Comparison with cyclobutadiene

In these discussions of benzene and cyclobutadiene we have compared MCVB level calculations of the n system with SCF level calculations of the core. We do not expect that using correlated wave functions for core energies would change the results enough to give a different qualitative picture. 

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Just as the unusual stability and reactivity of benzene are placed into their proper context by comparison with cyclobutadiene and cyclooctatetraene39, the 4 -electron homo-logues of benzene, it is instructive to compare the formally homoantiaromatic bicyclo [3.1.0]hexenyl/cyclohexadieny 1 cation systems with the homocyclopropenium and homo-tropenylium ions (Scheme 14). Such a comparison not only puts in context the properties of the latter two homoaromatic cations, but also reveals a different mode of cyclopropyl conjugation that occurs in the 4 -electron systems. 

Fig. 9 illustrates that the two acetylenic systems become nearly parallel at C1-C6 distances close to 3 A where the constructive overlap of the re-orbital with one of the re -nodes is compensated by a destructive overlap with the other rc -node (Fig. 9, bottom). From a conceptual point of view, the properties of the in-plane re-system at the 3 A threshold bear a striking resemblance to the interaction of the two re-bonds in D2h cyclobutadiene where the re-re interaction is zero and the re-re repulsion is considerable, thus accounting for the extreme instability of this antiaromatic molecule.41 Even more relevant is a comparison with the TS of the symmetry forbidden thermal [2S + 2S] cycloaddition (Fig. 10) which prompted us to call this region antiaromatic .42... 

There is much evidence that cyclic conjugated systems of An electrons show no special stability. Cyclobutadiene dimerises at extraordinarily low temperatures (>35K).28 Cyclooctatetraene is not planar, and behaves like an alkene and not at all like benzene.29 When it is forced to be planar, as in pentalene, it becomes unstable to dimerisation even at 0 °C.30 [12]Annulene and [16]annulene are unstable with respect to electrocyclic reactions, which take place below 0 °C.31 In fact, all these systems appear on the whole to be significantly higher in energy and more reactive than might be expected, and there has been much speculation that they are not only lacking in extra stabilisation, but are actually destabilised. They have been called antiaromatic 32 as distinct from nonaromatic. The problem with this concept is what to make the comparisons with. We can see from the arguments above that we can account for the destabilisation... 

The analogous dimerization of alkynes over Fe(C0)5 is not applicable, so clearly a different route towards alkynylated derivatives of 25 was needed. Comparison of 25 to cymantrene suggests that metallation of the hydrocarbon ligand should be the route of choice for the synthesis of novel substituted cyclobutadienes. In the literature, addition of organolithium bases (MeLi, BuLi) to the CO ligands with concomitant rearrangement had been observed [25]. But the utilization of LiTMP (lithium tetramethylpiperidide, Hafner [26]) or sec-BuLi as effectively non-nucleophilic bases led to clean deprotonation of the cyclobuta-... 

The reaction of (cyclobutadiene)metal complexes with X2 results in the oxidative decomplexation to generate either dihalocyclobutenes or tetrahalocyclobutanes. In comparison, substitution of (cyclobutadiene)MLn complexes 223 [MLn = Fe(CO)3, CoCp, and RhCp] with a variety of carbon electrophiles has been observed (equation 34)15. Electrophilic acylation of 1-substituted (cyclobutadiene)Fe(CO)3 complexes gives a mixture of regioisomers predominating in the 1,3-disubstituted product and this has been utilized for the preparation of a cyclobutadiene cyclophane complex 272 (equation 35)246. For (cyclobutadiene)CoCp complexes, in which all of the ring carbons are substituted, electrophilic acylation occurs at the cyclopentadienyl ligand. 

