Cyclobutadienes Cyclobutenes

Azete, azetine and azetidine are the aza analogues of cyclobutadiene, cyclobutene and cyclobutane, respectively (Box 8.1). 

An intriguing point is to be seen, however. Thus the Oosterhoff state correlation, in which St did not interact with the S0 and S2 configurations, results not only from Brillouin s theorem precluding interaction of S with So but also from the exact symmetry of the cyclobutadiene — cyclobutene reacting system which imparts different symmetries to the St and S2 configurations. With different symmetries, S and S2 also cannot interact. More generally, a reacting system will not have perfect symmetry. Even in the butadiene — cyclobutene case, one can expect molecular deformations to break up this symmetry. 

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... 

In some cases, double bonds add to triple bonds to give cyclobutenes, apparently at about the same rate that they add to double bonds. The addition of triple bonds to triple bonds would give cyclobutadienes, and this has not been observed, except where these rearrange before they can be isolated (see 15-63) or in the presence of a suitable coordination compound, so that the cyclobutadiene is produced in the form of a complex (p. 60). 

Fig. 5.7 Correlation diagram between cyclobutene-cyclobutadiene system in a disrotatory interconversion.

Cycloarsa(V)azanes, 28 216-223 Cyclobutadienes, 31 144, 166 Cyclobutene complexes with iron, 12 254,... 

Nonacarbonyldiiron in the presence of activated zinc as well as disodium tetracarbonylferrate have both been utilized to dechlorinate dimethyl cispransjransA,2,3,4-tetrachlorocyclobutane-1,2-dicarboxylate (41), which leads to [dimethyl l,2,3,4- 7-l,3-cyclobutadiene-l,2-dicarboxy-latejiron tricarbonyl (42) in 1 and 35-40% yield, respectively.15 As a result of the better leaving tendency of the bromide anion, the yield of 42 can be improved to 70% by reaction of the analogous dibromide with nonacarbonyldiiron in dimethylformamide.16 In another experiment, debromination with activated zinc in tetrahydrofuran converts cyclobutane-1,2-di-carboxylate 43 to cyclobutene-1,2-dicarboxylate 44 in 58% yield.15... 

A rare example of a ferracycloheptane 108 was obtained as the product of the photochemical reaction of a Petitt s cyclobutadiene iron complex with dimethyl-maleate [Eq. (43)].118 The ferracycloheptane arises from the insertion of a maleate into each of two Fe-C bonds and might therefore be considered a special case of alkene trimerisation (vide infra). The cyclobutene fragment in the final metallacycle remains coordinated to iron, as established crystallographically (Fig. 34). 

The practical exploitation of the proposed criterion can be very simply demonstrated by the example of the electrocyclic transformation of cyclobutadiene to cyclobutene, for which the structure of the intermediate can be quite reliably estimated from the available results of quantum chemical calculations [74] (Scheme 2). 

Cycloaddition of aza[2]ladderane 107 with DMAD in the presence of a ruthenium catalyst RuH2CO(PAr3)3 (Ar = Ph, />-FC6H4) yielded the exo-fuscA aza[3]ladderane diester 108, which underwent further cycloaddition with cyclobutadiene through the dienophilic cyclobutene Jt-bonds of 108 to yield a mixture of stereoisomers 109 and 110 in a ratio of 3 2 (Scheme 5). The two isomers were separated by HPLC. The reaction of 107 with cyclopentadiene has also been studied both experimentally and theoretically <1997SL38>. 

An example of this effect is provided by tri-/ r/-butylcyclobutadiene. This compound is stable at room temperature for a brief time because the bulky tert-butyl groups retard the dimerization reaction that destroys less hindered cyclobutadienes. The ring hydrogen of this compound appears at 5.38 8, a position somewhat upfield from that of the hydrogens of a nonaromatic model compound such as cyclobutene (5.95 8). 

The reaction of cyclobutene ring opening leading to 1,3-cyclobutadiene has been extensively studied by experimental and theoretical methods.64,66- 71... 

The ring opening of cyclobutadiene is predicted by symmetry rules to proceed as a conrotatory process. The kinetic product of the reaction is gauche- 1,3-butadiene with deviation from planarity of ca. 30°. The structures of the cyclobutene, TS, and gauche- 1,3-butadiene are shown in Figure 4-13. 

To explain the increase in the rate of the cyclobutene opening 6.292 —> 6.293, we need to remember that the conrotatory pathway will have a Mobius-like aromatic transition structure, not the anti-aromatic Hiickel cyclobutadiene that we saw in Fig. 1.38. We have not seen the energies for this system expressed in 8 terms, nor can we do it easily here, but the numbers are in Fig. 6.42, where we can see that a... 

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