Ethylene reaction pathways

Adams and Yang (10) have suggested that the S atom of methionine is recycled in the ethylene reaction pathway, as shown in Fig. 2. In this scheme, 5 -methylthioadenosine, the residual molecule which derives from the reaction converting SAM to ACC, is further metabolized to 5 -methylthioribose, which then transfers the S-methyl group to homoserine to form methionine. This scheme is hypothetical, and the enzymes necessary for all these reactions have not as yet been demonstrated. 

While diene metathesis or diyne metathesis are driven by the loss of a (volatile) alkene or alkyne by-product, enyne metathesis (Fig. 2) cannot benefit from this contributing feature to the AS term of the reaction, since the event is entirely atom economic. Instead, the reaction is driven by the formation of conjugated dienes, which ensures that once these dienes have been formed, the process is no longer a reversible one. Enyne metathesis can also be considered as an alkylidene migration reaction, because the alkylidene unit migrates from the alkene part to one of the alkyne carbons. The mechanism of enyne metathesis is not well described, as two possible complexation sites (alkene or alkyne) exist for the ruthenium carbene, leading to different reaction pathways, and the situation is further complicated when the reaction is conducted under an atmosphere of ethylene. Despite its enormous potential to form mul-... 

In summary, the results from the fixed bed reactor study provided evidence as to the effect of Au and KOAc on the performance of the catalyst, though, these experiments did not give any information on the perturbation of the reaction pathways with the addition of Au and KOAc. For this type of information, additional experiments were performed using the TAP reactor with 1,2 C-labeled ethylene used as an isotopic tracer of the kinetics. 

We will now discuss some very recent applications of the soft El ionization method for product detection in CMB experiments. We will first deal with two polyatomic reactions of ground state oxygen atoms with unsaturated hydrocarbons (acetylene and ethylene) these reactions are characterized by multiple reaction pathways and are of great relevance, besides being from a fundamental point of view, in combustion and atmospheric chemistry. 

An initial step of orthometallation probably also occurs when aniline is allowed to react with ethylene in the presence of a rhodium(I) catalyst. 2-Methylquinoline (10 turnovers relative to the metal) and JV-ethylaniline (30 turnovers) are formed after 72 h in what are probably two independent reaction pathways (Scheme 144).216 It is interesting to note that the intramolecular cyclization step in the proposed216 mechanism (Scheme 144) has precedent in the palladium-promoted quinoline synthesis reported by Hegedus et al.16 (see Scheme 142), but the transformation 118->119 is unusual in the chemistry of organometallic insertion reactions.106... 

In considering catalyzed olefin-cyclopropane interconversions, an important question arises concerning thermodynamic control and the tendency (or lack thereof) to attain a state of equilibrium for the system. Mango (74) has recently estimated the expected relative amounts of ethylene and cyclopropane for various reaction conditions and concluded that the reported results were contrary to thermodynamic expectation. In particular, the vigorous formation of ethylene from cyclopropane (16) at -78°C was stated to be especially unfavored. On the basis of various reported observations and considerations, Mango concluded that a reaction scheme such as that in Eq. (26) above (assuming no influence of catalyst) was not appropriate, because the proper relative amounts of cyclopropanes and olefins just do not occur. However, it can be argued that the role of the catalyst is in fact an important element in the equilibration scheme, for the proposed metal-carbene and [M ] species in Eq. (26) are neither equivalent nor freely interconverted under normal reaction conditions. Consequently, all the reaction pathways are not simultaneously accessible with ease, as seen in the published literature, and the expected equilibria do not really have an opportunity for attainment. In such a case, absence of thermodynamic control should not a priori deny the validity of Eq. (26). 

Fahey (16) suggests that intermediate 3 dissociates formaldehyde he finds supportive evidence in the rhodium-based system by observation of minor yields of 1,3-dioxolane, the ethylene glycol trapped acetal of formaldehyde. For reasons to be discussed later, we believe the formation of free formaldehyde is not on the principal reaction pathway. (c) We have also rejected two aspects of the reaction mechanism proposed by Keim, Berger, and Schlupp (15a) (i) the production of formates via alcoholysis of a formyl-cobalt bond, and (ii) the production of ethylene glycol via the cooperation of two cobalt centers. Neither of these proposals accords with the observed kinetic orders and the time invariant ratios of primary products. 

Scheme 13. Reaction pathway for the Ru-catalyzed reactions of disubstituted styrenyl ethers in the absence of ethylene atmosphere...

