Antara-supra cycloaddition

To summarize thermal supra-supra or antara-antara cycloadditions are allowed for systems having 4n + 2 electrons and supra-antara or antara-supra cycloadditions for systems having 4n electrons. 

The antara-supra processes, even if less studied than the more common supra-supra reactions are frequently encountered in recent works on cycloaddition reactions and confirm in their turn the utility of orbital symmetry considerations. 

Four stereocombinations of two molecules in the [2+2]-cycloaddition reaction are possible (Fig. 8.25) supra-supra (A), antara-antara (B), supra-antara (C) and antara-supra (D). C and D have the same allowedness, but may give different product stereochemistries depending on the nature of the reactants. 

Cycloadditions 4n+2 4n 4n + 2 Disrotatory Supra-Antara or Antara-Supra Supra-Supra or Conrotatory Supra-Supra or Antara-Antara Supra-Antara or... 

Therefore, if we derive or remember one rule for a pericyclic reaction, then any time an MO phase change is added the rule will reverse. Two reversals cancel each other. For example, 4n face to face (supra-supra) cycloadditions are not thermally allowed. If we add two electrons, we fill the next highest MO, which has a phase reversal. This means An+2 cycloadditions are thermally favored. Thermal electrocyclic reactions of 4n species go conrotatory, whereas thermal 4n+2 electrocyclic reactions go disrotatory. Thermal sigmatropic reactions of 4n species go supra-inversion or antara-retention. Count arrows to tell whether the pericyclic reaction is 4n or 4n + 2. Phase reversals occur between retention/inversion at the migrating center, between antarafacial/suprafacial migration, with 4n vs. 4n+2 electrons, and between thermal and photochemically excited species. 

For a successful cycloaddition to take place, the terminal tt lobes of the two reactants must have the correct symmetry for bonding to occur. This can happen in either of two ways, called supra facial and antara facial. Suprafacial cycloaddjtions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant. Antarafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on opposite faces of the other reactant (Figure 30.8). 

A G bond is considered to be involved in a cycloaddition in a supra manner if configuration is either retained or inverted at both of its termini in the course of the reaction. The retention of the configuration at one terminus and the inversion at the other will indicate an antara addition. 

Verify that the selection rules found for the two-component cycloadditions (Section 11.3, p. 594) agree with the general pericyclic selection rule. What can be said about all-antara 2 + 2 + 2 +. .. cycloadditions About all-supra 2 + 2 + 2 +. .. cycloadditions ... 

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. 

Obviously, an antara-antara reaction is allowed whenever the corresponding supra-supra reaction is, i.e. when the FO have the same symmetry (compare Figures 4.2a and 4.2d). When the FO of the two partners have different symmetry, the supra-supra reaction is forbidden (Figure 4.2c) but the supra-antara reaction is allowed (Figure 4.2e). An analogous reasoning shows that the supra-antara cycloaddition is allowed when the total number of electrons is p + q = 4n [Figure 4.1 (2)]. 

This type of condensation is of great interest in connection with the Woodward-Hoffmann selection rules for symmetry-allowed concerted suprafacial and antarafacial cycloaddition reactions.284 The generalized rules for cycloaddition of an m- to an n-electron system predict that the concerted supra-supra or antara-antara dimerization is allowed in the excited state (i.e., photochemically) when m + n = 4q, and in the ground state (i.e., thermally) when to + n = 4q + 2, where to and n are the numbers... 

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... 

Figure 29.23. [2 + 2] Cycloaddition. Supra,supra geometrically possible, but symmetry-forbidden. Supra,antara symmetry-allowed, but geometrically difficult.

Almost certainly, such a supra,antara process is impossible here on geometric grounds. But if the ring being formed is big enough, both supra,supra and supra, antara processes are geometrically possible in that case orbital symmetry determines, not whether cycloaddition occurs, but how it occurs (Table 29.2). 

Several photooxygenation mechanisms, such as concerted supra-antara [2 + 2] cycloaddition and stepwise additions involving charge-transfer complex (516), perepoxide... 

Also cis-trans ov supra,antara) cycloadditions of some 7r-electron systems are sterically feasible (ie. the top face of one component reacts with the bottom face of the other, at one side, but the two bottom faces react with each other at the other side of the cycloaddition). In such cases the above selection rules are exactly reversed. ... 

A more interesting field is that of the thermal 1,2-cycloadditions here concerted processes are forbidden, and experimental results can be compared with this prediction. A discrepancy arises in the case of keteiies, as there is considerable evidence in favour of a concerted mechanism for their thermal 1,2-cycloadditions (Section 6.1). However, it is possible to envisage the intervention of the perpendicular Tr-system of the C=0 bond of ketene, in such a way as to surmount the steric difficulties of an orthogonal approach of the reactants, required by a cis-trans (or supra,antara) cycloaddition the latter is symmetry-allowed as a thermal process when it is w + n = 4. 

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. 

The Woodward—Hofmann selection rules for the generalized case are the same as for cycloaddition reactions (1) For supra—supra or an antara—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p + q = 4n -I- 2, and it is photochemically allowed when p + q = 4n. (2) For supra—antara double group transfer, the reaction is symmetry allowed under thermal conditions when p -I- q = 4n, and it is photochemically allowed when p - - q = 4n - - 2. 

A simplest example of a cycloaddition reaction is provided by the dimerization of ethylene with the formation of cyclobutane, i.e., [2 + 2]-cycloaddition. According to the Woodward-Hoffmann rules, the supra-antara route I of the reactants approach is thermally allowed.The parallel supra-supra approach II is unfavored in the electron ground state but must materialize in an excited state. 

Thermal addition of ketene to olefin is antara on ketene and supra on olefin. Herert a-f-Ti scycloaddition is preferred over 7t s-l-3t s cycloaddition. ... 

Cycloaddition of two molecules of cis-2-butene may produce different stereoisomers. Write the structures of (i) supra-supra (ii) supra-antara and (iii) antara-antara products alongwith reaction conditions. 

FMO method Interaction of HOMO of one component with the LUMO of other involves either 7t s-i-Jt aor ji q+tc s cycloaddition. But structure of product indicates that process isrt a+7t s. Overlapping between HOMO of heptafulvalene and LUMO of tetracyanoethylene (TONE) makes the interaction possible. HOMO of heptafulvalene (11/7) possesses mirror plane (m) symmetry and LUMO of TCNE (vi/2) possesses C2-symmetry. Therefore, overlapping is not feasible in supra-supra-fashion. This cycloaddition is thermally feasible only if overlapping is antara on one of the component which is heptafulvalene. 

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