2- 2- cyclobutene

The thermal isomerization of cyclobutene to butadiene is one of the best known examples of a thermal unimolecular isomerization. Cooper 

A more recent study of this isomerization has been carried out over a very extended pressure range (Hauser and Walters, 1963). The high-pressure rate constants were obtained by extrapolation, whence 

At 5 mm the rate constant has already begun to decrease from the high-pressure value, and at 0-15 mm it is only about 13 % of the high-pressure value. Addition of inert gases in the fall-off region increases the value of the rate constant towards the high pressure limiting value. 

The very low value of the energy of activation for this isomerization is of considerable interest. Comparison with the decomposition of cyclobutane shows a reduction of 30 kcal mole caused by the presence of the double bond. If a similar transition state were involved in both reactions, then this difference would be a measure of the extra strain energy of the cyclobutene. This is quite unrealistically high. Thus we eliminate the possibility that the reaction path is as shown below  

A transition complex which is consistent with the low energy of activation of the isomerization involves the simultaneous deformation (twisting) of the cyclobutene ring with the stretching of the carbon-carbon bond opposite the double bond  

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Electi ocyclic reactions are examples of cases where ic-electiDn bonds transform to sigma ones [32,49,55]. A prototype is the cyclization of butadiene to cyclobutene (Fig. 8, lower panel). In this four electron system, phase inversion occurs if no new nodes are fomred along the reaction coordinate. Therefore, when the ring closure is disrotatory, the system is Hiickel type, and the reaction a phase-inverting one. If, however, the motion is conrotatory, a new node is formed along the reaction coordinate just as in the HCl + H system. The reaction is now Mdbius type, and phase preserving. This result, which is in line with the Woodward-Hoffmann rules and with Zimmerman s Mdbius-Huckel model [20], was obtained without consideration of nuclear symmetry. This conclusion was previously reached by Goddard [22,39]. 

The classic example is the butadiene system, which can rearrange photochemi-cally to either cyclobutene or bicyclobutane. The spin pairing diagrams are shown in Figure 13. The stereochemical properties of this reaction were discussed in Section III (see Fig. 8). A related reaction is the addition of two ethylene derivatives to form cyclobutanes. In this system, there are also three possible spin pairing options. 

SUBSTITUTED BUTADIENES. The consequences of p-type orbitals rotations, become apparent when substituents are added. Many structural isomers of butadiene can be foiined (Structures VIII-XI), and the electrocylic ring-closure reaction to form cyclobutene can be phase inverting or preserving if the motion is conrotatory or disrotatory, respectively. The four cyclobutene structures XII-XV of cyclobutene may be formed by cyclization. Table I shows the different possibilities for the cyclization of the four isomers VIII-XI. These structmes are shown in Figure 35. 

In a similar way Table II summarizes how the phase changes upon interconversion among the isomers. Inspection of the two tables shows that for any loop containing three of the possible isomers (open chain and cyclobutene ones), the phase either does not change, or changes twice. Thus, there cannot be a conical intersection inside any of these loops in other words, photochemical transformations between these species only cannot occur via a conical intersection, regardless of the nature of the excited state. 

Thus, to name just a few examples, a nucleophilic aliphatic substitution such as the reaction of the bromide 3.5 with sodium iodide (Figure 3-21a) can lead to a range of stereochemical products, from a l l mbrture of 3.6 and 3.7 (racemization) to only 3.7 (inversion) depending on the groups a, b, and c that are bonded to the central carbon atom. The ring closure of the 1,3-butadiene, 3.8, to cyclobutene... 

Thermal electrocyclizations of perhalogenated 1,3-butadienes yield perhalogenated cyclobutenes which can be solvolysed to 3,4-dihydroxy-3-cydobutene-l,2-dione ( squaric acid") and its derivatives (G. Maahs, 1966 H. Knorr, 1978 A.H. Schmidt, 1978). Double CO extrusion from fused cyclobutenediones has been used to produce cycloalkynes, e.g., benzyne from benzocyclobutenedione by irradiation in an argon matrix (O.L. Chapman, 1973) and cyc/o-Ci8, cyclo-Cn, etc. by laser desorption mass spectroscopy of appropriate precursors (see section 4.9.8). 

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

The thermal ring opening of l,2-bis(trimethylsiIoxy) cyclobutenes (from acyloin condensation of 1,2-dicarboxylic esters) was used in ring expansion prodecures (see p. 53f.). 

Cycloaddition of norbornadiene with allene takes place to yield the cyclobutene derivative 10[5], Cyclodimerization of 1,2-cyclononadiene (11) affords a mixture of stereoisomers of the cyclobutane derivatives 12[6,7],... 

Symmetry forbidden reaction (Section 10 14) Concerted re action in which the orbitals involved do not overlap in phase at all stages of the process The disrotatory ring opening of cyclobutene to 1 3 butadiene is a symmetry forbidden reaction... 

CycIoa.ddltlons. Cyclobutene adducts are formed from the reaction of acetylenic derivatives and maleic anhydride through a 2 + 2 cycloaddition (48). The reaction is photochemicaHy cataly2ed (see Photochemical technology). 

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