Tetra cyclobutadiene

A number of alkyl-substituted cyclobutadienes have been prepared by related methods Increasing alkyl substitution enhances the stability of the compounds. The tetra-/-butyl derivative is stable up to at least 150°C but is very reactive toward oxygen. This reactivity reflects the high energy of the HOMO. The chemical behavior of the cyclobutadienes as a group is in excellent accord with that expected from the theoretical picture of the structure of these compounds. 

Because of its great reactivity PTAD has found wide use in the interception of reactive, unstable dienes. For example, unstable isoindenes,226 3a//-indenes,146 1,3-divinylallenes,227 and benzene oxides228 have all been successfully trapped with PTAD. 4-(4-Bromophenyl)-l,2,4-triazole-3,5-dione (5, R = 4-Br—C6H4) is often used if the derivatives ai e required for X-ray structure determination.229 Azodicarboxylic esters have been used to trap tetra(trifluoromethyl)cyclobutadiene,230 and spiro[4.4]nonatetraene.231... 

The PE spectra of cyclobutadiene 281192, tetra-t-butylcyclobutadiene 282193, 1,3,5-tri-t-butylpentalene 283194 and aceheptylene 284195, all of which are subject to second-order bond localization, have been described in the literature. Further examples are 1,3,5,7-tetra-t-butyl-s-indacene 285 (which exhibits a first double band 1,2 at I 2 = 6.75 eV, followed by two bands at I = 8.50 eV and I = 9.30 eV) and the tetracyclic hydrocarbon 1,3,6,8-tetra-t-butylpentaleno[2,l-a]pentalene 286 (the first two bands of which are observed at 6.40 eV and 7.65 eV)196. 

It has to be pointed out, however, that additional valence isomers (CR)4 such as cyclopropenyl carbene, are found in local energy minima, if other cuts through the 54 dimensional hyperspace are calculated (30) - in agreement with compounds isolated recently e.g. from the reactions of both isomers, tetra (tert. butyl) tetrahedrane and cyclobutadiene, with tetracyanoethylene (32). 

Tetra(tert-butyl)tetrahedrane converts into tetra(tert-butyl)cyclobutadiene only when heated up to 140°C in vacuum. A barrier of 170 kJ mol separates these two isomers (Heilbronner et al. 1980). In the cation-radical state, the tetrahedrane structure converts into the cyclobutadiene structure without heating (Bock et al. 1980, Fox et al. 1982). From Scheme 6.34 it can be seen that by the action of aluminum chloride on methylene chloride, tetrahedrane forms the cation-radical of its isomer—the cyclobutadiene cation-radical and not the cation-radical of the same skeleton. The latter is more stable than the former because of more effective delocalization of the unpaired electron and positive... 

Tetrakis(trifluoromethyl)-5-thiabicyclo[2.1.0]pent-2-ene A-oxide 48, a cyclobutadiene episulfoxide, has been prepared by selective oxidation of the corresponding episulfone 47 with peroxytrifluoroacetic acid (Scheme 14) <1976JA4325, 1977JA629>. In an analogous manner, the episulfoxide of type 48 bearing four 2-chloro-l,1,2,2-tetra-fluoroethyl groups was obtained by oxidation in dichloromethane <2000JFC(102)323>. 

Matrix irradiation (240nm) of tricyclopentanone (111) yields cyclobutadiene, CO, cyclopentadienone, and acetylene.No evidence for the formation of tetrahedrane (112a) was obtained, in contrast with the previously reported behaviour of the tetra-t-butyl derivative of (111), which gave the tetrahedrane... 

Since these two initial reports, several other complexes possessing tetra-substituted, cyclobutadiene ligands have been reported, notably by Maitlis and co-workers ( ), who have demonstrated that one can transfer a cyclobutadiene ligand from one metal to another to produce new cyclobutadiene-metal complexes. 

The smallest conceivable conjugated monocyclic diene is 1,3-cyclobutadiene. Several complexes involving cyclobutadiene are known. The compound itself is unstable and has not been studied by structural methods. It will therefore not be included in the present discussion. 1,3-Cyclobutadiene has, however, been isolated in argon matrices, and it has been established that the molecule has symmetry. For the tetra-tert-butyl derivative an envelope conformation (twist angle 7°) was found by X-ray methods, however the distances in the ring were obviously too similar for an antiaromatic system [1.464(3) and 1.483(3) A]. A redetermination at even lower temperatures gave more reasonable results (1.441 and 1.527 A) and a further analysis of the anisotropic parameters revealed that some residual disorder is still responsible for some equilibration and distances of 1.34 and 1.60 A were assumed to be the correct ones. ... 

It has been predicted21-23 that tetra-substitution of cyclobutadiene with donor (D) and acceptor (A) groups, as in 11, will produce an optimal stabilizing effect, due to contributing forms 11c of tetramethylenecyclobutane structure. 

In 1976, Chapman repeated our experiment at 8K and reported that tetrakis-(trifluoromethyl)tetrahedrane is formed. Some time later, Masamune and his coworkers detected cyclobutadiene in the IR and UV spectrum but not the tetra-hedrane 9S),... 

The reaction of 1,2-diphenylcyclobutadiene (generated in situ by oxidation of its iron tricarbonyl complex) with /j-benzoquinone yields adduct 1-A as the exclusive product. A completely analogous structure is obtained using maleimide as the dienophile. However, with the more reactive dienophiles tetra-cyanoethylene and dicyanomalemide, two isomeric adducts of type 1-B and 1-C are found in a 1 7 ratio in each case. Discuss these results and explain how they relate to the issue of a square versus a rectangular structure for the cyclobutadiene ring. 

DIELS-ALDER REACTIONS Benzocyclopropene. 1,2-BisC3-tosyIethoxycarbonyl)-diazene. Cyclobutadiene iron tricarbonyl. 1,2-Dicyanocyclobutene. Diethyl ketomalonate. 1,3,4,6-Heptatetraene. 2-Hydroxy-5-oxo-5,6-dihydro-2/f-pyrone. 3-Hydroxy-2-pyrone. Isopropylidene isopropylidenemalonate. Lithium tetra-methylpiperidide. 4-Methoxy-5-acetoxymethyl-o-benzoquinone. 4-Methoxy-5-methyl-o-benzoquinone. frans-l-Methoxy-3-trimethyl-silyloxy-l,3-butadiene. Per-fluorotetramethylcyclopentadiene. 4-Phenyl-l, 2,4-triazoline-3,5-dione. Potassium f-butoxide. 

Excited State Polarizabilities. - Jonsson et al.156 have attempted to simulate excited state polarizabilities by means of the optimization of a single determinant ground state. These excited state polarizabilities are given by the double residues of the cubic response functions. The method has been applied to H20, 03, HCHO, C2H4,C4H6, cyclobutadiene, pyridine, pyrazine and s-tetra-zine and the results compared with others obtained from multi-determinant optimized excited states. 

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