Olefins, proton affinities

This describes the process during which the monomer is transferred into its cation. This process has proven itself to be the rate determining factor (see part 4.1.3). An extensive collection of proton affinities of relevant olefines is given in Ref.108). 

The variation in the AH of this reaction for a series of olefins is essentially governed by the differences in the proton affinities of the olefins, and these are steeply graded according to the degree of substitution and the nature of the substituents in the vicinity of the double bond (Table 1). 

Since the proton affinity P(Rn=) of the branched olefin Rn= is certainly greater than P(C2H4), the reaction would be endothermic a similar argument applies to the transfer of carbonium ions. This may account for the low yields observed even in the heterogeneous cationic oligomerisation of ethylene. The most likely transfer reactions are hydride ion and methide ion transfers between oligomer ions and molecules (see below), leading to conjunct polymerisation [12]. 

Am (PIB=). Of the four isomeric olefinic groups which can be formed by removal of CH3 from the polyisobutyl ion and subsequent isomerisation, the structure -CMe2 CMe = CH2 will have the largest AMe+ which is taken as the same as that of isobutene this can be estimated as -70 kcal less than the proton affinity, i.e., -130 kcal. 

In every case an olefin is one of the products of the primary cracking step. Now by considering each reaction in reverse, a common denominator for all the designated cracking systems can be found in the chemistry of olefins. The answer lies in the character of the olefinic double bond, which comprises the normal valence pair electrons, and in addition two extra or pi electrons, which endow the double bond with the ability to attract positively charged groups, especially protons. This ability is expressed quantitatively by the proton affinity, which is shown below for propylene and isobutylene ... 

Theory helps the experimentalists in many ways this volume is on chemical shift calculations, but the other ways in which theoretical chemistry guides NMR studies of catalysis should not be overlooked. Indeed, further theoretical work on two of the cations discussed above has helped us understand why some carbenium ions persist indefinitely in zeolite solid acids as stable species at 298 K, and others do not (25). The three classes of carbenium ions we were most concerned with, the indanyl cation, the dimethylcyclopentenyl cation, and the pentamethylbenzenium cation (Scheme 1), could all be formally generated by protonation of an olefin. We actually synthesized them in the zeolites by other routes, but we suspected that the simplest parent olefins" of these cations must be very basic hydrocarbons, otherwise the carbenium ions might just transfer protons back to the conjugate base site on the zeolite. Experimental values were not available for any of the parent olefins shown below, so we calculated the proton affinities (enthalpies) by first determining the... 

B3LYP/TZVP geometries of all species shown in Scheme 1, followed by MP4(sdtq)/6-311+G single-point energy calculations. The theoretical proton affinities are shown for comparison, the experimental PA for pyridine is 222 kcal/mol. It would appear that olefins that protonate on a zeolite to yield persistent carbenium ions are very basic indeed. 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

Direct measurement of the proton affinity is possible for only a few molecules, mainly olefins and carbonyl compounds. However, these measurements have been used to establish a scale of E... 

Scheme 9.51, which shows the cyclization by the LBA in a single step to the tricyclic product in 75% yield with 87% ee [21b]. This process is clearly initiated by protonation of the internal olefinic jt-bond to form a bicyclic acetylene, which then undergoes a second (and slower) cyclization to generate the close core analog of pseudopterosin tricycle. The successful employment of this scenario is based on the lower proton affinity of C=C triple bond relative to C=C double bond. 

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