Cyclobutadiene reactivity

Cyclobutadiene escaped chemical charactenzation for more than 100 years Despite numerous attempts all synthetic efforts met with failure It became apparent not only that cyclobutadiene was not aromatic but that it was exceedingly unstable Beginning m the 1950s a variety of novel techniques succeeded m generating cyclobutadiene as a transient reactive intermediate... 

Shielding and Stabilization. Inclusion compounds may be used as sources and reservoirs of unstable species. The inner phases of inclusion compounds uniquely constrain guest movements, provide a medium for reactions, and shelter molecules that self-destmct in the bulk phase or transform and react under atmospheric conditions. Clathrate hosts have been shown to stabiLhe molecules in unusual conformations that can only be obtained in the host lattice (138) and to stabiLhe free radicals (139) and other reactive species (1) similar to the use of matrix isolation techniques. Inclusion compounds do, however, have the great advantage that they can be used over a relatively wide temperature range. Cyclobutadiene, pursued for over a century has been generated photochemicaHy inside a carcerand container (see (17) Fig. 5) where it is protected from dimerization and from reactants by its surrounding shell (140). 

Extrapolation from the known reactivity of cyclobutadiene would suggest that azetes should be highly reactive towards dimerization and as dienes and dienophiles in cycloaddition reactions and the presence of a polar C=N should impart additional reactivity towards attack by nucleophiles. Isolation of formal dimers of azetes has been claimed as evidence for the intermediacy of such species, but no clear reports of their interception in inter-molecular cycloaddition reactions or by nucleophiles have yet appeared. 

DE, since the 7c-electron energy is 4a + 4/3, the same as that for two independent double bonds. Thus, at this level of approximation, HMO theory prediets no stabilization for eyelobutadiene from delocalization and furthermore predicts that the molecule will have unpaired electrons, which would lead to very high reactivity. In addition, cyclobutadiene would suffer angle strain, whieh is not present in benzene. The extreme instability of eyelobutadiene is then understandable. Higher-level MO ealculations modify this picture somewhat and predict that eyelobutadiene will be a rectangular molecule, as will be diseussed in Chapter 9. These ealculations, nevertheless, agree with simple HMO theory in... 

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. 

Cyclo butadiene is highly reactive and shows none of the properties associated with aromaticity. In fact, it was not even prepared until 1965, when Rowland Pettit of the University of Texas was able to make it at low temperature. Even at —78 °C, however, cyclobutadiene is so reactive that it dimerizes by a Diels-Alder reaction. One molecule behaves as a diene and the other as a dienophile. 

Conjugated chains, 14, 46 Correlation diagrams, 44, 50 Cyclobutadiene, 171 Cyclobutane, 47, 222 orbital ordering, 26 through-space interactions, 26 Walsh orbitals, 27 Cyclobutene, 200 Cyclohexane, 278 Cyclohexene (half-boat), 274 Cyclopen tadiene, 225 Cvclopen tadienone, 269 Cyclopentadienyl anion, 237 Cyclopentane, 254 Cyclopen ten e, 241 Cyclopropane, 41, 47, 153 construction of orbitals, 19, 22 Walsh orbitals, 22, 36, 37 Cyclopropanone, 48, 197 bond lengths, 38 Cyclopropen e, 49, 132 reactivity, 40... 

This procedure is illustrative of the synthetic use of cyelobuta-dieneiron tricarbonyl2 as a source of highly reactive cyclobutadiene. Cyclobutadiene has been employed, for example, in the synthesis of oubane, Dewar benzenes, and a variety of other systems.3,1... 

It is clear that simple cyclobutadienes, which could easily adopt a square planar shape if that would result in aromatic stabilization, do not in fact do so and are not aromatic. The high reactivity of these compounds is not caused merely by steric strain, since the strain should be no greater than that of simple cyclopropenes, which are known compounds. It is probably caused by antiaromaticity. ... 

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... 

Secondly, the carbon framework holding the exocyclic double bonds could be extended. This is demonstrated by naphtharadialene 5, a highly reactive intermediate which has been generated by thermal dehydrochlorination from either the tetrachloride 178 or its isomer 179106. Radialene 5 has not been detected as such in these eliminations rather, its temporary formation was inferred from the isolation of the thermolysis product 180 which was isolated in 15% yield (equation 25). Formally, 5 may also be regarded as an [8]radialene into whose center an ethylene unit has been inserted. In principle, other center units—cyclobutadiene, suitable aromatic systems—may be introduced in this manner, thus generating a plethora of novel radialene structures. 

Besides the above configurational and steric factors electronic effects of substituents have also been studied on Diels-Alder reaction. How the electronic and steric factors both operate is best afforded by cyclobutadiene which is a highly reactive species and undergoes Diels-Alder reaction even at very low temperature to give a mixture of the following two products. 

Transient intermediates like benzynes or cyclobutadiene are extremely reactive dienophile. 

Unlike pyridine (85MI1), tri-rm-butylazete (101) has, as judged from its reactivity, olefinic character [88AG(E)(27)272]. As in the case of cyclobutadiene, the push-pull substitution [69CC240 88AG(E)(24)1437] promotes the stabilization of azete, as has been demonstrated by the isolation of the first thermodynamically stable substituted azete, tris(dimethylamino)azete... 

The reactions of acetylenes and of unsaturated metallacycles present a large and difficult field The high reactivity of the species involved gives rise to a rich and diverse chemistry but this does not make mechanistic interpretation any easier. As is well known , the bis (acetylene) - cyclobutadiene interconversion is difficult to rationalise, but nonetheless it does take place ... 

Rehek J, Gavina F. 3-Phase test for reactive intermediates—cyclobutadiene. J Am Chem Soc 1974 96 7112-7114. 

Cyclobutadienes represent very reactive alkenes that undergo both [2 + 2] as well as [4 + 2] cycloadditions. Both the cyclodimerizations, mixed [2 + 2] cycloadditions and Diels-Alder reactions of these reactive species have been reviewed (see Houben-Weyl, Vols. 4/4, p 231 and E 17 f, Section 10B). In most instances the initially formed cyclodimer is tricyclo[4.2.0.02-5]octa-3,7-diene (36) and has the all cis-syn configuration. This is attributed to the concerted [4n -I- 2n] cycloaddition mechanism in which stereochemical control is affected by secondary orbital interactions. 

Other reactive cyclobutadienes including the parent derivative have been reported to undergo cycloadditions with cycloalkenes to give tricycloalkenes. In these instances both syn-and anti-isomers are obtained.11-15... 

Compounds with a narrow HOMO-LUMO gap (Figure 5.5d) are kinetically reactive and subject to dimerization (e.g., cyclopentadiene) or reaction with Lewis acids or bases. Polyenes are the dominant organic examples of this group. The difficulty in isolation of cyclobutadiene lies not with any intrinsic instability of the molecule but with the self-reactivity which arises from an extremely narrow HOMO-LUMO gap. A second class of compounds also falls in this category, coordinatively unsaturated transition metal complexes. In transition metals, the atomic n d orbital set may be partially occupied and/or nearly degenerate with the partially occupied n + 1 spn set. Such a configuration permits exceptional reactivity, even toward C—H and C—C bonds. These systems are treated separately in Chapter 13. 

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