Triafulvenes

Identify the most negatively-charged and most positively-charged regions in each molecule. (Ignore the phenyl rings attached to triafulvene.) The dipole moments of these molecules have been measured as 6.3, 0 and 0.8 debyes. Which molecule is responsible for which dipole moment Explain the trend in dipole moments. 

Some derivatives of triafulvene undergo rotation about the carbon-carbon double bond even at room temperature. Given that cis-trans isomerization about double bonds is normally very difficult (see Chapter 7, Problem 1), how would you rationalize this Examine the electrostatic potential map for perpendicular hexaphenyltriafulvene (the rotational transition state).Would polar solvents tend to lower or raise the rotation barrier Explain. 

Library of Congress Cataloging in Publication Data. Main entry under title Cyclic compounds. (Topics in current chemistry 57). Bibliography p. Includes index. CONTENTS Eicher, T. and Weber, J. L. Structure and reactivity of cyclopropenones and triafulvenes. — Sargent, M. V. and Cresp, T. M. The higher annulenones. 1. Cyclic compounds—Addresses, essays, lectures. I. Eicher, Theophil, 1932- II. Series. 

Structure and Reactivity of Cyclopropenones and Triafulvenes Theophil Eicher and Josef L. Weber 1... 

In this article it is intended to give a collective and comparative survey of cyclo-propenone and triafulvene chemistry in the following areas ... 

The origin of cyclopropenone chemistry goes back to the successful preparation of stable derivatives of the cyclopropenium cation <5 3), the first member of a series of Huckel-aromatic monocyclic carbo-cations possessing a delocalized system of (4n + 2)-7r-electrons. This experimental confirmation of LCAO-MO theory stimulated efforts to prepare other species formally related to cyclopropenium cation by a simple resonance description of electron distribution, namely cyclopropenone 7 and methylene cyclopropene (triafulvene) 8 ... 

Methylene cyclopropene (5), the simplest triafulvene, is predicted to be of very low stability. From different MO calculations5 it has been estimated to possess only minor resonance stabilization ranging to 1 j3. Its high index of free valency4 at the exocyclic carbon atom causes an extreme tendency to polymerize, a process favored additionally by release of strain. Thus it is not surprising that only one attempt to prepare this elusive C4H4-hydrocarbon can be found in the literature. Photolysis and flash vacuum pyrolysis of cis-l-methylene-cyclopropene-2,3-dicarboxylic anhydride (58), however, did not yield methylene cyclopropene, but only vinyl acetylene as its (formal) product of isomerization in addition to small amounts of acetylene and methyl acetylene65 ... 

Stabilization of the methylene cyclopropene system has to be expected according to the introductory concept of triafulvene stabilization, if (a) the exocyclic C-atom... 

It should be pointed out that there are some methylene cyclopropene derivatives, whose stability is ascribed mainly to inductive effects brought about by strongly electron-withdrawing substituents. Thus, l,2-bis(p-tolyl)-4,4-(bis-trifluoromethyl)-triafulvene (63) synthesized recently by Agranat66 is a perfectly stable molecule with a dipole moment (7.42 D) comparable to that of l,2-diphenyl-4,4-dicyano-triafulvene (64) of the resonance-stabilized type (l)67 (7.9 D). 

Likewise, triafulvenes 65/66 substituted only by fluorine are reported to be stable68,24). 

One of the first syntheses of a triafulvene utilized the Wittig reaction, when diphenyl cyclopropenone was reacted with triphenyl carbomethoxymethylene phos-phorane giving 696 ) ... 

Triafulvene 69 was synthesized by an interesting route starting with diphenyl cyclopropenium cation and lithio ethyl acetate72. Although widely used in calicene chemistry (see later) this reaction principle — like the Wittig reaction — did not find general application in methylene cyclopropene synthesis. 

The reaction of the anion ICHXY with 75 shows a marked solvent dependence60. Addition of nitriles, e.g. CH3CN, suppresses the addition at C1 — sometimes observed as a side reaction (77) — in favor of addition at C3 resulting in triafulvene formation... 

Triafulvenes derived from cyclic 0-dicarbonyl systems like 84/85ia 861S and S779 are conveniently prepared from cation 75 by the DIPEA method and in some cases (5780,81), 8862 8983 ) from diphenyl cyclopropenone by condensation in acetic anhydride. 

The reaction of diphenyl cyclopropenone with aryl malononitriles75 or aryl cyano acetone84 unexpectedly gave rise to 4-cyano-4-aryl triafulvenes 90, as well as the formation of quinocyclopropenes (see later) ... 

Special cases of triafulvene formation were found in the base-induced reaction of the nitroso compound 91 with dimethyl fumarate85, in the thermolysis of tetra-fluorocyclopropene reported to give the perfluorinated triafulvenes 65/6668 and in the addition of bis(trifluoromethyl) ketene to bis (p-tolyl) cyclopropenone66 which gave rise to triafulvene 63 by elimination of C02 ... 

Dicyanomethylene compounds react with ethoxy cation 75 in the presence of DIPEA to form triafulvenes 95 87, 88. The same is true for 1,3-bis(dicyanomethylene)-indane and cyanomethylene dimedone, from which the sterically crowded vinylogous triafulvenes 96 and 97 were prepared88). 

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