Antarafacial modes

Suprafacial attack of me ethene molecule on anotlier (left) is not permitted by the Woodward-Hoffmann id the alternative antarafacial mode of attack is sterically unfavourable. Suprafacial attack is however permitted Diels-Alder reaction between butadiene and ethene (right). 

A bonding interaction can be maintained only in the antarafacial mode. The 1,3-suprafacial shift of hydrogen is therefore forbidden by orbital symmetry considerations. The allowed... 

A similar analysis of the 1,5-sigmatropic shift of hydrogen leads to the opposite conclusion. The relevant frontier orbitals in this case are the hydrogen Is orbital and ij/j of the pentadienyl radical. The suprafacial mode is allowed whereas the antarafacial mode is forbidden. The suprafacial shift corresponds to a favorable six-membered ring. 

The selection rules for cycloaddition reactions can also be derived from consideration of the aromaticity of the transition state. The transition states for [2tc -f 2tc] and [4tc -1- 2tc] cycloadditions are depicted in Fig. 11.11. For the [4tc-1-2tc] suprafacial-suprafacial cycloaddition, the transition state is aromatic. For [2tc -F 2tc] cycloaddition, the suprafacial-suprafacial mode is antiaromatic, but the suprafacial-antarafacial mode is aromatic. In order to specify the topology of cycloaddition reactions, subscripts are added to the numerical classification. Thus, a Diels-Alder reaction is a [4tc -f 2 ] cycloaddition. The... 

Particular interactions between energy levels could stabilize supra-facial-suprafacial [2+2] concerted reactions, [2.ns- - ig, 57) or supra-facial-antarafacial modes, [2Jts-hn a]- Other interactions might be favorable toward biradical reactions. These diagrams will be discussed in detail in Section VI. 

Shifts should occur in allylic cations as in the neutral four-electron systems, the rules require the suprafacial-antarafacial mode in the ground state. In the bicyclo[3.1.0]hex-3-en-2-yl cation (67), the ring fusion prevents access of... 

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). 

Figure 8.13 Suprafacial and antarafacial modes in a cycloaddition reaction.

In electrocyclic reactions, the suprafacial mode involves each component of the new CT-bond being formed from the same face of the reactant Tr-system (this is equivalent to disrotation). The antarafacial mode involves twisting of the orbitals so that the two components of the new a-bond form from the opposite face of the reactant TT-system (this is equivalent to conrotation) (Fig. 8.14). 

The formation of the new o-bond(s) must occur by an appropriate overlap of the same phases of these orbitals. If only one a-bond is forming, as in electrocyclic reactions, then only the overlap of the HOMO of the open chain reactant is considered. Such an overlap can occur in one of the two fundamental ways suprafacial mode or antarafacial mode (see Fig. 8.14). If two or more a-bonds are formed during the reaction, as in cycloaddition reactions, then the overlap of the HOMO of one reactant with the LUMO of the second reactant must be considered (see section 8.3). 

Additions are somewhat less common than [4+2]-cycloadditions because they are allowed only when one component reacts in the antarafacial mode. Most examples of [2+2]-additions that are antarafacial in one component have been observed with ketenes. Ketenes are ideal antarafacial components in reactions of this type, since they offer a minimum of steric hindrance to antarafacial addition because one of the carbon atoms involved is s p-hybridized. This results in a substantial decrease in the degree of crowding in the transition state. 

However, compound II can be converted into toluene by [1,3] or [1,7] hydrogen shift. Under thermal conditions, both of these rearrangements are symmetry forbidden in the suprafacial mode and geometrically forbidden in the antarafacial mode. It should be noted that antarafacial [1,7] hydrogen shift can occur in flexible systems. But in rigid systems it also becomes geometrically forbidden. 

A bonding interaction can be maintained only in the antarafacial mode. The 1,3-suprafacial shift of hydrogen is therefore forbidden by orbital symmetry considerations. The allowed antarafacial process is symmetry-allowed, but it involves such a contorted geometry that this shift, too, would be expected to be energetically difficult. As a result, orbital symmetry considerations reveal that 1,3-shifts of hydrogen are unlikely processes. 

Thermal [2+2]-cycloaddition reactions are less common, but photochemical [2+2]-cycloaddition reactions are very common. This fact can be explained by analyzing these cycloaddition reactions using Woodward-Hofifmann selection rules. In frontier orbital approach, the thermal reaction of two ethene molecules (one is HOMO and other is LUMO) is orbital symmetry forbidden process for its suprafacial-suprafacial [7t s+7t s]-cycloaddition, but a suprafacial-antarafacial [jt s+jt a]-cycloaddilion reaction is symmetry allowed process (Fig. 3.1). It signifies that the cycloaddilion of one two-7t electron system with another two-ji electron system will be a thermally allowed process when one set of orbitals is reacting in a suprafacial mode and other set in an antarafacial mode ( s means suprafacial and a means antarafacial). Thermal [7t s+Ji a]-reactions usually occur in the additions of alkenes to ketenes, when alkene is in the ground state and ketene in the excited state [1] (Fig. 3.2). 

It is seen from the figure that in thermal reaction, a bonding interaction (in the same phase) can be maintained only in the antarafacial mode of shift. Therefore, thermal 1,3-suprafacial shift of hydrogen is forbidden from orbital symmetry considerations. The antarafacial shift is orbital symmetry allowed process and will be a concerted process. Photochemically, [l,3]-suprafacial shift of hydrogen is a symmetry allowed process because the bonding intemction takes place in the same phase of allyl group [1, 2]. 

Calculations at the PM3 or B3LYP/6-31G(d) SCF-MO levels of theory have revealed that pairs of electron-withdrawing substituents (R,R ) cause trapezoid geometric distortion during 2s -t- thermal cycloaddition-elimination, such that a fully synchronous transition state (36) becomes favoured for conversion of (35) to (37) the alternative Woodward-Hoffman antarafacial mode or stepwise biradical pathways need not be invoked. ... 

COD)Cu2Cl2 is the first intermediate. Irradiation of ct-COD gave mostly TCO along with some rr-COD. From experiments at different wavelengths they conclude, however, that cr-COD is the isomer which undergoes the cyclization to TCO. Since this latter can be formed from ct-COD in a concerted fashion only in the thermally allowed supra-antarafacial mode, distortion of the orbitals by the Cu-ion to make this reaction photochemically feasible was offered as an explanation. 

A bonding interaction can be maintained only in the antarafacial mode therefore, the 1,3-sigmatropic suprafacial hydrogen shift is considered forbidden. Since the geometry required for the orbital symmetry-allowed antarafacial shift is very contorted, this shift, too, is of high energy, and the concerted process in unlikely under conditions of thermal activation. 

Additions are less common than [4 + 2] cycloadditions because they are allowed only when one component reacts in the antarafacial mode. Most examples of [2 -f 2] additions that are antarafacial in one component have been observed with... 

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