C5 cyclics

Referring first of all to the reactions over 0.2% platinum/alumina (Table V) the major features of the product distributions may be explained by a simple reaction via an adsorbed C5 cyclic intermediate. For instance, if reaction had proceeded entirely by this path, 2-methylpentane-2-13C would have yielded 3-methylpentane labeled 100% in the 3-position (instead of 73.4%) and would have yielded n-hexane labeled 100% in the 2-position (instead of 90.2%). Similarly, 3-methylpentane-2-I3C would have yielded a 2-methylpentane labeled 50% in the methyl substituent (instead of 42.6%), and would have yielded n-hexane labeled 50% in the 1- and 3-positions (instead of 43.8 and 49% respectively). The other expectations are very easily assessed in a similar manner. On the whole, the data of Table V lead to the conclusion that some 80% or so of the reacting hydrocarbon reacts via a simple one step process via an adsorbed C5 cyclic intermediate. The departures from the distribution expected for this simple process are accounted for by the occurrence of bond shift processes. It is necessary to propose that more than one process (adsorbed C6 cyclic intermediate or bond shift) may occur within a single overall residence period on the catalyst Gault s analysis leads to the need for a maximum of three. The number of possible combinations is large, but limitations are imposed by the nature of the observed product distributions. If we designate a bond shift process by B, and passage via an adsorbed Cs cyclic intermediate by C, the required reaction paths are... 

A more detailed analysis of the results obtained over 10% platinum/ alumina (115) leads to an extended array of parallel, multistep reaction paths, and it was concluded (for 273°C) that an adsorbed species had a chance of reacting via an adsorbed C5 cyclic intermediate of about 0.3, of reacting via a bond shift of about 0.2, and a chance of desorption of about 0.5. One would expect these probabilities to be temperature dependent, but to different extents, so that the nature of the product distributions should also be temperature dependent. 

When one considers the various results from the reactions of labeled and unlabeled hexanes over supported catalysts and over thick and ultra-thin films, the conclusion emerges that catalysts with very small platinum particles (ultrathin films or 0.2% platinum/alumina) strongly favor reactions via an adsorbed C5 cyclic intermediate, but at large particle size... 

Direct ( hydrogenative ) Cj cyclization of alkanes to give saturated C5 cyclic products. This is a typical metal-catalyzed reaction occurring in a hydrogen-rich atmosphere over a narrow group of metals (25). 

Rhodium can be added as the fourth member of this group—particularly since skeletal isomerization over rhodium also involves C5 cyclic intermediates 42). Its alloying with copper results in the appearance of C5 cyclic products 43). 

C5 Cyclization of various alkanes 38, 38a) over platinum on carbon was first observed in 1954. Barron et al. 15a) postulated the formation of a surface C5 cyclic intermediate, which may desorb as a cyclopentanic hydrocarbon or may produce skeletal isomers without desorption. 

The surface complex shown in Scheme V may be identical with the common C intermediate of C5 cyclization and C5 cyclic isomerization 15a). Its degree of dissociation may not be too high (57), and the bond between it and the metal may have some sort of fluxional character. 

Kazansky et al. reported (over platinum on carbon under identical conditions) about 5% C5 cyclic yield from -alkanes 38), 12% from 3-ethylpentane (79), and 35% from 2,2,4-trimethylpentane 38a, 80). The Cj cyclization of 2,5-dimethylhexane did not take place, presumably because the end methyl group removes carbon atom No. 2 from the surface 81) (Scheme VII). The... 

Such a mechanism is a reality. Methylpentenes are able to form an unsaturated C5 cyclic compound 1-methylcyclopentene, even under conditions when 3-methylpentane does not cyclize. Its amount is highest from 3-methyl-1 -pentene (having a terminal olefin bond) (25). This points to another, dehydrogenative route. 

The dehydrogenative route is probably identical with the alkene-alkyl insertion mechanism (I5a) (Scheme IVA) rather than with the dicarbyne cyclization (85a). The latter was based on the unreactivity of -hexane in C5 cyclic reactions over iridium (4Ia). 

Fig. 7, Percent selectivity of hydrogenative C, ring closure as a function of the hydrogen content of the carrier gas. Pulse system catalyst, 0.4 g platinum black T = 360°C. Starting hydrocarbons ( ) 3-methylpentane ( ) 3-methyl-1-pentene (T) tra . -3-methyl-2-pentene (A) di-2-methyl-2-pentene. Selectivity is expressed as methylcyclopentane (MCP) % in the total C5 cyclic product (MCP + MCPe) (55).

Two main pathways of metal-catalyzed skeletal rearrangement have been distinguished bond shift mechanism and C5 cyclic isomerization (7, 8). 

Isomerization—wherever the structure of the reactant permits the C5 cyclic pathway—is accelerated by hydrogen (27, 27a). The parallelism between the isomer and C5 cyclic yields (27a, 62), and also the composition of isomers (91-92) indicate a prevailing C5 cyclic pathway of isomerization in the presence of hydrogen. 

