Suprafacial modes

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... 

Photochemically, the [l,3]-rearrangement is allowed in the sterically easily accessible suprafacial-suprafacial mode. A few of the many examples known are shown in Equations 12.80-12.83. 

The majority of known [m, -rearrangements are six-electron processes, thermally allowed in the suprafacial-suprafacial mode. They take a variety of forms, of which we shall consider only the most important.149... 

An even milder cycloelimination uses a ring of five atoms 6.28 instead of six, but still involves six electrons. This is no longer a retro-ene reaction, but it is still a retro group transfer and it is allowed in the all-suprafacial mode 6.29. The pyrolysis of N-oxides 6.30 is called the Cope elimination, and typically takes place at 120°, the corresponding elimination of sulfoxides 6.31 (X=S) typically takes place at 80°, and, even easier, the elimination of selenoxides takes place at room temperature or below. All these reactions are affected by functionality making these numbers only rough guides, but they are all reliably syn stereospecific. 

Then there is the problem of assessing whether the reaction is symmetry-allowed or not using the Woodward-Hoffmann rule. All reactions using (An + 2) electrons (an odd number of curly arrows) are allowed in the all-suprafacial mode, and so it is helpful to draw the dashed or solid lines (or better still use a line with a distinctive colour) to show the developing overlap with only suprafacial components. The (4q + 2)s components will then add up to an odd number, and the task is done. 

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). 

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). 

The metal s role in cycloaddition processes can be extended beyond [2 -)-2]. The symmetry of the atomic orbital used by the metal to donate or withdraw the electron pair will be dictated by the Woodward-Hoffmann symmetry rules b. As in the [2- -2] case, the metal is looked upon as a pseudo-organic participant. When an orbital reacting in a suprafacial way is called for by the symmetry rule, a metal symmetric orbital is used. Similarly, when an antarafacial participant is called for, a metal antisymmetric orbital is used. In the [2 -f-2 -f-2] cycloaddition process, for example, where the ligand participants must react along the suprafacial mode, the... 

Hydrogen is not the only group which can migrate, and there are therefore many other kinds of sigmatropic rearrangement. As usual, those involving a total of (4n + 2) electrons are allowed in the all-suprafacial mode, and these are the common reactions. Here are some examples.136-142... 

Since its discovery over sixty years ago [1] the Diels-Alder reaction has lost none of its attraction. [2, 3] It enables, in a one-step inter- or intramolecular reaction, the rapid preparation of cyclic compounds having a six-membered ring. During the course of the [4 -I- 2] cycloaddition four new stereocenters can be introduced directly, and their stereo-control is a topic of major interest in modem synthetic chemistry. [4-6] In addition, in intermolecular reactions, the relative positions of the reaction partners (regiochemistry) must be taken into account. If a concerted reaction is assumed, both a cis addition (suprafacial mode) and a preferred endo orientation (Alder rules) can be expected. But how can the absolute configuration of the desired product be controlled There are three basic possibilities the use of a chirally modified diene, a chirally modified dienophile, or a chiral catalyst. Although the first successes resulted from the attractive, hut difficult, catalytic route, [4b, 7] the majority of the investigators are concerned with the stoichiometric... 

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. 

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). 

Photochemical [2+2]-cycloaddition in suprafacial mode is symmetry allowed process and occurs in dimerization of alkenes, intermolecular additions of alkenes, and intramolecular cycloadditions of dienes and alkynes. The following examples are illustrative ... 

Now, we may consider the TS structure for a Diels-Alder cycloaddition reaction. The Huckel TS structure of zero node is aromatic in nature. Thus, the reaction takes place in suprafacial mode following the Huckel topology, whereas in Mobius... 

In thermal reaction, bonding interaction is maintained in the suprafacial mode of 1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is symmetry forbidden. The suprafacial shift also corresponds to a favorable six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type TS for suprafacial [l,3]-sigmatropic hydrogen shift is antiaromatic and is a forbidden process (Fig. 4.2) [1, 2]. Photochemically, [l,5]-hydrogen shift in the suprafacial mode is a symmetry forbidden process, but antarafacial shift is a symmetry allowed process (Fig. 4.3). 

Cycloaddition processes can be described by a symbolism which describes the type and number of electrons involved in the reaction and the topology of the reaction. Thus a Diels-Alder reaction is a [477 + 277 ] process, signifying addition of a four-77-electron and a two-77-electron system, with both sets of orbitals reacting in a suprafacial mode. The allowed 2 + 2 addition would be described as 2iTs + 277 ]. 

But, (1, 4]-sigmatropic shifts are sterically prohibited. Thermal (1, 4]-shifts are thermally allowed through suprafacial mode of migration this process involves inversion at the migrating centre. 

Sigmatropic shift in contrary proceeds with retention of configuration through suprafacial mode. PMO-analysis also reveals that [1, 4]-shift with inversion would be thermally allowed since transition state here is with four electrons, one node and is aromatic. 

---