Cyclobutene, from 1,3-butadiene

It should therefore be possible to bring about such processes photo-chemically by direct conversion of the excited form of the reactant to the ground state of the product via the BO hole corresponding to the antiaromatic transition state. A classic example is the photochemical formation of cyclobutenes from 1,3-butadienes by disrotatory ring closure e.g.,... 

Ring closure reactions. These are invariably accompanied by transfer of an atom from one location in the molecule to another. (148) is an example of this type of process and so would be the formation of cyclobutene from butadiene. 

Simple examples of electrocyclic reactions are the formation of cyclobutene from butadiene and cyclohexadiene from hexatriene ... 

The synthesis of 2-chloro-2,3,3-trifluorocyclobutyl acetate illustrates a general method of preparing cyclobutanes by heating chlorotrifluoroethylene, tetrafluoroethylene, and other highly fluorinated ethylenes with alkenes. The reaction has recently been reviewed.11 Chlorotrifluoroethylene has been shown to form cyclobutanes in this way with acrylonitrile,6 vinylidene chloride,3 phenylacetylene,7 and methyl propiolate.3 A far greater number of cyclobutanes have been prepared from tetrafluoroethylene and alkenes 4,11 when tetrafluoroethylene is used, care must be exercised because of the danger of explosion. The fluorinated cyclobutanes can be converted to a variety of cyclobutanes, cyclobutenes, and butadienes. 

Ring-opening of cyclobutenes to butadienes is very common a recent example is the formation of the aldehyde 6 in greater than 97% diastereomeric purity from the cyclobutene 5 (R = 4-methoxybenzyl) above —78 °C (equation 5)5. 

The present procedure offers in good yields a simple and ready preparation of pure cyclobutene from the easily available cyclopropylcarbinol. The product is free of the impurities (e.g., 1,3-butadiene, bicyclobutane, methylenecyclopropane) usually obtained with the various methods so far reported. The procedure described for the synthesis of... 

In the case of conrotatory mode, the symmetry is preserved with respeo to C2 axis of rotation. On 180° rotation along this axis, F goes to H. and H2 to H, and the new configuration is indistinguishable from the original. An orbital symmetric with respect to rotation is called a and antisymmetric as b. On the other hand, in the case of disrotatory moot-the elements of symmetry are described with respect to a mirror plane. Tilt symmetry and antisymmetry of an orbital with respect to a mirror plant of reflection is denoted by a and a" respectively (Section 2.9). The natun of each MO of cyclobutene with respect to these two operations is shov. n in the Table 8.4 for cyclobutene and butadiene. 

In this section we will present results33 from the MD simulations along the IRC performed for five model reactions the HCN CNH isomerization reaction, the conrotatory ring opening of cyclobutene, ethylene-butadiene cycloaddition, the prototype SN2 reaction Cl +CII3CI C1CH3- -Cr, and the chloropropene isomerization Cl - CH2 - CH=CH2 - CH2=CH - CH2C1. 

Let us first examine how the relevant MOs for two interconverting molecules, for example, cyclobutene and butadiene, change when one molecule is converted into the other. Note particularly that we need to examine only the MOs explicitly involved in the reaction most of the a framework remains unchanged and no orbitals derived from this need to be considered. 

Figure 4.8. Calculated state correlation diagram for the disrotatory ring opening of cyclobutene to butadiene (by permission from Grimbert et al., 1975).

Figure 4.11. Derivation of the orbital correlation diagram for the conrotatory ring opening of cyclobutene a) intended correlation, b) correlation including interaction between a and n and between and o MO s, respectively. Orbiial symmetry labels n and a apply strictly only at the planar cyclobutene and butadiene geometries. Labels S and A, or solid and broken correlation lines respectively, indicate the symmetry behavior with respect to the twofold-symmetry axis (by permission from Michl, 1974b).

Figure 6.4 The butadiene cyclobutene ring closure reaction, (a) For the conrotatory reaction, the loop constructed from butadiene, cyclobutene, and bicyclobutane is p (or pi ), and no conical intersection is enclosed, (b) For the disrotatory reaction, the loop is ip (or i ) and encloses a conical intersection.

Figure 7.6 Plot of experimental and RRKM calculated k(E) vs. E for the isomerization of cyclobutene to butadiene. The dashed line is an RRKM calculation using the transition state parameters of Frey et al. (1966) while the solid line is an RRKM calculation of Jasinski et al, (1983). Taken with permission from jasinski et al. (1983).

Table 4 Total energy (in a.u.), point-group symmetry, mean value of the polarizability of Eq. (58) (in a.u.), and hardness (in eV) for the cyclobutene cis-butadiene reaction via either the conrotatory or the disrotatory transition state. The results are from ref. 54...

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. 

If we place four electrons in the lowest energy orbitals of the butadiene, it is clear from Figure 15.16 B that the conservation of orbital symmetry predicts that the conrotatory process (black lines) is preferred. The disrotatory (colored dashed lines) process leads from butadiene to an excited state of cyclobutene. The same conclusions are reached by considering the reaction in the reverse direction. 

This is the same as that for union to hexatriene. Thus (190), and so likewise the boat transition state, is neither aromatic or antiaromatic but nonaromatic. The difference in energy between it and the aromatic chair (186) should therefore be about one-half that between an aromatic transition state and an analogous antiaromatic one. Recently Goldstein has measured the difference in activation energy between the rearrangements of an analog of (181) with deuterium in place of methyl (to avoid complications from steric effects). They found it to be 10 kcal/mole, rather more than half the corresponding difference (14 kcal/mole p. 345) between the aromatic conrotatory and antiaromatic disrotatory transition states for conversion of cyclobutene to butadiene (Fig. 5.37). 

Fig. 10.3. Variation of principal geometry parameters for the conrotatory conversion of cyclobutene into butadiene (Adapted from Ref. [38])...

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