With platinum and palladium supported on acidic alumina, cyclopentanes are important intermediates of aromatization (44, 123-124). For example, n-heptane gave about 2-3 times more aromatic product than 2,4-dimethyl-pentane, whereas the formation of C5 cyclic products was about the same from both alkanes. Alkylcyclopentanes aromatized at a reasonable rate (123a). 

Kazansky et al. (5) estimated the role of C5 cyclic intermediates in aromatization to be about 5% over platinum on carbon. Dautzenberg and Platteeuw found about 11% C5 cyclic pathway with nonacidic platinum on alumina (23). 2,2,4-Trimethylpentane is forced to produce aromatics via C5 cyclization because of its structure here the quaternary carbon atom facilitates ring enlargement (5, 23). 

Davis also estimated the contribution of the C5 cyclic mechanism to be about 5% in the aromatization of 1-[ C]- and 4-[ C]- -heptane over nonacidic Pt-alumina (125) ... 

C5 cyclization requires stricter geometric conditions than aromatization. This is in favor of the dual-site mechanism of C5 cyclic reactions (25). All metals catalyzing it have an fee lattice, and their atomic diameter lies between 0.269 and 0.277 nm. These two criteria must be fulfilled simultaneously. With such a distance between the two sites, the screening of the C—C bond adjacent to the preferably adsorbed tertiary C atom becomes evident. Figure... 

The assumption of reactive chemisorption may be useful for the surface intermediate of C5 cyclic reactions. It may well be possible that a competition occurs between a reactive and a dissociative chemisorption the former giving C5 the latter cyclic products. There is a thermodynamic relationship between these two surface species (see Section II,A,2). Scheme XIII summarizes all the above-mentioned facts about hydrogen effects and various surface intermediates (31). 

If hydrogen occupies all sites, the dual-site mechanism may operate over two adjacent /2g sites 42). The importance of active site periodicity and the screening of the adjacent C—C bond is valid in this case, too. This (assumedly adsorbed) hydrogen does not participate in C5 cyclic reactions. There is some indication, however, that it might be mobilized for cyclobutane ring opening 97, 97a). 

One carbon atom in a wrong interstice may block the C5 cyclization activity of several surrounding sites. Therefore, C5 cyclic reactions are suppressed first during catalyst deactivation, while aromatization activity lasts much longer 159). This again supports the reactive adsorption mechanism 154). A different type of deactivation was reported as being due to disordered and ordered surface carbonaceous deposits 138,148). 

Different approaches to the C5 cyclic mechanism have been put forward. Comparative studies of the isomerization of 2,2,3-trimethylpentane, 2,2,4-trimethylpen-tane, and 2,2,4,4-tetramethylpentane indicated161 that the latter did not display any appreciable isomerization on palladium below 360°C. Since this compound cannot undergo dehydrogenation without rearrangement to form an alkene, the cyclic mechanism on palladium is suggested to occur via the 1,2,5 triadsorbed species (20) bonded to a single surface atom. 

C5-cyclic intermediates are involved in the l-methyl-2-ethylbenzene n-propylbenzene isomerization ... 

An efficient a-sialylation that took advantage of the highly reactive sialyl donors having C5 cyclic imides, especially phthalimide (NPhth), by virtue of the fixed-dipole moment effects, was also reported [42], For example, the use of the sialyl donor 4 and acceptor 6 at -78°C supplied the sialoside 7 as a only with 92% yield on 50 mg scale. The scale-up in batch process, however, significantly decreased the yield and stereoselectivity. This decrease in sialylation efficiency might be a result of... 

Along with typical acid catalyzed reactions such as cracking, the metal catalyzed reactions such as hydrogenolysis and isomerization could also take place over Pt-HZSM-5 catalysts [4, 5]. An earlier study of us [11] showed, however, that metal catalyzed isomerization over a Pt catalyst on a strong acidic support can manifest themselves under specific experimental conditions at low temperatures (<573 K) and with higher Ho/nH ratios only. While n-hexane underwent aromatization also on HZSM catalyst without Pt [11], the presence of Pt was indispensable for skeletal isomerization. The appearance of methylcyclopentane (MCP) - as a possible intermediate [12] - was crucial to assume that the so-called C5-cyclic metal catalyzed isomerization took place. 

The conversion of n-hexane over various Pt-zeolite catalysts responded strongly to changes in hydrogen pressures [13-15]. Increasing the hydrogen/n-hexane ratio promoted metal catalyzed skeletal isomerization, mainly by the C5-cyclic pathway and shifted also the fragment composition towards vmues typical of metal-catalyzed hydrogenolysis. 

Sen, selectivity to lower alkanes 5j,to skeletal isomers 5cs,to C5 cyclic molecules Sc6 to 6 cyclic molecules 5an>ni, to benzene or toluene. 

There are two separate and distinct mechanisms by which skeletal isomerisation can occur (i) the bond shift mechanism, and (ii) the C5 cyclic mechanism. The first is clearly the only possibility when there are less than five carbon atoms in the chain so the way of isomerisation of n-to isobutane has to be by bond-shift. Two somewhat different mechanisms with a number of minor variations have been proposed. The first involves an actual or virtual cyclopropanoid species formed... 

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