Styryl ions

The mechanism is outlined in Scheme 1. The formation of triple ions increases the concentration of Cs+ ions and buffers, therefore, the dissociation of ion-pairs into free styryl ions which are the main contributors to the propagation. 

It seems to this writer that the first alternative is the correct one. A proton transfer from NHS to styrene- ion is unlikely to be faster than a proton transfer from NH3 to poly-styryl- ion, and it was shown that the latter reaction is not too rapid. Hence, if an electron transfer does take place one might expect dimerization of styrene ions and eventually initiation of polymerization. This might be an alternative explanation for the formation of a small amount of polymer during the reduction, but nevertheless this still remains to be only a minor reaction. On the other hand, in the reduction of 1,1-diphenyl ethylene, the electron affinity of which is higher than that of styrene, the dimeric di-ion, Ph2 C. CH2. CH 2. C. Ph2 is formed in comparable amounts with the monomeric Ph2 C. CH3ion (17). 

The ethylenebenzenium ion 111 was found to undergo isomerization to yield the a-phenylethyl (styryl) ion with an activation energy of /ia = 13 kcal mol-1.295 In a recent study296 a value of 26.7 kcal mol 1 was computed [B3LYP/6-311 lG(d,p) level] and the styryl ion was found to be more stable by 13.9 kcal mol-1. 

Although styrene polymerized by ionic mechanism is not utilized commercially, much research was devoted to both cationic and anionic polymerizations. An investigation of cationic polymerization of styrene with an A1(C2H5)2C1/RC1 (R = alkyl or aryl) catalyst/cocatalyst system was reported by Kennedy.The efficiency (polymerization initiation) is determined by the relative stability and/or concentration of the initiating carbocations that are provided by the cocatalyst RCl. A/-butyl, isopropyl, and j c-butyl chlorides exhibit low cocatalytic efficiencies because of a low tendency for ion formation. Triphenylmethyl chloride is also a poor cocatalyst, because the triphenylmethyl ion that forms is more stable than the propagating styryl ion. Initiation of styrene polymerizations by carbocations is now well established. 

The electron impact mass spectrometric fragmentations of (E)-3- and ( )-4-styryl-pyridazines show that the intensity ratio of the M and (M -1)" ions, the general degree of fragmentation and the elimination pathways of nitrogen are the most characteristic features distinguishing between the two isomeric compounds (81JHC255). 

A similar stereospecific conjugate addition to epoxysulfone 323 was also Observed S , with Grignard reagents in the presence of nickel ion or palladium catalysts Methyllithium and n-butyllithium did not add to 1-propenyl phenyl sulfone nor to phenyl -styryl sulfone but underwent lithiation at — 95 However methyllithium, n-... 

Interesting results concerning phenyl group participation were observed with ( )-styryl(phenyl)iodonium tetrafluoroborate (26) using a deuterated substrate (eq 12)16 When 26-ad was heated in trifluoroethanol (TFE) at 60 °C, slow reaction gave die E isomer of substitution product 28 quantitatively, but the deuterium was completely scrambled between the a and p positions. This strongly indicates that a symmetrical intermediate is involved during the reaction and the most reasonable one is vinylenebenzenium ion (27) formed by phenyl participation. This intermediate also explain the exclusive formation of the retained ( )-28. 

A side-effect was the spectroscopic hunt for the elusive styryl cation which resulted in the identification of the principal cations formed by the action of acids on styrene the subsequent quest for a non-spectroscopic method to identify organic cations led to the development of the polarography of carbenium and oxonium ions. 

Detailed studies led Gandini and Plesch to formulate the concept of pseudocationic polymerisations. These are reactions which show many of the characteristics of cationic polymerisations, but do not involve ions. Since they could see no other alternative compatible with general chemical knowledge, they formulated the reactive species as an ester, and they were able to support this view by direct experiments (formation of the ester in the styrene solution by metathesis). The evidence indicates that in the system styrene, perchloric acid, methylene dichloride, the poly(styryl perchlorate) ester requires four molecules of styrene for its stabilisation. When these are no longer available, the ester ionises, and the residual styrene is consumed by a very fast, truly cationic polymerisation ionisation of the ester is a complicated reaction which has been only partly elucidated. The initiation and propagation of the pseudocationic polymerisation can be represented thus ... 

We report spectroscopic and conductimetric studies on the reactions of styrene in the presence of perchloric acid (most in methylene dichloride but some in ethylene dichloride), both during and after the polymerisation. During polymerisation no ions are detectable, but at the end aralkyl (and subsequently allylic) ions are formed. Quantitative results show that ion formation sets in when the styrene concentration has fallen to four times the acid concentration we interpret this finding as showing that the ester (oligo-)styryl perchlorate requires four molecules of styrene for stabilisation in solution. 

Most of the facts in Sections 2.2.1 to 2.2.10 are actually sufficient to dispose of this view and, taken together, they form an array of concordant and compatible observations which are all incompatible with it. In addition, it must be noted that amongst reagents of the type which interest us here, equilibria between ions and esters are unknown, and that attempts to prepare some of the active esters, e.g., styryl triflate, in the absence of a stabilising agent, always produce intractable mixtures [49]. 

The electron impact mass spectra of 3-methyl-4-nitro-5-styryl-isoxazoles exhibit, on the contrary, only negligible loss of OH"80. This has been interpreted in terms of an isoxazole-to-azirine rearrangement80. The latter fragments directly to an abundant cinnamoyl ion as well as rearranges to oxazole and an epoxide through an intramolecular oxidation of the ethylenic bond by the nitro group80 see Scheme 10. 

A comparison of the spectra of the ortho isomers, 5 with 7, reveals that fragmentation due to oxidation of the styryl double bond apparently is absent in 7. Instead, the MS of the latter exhibits ions corresponding to the formation of a benzopyrylium ion see Scheme 15. 

The (Z)- and ( )-styrylpyrazine structures 20j and 20k were assigned on the base of the mass, NMR, and UV spectral data. The mass spectrum of Z isomer (20j) shows a base peak (the molecular ion) at m/z 210 with a peak at m/z 133 formed by the loss of a phenyl group firom 20j. The H-NMR spectrum shows the presence of five aromatic and two olefinic protons in addition to one heteroaromatic proton and two methyl groups attached to the heteroaromatic nucleus. Ozonolysis of the Z isomer (20j) yields 3-formyl-2,5-dimethylpyrazine (487) and benzaldehyde, confirming the styryl moiety in 20j. The ( )-styryl derivative (20k) is readily isomerized to the Z isomer (20j) on exposure to sunlight (Scheme 60). Extraction of the pyrazines from I. humillis in the dark indicates that E isomer 20k is the naturally occurring product 144,145). 

---