Mechanism, dehydrocyclization

Our interest in the forgoing dehydrocyclization mechanisms stems from some superacid-based studies we carried out some years ago, in which an observable cyclodecyl cation 1 loses H2 concomitant with C-C bond formation (shown in bold) to give the 9-decalyl cation 2, and the possible analogy with a 2-octyl cation 3 (as 3 ) giving H2 and 1,2-dimethyl cyclohexane 4 is shown in Scheme 1. The background to this work is reviewed in the next section. 

This protonated cyclopropane is found at B3LYP/6-31G to be 15.99 kcal/mol above the 1,5-p-H-bridged 23, the most stable structure of the 2-octyl cation. This transition-state is thus significantly smaller than the calculated transition-states that we have obtained for the dehydrocyclization mechanisms (29.60 kcal/mol for structure 24), and therefore in at least qualitative agreement with the observation of Davis (8) that an equilibration of n-octane with at least some other isooctanes is set up prior to significant dehydrocyclization of the feedstock. 

Platinum catalyzes at least two types of C6- dehydrocyclization, one of which involves olefinic intermediates (13, 28, 29). In the case of paraffins, this latter reaction involves the ring-closure of hexatrienes (30, 31). In the C6-dehydrocyclization of n-butylbenzene and n-pentylbenzene, phenyl-butadiene and phenylpentadiene could correspond to these triene intermediates (13, 14). The second C6-dehydrocyclization mechanism is similar to C5-dehydrocyclization, and may not involve olefinic intermediates. 

There are at least two C6-dehydrocyclization mechanisms one of these proceeds through arylalkene intermediates and corresponds to the hexatriene-type C6-dehydrocyclization of paraffins. The other pathway is direct ring closure. It is probably related to C5-dehydrocyclization. 2-Butylnaphthalene may differentiate between the two mechanisms phenanthrene is probably formed by the first reaction, anthracene by the second. 

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

At this stage, one could question whether, in the case of simpler molecules (hexanes, heptanes, etc,), the dehydrocyclization mechanism could not also involve the simple ring closure of a 1,5 di-a-adsorbed species (Scheme 87). 

The second aromatization reaction is the dehydrocyclization of paraffins to aromatics. For example, if n-hexane represents this reaction, the first step would be to dehydrogenate the hexane molecule over the platinum surface, giving 1-hexene (2- or 3-hexenes are also possible isomers, but cyclization to a cyclohexane ring may occur through a different mechanism). Cyclohexane then dehydrogenates to benzene. 

The mechanism of the dehydrocyclization reaction is not completely understood. Two alternatives were proposed (11), one which proceeds via a silaimine mechanism, exemplified by reaction (25), and one wherein ring closure occurs by nucleophilic displacement of hydrogen, reaction (26). 

Muller (ISO) has recently shown that the dehydrocyclization of 2,2,4,4-tetramethylpentane to 1,1,3,3-tetramethylcyclopentane occurs on thick polycrystalline platinum film catalysts with a rate that is comparable to the formation of 1,1,3-trimethylcyclopentane from 1,2,2-trimethyl-pentane. As Muller points out, reactions (10)—(12) cannot occur from 2,2,4,4-tetramethypentane, and it is clear that either these mechanisms are inadequate, or at least there must be an alternative mechanism available. Muller suggests mechanism (14) which requires two adjacent platinum sites. 

The focus of the next four chapters (Chapters 14-17) is mainly on the theoretical/computational aspects. Chapter 14 by T. S. Sorensen and E. C. F. Yang examines the involvement of p-hydrido cation intermediates in the context of the industrially important heptane to toluene dehydrocyclization process. Chapter 15 by P. M. Esteves et al. is devoted to theoretical studies of carbonium ions. Chapter 16 by G. L. Borosky and K. K. Laali presents a computational study on aza-PAH carbocations as models for the oxidized metabolites of Aza-PAHs. Chapter 17 by S. C. Ammal and H. Yamataka examines the borderline Beckmann rearrangement-fragmentation mechanism and explores the influence of carbocation stability on the reaction mechanism. 

In our calculations we will first discuss our results starting with both the 2-and 3- octyl cations (the 4- octyl cation cannot form a 1,6-p-H-structure). The n-octane conversion to aromatics, as described by Davis (8), is a good test of our proposed mechanisms, for several reasons (1) his experimental observation would require the formation of approximately equal amounts of 1,2-dimethylcyclohexane (o-xylene) and ethylcyclohexane (ethylbenzene), even though in our mechanism the structure of the needed 1,6-p-H cation intermediates are quite different, and (2) the formation of to- and p-xylene requires a prior isomerization of n-octane to 2- and 3- methylheptane, and this must be a faster reaction than the dehydrocyclization (or at least competitive with it). If our mechanisms are valid, we should be able to reproduce some aspects of the above results. 

