Suprafacial addition mode

Formation of these products is rationalized in terms of suprafacial addition of the elements of Pd—Cl to a ring that is opening stereospecifically in a disrotatory mode, with the breaking bond bending away from the metal (dis-out) (equation 326). The absence of rj to rj to rj interconversions between the isomeric complexes is rationalized in terms of a severe steric blocking of the anti (to metal) allylic face of the complex by the nine-membered ring. This is evident from X-ray crystal structure analysis of the mononuclear acetylaceto-nate allylic complex obtained from chloropalladation of the cw-bicyclononane ". ... 

Woodward and Hoffman described addition modes based on the facial approach of the reactive termini as suprafacial and antarafacial. "A suprafacial process is one in which bonds made or broken lie on the same face of the system undergoing reaction", as in 11. 2 antarafacial process has the "newly formed or broken bonds on opposite sides of the reaction system," as in 12. 2 Woodward and Hoffman then described several... 

As both the n-systems are involved in the cyclo addition, it is essential to specify the modes with respect to each of them. Specification is made by placing subscript S or a after the number refering to n-component. For instance, suprafacial addition with respect to each component of Diels-Alder reaction is specified as n] +n J cycloaddition. Another way is simply writting it as 2s+4s cycloaddition. 

The two modes of addition and the associated stereochemistry resemble other addition reactions we studied earlier. The suprafacial addition is a concerted syn addition to one of the ti systems. The antarafacial addition corresponds to a concerted anti addition. Although anti addition reactions are common in the chemistry of alkenes, the two groups that add to the alkene are not bonded to each other in the transition state. In cycloaddition reactions, both atoms of the molecule that bond to the terminal atoms of the second molecule are also connected to each other. Thus, only if the number of atoms in each of the two molecules is quite large can one molecule add to the other in an antarafacial process. 

Woodward and Hoffmann refer to this geometry of approach as the nonlineary to distinguish it from the alternative, more symmetrical linear approach (5), in which the sp orbital enters suprafacially. Although Hoffmann has reported calculations by the extended Hiickel method that are in agreement with the mode of addition 4 predicted by the pericyclic theory,6 there is little experimental information. 

Cycloadditions, which should be allowed in the suprafacial-supra-facial mode, are known. An example is the addition of tropone to cyclopentadiene... 

There is another aspect of cycloaddition TS structure that must be considered. It is conceivable that some systems might react through an arrangement with Mobius rather than Hiickel topology (see p. 716). Mobius systems can also be achieved by addition to opposite faces of the tt system. This mode of addition is called antarafacial and the face-to-face addition is called suprafacial. In order to specify the topology of cycloaddition reactions, subscripts s and aare added to the numerical classification. For systems of Mobius topology, as for aromaticity, 4n combinations are favored and 4n- -2 combinations are unfavorable... 

There are two modes of orbital overlap for the simultaneous formation of two cr bonds—suprafacial and antarafacial. Bond formation is suprafacial if both cr bonds form on the same side of the tt system. Bond formation is antarafacial if the two cr bonds form on opposite sides of the tt system. Suprafacial bond formation is similar to syn addition, whereas antarafacial bond formation resembles anti addition (Section 5.19). 

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... 

As cycloadditions occur between two Ti-systems, each can undergo either suprafacial (s) or antarafacial (a) addition to give four different possibilities, s+s,a+a,s+a and a+s. The Woodward-Hoffmann rules for cycloadditions distinguish between these four modes according to the numbers of electrons involved. 

As is the case for other pericyclic reactions, the selection rules for a thermal [i, ] sigmatropic reaction are reversed for the photochemical reaction. If irradiation of a 1,5-hexadiene produces the electronically excited state of one and only one of the two allyl components, then the HOMO of one component is (/f3, and the HOMO of ihe other component is suprafacial-suprafacial reaction (Figure 11.46) is forbidden (as is the antarafacial-antar-afacial pathway), but the antarafacial-suprafacial and suprafacial-antarafacial pathways are allowed (Figure 11.47). Analysis of higher sigmatropic reactions shows that the selection rules also reverse with the addition of a carbon-carbon double bond to either of the n systems. Thus, the [3,5] sigmatropic reaction is thermally allowed to be suprafacial-antarafacial or antarafacial-suprafacial and photochemically allowed to be suprafacial- suprafacial or antarafacial-antarafacial. Two of these reaction modes are illustrated in Figure 11.48. 

Since in a typical cycloaddition reaction there is addition of two unsaturated systems, it is logical to expect the addition to occur on the same or the opposite faces of the system involved. Furthermore, as both the Tu-systems are undergoing addition, it is necessary to specify these modes of addition on each of them especially to discuss and understand the stereochemistry of the product. These different modes have been termed as suprafacial (on the same side) and antarafacial (on the opposite side) and are shown in Figure 4.1. 

The ene reaction can also be analyzed by using PMO method. As shown in Figure 6.10, in the transition state of ene reaction, addition to the double bond takes place from the same side (suprafacial), therefore, a supra—supra mode of addition leads to a Htickel array that is aromatic with (4n + 2) TT-electrons. Hence, the reaction is thermally allowed and photochemically 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 ... 

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 ]. 

Suprafacial and Antarafacial Processes These are two stereochemical modes of cycloaddition. Here two systems containing double bonds add up, therefore it is logical to expect that addition occurs either at the same side of opposite side of the system. As both the Jt-systems are undergoing addition, it is... 

We have seen that the stereochemically distinguisliable modes of addition on a single olefin component are but two - suprafacial and antarafacial. For a two component +, rp] cyclo-addition the maximum number of distinguishable modes is therefore 2, for a three component addition it is 2, ... 

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