1,3-cyclobutadiene structure

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... 

The cyclobutadiene structure (36), like cycloalkyne (37), also formed an adduct with singlet oxygen (dioxetane 143), which rearranged to a dione product (144 equation 31) (79LA473). 

Particularly interesting is the dimer formed from didehydrobullvalene (51) in which the two fluctuating bullvalene systems are fused to each other by a four-membered ring. According to H-NMR data (52), with the three-membered rings in the anticonfiguration, is the thermodynamically most stable of the 17 possible valence isomers, whereas the cyclobutadiene structure (55) is not present in the equilibrium mixture 200) in addition to these dimers, a trimer is also obtained 200). 

In the gas phase, 2 is a thermally very stable compound up to 850 °C. Pyrolysis at 880°C/10 Torr generates styrene (55-62%) and o-xylene (6%) along with small amounts of phenylacetylene, benzene, toluene and unidentified hydrocarbons. Cycloaddition reactions with dienophiles were among the first reactivity studies on 2 they were of course driven by the expectation to generate a cyclobutadiene structure by a twofold (4 + 2) cycloaddition. However, while 2 reacts readily with electron-deficient alkenes such as TCNE, A -phenyhnalermide, 4-phenyl-l,2,4-triazolinedione and diethyl azodicarboxylate to form 1 1 adducts 115, a second Diels-Alder reaction... 

Stabilization of cyclobutadienes may be achieved, if the cyclobutadiene structure is only one of the important resonance forms. This is the case in the so-called push-pull cyclobutadienes. 

The biphenylene system has interested theoretical chemists for over 20 years. LCAO-MO calculations93 c 18>62,63,69 predicted that the four-membered ring is closer in structure to tetramethylenecyclobutane than to cyclobutadiene. Structural analyses70 have supported this prediction and have shown 56 to be completely planar with the bond-lengths and angles shown in Fig. 2. Evaluation of the proton NMR spectrum of biphenylene indicated that the partial bond fixation shown by... 

A more general classification considers the phase of the total electronic wave function [13]. We have treated the case of cyclic polyenes in detail [28,48,49] and showed that for Hiickel systems the ground state may be considered as the combination of two Kekule structures. If the number of electron pairs in the system is odd, the ground state is the in-phase combination, and the system is aromatic. If the number of electron pairs is even (as in cyclobutadiene, pentalene, etc.), the ground state is the out-of-phase combination, and the system is antiaromatic. These ideas are in line with previous work on specific systems [40,50]. 

Thus cyclobutadiene like cyclooctatetraene is not aromatic More than this cyclo butadiene is even less stable than its Lewis structure would suggest It belongs to a class of compounds called antiaromatic An antiaromatic compound is one that is destabi lized by cyclic conjugation... 

One of molecular orbital theories early successes came m 1931 when Erich Huckel dis covered an interesting pattern m the tt orbital energy levels of benzene cyclobutadiene and cyclooctatetraene By limiting his analysis to monocyclic conjugated polyenes and restricting the structures to planar geometries Huckel found that whether a hydrocarbon of this type was aromatic depended on its number of tt electrons He set forth what we now call Huckel s rule... 

Early structures, proposals and refutations included benzoazetenone, cyclobutadienes (B-65MI41600) Or an oxarirane (04CB966, 26LA(437)162, B-35MI41600), and these references should be consulted for this early work. 

The rectangular structure is calculated to be strongly destabilized (antiaromatic) with respect to a polyene model. With 6-3IG calculations, for example, cyclobutadiene is found to have a negative resonance energy of—54.7 kcal/mol, relative to 1,3-butadiene. In addition, 30.7 kcal of strain is found, giving a total destabilization of 85.4 kcal/mol. G2 and MP4/G-31(d,p) calculations arrive at an antiaromatic destabilization energy of about 42kcal/mol. ... 

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. 

Cyclobutadiene is a rectangular molecule with two shorter double bonds and two longer single bonds. Why do the following structures not represent resonance forms ... 

McWeeny, R., Proc. Roy. Soc. (London) A227, 288, The valence-bond theory of molecular structure. III. Cyclobutadiene and benzene." f. 

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... 

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