Olefins ionization potential

Scheme 35 Formation of the arene-olefin (Ar-Ol) exciplex and its deactivation by charge-transfer effects induced by changes of olefin ionization potential (IP) and solvent polarity according to Leismann et al. (From Ref. 182.)...

The lack of reactivity of the aryloxazolinones (65) in photocycloaddition to many of the olefins other than 1,1-dimethoxyethene and furan probably results from efficient decay of E2 or D. Exciplex E2 and diradical D are proposed as intermediates in these cases for several reasons. Exciplex formation is most likely dependent on olefin ionization potential, and the ionization potential of many of the un-reactive olefins are intermediate between the ionization potential of furan and 1,1-dimethoxyethene as determined from the maxima of tetracynoethylene olefin charge transfer bands60 66,67. Although ds-2-butene does not form a cycloadduct with 2-phenyl-2-oxazolin-4-one (65a), ds-2-butene is isomerized to rram-2-butene during the irradiation52. Cis-trans isomerization is expected from decay of a triplet diradical. Decay of the exciplex and diradical intermediates in competition with reaction presumably results from steric hindrance from the aryl substituent. The olefins which give cycloadducts, furan and 1,1-dimethoxyethene, are expected to produce low steric hindrance with the aryl substituent in an exciplex or diradical. 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

When both reactants have comparable ionization potentials, there is a close relationship between the H2 and H2 transfer reactions. For instance, a low efficiency of the H. transfer reaction from the alkane to a neutral olefin molecule (at least in the case of cyclohexane) is paralleled by a low efficiency of the corresponding H2 transfer process. Such a relationship can be accounted for by resonance phenomena. Unfortunately, not enough information is available on those systems where the... 

Reactions between much stronger donors and acceptors belong to the electron tranter band. Such olefins do not form cyclobutanes but ion radical pairs or salts of olefins. refrato(dimethylamino)elhylene has an ionization potential as low as Na. The olefin with extraordinary strong electron-donating power is known not to undergo [2+2]cycloaddition reaction, but to give 1 2 complex with TCNE (transfer band in Schane 3) [23]. 

The spectra and calculations all led to the conclusion that there is an usually large interaction between both the 7T and lone-pair orbitals in the carbonyl portion of the molecule with the n and a orbitals of the olefin portion. The first ionization potential (9.57 eV) involves ionization of an electron from the oxygen lone pair, whereas the second (11.19 eV) involves ionization of an electron from the olefin rr-bond. The most vertical ionization is from the 7 ax MO (16.11 eV), the second lone-pair orbital on oxygen. 

It is well known that the rate of electrophilic addition to olefins generally increases as the HOMO of the alkene is raised energetically, i e its ionization potential is lowered. Pertinent experimental data have recently been reviewed308). 

Collin, J. and Tossing, F.P. Ionization potentials of some olefins, diolefins and branched paraffins, J. Am. Chem. Soc., 81(5) 2064-2066, 1959. 

The ionization potential, 8.69 eY, is lower than in the case of -cyclooctene (8.98 eV) or cyclohexene (9.12 eV), as expected. The highly strained anti-Bredt olefin, 11-bromo-e/ -9-chloro-7-ethoxybicyclo[5.3.1]undec-l(ll)ene has been synthesized and its struc-... 

These energy values are calculated from thermochemical tables (11) and the ionization potentials of hydrocarbons obtained by Stevenson (15) using mass spectrometric methods. The union of an olefin and a proton from an acid catalyst leads to the formation of a positively charged radical, called a carbonium ion. The two shown above are sec-propyl and fer -butyl, respectively. [For addition to the other side of the double bond, A 298 = —151.5 and —146 kg.-cal. per mole, respectively. For comparison, reference is made to the older (4) values of Evans and Polanyi, which show differences of —7 and —21 kg.-cal. per mole between the resultant n- and s-propyl and iso-and tert-butyl ions, respectively, against —29.5 and —49 kg.-cal. per mole here. These energy differences control the carbonium ion isomerization reactions discussed below.]... 

