Wilfried Franke

Eckenprotoniertes cyclopropan in der gasphase: kopplung von unimolekularer [1.1]-H2-eliminierung aus C3H7+ und ringöffnung des cyclopropylkations

Abstract Based on energetic data (activation energy), kinetic isotope effects and MINDO/3 calculations it is suggested that unimolecular loss of H2 from C3H7+ proceeds via a transition state (10) involving synchronous loss of a hydrogen molecule (symmetry allowed [1.1]-elimination) and ring opening of the cyclopropyl to the allyl cation. Alternative mechanisms are discussed.

Carbon Skeletal Rearrangements Via Pyramidal Carbocations

The scrambling of carbon atoms in gaseous carbocations is readily explicable by invoking transition states or intermediates of pyramidal structure, which are now be seen as the logical bridge between organic and organometallic chemistry. The electronic structure of this species is treated with respect to some of their physical and chemical properties (energy, charge distribution, geometry). Specific examples discussed include the following hydrocarbon ions, the gas phase chemistry of which proceeds via pyramidal intermediates: C5H5+ C5H9+ C5H11+, C5H5+. Reasons are provided why, in contrast, the carbon scrambling in some other systems, as for example tropylium ion ⇄ benzyl cation, does not involve pyramidal cations but proceeds via a sequence of orbital symmetry allowed isomerization, or why in the case of simple saturated hydrocarbon ions, as for example C4H9+, the C-skeleton reorganization is achieved by the well-known Wagner/Meerwein type rearrangement. The carbon scrambling in ionised cyclopentadiene, which precedes the formation of aliene cation radical and acetylene, is due to the intermediacy of ionised bicyclo[2.1. 0]pent-2-ene, a species which is also involved in the phototransposition of carbon atoms in neutral cyclopentadiene. Pyramidal-like structures are much too high in energy to play a role in the degenerate isomer ization of C5H6+.

Eliminierung von Aceton und Bildung von protoniertem Aceton bei der chemischen Ionisation von [2.2.6.6]-Tetramethylcyclohexanon/ Elimination of Acetone and Formation of Protonated Acetone from [2,2,6,6]-Tetramethylcyclohexanone under the Condition of Chemical Ionisation

Abstract By means of appropriately D and 13C labelled precursors it is shown that the proton catalysed degradation of [2,2,6,6]-tetramethyl cyclohexanone {e.g. elimination of acetone and formation of protonatecl acetone from [MH]+) proceeds via two distinct pathways. The energetically favoured one (pathway A, Scheme 1) involves a sequence of [1,2]-methyl migration, ring contraction and methyl migration, whereas the energetically less attractive path B commences with a Wagner-Meerwein ring contraction, followed by hydroxyl and methyl migration. Semi-empirical quantum mechanical calculations (MNDO) are em-ployed to explore computationally relevant parts of the potential energy surface. The syntheses of the specifically 13C labelled title compounds (1 a, b) are described in detail.

Elf stabile C5H9+-Kationen in der Gasphase. Zur dissoziativen Ionisierung von 31 isomeren C6H9Br-Verbindungen / Eleven Stable C5H9+ Cations in the Gas Phase. On the Dissociative Ionization of 31 Isomeric C5H9Br Compounds

Abstract Collisional activation mass spectrometry (CA) reveals the existence of 11 stable C5H9+ cations in the gas phase, e. g. the substituted allyl cations a, b, c, d,and e, the sub-stituted vinyl cations f, g, h, and i, the methyl cyclobutyl cation j and the cyclopentyl cation k, respectively. The ethyl substituted allyl cation a is formed via dissociative ionization of the isomeric precursors 1, 3, 4, 5, 18, 19, 20, 22, and 28 by means of various mechanistic processes, whereas the 1,3-dimethylallyl cation b is generated from both 2 (by allylic cleavage) and in part from the stereoisomeric cyclopropan derivatives 25, 26 and 27. 6+ -gives a mixture of the vinyl cations 1 and g. From 13 and 14 the main product generated is the 1,2-dimethylallyl cation d, which is formed directly from 11 and also by quite complicated processes from 13, 14 and to a certain extent from 25, 26 and 27. The dissociative ionization of 9, 15, 16, 21, 24 and (in part) 23 give rise to the formation of the substituted vinyl cation h. Decomposition of 23+ • results not only in formation of h but generates also the 1,1-dimethylallyl cation e. From 29+-and 30+-both the methylcyclo-butyl cation j and cyclopentyl cation k are produced, whereas the isomeric precursor 28 gives mainly the substituted allyl cation a and a second, as yet, unidentified C5H9 cation. In general, it can be stated that the gas phase chemistry of cation radicals of substituted cyclopropanes is characterised by multistep-reactions, commencing with spontaneous ring opening. The so formed intermediates undergo various rearrangements (including hydrogen and alkyl shifts) prior to expulsion of Br·. Direct elimination of Br· from intact cyclopropan-like structures, followed by ring opening of the intermediate cyclopropyl cation, cannot compete with the above-mentioned multistep-sequences.

