Propene pathways

Fig. 12-5. Ozone-propene reaction pathways showing oxidation products.

In the following scheme, an oxidation pathway for propane and propene is proposed. This mechanism, that could be generalized to different hansition metal oxide catalysts, implies that propene oxidation can follow the allylic oxidation way, or alternatively, the oxidation way at C2, through acetone. The latter easily gives rise to combustion, because it can give rise to enolization and C-C bond oxidative breaking. This is believed to be the main combustion way for propene over some catalysts, while for other catalysts acrolein overoxidation could... 

It also explains the /Z selectivity of products at low conversions (kinetic ratio. Scheme 19). In the case of propene, a terminal olefin, E 2-butene is usually favoured (E/Z - 2.5 Scheme 19), while Z 3-heptene is transformed into 3-hexene and 4-octene with EjZ ratios of 0.75 and 0.6, respectively, which shows that in this case Z-olefins are favoured (Scheme 20). At full conversion, the thermodynamic equilibriums are reached to give the -olefins as the major isomers in both cases. For terminal olefins, the E olefin is the kinetic product because the favoured pathway involved intermediates in which the [ 1,2]-interactions are minimized, that is when both substituents (methyls) are least interacting. In the metathesis of Z-olefins, the metallacyclobutanes are trisubstituted, and Z-olefins are the kinetic products because they invoke reaction intermediates in which [1,2] and especially [1,3] interactions are minimized. 

Which reaction pathway dominates is dependent on the O s) concentration and the relative rates of carbonate formation and desorption of the gaseous C4, C6 and C7 gaseous products some control of these is possible38 by varying the propene-to-oxygen ratio and also the oxidant, such as substituting N20 for 02. 

Fig. 30. Mechanism for C-C activation of propene. Decay of the allyl hydride complex may proceed via migration of the metal-bound H atom to the /3-carbon atom in the allyl moiety (i.e. reverse /3-H migration), leading to formation of the same metallacyclobutane complex implicated in the Y + cyclopropane reaction. The dynamically most favorable decay pathway is to YCH2 + C2H4.

The simple hydrocarbon substrates included ethene, 1,2-propa-diene, propene and cyclopropane (22). Their reactivity with Sm, Yb and Er was surveyed. In contrast to the reactions discussed above, lanthanide metal vapor reactions with these smaller hydrocarbons did not provide soluble products (with the exception of the erbium propene product, Er(C H ) ). Information on reaction pathways had to be obtained primarily by analyzing the products of hydrolysis of the metal vapor reaction product. 

That the sequence shown in Scheme 3 is not the only pathway available for H—NiY formation is indicated by the isolation of 1,3-cyclooctadiene from the reaction products of the dimerization of propene with the n-cyclooctenylnickel system (25) (80) it seems reasonable that the H—NiY species 22 in this case is at least in part formed through direct elimination from 25 without prior monomer insertion into the Ni—C—bond [Eq. (6)] ... 

Various alkyl- and aryl-substituted [3]radialenes could be prepared from 1,1-dihaloal-kenes using organometallic pathways. Hexamethyl-[3]radialene (25), the first [3]radialene to be synthesized, was obtained in a very low yield by treatment of l,l-dibromo-2-methyl-1-propene (22) with butyllithium8,9. The lithium carbenoid 23 and the butatriene 24 are likely intermediates of this transformation (Scheme 2), the former being the source of an unsaturated carbene moiety which is transferred onto the latter. However, the outer double bonds of 24 are more readily cyclopropanated than the central one. 

The dissociation of a coordinated allene by hydrogen was evidenced for [(// -al-lyl) Pd(PR3)Cl] complexes [11]. The hydrogenation of allene to propene was then invoked to follow the pathway depicted in Scheme 4.7 ... 

Finally, an additional reaction pathway exists and this does not seem to be operative with SAPO-34 and Beta under regular processing conditions. This path seems to be operative with ZSM-5 and that may involve successive methylations of propene, followed by cracking to yield higher alkenes [111]. A similar mechanism that involves successive methylations of ethylene followed by cracking to yield higher alkenes over ZSM-5 does not seem to be as important [125]. It is conceivable that this mechanism may be partly operative during the MTO experiments over SAPO-34 described above that used co-fed ethylene or co-fed propylene [126]. 

The degradation products of GOS were 1,3-dimethyl pyrogallol (HI), 2-(2 ,6 dimethoxy phenoxy)-2-propenal (Vni), 2-(2, 6 -dimethoxy phenoxy)-3-hydroxypropanal (XII), and GOS-Dimer. These products show that the reaction includes oxidative polymerization and the cleavage of -0-4 ether linkage following the alkyl-phenyl cleavage. This depolymerization pathway of GOS is also similar to that of SOS (Table I). 

Unhke the homoalkyls, the tetraaUyluranium complex does not decompose via the /5 elimination pathway. The elimination of R—H occurs instead. The product ratios as determined by mass spectroscopy are propene, 81.5 %, and propane, 18.5 % 143). 

---