In view of the electrophilic character of ground state oxygen atoms in their reactions with olefins, there should be an approximate correlation of the rates with the ionization potentials of the olefins. Such a correlation does exist (27) but it is only approximate (28). Approximate correlations exist also with some other related physical properties (27), such as heats of hydrogenation and spectroscopic excitation energies, and with theoretically derived properties (28) (obtained by molecular or-... 

Rates of phenyl azide addition have been correlated with ionization potentials for a number of strained olefins. The results show that about... 

Alkylphosphines have been studied (99, 231), as have cyclopolyphos-phines (120). The reaction products of phosphorus with various olefins were investigated using mass spectral methods (108), and a large number of phospholines have been reported (181, 237). The mass spectrum of the trimer of tetramethylphosphinoborine showed that B—P, C—B, and C—P bonds were broken first, and with the pentamer of the dimethyl compound, B—H, B—P, and C—P bonds were broken (101, 232). The spectra of thiophosphorustri-A -methylimide and its arsenic analog were reported by Holmes and Forstner (130). Ionization potentials have been recorded for many perfluoroalkylarsines and some of the related alkyl compounds (79). 

A schematic representation of the possible processes in arene-olefin photocycloaddition according to Leismann et al. [182] is given in Scheme 35. Increasing charge separation in the exciplex, caused by decreasing ionization potentials of the olefins, can lead to zwitterionic intermediates, which are able to form ortho adducts. Increasing solvent polarity can reduce the formation of adducts by propagating the formation of radical ion pairs. The possibility that the ortho adduct may be formed directly or via a different short-lived intermediate was not incorporated in the scheme. 

For photocycloaddition, to benzene the following conclusions were drawn from this empirical correlation [124], Olefins with poor electron-donor or poor electron-acceptor abilities yield mainly meta adducts with benzene (i.e., if AG > 1.4-1.6 eV, all other olefins yield mainly ortho adducts). Even ethene, which had seemed to behave exceptionally, fits into this correlation provided that it acts as the acceptor. The transition area from ortho to meta cycloaddition (i.e., the AG region where ortho meta = 1 1) is relatively large ( 0.2 eV). This is considered not to be surprising because the AG correlation is based on many different types of olefins. When only AG values for derivatives of 1,3-dioxole and for 1,4-dioxene were used, the transition area was narrowed to 0.03 eV. Not only ethene but also vinylene carbonate now fit into the correlation. According to the ionization potential rule, this compound should give only ortho photocycloaddition with benzene. Mattay s empirical rule predicts mainly meta addition, which is indeed found experimentally. 

It is evident that the electron density on the olefin and the resultant changes in bond length, nuclear shielding, and vibrational modes arc related to the ionization potential of the metal atoms. It is not clear whether this effect is caused by inhibition of the a forward donation or enhancement of the 7r back donation as the ionization potential is lowered. When electronegative substituents are placed on the olefin to activate it, there is also the question of the extent to which the backbonded electron density remains localized on the double bond or is delocalized to the substituent groups. Any means of probing the electron density on the metal or olefin would be useful in ascertaining the relative importance of the [Pg.44]

A further variant is the oxidation of olefins by Mn(III) acetate in the presence of halide ions. Thus, oxidation of cyclohexene by Mn(III) acetate in acetic acid at 70°C is slow, but addition of potassium bromide leads to a rapid reaction. Cyclohexenyl acetate was formed in 83% yield.223 In contrast to what would be expected for an electron transfer mechanism, norbomene (ionization potential 9.0 eV) was unreactive at 70°C, whereas cyclohexene (ionization potential 9.1 eV) and bicyclo[3,2,l] oct-2-ene reacted rapidly. The low reactivity of norbomene can be explained, if oxidation involves attack at the allylic position... 

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