Propene radical cation

Electron impact mass spectrometry of the cyclobutanedione (24) gives rise to dimethylcarbene radical cation.35 Appearance energy measurements and ah initio calculations indicated that the radical cation lies 84 kJ mol 1 above the propene radical cation and is separated from it by a barrier of 35 kJ mol-1. Diarylcarbene radical cations have been generated by double flash photolysis of diaryldiazomethanes in the presence of a quinolinium salt (by photo-induced electron transfer followed by photo-initiated loss of N2).36 Absolute rate constants for reactions with alkenes showed the radicals to be highly electrophilic. In contrast to many other cation radicals, they also showed significant radicophihc properties. 

When an alkene molecule loses an electron, a cation radical is formed. The very reactive cation radical (CH3)2C—CHJ is generated from 2-methyl-propene in light in the presence of TiCl4. It can be detected by ESR in the frozen parent compound at 123 K [172], We assume that at higher temperatures these formations are dimerized to dications. The existence of a donor-acceptor complex is a necessary condition for the mechanism generating cation radicals (see Chap. 3, Sect. 5). a-Methylstyrene is cationically polymerized when illuminated in the presence of tetracyanobenzene in methylene chloride. From the two compounds, of which a-methylstyrene is the donor (D) and tetracyanobenzene the acceptor (A), the donor-acceptor complex is generated in the singlet and triplet states it dissociates to solvated ion radicals [173]... 

In contrast with the ethylene cation and the propene cation, having non-planar twisted structures, the fluorinated ethylene and propene cations are concluded to have planar structures. The cleavage of the C-H bond of alkyl aromatic radical cations usually occurs in a heterolytic fashion in solution, however, homolytic cleavage is equally possible [226] (Scheme 7). 

In a series of papers Gross et provide experimental evidence that ionization of cyclopropane in the gas phase results in the formation of a trimethylene cation radical (2) isomerization of cold 2 to ionized propene (3) can be ruled out. The authors, moreover, stress that the structural changes of ionized cyclopropane in both the gas phase and in condensed phase are entirely analogous. 

An example of the application is the oxidation of tran -anethole [trans- - A-methoxyphenyl)propene], the radical cation of which by LSV was found to undergo rapid RR dimerization (through C-2). The rate constant measured by SECM is as high as 4 X 10 M s [199]. The fit of the experimental data to the working curve is shown in Fig. 25. The SECM technique has also been used to determine the rate constant for the dimerization of the radical anions of acrylonitrile, the prototype example of electrohydrodimerization (Chapter 21). The value of k was found to be 6 x 10 M s [201]. 

Structural isomerizations of arylcyclopropane cation radicals to propene derivatives are also frequently observed. Trimethylene cation radicals are the most probable intermediates in these processes. However, in some cases the nature of the mode used to promote the SET event is crucial in determining the type of rearrangement sequence that is followed. For instance, DCN-sensitized photoreaction of 24 gives 25, whereas photoexcitation of the EDA complex of 24 and TCNE leads to production of 26. Oxygenation reactions to form dioxolanes are also typical reactions of arylcyclopropane cation radicals formed under aerobic conditions. Of particular interest are the DCA-sensitized oxygenation reactions of cis-27 and trans-27, both of which give cis-2S as a major product. Addition of biphenyl as a cosensitizer or magnesium perchlorate is known to accelerate these processes. Trimethylene cation radicals are plausible intermedi-... 

Another kind of metalloporphyrin catalyzed olefin epoxidation utilizes aldehydes, which generate peroxy radicals and subsequently peroxy acids [95,98,258]. The latter are complexed by the metalloporphyrin and serve as sources of 0-atoms for epoxidation either as the peroxy acid complex or as the oxidized ir-cation radical (oxometal species) [96,97]. The catalysis of propene epoxidation by... 

Cao, J.R. George, M. Holmes, J.L. Fragmentation of 1- and 3-Methoxy-propene Ions. Another Part of the [C4H80] Cation Radical Potential Energy Surface. J. Aon. Chem. Soc. Mass Spectrom. 1992,3,99-107. 

Dimerization in concentrated sulfuric acid occurs mainly with those alkenes that form tertiary carbocations In some cases reaction conditions can be developed that favor the formation of higher molecular weight polymers Because these reactions proceed by way of carbocation intermediates the process is referred to as cationic polymerization We made special mention m Section 5 1 of the enormous volume of ethylene and propene production in the petrochemical industry The accompanying box summarizes the principal uses of these alkenes Most of the ethylene is converted to polyethylene, a high molecular weight polymer of ethylene Polyethylene cannot be prepared by cationic polymerization but is the simplest example of a polymer that is produced on a large scale by free radical polymerization... 

Polyethylene (Section 6 21) A polymer of ethylene Polymer (Section 6 21) Large molecule formed by the repeti tive combination of many smaller molecules (monomers) Polymerase chain reaction (Section 28 16) A laboratory method for making multiple copies of DNA Polymerization (Section 6 21) Process by which a polymer is prepared The principal processes include free radical cationic coordination and condensation polymerization Polypeptide (Section 27 1) A polymer made up of many (more than eight to ten) amino acid residues Polypropylene (Section 6 21) A polymer of propene Polysaccharide (Sections 25 1 and 25 15) A carbohydrate that yields many monosacchande units on hydrolysis Potential energy (Section 2 18) The energy a system has ex elusive of Its kinetic energy... 

In Section 6.21 we listed three main methods for polymerizing alkenes cationic, free-radical, and coordination polymerization. In Section 7.15 we extended our knowledge of polymers to their stereochemical aspects by noting that although free-radical polymerization of propene gives atactic polypropylene, coordination polymerization produces a stereoregulai polymer with superior physical properties. Because the catalysts responsible for coordination polymerization ar e organometallic compounds, we aie now in a position to examine coordination polymerization in more detail, especially with respect to how the catalyst works. 

The initiator can be a radical, an acid, or a base. Historically, as we saw in Section 7.10, radical polymerization was the most common method because it can be carried out with practically any vinyl monomer. Acid-catalyzed (cationic) polymerization, by contrast, is effective only with vinyl monomers that contain an electron-donating group (EDG) capable of stabilizing the chain-carrying carbocation intermediate. Thus, isobutylene (2-methyl-propene) polymerizes rapidly under cationic conditions, but ethylene, vinyl chloride, and acrylonitrile do not. Isobutylene polymerization is carried out commercially at -80 °C, using BF3 and a small amount of water to generate BF3OH- H+ catalyst. The product is used in the manufacture of truck and bicycle inner tubes. 

As mentioned above, the conversion of cyclopropane to propene radical cation has been investigated by ab initio calculations. The general course of this reaction was confirmed, or anticipated, by product studies in the electron transfer-sensitized conversion of 1,1,2,2-tetraphenylcyclopropane (37) to 1,1,3,3-tetra-phenylpropene (38). The sequence of the key steps, migration versus ring opening cannot be derived from the results. In the case of 37, the four phenyl substituents may actually favor a ring-opened bifunctional radical cation. 

Ab initio calculations (UHF/6-31G //MP2/6-31G ) on the CsHg " potential surface confirm the structure, but fail to support the existence of 7 + 4i,i42 conversion of 6 + ( Ai) to propene radical cation, 8 + ( 10 kcal/mol below 6 +), has a barrier of 30 kcal/mol (UMP2/6-31G ), " approximately one-half that... 

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