Transition state pyrolytic

Enby 6 is an example of a stereospecific elimination reaction of an alkyl halide in which the transition state requires die proton and bromide ion that are lost to be in an anti orientation with respect to each odier. The diastereomeric threo- and e/ytAra-l-bromo-1,2-diphenyl-propanes undergo )3-elimination to produce stereoisomeric products. Enby 7 is an example of a pyrolytic elimination requiring a syn orientation of die proton that is removed and the nitrogen atom of the amine oxide group. The elimination proceeds through a cyclic transition state in which the proton is transferred to die oxygen of die amine oxide group. 

Several types of compound undergo elimination on heating, with no other reagent present. Reactions of this type are often run in the gas phase. The mechanisms are obviously different from those already discussed, since all those require a base (which may be the solvent) in one of the steps, and there is no base or solvent present in pyrolytic elimination. Two mechanisms have been found to operate. One involves a cyclic transition state, which may be four, five, or six membered. Examples of each size are... 

Both 1st- and 2nd-order rate expressions gave statistically good fits for the control samples, while the treated samples were statistically best analyzed by 2nd-order kinetics. The rate constants, lst-order activation parameters, and char/residue yields for the untreated samples were related to cellulose crystallinity. In addition, AS+ values for the control samples suggested that the pyrolytic reaction proceeds through an ordered transition state. The mass loss rates and activation parameters for the phosphoric acid-treated samples implied that the mass loss mechanism was different from that for the control untreated samples. The higher rates of mass loss and... 

The first reaction step of thermal degradation is scission of an ester bond. Esters containing at least one ft-hydrogen atom decompose pyrolytically to give olefins and acids via a cyclic transition state (Figure 2.14). 

A general reaction of 1 -acyl-2- )r m-alkylaairidin B fa the pyrolytic rearrangement to an uneaturated amide1 . -SM (Eq, 72). Stereochemical evidence supports the view that the reaction involves the intramolecular aVdinination of a proton concerted with the opening, of the aziridine dug (transition state, XLVI). So... 

The slopes of the lines obtained from the Taft correlations of aliphatic primary, secondary and tertiary chlorides obtained at different temperatures by the extrapolation p%2lP%i T IT2 indicate that the positive nature at the carbon reaction center of the C—Cl bond in the transition state increases from a primary to a tertiary carbon atom (Table 8)70. An additional fact is that for each type of alkyl halide, the degree of positive charge at the carbon reaction center tends to decrease as the temperature increases. This means that the pyrolytic eliminations tend to be more concerted and less polar at very high temperatures. These data support Maccoll s theory on the heterolytic character of the alkyl halides pyrol-yses in the gas phase1. 

In connection with the methoxy participation, the gas-phase pyrolytic elimination of 4-chloro-1 -butanol was investigated177. The products are tetrahydrofuran, propene, formaldehyde and HCl. It is implied that the OH group provides anchimeric assistance from the fact that, besides formation of the normal unstable dehydrochlorinated intermediate 3-buten-l-ol, a ring-closed product, tetrahydrofuran, was also obtained. The higher rate of chlorobutanol pyrolysis with respect to chlorethanol and ethyl chloride (Table 27) confirmed the participation of the OH group through a five-membered ring in the transition state. 

A later synthesis from Arigoni s laboratory uses a very ingenious, completely chemical process [124]. There are two crucial steps in which the geometries of two transition states in a pyrolytic reaction determine the overall outcome. In the first of these (which is an ene reaction ) a triple bond, -C = C-3H acquires H from one direction only, forming... 

Heine ° has pointed out that this reaction represents an example of the reverse of the reaction on pyrolysis of imido esters. Heine has recently shown that the pyrolytic isomerization of cis- and rra j-l-p-nitrobenzoyl-2,3-diphenylaziridines into 2-p-nitrophenyl-4,5-diphenyl-2-oxazolines is a stereospecific process. These results are consistent with a mechanism that involves either a four-membered transition state or a short-lived tight ion-pair intermediate that collapses to the oxazoline before racemisation can occur . The pyrolysis of l,3-diaroyl-2-aryl-aziridines results in a different kind of reaction, in which a-benzamidobenzal-acetophenones are produced, viz. 

The chain scission can be seen as a pyrolytic elimination reaction. All mechanisms described in Section 2.2 may take place during chain scission. A reaction of chain scission with a cyclic transition state may take place, for example, during cellulose pyrolysis ... 

In the gas phase, when the reaction is normally initiated by heat, and so is called pyrolysis, there are two common pathways. The first is via a cyclic transition state and the second involves a free radical pathway. These two pyrolytic eliminations are rather different in nature from those that occur in solution, and so we will discuss them separately. 

The trans isomer of the alkene stilbene is formed, as indicated by the presence of the deuterium atom in the product. In this case, elimination is via a cyclic transition state, and so the opposite stereochemistry is obtained than was achieved in the E2 mechanism. This cyclic pyrolytic elimination is labelled the Ei mechanism, which stands for intramolecular, or internal, elimination. 

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