DISSOCIATIVE IONIZATION OF ARYL-SUBSTITUTED VINYL BROMIDES IN THE GAS PHASE: EXPERIMENTAL AND COMPUTATIONAL EVIDENCE FOR THE FORMATION OF STABLE α-ARYLVINYL CATIONS BOTH BY DIRECT MEANS AND BY SPONTANEOUS EXOTHERMIC ISOMERIZATION OF U

The kinetic energy release T which accompanies the Brloss from ionized ( E ) and (Z)-0-bromostyrenes (5 and 6) in the gas phase is higher by 0.7 * 0.03 kcal mol-’ than that from the molecular ion of a-bromostyrene (4). Together with both collisional activation (CA) spectra and MO calculations this is interpreted as evidence for the direct formation of the a-phenylvinyl cation 7 from 4’. and exothermic isomerization of the incipient [M Br]’ species formed from 5’. and 6’s. Except for collision-induced CH2 loss, the CA spectra of the C8H7+ ions formed from 4’-, 5’., and 6’. and by chemical ionization of henylacetylene are nearly identical. In the collision-induced methylene loss from the C8H7+ ions derived from aand a partial degenerate isomerization prior to the CH2 loss. The various possible cationic intermediates from the dissociative ionization of 4-6 and the pathways connecting them were investigated computationally by MIND0/3 and ab initio methods. Only 7 and the phenyl-bridged ion 9 are stable species, while the 0-phenylvinyl cation 11 rearranges without activation energy via the hydrogen-bridged ion 10 to 7. The corrected STO-3G calculated relative energies in kcal mol-’ are (7) 0, (9) 16, (10) 25, and (11) 42. The calculated barriers for the 11 9 and the 7 9 rearrangements are 4.4 and 32 kcal mol-’. MIND0/3 calculations show that p-OH and p-Me substituents have little effect on the energy differences between the a-aryl and the aryl-bridged vinyl cations. The T values associated with Br. loss from M’. of (E)-o-bromo-2,6-, -2,5-, and -3J-dimethylstyrenes are on the average 0.43 kcal mol-’ higher than from the a-bromo analogues, indicating a similar isomerization to the more stable a-arylvinyl cations. The Tvalue for Br. loss from M’. of 2,2-dianisyl-l-phenylvinyl bromide (25) is 0.23 kcal mol-’ higher than that for M+. of (E) and (Z)-1,2-dianisyl-2-phenylvinyl bromides (23 and 24). Similar results are obtained for the 2,2-dianisyl1-tolylvinyl and (Z)1,2-dianisyl-2-tolylvinyl bromides (27 and 26). Exothermic isomerization of the a-phenyl and a-tolylvinyl cations to the a-anisylvinyl cations 28 and 29 is corroborated by CA spectra of the [M Br]’ ions. The loss of the An2C+fragment from these ions is ascribed to a competition between the direct, collision-induced high-energy dissociation of 28 and 29, with a 0-aryl rearrangement prior to decomposition. Similarities and differences between the reactions of the vinyl cations in the gas and the condensed phase are discussed. 8-l P C-labeled 4 and 5, C, contributes 21.5% and C, 78.7% starting from 4, but C, contributes 29.9% starting from 5, indicating Many intrinsic properties of ions can be examined advantageously in the gas phase? For example, the number and structures of ionic species capable of existing in significant minima on the potential energy surface can be revealed by collisional activation (CA) mass ~pectrometry.~ Complementary information may be obtained from the kinetic energy release, T, which accompanies unimolecular dissociation of a metastable ion of i n t e r e ~ t . * ~ , ~ * ~ ~ * ~ The shape of these metastable peaks constitutes a “direct picture” of what happens when dissociation takes place. As has been shown for several systems, when ions lose a neutral X (X = BP, CO, CH20, or HzO) to form a carbenium ion R’, exothermic isomerization of the incipient caton is indicated by an increase in the (1) (a) Technion Haifa. (b) Technische Universitat Berlin. (c) Hebrew University Jerusalem. (d) EPFL-Ecublens Lausanne. (2) (a) Williams, D. H. Acc. Chem. Res. 1977, 10, 280. (b) Bowen, R. D.; Williams, D. H.; Schwarz, H. Angew. Chem., Int. Ed. Engl. 1979,18,451. (c) Arnett, E. M. Acc. Chem. Res. 1973, 6, 404. (d) Kebarle, P.; Davidson, W. R.; Sunner, J.; Meza-Hajer, S. Pure Appl. Chem. 1979, 51, 63. (3) For reviews see: (a) Levsen, K.; Schwarz, H. Angew. Chem., Inr. Ed. Engl. 1976,15, 509. (b) McLafferty, F. W. “Chemical Applications of High Performance Mass Spectrometry”; American Chemical Society: Washington, D. C., 1978. (c) “Collision Spectroscopy”; Cooks, R. G., Ed.; Plenum Press: New York, 1978. (d) Levsen, K. “Fundamental Aspects of Organic Mass Spectrometry”; Verlag Chemie: Weinheim/Bergstr., Germany, 1978. (4) Williams, D. H.; Stapleton, B. J.; Bowen, R. D. Tetrahedron Lett. 1978, 2919. (b) Bowen, R: D.; Williams, D. H.; Schwarz, H.; Wesdemiotis, C. J. Am. Chem. Soc. 1979,101,4681; J . Chem. Soc., Chem. Commun. 1979, 261. (c) Stapleton, B. J.; Bowen, R. D.; Williams, D. H. J. Chem. Soc., Perkin Trans. 2 1979, 1219. (d) Schwarz, H.; Stahl, D. Int. J. Mass Spectrom. Ion Phys., in press. 0002-7863/81/1503-2770

Dissociative ionization of aryl-substituted vinyl bromides in the gas phase: experimental and computational evidence for the formation of stable .alpha.-arylvinyl cations both by direct means and by spontaneous exothermic isomerization of unstable isomeric ions

01.25/0 Scheme I