Suprafacial motion

FIGURE 20.43 Suprafacial motion Bottom to top involves bond breaking and bond... 

FIGURE 20.48 Suprafacial motion of a hydrogen is possible in O3, the photochemical HOMO for this reaction. 

The linear cheletropic reactions in which the polyene is a suprafacial component (i.e., involving disrotatory motion of the termini) will be allowed if it has a total of (4n + 2) electrons. But linear cheletropic reactions in which the polyene is an antarafacial component (i.e., involving conrotatory movement of the termini) are allowed if it has a system of 4n electrons. 

Depending on whether the starred carbon of 11 pivots so as to bond backside or frontside, there will be a 60° (82) or 120° (83) periodicity in the motion around the remaining carbons. The 60° pattern allows for raeemization in two ways if the starred carbon passes over the marker substituent M, the species develops a plane of symmetry if the starred carbon never passes over M, it still leads to dZ-pairs of nocaradiene or tropilidene. The 120° pattern does not allow for raeemization so that optical activity in the isomers of 11 would be preserved. According to the orbital symmetry rules (Fig. 20a), the [1,5] suprafacial shift is allowed the 120° pattern with retention should be the one observed, if the proposed mechanism is correct. 

The kinetic isotope effect (Equation 1) depends on the properties of the transition state. Since there is no way to determine these properties experimentally, a "reasonable model is assumed. Calculations were performed on two different models for the structure of the activated complex. One of these consisted of the suprafacial, least-motion approach of the S( P) atom to yield a symmetrical thiirane molecule, while the other was an asymmetrical approach at 100° to the C-C bond to give a ring-distorted diradical-like intermediate vide infra). Unfortunately, the calculation is not sensitive to the models chosen and does not distinguish the two structures for the activated complex. The experimental isotope effects for C2D4, CH2CD2 and cts-CHDCHD with the same geometrical structure and set of force constants were reproduced in a self-consistent manner for either models. Nonetheless, these calculations were useful. 

The conclusion would appear to be that the vinylcyclobutane rearrangement involves biradicals, possibly formed by conrotatory-level motions (see Chapter 7, Section 3.14), Scheme 7.103, which are incompletely equilibrated before cyclization to cyclohexene because there is always a preference for utilization of the allylic moiety in a suprafacial way. 

Second, since neither rotation about the C2-C3 bond nor ring flexing motion can be effected and the P-endo hydrogen (H) at C3 cannot be forced to a 180° dihedral angle with respect to chlorine (Cl) at C2, only syn (cis or suprafacial) is possible. 

Let s set about finding an experiment to test the requirement of the theory for suprafacial [1,5] motion. What will we know at the end of the experiment If we find that the [1,5] shift is indeed strictly suprafacial, the theory will be supported (not proved ), and we will certainly feel better about the mechanistic hypothesis. If we find antarafacial motion, or both suprafacial and antarafacial motions, the theory will be proved (yes, proved) wrong. There is no way our hypothesis can accommodate antarafacial motion there is an absolute demand for suprafaciality, which nicely illustrates the precarious life of a theory. It can always be disproved by the next experiment, and it can never become free of this state of affairs. 

The conrotatory motion involves an antarafacial interaction between the termini, and disrotation involves a suprafacial interaction between these centres. There are two distinct possibilities for each mode of rotation, and therefore four possible products in aU. In many cases, however, the inherent mmetry of the system may not allow such distinction to be made. Even in cases where the two forms of one mode are distinguishable, it does not necessarily follow that both will occur the geometry of the system and the steric factors can be decisive. For example, the conrotatory ting-opening of trans-3,4-dimethylcyclobut-l-ene should, in principle, yield a 1 1 mixture of trans, /nms-hexa-2,4-diene and as, cis-hexa-2,4-diene (Equation 3.17). However, the inward conrotation of the two methyl groups is unfavourable because of the rapid increase in steric compression between these two substituents, and the 78... 

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