Hydrocarbons methoxylation

A carbenoid-type mechanism with free or surface-bound species formed by a elimination from methanol promoted by the strong electrostatic field of zeolites was proposed first.433,456,457 Hydrocarbons then can be formed by the polymerization of methyl carbene, or by the insertion of a surface carbene (8) into a C-O bond453-455,458,459 (Scheme 3.2, route a). If surface methoxyl or methyloxonium species are also present, they may participate in methylation of carbene454,455,460,461 depicted here as a surface ylide (9) (Scheme 3.2, route b). A concerted mechanism with simultaneous a elimination and sp3 insertion into methanol or dimethyl ether was also suggested 433,454,457... 

Since the extreme oxidizing power of the oxyl spin centers is successfully employed in waste water treatment, an application of these intermediates seems to be self-contradictory in terms of synthetic use. However, alkoxylation of hydrocarbons is a very important technical field since it allows the installation of functionalities without using the detour via halogenations. The selective introduction of functional groups on a completely nonactivated hydrocarbon has not yet been realized by BDD technology. In contrast, the direct anodic methoxylations of activated carbons exhibiting benzylic or allylic moieties can be performed at BDD anodes. The results obtained with BDD electrodes are quite similar to those when graphite serves as anode [57]. The anodic synthesis of benzaldehyde dimethyl ketals is industrially relevant and performed on the scale of several thousand tons. A detailed study of the anodic methoxylation of 4-tert-butyltoluene (10) at BDD was performed [58]. Usually, the first methoxylation product 11 and the twofold functionalized derivative 12 are found upon electrochemical treatment (Scheme 5). 

Oxidations. Silyl ketene acetals undergo methoxylation to give a-methoxycarboxylic esters on treatment with Phl=0 in methanol. Aryl ketones are obtained from the corresponding hydrocarbons in an oxidation catalyzed by a (salen)Mn(III) complex, using Phl=0 as oxidant. ... 

It would appear that most of the benzene ring structures in humic substances have two or more substituents, such as carbonyl, carboxyl, hydrocarbon, hydroxyl, and methoxyl and other ether functional groups. The evidence from identification of the products of degradation reactions, and more recently from NMR data, suggests that aliphatic hydrocarbons and aliphatic and aromatic ether groups link the core components in the macromolecules, and that carbonyl, carboxyl, and hydroxyl substituents are likely to be attached to some of the aliphatic hydrocarbons (Hayes and Swift, 1978). 

Carbenic mechanisms. Venuto and Landis [10] were the first to address the question of mechanism of hydrocarbon formation from methanol over zeolites, in this case zeolite X [11]. These workers proposed a scheme involving a-elimina-tion of water and polymerization of the resultant methylcarbenes to olefins. Swabb and Gates [12], elaborating on Venuto-Landis, proposed that concerted action of acid and basic sites in the zeolite (mordenite) facilitates a-elimina-tion of water from methanol. According to Salvador and Kladnig [13], carbenes are generated through decomposition of surface methoxyls (a-el imination of silanol) formed initially upon chemisorption of methanol on the zeolite (zeolite Y). Hydrocarbons are assumed to form, in the latter two schemes, also by carbene polymerization. 

It is apparent that much resourceful, imaginative experimentation has been done to resolve the question of C-C bond formation from methanol. Although the answer remains elusive, these experiments tell us at least what is probably not involved in the bond formation, particularly in the presence of zeolite catalysts. The Stevens rearrangement of oxonium ylide can be ruled out, as well as the carbocationic route invoking hypervalent carbon transition states. Not excluded are surface-bound species such as carbenoids and ylides. Again there seems to be a consensus that surface methoxyls are precursors to these reactive C- intermediates, which seems somehow to be "coming full circle", since surface methoxyls were early shown to be intermediates in the formation of DME, which is itself an intermediate in hydrocarbon formation. Finally, if the free radical character of the initiation step proves correct, the implications to zeolite catalysis will be far-reaching. 

The Al-0 bond is therefore more nucleophilic than basic. These same workers then studied the decomposition of various methylating agents MeX (X = Oil, I, 0S03Me) over ZSM-5 [29] (Table 5). The fact that hydrocarbons were formed from dimethyl sulfate argues against TMO ion as a key intermediate, since the oxygen in dimethyl sulfate is too weakly nucleophilic to form an oxonium ion. Hunter and Hutchings propose the initial formation of a surface—bound methoxyl, which is deprotonated to a surface-bound ylide. This surface-bound ylide is isoelectronic with surface-bound carbene [13,30],... 

Salvador and Kladnig used H-Y and Na-Y zeolites between 20 and 350 °C. They showed that at 120 °C the surface methoxyl groups react with methanol in the gas phase to give dimethyl ether. Light hydrocarbons, predominantly butane and propylene, were detected at 250 "C. Na-Y zeolite was not active for methanol conversion in the temperature range studied. The higher activity of H-Y is due to the high Bronsted acid character of its surface. 

The easiness of one-electron transfer is directly related to the ionization potential of aromatic hydrocarbons and to electron donor substitutes, such as methoxyl, which decreases the oxidation potential of lignins dissolved in acetic acid [1]. 

Hawthorne, S. B., Miller, D. J., Langenfeld, J. J., and Krieger, M. S. (1992) PMi0 high volume collection and quantification of semi- and nonvolatile phenols, methoxylated phenols, alkanes, and polycyclic aromatic hydrocarbons from winter urban air and their relationship to wood... 

C-H Oxygenation. The combination of PhI(OAc)2 and catalytic Pd(OAc)2 can be used with functionalized aromatic and aliphatic hydrocarbons for the directed oxygenation of C-H bonds (eq 64). 66 6 Substrates containing pyridine, azole, imine, and oxime groups fuction under these reaction conditions to afford acetoxy lated or methoxylated compounds. Oxidation of 1 ° methyl centers is strongly preferred over 2° and 3° C-H sites. 

The direct alkoxylation of hydrocarbons allows the installation of functionalities without using the detour via halogenations. Consequently, this represents a very important technical field. The anodic synthesis of benzaldehyde dimethyl ketales is industrially relevant and performed on a several thousand ton scale [22, 23]. The direct anodic methoxylation of benzylic or allylic moieties can be performed at BDD anodes. The results obtained on BDD electrodes are quite similar to the ones when graphite serves... 

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