45 neatly explains why the rates of dehydrocyclization of 1, 2, and 3 are so similar. However, since 2-methylhexane also undergoes 1-5 dehydrocyclization, involvement of methylenic carbon atoms and not simply terminal carbon atoms must also be possible. The pathway for the C7-alkanes must be the reverse of nonselective hydrogenolysis of methylcyclopentane (Mechanism A), since it also results in isomerization to 2,4-dimethylpentane and 3-methylhexane, most likely via adsorbed 1,3-dimethylcyclopentane (scheme 46). It is... 

Hence, three different dehydrocyclization mechanisms, corresponding to the three mechanisms of hydrogenolysis of methylcyclopentane, may be characterized ... 

On the other hand, the selective dehydrocyclization, which does not allow the formation of secondary-primary C-C bonds, must involve only two methylic carbon atoms in the 1 and 5 positions. Although the reverse reaction (selective hydrogenolysis of methylcyclopentane) could be observed on platinum catalysts of low dispersion at 220°C (86), the selective dehydrocyclization of methylpentanes on these catalysts is detectable only at higher temperatures (280°-300°C), where it competes with another process, ascribed to Mechanism C (33). Fortunately, it was found recently that iridium supported on AI2O3 or SiOj selectively catalyzes at 150°C the cyclic type interconversion of 2-methyl- and 3-methylpentanes (88). n-Hexane under the same conditions yields only cracked products (702) (Scheme 52). Similarly,... 

Adsorbed carbenes formed by a-hydrogen elimination are the most likely precursors of dicarbynes on iridium. On this metal, further dehydrogenation of metallocarbenes to metallocarbynes might also be easy, thus explaining why the selective Mechanism B for both dehydrocyclization and hydrogenolysis is the only one operating. 

Finally, the partially selective Mechanism C in hydrogenolysis of cyclopentanes has a counterpart in dehydrocyclization of methylpentanes and n-hexane. The intervention of this mechanism, involving metallocyclobutane intermediates, is strongly supported by studies of aromatization (see Section V). 

The sextet-doublet model was adapted for 1-5 dehydrocyclization, the reverse of cyclopentane hydrogenolysis, and it was proposed that physically adsorbed alkane reacts with chemisorbed hydrogen according to a push-pull mechanism (772) (Scheme 55). 

Besides isomerization and dehydrocyclization, substituted benzenes also undergo hydrogenolysis of the side chain and homologation. This very interesting reaction, which consists in the growth of the side chain by one carbon atom, was demonstrated unambiguously by Csicsery and Burnett (160). o-Ethyltoluene leads to o-n-propyl- and o-isopropyltoluenes, o-di-ethylbenzene and cymenes, and the occurence of this reaction is, in our opinion, an argument in favor of carbene mechanisms. ... 

---