Further evidence for the Aa11 mechanism was obtained from a solvent kinetic isotope study. The theoretical kinetic isotope effects for intermediates in the three reaction pathways as derived from fractionation factors are indicated in parentheses in Scheme 6.143,144 For the Aa11 mechanism (pathway (iii)) a solvent KIE (/ch2o A d2o) between 0.48 and 0.33 is predicted while both bimolecular processes (pathways (i) and (ii)) would have greater values of between 0.48 and 0.69. Acid-catalysed hydrolysis of ethylene oxide derivatives and acetals, which follow an A1 mechanism, display KIEs in the region of 0.5 or less while normal acid-catalysed ester hydrolyses (AAc2 mechanism) have values between 0.6 and 0.7.145,146... 

Scheme 5 Proposed reaction pathway for the partial oxidation of ethanol. The dehydration into ethylene intermediate is not included. 

The first mode of the high resolution C-NMR of adsorbed molecules was recently reviewed Q-3) and the NMR parameters were thoroughly discussed. In this work we emphasize the study of the state of adsorbed molecules, their mobility on the surface, the identification of the surface active sites in presence of adsorbed molecules and finally the study of catalytic transformations. As an illustration we report the study of 1- and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides (4>3). Another application of this technique consists in the in-situ identification of products when a complex reaction such as the conversion of methanol, of ethanol (6 7) or of ethylene (8) is run on a highly acidic and shape-selective zeolite. When the conversion of methanol-ethylene mixtures (9) is considered, isotopic labeling proves to be a powerful technique to discriminate between the possible reaction pathways of ethylene. 

Conversion of methanol-ethylene mixtures The C isotopic labeling is a powerful technique to discriminate between the possible reaction pathways of ethylene (9,50). 

Four different experiments were realized by labeling either the methanol or the ethylene molecules (Figure 7). The reactions were studied in static conditions adsorbing either methanol (A and B) or ethylene (C and D) prior to the second reactants. The l C-NMR spectra of Figure 7 reveals that the order of adsorption of the reactants is very important for the reactivities at first, surface alkylation occurs and is followed by separated reaction pathways for CH3OH and C2H/. 

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]. 

Reaction pathways for the addition of ethylene to butadiene radical cation involving H-shifts have been investigated at the coupled cluster UCCSD(T)/DZP//UMP2(fc)/DZP-b ZPE level of theory.Several rearrangement reactions have been found to occur below the energy limit of separated ethylene and butadiene radical cation. The cyclopentenyl cation ( 5117)+ in the gas phase may originate from various pathways. 

Mg+" reacts with alkyl halides in the gas phase via a range of substrate-dependent pathways Not all halides are reactive—examples of unreactive substrates include methyl chloride, vinyl chloride, trichloro and tetrachloro ethylene. Reaction with ethyl chloride proceeds via an elimination reaction (equation 18) followed by a displacement reaction (equation 19). For larger alkyl halides, such as isopropyl chloride, chloride abstraction also occurs (equation 20). For multiply halogenated substrates such as carbon tetrachloride, oxidative reactions occur (equations 21 and 22), although organometallic... 

Somewhat analogous reactions would be expected for the reaction of ethylene with 02 ions but the observed reaction rate is lower than for propene, suggesting that the reaction pathway may be controlled by the C—H bond energies. For reactions of propane and 1-butene with 02, oxygenated compounds of the same carbon number as the reactants were produced. The initial step is thought to involve a hydrogen atom abstraction from a secondary carbon atom. 

From the ethylene results, and similar results on acetylene [306], it is evident that interdimer reactions play an important role in the chemistry of organic molecules on Ge(100)-2 x 1. The simple picture of reaction across a single Ge-Ge dimer, while capturing a number of important reaction pathways, is incomplete. Even small C2 molecules such as ethylene and acetylene can bridge across dimers along a dimer row. Other molecules are found to bridge across the wider trench. Furthermore, these studies indicate that multiple reaction products can form even for simple systems. 

Yu and Wang431 considered that indole-3-acetic acid exerts its stimulating effect on expansion growth by inducing the synthesis of the enzyme catalyzing the conversion of S-adenosylmethionine into ACC, a conclusion at variance with the suggestion of Vioque and coworkers432 that indoleacetic acid oxidase and its substrate (IAA) participate in the last reaction in the ethylene biosynthesis pathway, namely, the formation of ethylene from ACC. 

All types of olefins can serve as substrates. Suitable acyclic olefins include ethylene, terminal and internal monoenes up to and including tetrasubstituted-double bonds, and aryl-substituted olefins. With dienes (and polyenes) an additional, intramolecular reaction pathway becomes available which leads to cyclic olefins (Reaction 2). 

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