Bunz et al. explored the possibility of doping PPE chains covalently with small amounts of fluorescence-quenching cyclobutadiene complexes, in order to endow their optical properties to the base polymer, PPE [80]. Due to their extensive experience of cyclobutadiene complexes in polymer synthesis [81], the authors prepared several polymers PAE-CoCpl-5 (Table 4) containing different contents of CoCp complexes. The quantum yields were determined by simple comparison of the intensities of the emitted light to that of a standard... 

Later in 1978, Masamune reported the first matrix Fourier transform IR spectrum of cyclobutadiene that now allowed a direct comparison of the observed spectrum of 1 with the calculated spectra. In Figure 6, Masamune s experimental spectrum is compared with the computed IR spectra (STO-4G and 4-3IG basis sets). It is seen that both calculated spectra, which were performed with relatively small basis sets, are in good qualitative agreement with the observed spectrum of 1. One could therefore conclude that the ground state structure of cyclobutadiene is indeed rectangular. 

The scheme for treating these subjects is as follows. We compare typical aromatic and antiaromatic compounds. Similar to the comparison between benzene and cyclobutadiene, we concern ourselves here with pyridine (34) and azete (57). Such an approach provides, apart from other advantages,... 

Table 4 shows Bo values calculated for hexagons and squares using Kollmar s method.157 Comparison of entries 1—4 to entries 5—8 reveals that the benzene analogues, with the 4/V+ 2 electrons, have considerably larger Bo values than the 4 TV analogues of square cyclobutadiene. This trend is related to the accepted view that cyclic delocalization of 47V + 2 electrons possesses a higher resonance energy than cyclic delocalization of 47V electrons. 

Comparison of the tropone ring geometry in iron complex 282 with that of the parent tropone shows that the double bond character of the carbonyl group and the bond between C-2 and C-3 in 282 is even higher, probably as a result of dominant contributions from two cyclobutadiene resonance... 

A comparison of the three dimerizations discussed in the latter part of this chapter illustrates nicely the interplay between symmetry and energy The presence of an additional tt bond in cyclobutadiene offers enough energetic advantage to concerted closure of the four-membered ring to syn-TCOD for it to take precedence over the more general - and no less allowed - stepwise pathway via a transoid biradical. Cyclopropene, with just the one 7r-bond, behaves like a normal alkene and finds the latter pathway more convenient. Silacyclopropene starts off along a similar pathway, but makes use of the relative weakness of the CSi bonds to react in an entirely different manner, but one that is still consistent with the requirements of orbital symmetry conservation. 

As can be seen from this comparison, the resulting values are affected by the choice of the critical structure and on going from X(n/4) to X(-7t/4), the systematic shift of the dominant similarity from the zwitterionic state Z + Z2 to the state Zj -Z2 is observed. We can thus see that the predictions for both types of critical structures differ and the problem thus appears which of the above two critical structures should be regarded as a true model of the transition state in forbidden reactions. Similarly as in the case of allowed reactions such a decision does not arise from the approach itself, but some external additional information is generally required. This usually represents no problem since the desired information can be obtained, as in the case of allowed reactions, from the simple qualitative considerations based on the least motion principle [80,81], or from the direct quantum chemical calculations.This is also the case with us here, where the desired information is provided by the quantum chemical study [63] of the thermally forbidden cyclization of the butadiene to cyclobutene. From this shufy it follows that the ground state of the transition state should correspond to the ground state of the cyclobutadiene which is the Zj - Z2 state. 

The Dewar isomer, 123, of 3-methyl-4(3H)-pyrimidinone (122) was generated in argon matrices by 308 nm irradiation of the parent molecule and identified by comparison of the experimental matrix IR spectrum with spectra computed for 123 and other possible products. " For several 4(3H)-pyrimidinones not methylated at N3, two other types of matrix photoreactions were observed in addition to the isomerization to the Dewar isomer phototautomerism and ring opening. Somewhat later, matrix isolated Dewar pyridine (124) was produced by UV-irradiation of pyridine in solid argon and was identified with the aid of DFT computations. On further photolysis, 124 produced cyclobutadiene (118) and HCN. 

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