Partial dehydrogenation

Partial hydrogenation of pyrrole derivatives and partial dehydrogenation of pyrrolidines afford /I -pyrrolines (80-82). However, because of the complex nature of the reaction, it is of little preparative value. The same is true for isomerization of /) -pyrrolines to /) -pyrrolines (83). A photodehydrogenation of 2,6-dimethylpiperidine (26) has been observed recently, affording 2,6-dimethyl-3,4,5,6-tetrahydropyridine (27) in a good yield (84)-... 

Partial dehydrogenation of a tetrahydro-j3-carboline derivative has been postulated to account for the origin of 7-hydroxy-l-... 

Cyclohexadiene and benzene form identical structures on Pt(l 1 1) at low pressures (Figures 7.23 and 7.24). 1,3-Cyclohexadiene dehydrogenates to form benzene on the surface, while benzene adsorbs molecularly. Figure 7.24b schematically shows the adsorbed benzene structure at low pressure. The STM images of the C6 cyclic hydrocarbons show three different adsorbed structures on Pt(l 1 1). Cyclohexene and cyclohexane partially dehydrogenate to form rc-allyl, 1,4-cyclohexadiene adsorbs in a boat configuration, and both 1,3-cylohexadiene and benzene adsorb as molecular benzene on the surface. 

Partial dehydrogenation of methanol can yield formaldehyde and this can potentially react with itself or unconverted methanol to yield another commonly reported by-product of methanol decomposition, methyl formate 1,12... 

Biological. A mutant of Pseudomonas putida dihydroxylyzed benzene into cA-benzene glycol, accompanied by partial dehydrogenation, yielding catechol (Dagley, 1972). Bacterial... 

Treatment of 73 with selenium metal at 360° yields dibenzothio-phene. Oxidation of 73 with hydrogen peroxide was accompanied by partial dehydrogenation yielding 1,2,3,4-tetrahydrodibenzothiophene 5,5-dioxide (68). ... 

Linear alkylbenzenes are made from -paraffms (Cio-Cu) by either partial dehydrogenation to olefins and addition to benzene with HF as catalyst (60%) or chlorination of the paraffins and Friedel-Crafts reaction with benzene and an aluminum chloride catalyst (40%). See Chapter 24 for more information. 

However, the considerable decrease in methane yield in the above case does not indicate the contribution of the hydroaromatic structure towards methane formation. It has been established (19, 23) that complete or even a partial dehydrogenation inhibits tar formation completely (though a minimum dehydrogenation is necessary for this effect) and retains all the hydroaromatic carbon in the char. The decrease in methane yield on pyrolysis of... 

Here M and T represent methylcyclohexane and toluene in the gas phase, and Ttt represents adsorbed toluene. The first step in the above reaction sequence represents the adsorption of methylcyclohexane with subsequent reaction to form toluene, while the second step is the desorption of toluene from the surface. Very likely the first step represents a series of steps involving partially dehydrogenated hydrocarbon molecules or radicals. However, at steady-state conditions the rates of the intermediate steps would all be equal, and the kinetic analysis is, therefore, not complicated by this factor. To account for the near zero-order behavior of the reaction, it was suggested that the active catalyst sites were heavily covered with... 

The chemisorption of over 25 hydrocarbons has been studied by LEED on four different stepped-crystal faces of platinum (5), the Pt(S)-[9(l 11) x (100)], Pt(S)-[6(l 11) x (100)], Pt(S)-[7(lll) x (310)], and Pt(S)-[4(l 11 x (100)] structures. These surface structures are shown in Fig. 7. The chemisorption of hydrocarbons produces carbonaceous deposits with characteristics that depend on the substrate structure, the type of hydrocarbon chemisorbed, the rate of adsorption, and the surface temperature. Thus, in contrast with the chemisorption behavior on low Miller index surfaces, breaking of C-H and C-C bonds can readily take place at stepped surfaces of platinum even at 300 K and at low adsorbate pressures (10 9-10-6 Torr). Hydrocarbons on the [9(100) x (100)] and [6(111) x (100)] crystal faces form mostly ordered, partially dehydrogenated carbonaceous deposits, while disordered carbonaceous layers are formed on the [7(111) x (310)] surface, which has a high concentration of kinks in the steps. The distinctly different chemisorption characteristics of these stepped-platinum surfaces can be explained by... 

Since 1977 several papers on rhodium and iridium systems relevant to the present work have appeared (10-16). One of these by Vrieze s group (15), which reported partial dehydrogenation of tricyclohexylphosphine coordinated to iridium(I) and rhodium(I), overlapped with some of our studies which were reported almost simultaneously at a conference (17). 

Successful cyclizations involving attack on reduced rings have also been reported in the presence of polyphosphoric acid the originally formed hexahydrophenanthridines (15) all undergo spontaneous partial dehydrogenation to the corresponding 7,8,9,10-tetrahydro-phenanthridines (16).48... 

The apparently simple procedures of partial dehydrogenation of pyrrolidines and partial hydrogenation of pyrroles afford Zl1-pyr-rolines. However, the reaction is complex and is of little preparative value.97-98 A 1-Pyrrolines may be obtained by isomerization of A 3-pyrrolines.100 From the preparative point of view, partial hydrogenation of quaternary pyridine salts in strongly alkaline media to give 1-alkyl-id 2-piperideines is more important.101 Formation of heterocyclic enamines was observed in the reduction of i -methyl-pyrrolidone with lithium aluminum hydride,102 -alkylpiperidones with sodium in ethanol,103,104 and in the electrolytic reduction of N-methylglutarimide.106... 

Acetyl-l-benzyl-3,4,4a,5,6,7-hexahydro-l,5-naphthyridine-2,7(l//)-dione (15) (a byproduct from a primary synthesis) appears to have undergone partial dehydrogenation and N— (9-transacylation to afford 7-acetoxy-l-benzyl-3,4-dihydro-1,5-naphthyridin-2( 1 //)-one (16) (Pd/C, xylene, 130°C, 30 h ... 

Corma el al. (126) found that PtNaY was an active but rather unstable catalyst for methylcyclohexane dehydrogenation to toluene. These workers studied both the dehydrogenation and the catalyst decay kinetics. It was concluded that the reaction occurs via a series of consecutive partial dehydrogenation steps, the first of which was rate determining. Further, catalyst deactivation was caused by coke deposition from partially unsaturated precursor molecules. 

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