Supra-antara mode

It is seen from Fig. 3.10 that conrotatory cleavage is equivalent to a [(j2s + 2a] or a [c2a + 2s ] cyclo-addition, whereas disrotatory cleavage coincides with a [ 2s +, j2s] or a [ 2a + 2al process. An important point to note here is the effective placing of the supra-supra and antara-antara interactions into one category, and the supm-antara and antara-supra modes into a separate category. The members of the first set each contain an even number of inversions at carbon centres (i.e. 0 or 2), whilst the members of the second set each contain an odd number of inversions (i.e. 1). This is a general result, as will be seen in Section 3.3.3. 

An antarafacial interaction on one, or both, of the molecules requires that the approach be orthogonal. Because of the high symmetry of the system the supra-antara and antara-supra modes are identical. Correlation diagrams for the [ 2j + 2, ] addition are to be found in the literature (Woodward and Hoffmann, 1965, 1969). We will concentrate here on the other two modes of addition. 

In the antam-antara case (Fig, 4.8) there are two C2 axes - one bisects the molecular axes of the two ethylenes ( 22) and the other lies in the xy plane (which is a horizontal plane perpendicular to the page) and at 45 to the y and z co-ordinates (C2j j,) such that it bisects the two o-bonds that are being formed. The antara-supra mode is of lower symmetry, and the only useful element is the C2Z axis. There are no symmetry planes for the system because of the non-planar nature of the first-formed product molecule. 

In case of [tt s + tu s] cycloaddition (4n 7r-electron system), a supra—supra mode of addition leads to a Huckel array, which is antiaromatic with 4n 7u-electrons (Figure 4.7). Therefore, the supra—supra mode of reaction is thermally forbidden and photochemically allowed. However, a supra—antara mode of addition uses a Mobius array, which is aromatic with 4n 7t-electrons. Therefore, [rt s + rc a] cycloaddition reaction is thermally allowed and photochemically forbidden. Similarly, we can analyze the [tu" s + Tt s] cycloaddition having (4n + 2) 7t-electrons (Figure 4.7). In this case, a supra—supra mode of addition leads to a Huckel array, which is aromatic with (4n + 2) 7C-electrons. Therefore, [7t" s + tu s] cycloaddition reaction now becomes thermally allowed and photochemically forbidden. However, a Itch s + Tu a] cycloaddition uses a Mobius array, which is antiaromatic with (4n + 2) 7u-electrons. Therefore, the reaction is thermally forbidden and photochemically allowed in this mode. 

Figure 3.10 (b) - (e) illustrates the topology of the interactions between cr-bond and rr-system for the electrocyclic ring opening of a cyclobutene (cf. Equation 3.3). We have the usual four possible modessupra-supra. supra-antara, antara supra, and antara-antara (cf. Fig. 3.2), and these modes are related to the conrotatory and disrotatory ways of ring cleavage. There are in fact four possible pairs of interactions in Fig, 3.10(c) and (d) the alternatives which, in so far as the orbital-orbital interactions are concerned are enantiomeric with those shown, and are therefore omitted. 

It is seen that the placing of the supra-supra and antara-antara interaction modes into one class, and the supra-antara and antara-supra interactions into... 

Consider Fig. 5.2 it is at once apparent that the supra-supra and antara-antara interactions should lead to transition state complexes of high energy since they are of the —H type. The concerted thermochemical dimerization of two ethylenic molecules should therefore proceed by way of supra-antara or antara-supra interactions (if geometrically possible) because they provide pathways to the low energy + A-H transition state. Conversely, the first excited state photochemical cyclo-addition should occur by either supra-supra or antara-antara modes because once again we have a + A-H transition state available (i.e. the excited form of a — H transition state). 

This category corresponds with the formation or cleavage of three-membered rings. In thermo chemical reactions the antara-supra and supra-antara modes are to be expected and of the two the linear process (Fig. 3.16(b) (i)), the supra-antara mode, may be ruled out for reasons based on geometry and orbital overlap. The low energy pathway is therefore the non-linear process (Fig. 3.16(a) (ii)). 

The [2+2] cycloadditions can be concerted under thermal conditions provided that the interaction between the Ji-systems takes place in a supra-antara mode (Fig. 1). This [27is + 27+] mechanism [20] is sterically very demanding and, therefore, it should be facilitated by cumulenes possessing s/ -hybridized electrophilic carbon atoms. This makes ketenes and isocyanates suitable candidates for concerted symmetry-allowed thermal [2+2] cycloadditions. However, the presence of heteroatoms in both possible [2+2] reactions leads in turn to different stepwise mechanisms in which the electrophilic nature of the v/ -hybridized carbon atoms of ketenes and isocyanates plays a crucial role (Scheme 2). According to these mechanisms, zwitterionic intermediates (6) and (7) are plausible via formation of C-N or C-C bonds, respectively. 

In the course of a pericyclic cycloaddition, the interacting terminal lobes of each component may overlap either in a suprafacial mode or in an antarafacial mode. If both the new bonds form from the same face of the molecule it is known as a suprafacial mode (also known as supra-supra). It is antarafacial if one bond forms from one surface and the other bond forms from the other surface (also known as supra-antara) (Fig. 8.13). 

The FMO argument for the [2+2]-cycloaddition in the ground state (i.e. under thermal conditions) is that the LUMO (rt ) of one ethene and the HOMO (tt) of another ethene are phase mismatched for the supra-supra [2s+2s]-cycloaddition (Fig. 8.26). Thus, it is symmetry forbidden under thermal conditions (two molecules in the ground state). Similarly, the antara-antara mode is symmetry forbidden. The anatra-supra or supra-anatara mode... 

By using molecular models it may be readily seen that the orbital overlap is the most efficient for the [,j2s + 2j] combination, and the stericinteractions are less pronounced than in the alternative modes of interaction. The orbital overlap in the supra-antara and mtara-antara geometrical arrangements are improved if one or both of the ethylene molecules is twisted about its O-C axis to that the coplanar relationship of the CH —CHj atoms is destroyed. It therefore follows that the [ 2j + 2g] and [ 2, + 2a] combinations are less likely, other things being equal, than the [ 2s+ 2s] mode in simple unstrained systems. 

The three possible topological interactions in [ 2 + 2] cyclo-addition reactions are shown in Fig. 5.2 again the basis molecular orbitals of the ethylene components are considered. In the supra-supra and i v antara-antara combinations there are no out of phase orbital overlaps (or two if the signs are reversed on one ethylene component). In the supra intara mode there is one out of phase overlap. Since there are four electrons involved, the Mobius type interaction (i.e. supra-antard) should be preferred the other combinations should therefore be possible under photochemical control. These results accord with the previous findings of orbital symmetry theory. 

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