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... 

In contrast to this mechanism, the one proposed in our work operates direct from the oxidation state of the alkane feedstock. The same alkyl cation intermediate can lead to both alkane isomerization (an alkyl cation is widely accepted as the reactive intermediate in these reactions) and we have shown in this paper that a mechanistically viable dehydrocyclization route is feasible starting with the identical cation. Furthermore, the relative calculated barrier for each of the above processes is in accord with the experimental finding of Davis, i.e. that isomerization of a pure alkane feedstock, n-octane, with a dual function catalyst (carbocation intermediate) leads to an equilibration with isooctanes at a faster rate than the dehydrocyclization reaction of these octane isomers (8). 

Q dehydrocyclization, 29 311 ring enlargement, 29 311-316 Bifunctional Fisher-Tropsch/hydroformylation catalysts, 39 282 Bifunctional mechanism, 30 4 Bifurcation diagram, oscillatory CO/O, 37 233-234... 

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... 

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

M(CxC) matrix, 32 290-291, 311-313 Measurements, interpretation of, in experimental catalysis, 2 251 Mechanism see also specific types cobalt catalysis, 32 342-349 dehydrocyclization, 29 279-283 rhodium catalysis, 32 369-375 ruthenium catalysis, 32 381-387 space, 32 280... 

A comparison of the cyclization rates of alkanes and alkenes may help to distinguish between associative and dissociative ring closure mechanisms, just as in the case of Cg dehydrocyclization of hexane and hexenes. 

The stepwise dehydrocyclization of hydrocarbons with quaternary carbon atoms over chromia was interpreted by Pines 94). Here a skeletal isomerization step prior to cyclization was assumed. This is not of a cationic type reaction, and the results were explained by a free radical mechanism accompanied by vinyl migration (Scheme IXA). Attention is drawn to the fact that... 

Cg Dehydrocyclization. Arguments have been put forward that primary ring closure produces six-membered rings over three important catalyst types oxides, supported platinum, and bimetallic catalysts (107). The postulation of metal catalyzed Cg ring closure does not involve any definite suggestion whether its mechanism is direct or stepwise. ... 

Eischens and Selwood 176) have made a study of the activity of reduced chromia-on-alumina catalysts for the dehydrocyclization of n-heptane. The activity per unit weight of chromium was found to increase sharply at concentrations below about 5 wt. % Cr activities were measured down to 1.9 wt. % Cr concentration at which point the highest activity was observed. Selwood and Eischens concluded that this effect is due to the fact that the chromia is most dispersed at these low concentrations, in agreement with the present EPR data. However, if a two-site mechanism 177) is necessary for dehydrocyclization, the activity may drop at even lower chromium concentrations due to isolation of individual chiomium spins. 

Recent data on other alloys confirm the overall classification presented above, but at the same time lead to some refinement of the picture. For example, the most diluted Pt-Au alloys revealed isomerization, identified as running via 3C intermediates. This evidence was obtained (248) by establishing the fact that pentane isomerizes on most diluted Pt-Au alloys with 100% selectivity, whereas this molecule can only isomerize via the 3C complexes. This conclusion has been confirmed by the isotopic labeling method (269). It is therefore reasonable to assume that this isomerization can also proceed on isolated Pt sites, as can a part of dehydrocyclization and the dehydrogenation. We must conclude on the basis of this information that on metals like Pt, the fast multisite and the slow one-site mechanisms of hydrocarbon reactions may operate in parallel with each other. 

Furthermore, it is likely that the mechanism of catalytic dehydrocyclizations, studied by Steiner (83) on Cr2O3 with and without additions of foreign oxides and on M0S2, will be better understood by applying the theory of electron defects and of space-charge layers. Also, it will be fruitful to use isotopes for such studies, as has been done by Winter (86,87). 

Additional evidence to this scheme was reported applying temporal analysis of products. This technique allows the direct determination of the reaction mechanism over each catalyst. Aromatization of n-hexane was studied on Pt, Pt—Re, and Pd catalysts on various nonacidic supports, and a monofunctional aromatization pathway was established.312 Specifically, linear hydrocarbons undergo rapid dehydrogenation to unsaturated species, that is, alkenes and dienes, which is then followed by a slow 1,6-cyclization step. Cyclohexane was excluded as possible intermediate in the dehydrocyclization network. 

Since dehydrocyclization and ring cleavage are reversible processes, the same mechanisms may be operative in both transformations. Two basic cleavage patterns—a selective and a nonselective ring opening—are known. In the selective process only secondary-secondary carbon-carbon bonds are broken. Conversely, only the formation of carbon-carbon bonds between methyl groups is allowed in ring formation. In contrast, all bonds are cleaved equally in the nonselective reaction. Bonds adjacent to quaternary carbon atoms are exceptions, since they are not cleaved at all. 

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