Suprafacial, defined

Dihydrothiophene-1,1-dioxides (42) and 2,17-dihydrothiepin-1,1-dioxides (43) undergo analogous 1,4 and 1,6 eliminations, respectively (see also 17-38). These are concerted reactions and, as predicted by the orbital-symmetry rules (p. 1067), the former is a suprafacial process and the latter an antarafacial process. The rules also predict that elimination of SO2 from episulfones cannot take place by a concerted mechanism (except antarafacially, which is unlikely for such a small ring), and the evidence shows that this reaction occurs by a non-concerted pathway.The eliminations of SO2 from 42 and 43 are examples of cheletropic reactions, which are defined as reactions in which two a bonds that terminate at a single atom (in this case the sulfur atom) are made or broken in concert. 

The suprafacial shift along the carbon framework is not restricted to cyclic systems but may also prevail in acyclic cases. In the example given in Scheme 11, minimization of dipolar repulsion between the two C-0 bonds mandates a preferred conformation of the initial radical, leading to a stereo-chemically defined alkene radical cation and, ultimately, to a single diastere-omer of the product [119]. 

Due to the formal analogy to the classical Diels-Alder reaction, the mechanism of cyclic peroxide formation through cycloadditions of 1,3-dienes with O2 was considered for a long time to involve a concerted suprafacial [4 4- 2]-cycloaddition of a super-dienophile, namely a singlet oxygen to 1,3-dienic system In such a case, the concerted or almost concerted cycloaddition must be c -stereospecific and the stereochemical properties of the diene must be reflected in the three-dimensional structure of cyclic peroxide according to well-defined rules. Indeed, it was found in early stereochemical... 

In the first step, a cyclic intermediate is generated by a suprafacial addition, followed by a SN2-type ring opening (e.g., halogenation or epoxidation ring opening). In this manner, olefin 1 may either be converted into diastereomer 2 or 3, which may be optically active or racemic. Mechanism control thus means that the relative, but not the absolute, configuration of the two vicinal centers is defined. 

The only difference to the simple diastereoselection lies in the fact that the olefmic components now have two reactive sites each, namely the two olefmic sp1 centers, which are all converted into sp3, and hence potentially stereogenic centers. However, the configurations of these stereogenic centers are not independent, but mutually correlated by the principle of suprafacial addition (see Section 2.3.1). Thus, although a maximum of 24 stereoisomers could be formed, this number must be divided in half for each olefmic reactant with a defined (E)) Z) configuration. This results in only four stereoisomers in the form of two enantiomeric pairs of ejeo/enrfo-diastereomers. 

So far we have only defined what suprafacial and antarafacial mean on rc-systems (Fig. 2.7), but we need to see how o-bonds are treated by the Woodward-Hoffmann rules. Just as a suprafacial event on a rc-bond has overlap developing to the two upper lobes that contribute to the bond, so with o-bonds (Fig. 3.5a), overlap that develops to the two large lobes of the sp3... 

Another anomalous cycloaddition is the insertion of a carbene into an alkene. 6-Electron cheletropic reactions (p. 28) are straightforward allowed pericyclic reactions, which we can now classify with the drawings 3.47 for the suprafacial addition of sulfur dioxide to the diene 2.179 and its reverse. Similarly, we can draw 3.48 for the antarafacial addition of sulfur dioxide to the triene 2.180 and its reverse. The new feature here is that one of the orbitals is a lone pair, which is given the letter co to distinguish it from o- and n-bonds, with suprafacial and antarafacial defined by the drawings 3.45 and 3.46, which apply to all sp3 hybrids and p orbitals, filled or unfilled. 

In order to describe the ring opening of the aziridine 6.55, we need to define what suprafacial and antarafacial mean when applied to a p orbital. This is shown in Fig. 6.9, and applied there to the conrotatory aziridine opening. When both lines are drawn into the same lobe it is suprafacial, and when there is one line drawn into the top lobe and one into the bottom, it is antarafacial. Since this is neither a n nor a a orbital, it is given the Greek letter uj. The same designations apply whether the orbital is filled (on the left) or unfilled (on the right), and whether it is a p orbital or any of the sp" hybrids. 

Fig. 6.9 Suprafacial and antarafacial defined for a p orbital, and the allowed conrotatory interconversion of an aziridine with an azomethine ylid...

Table 4 shows examples for the sulfoxide rearrangement of different 2-cycloalkenyl sulfoxides 22"32-44-76 78, mostly obtained by oxidation of a stereochemically defined precursor (diastereomerically or enantiomerically pure, entries 3-82 t 2, 1076). In all cases complete chirality transfer was observed due to the suprafacial character of the rearrangement. 

The Diels-Alder reaction is one of the most powerful and efficient processes for formation of six-membered rings with the potential of controlling the relative and absolute stereochemistry at four newly created stereogenic centers [1]. Relative stereochemistry is usually well-defined because of the formation of a cyclic transition state arising from suprafacial-suprafacial interaction, with endo approach [2]. The reaction can be accelerated by Lewis acids, high pressure, or radical cations. Diels-Alder reactions catalyzed by Lewis acids are generally more regio- and stereoselective than their thermal counterparts [3]. 

Cycloaddition reactions are transformations involving the fusion of open-chain substrates to cyclic products. Woodward and Hoffmann have divided all concerted cycloaddition reactions into allowed and forbidden categories defined by a complete set of selection rules (5). We address ourselves here to the catalytic operations required of a transition metal to switch the forbidden transformations to allowed. Our attention, therefore, will be directed exclusively to the forbidden reactions. Forbidden-to-allowed catalysis will be discussed as it applies to the simplest, and perhaps most important cycloaddition, the concerted, suprafacial, 1,2-addition of two olefins. 

A generalization allows easy application of the Rules to any concerted pericyclic reaction involving two-electron bonds for the allowed reaction there must always be an odd number of suprafacial (as opposed to antarafacial) uses of bonds. Here, suprafacial is defined as utilizing both atoms of a C7 bond with retention of configuration or both with inversion with a Trbond, the p orbitals must be used from the same face of the bond. An antarafacial use of a cr bond would give retention at one atom and inversion at the other with a tt bond, the p orbitals must be used from opposite faces of the bond. 

We have already mentioned the binary nature of the pericyclic reaction rules. If we change the mode of interaction of one of the reactants, we will reverse the allowed / forbidden nature of the reaction. For example, if we change the interaction mode of one of the reaction partners in the cycloaddition from suprafacial to antarafacial, now the [4+2) cycloaddition is forbidden, and the [2 + 2] cycloaddition is actually allowed. The [2+2] cycloaddition is now designated as [ 2 + T 2a]. In Figure 15.10 A we define the [ 2 + 2j] reaction, and in Figure 15.10 B we show a realistic geometry that could achieve the necessary orbital interactions. The two TT systems approach in a perpendicular orientation, and the lines define the suprafacial and antarafacial interactions. 

Figure 15.9 defined suprafacial and antarafacial interactions of a lone pair in a p orbital (called an uj component). Using these definitions, predict if the ring-opening of the cyclopropyl anion shown below will occur in a conrotatory or disrotatory fashion. What will be the stereochemistry of the product ... 

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