Suprafacial addition

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

When the orbitals have been classified with respect to symmetry, they can be arranged according to energy and the correlation lines can be drawn as in Fig. 11.10. From the orbital correlation diagram, it can be concluded that the thermal concerted cycloadditon reaction between butadiene and ethylene is allowed. All bonding levels of the reactants correlate with product ground-state orbitals. Extension of orbital correlation analysis to cycloaddition reactions involving other numbers of n electrons leads to the conclusion that the suprafacial-suprafacial addition is allowed for systems with 4n + 2 n electrons but forbidden for systems with 4n 7t electrons. 

Fig. 6.13. HOMO-LUMO interactions in the [2 + 2] cycloadditions of an alkene and a ketene (a) frontier orbitals of the alkene and ketene (b) [2tts + 2ttJ representation of suprafacial addition to the alkene and antarafacial addition to the ketene (c) [2tts + (2tts + 2tts)] alignment of orbitals.

Numerous examples of intermolecular and intramolecular photocycloaddition to heterocyclic systems (including the dimerization of individual heterocycles) have now been reported. Two types of cycloaddition can readily be effected photochemically, namely, [n2 + 2] and [ 4 + 4] additions. Although concerted suprafacial additions of this type are allowed photochemical processes, in reality many cycloadditions occur via diradicals, zwitterions or exciplexes. 

Restricted Hartree-Fock calculations of the suprafacial addition of ethylene to l,4-dithioniabicyclo[2.2.0]hexane with the 6-31G basis failed to locate the transition state for this process. Synchronous suprafacial addition should... 

Thus we find that the reaction is a syn (suprafacial) addition with respect to both the diene and dienophile. The frontier orbitals involved shows that the reaction occurs by interaction of HOMO and LUMO. So there is no possibility of substituents to change their position. Substituents which are on the same side of the diene or dienophile will be cis on the newly formed ring as is seen between the reaction of dimethyl maleate (a cis dienophile) with 1,3 butadiene. The product formed is cis 4,5 dicarbomethoxy cyclohexane. 

The molecule of S02 must lie in a plane which bisects the suprafacial cis conformation of the diene. This attack of S02 will be from below, so that a concerted suprafacial, suprafacial addition is allowed. Such reactions are called linear cheletropic processes. 

The concerted suprafacial, suprafacial addition to an alkene as shown above is a disallowed process. So in linear cheletropic process the interaction is suprafacial for both orbitals of the diene and also for HOMO of S02, but it is antarafacial for the LUMO of S02. 

The procedure described here serves to illustrate a general [3+2] annulation method for the synthesis of cyclopentane derivatives. A unique feature of this one-step annulation is its capacity to generate regio-specifically five-raembered rings substituted at each position, functionally equipped for further synthetic elaboration. As formulated in the following equation, the reaction proceeds with remarkably high stereoselectivity via the effective suprafacial addition of the three-carbon allene component to an electron-deficient olefin ("allenophile"). ... 

Some representative examples of the [3+2] annulatlon are listed in Table 1. Both cyclic and acyclic allenophiles participate in the reaction, a-Alkylidene ketones undergo annulation to provide access to spiro-fused systems, and acetylenic allenophiles react to form cyclopentadiene derivatives. The reactions of (E)- and (Z)-3-methy1-3-penten-2-one illustrate the stereochemical course of the annulation, which proceeds with a strong preference for the suprafacial addition of the allene to the two-carbon allenophile. The high stereoselectivity displayed by the reaction permits the stereocontrol led synthesis of a variety of mono- and polycyclic systems. 

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. 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, these may be lowered by polar effects (74AF(3)75i). [2 + 2]... 

The mechanism of Figure 10-7 cannot be correct for bromine addition to alkenes in solution for two important reasons. First, notice that this mechanism requires that the two C-Br bonds be formed on the same side of the double bond, and hence produce suprafacial addition. However, there is much evidence to show that bromine and many other reagents add to alkenes to form antarafacial addition products (Figure 10-8). 

Diimide is generated in situ by oxidation of H2NNH2 suprafacial addition occurs (Section 11-5). 

This is an alternative scheme for the hydroxylation of alkenes (see Section 11-7C). However, the overall stereochemistry is opposite to that in permanganate hydroxylation. For instance, cyclopentene gives trans-1,2-cyclopen-tanediol. First the oxirane forms by suprafacial addition and then undergoes ring opening to give the trans product ... 

In the hydrogenation of 1,2-dimethylcyclohexene over a platinum catalyst, the suprafacial addition product is formed. Assuming that the mechanism of this hydrogenation is as shown in Figure 11-2, what conditions must be put on the stereochemistry of each of the postulated steps in order that the overall reaction be suprafacial ... 

Further evidence of stereospecificity in [4 + 2] additions is that the configurations of the diene and the dienophile are retained in the adduct. This means that the reactants (or addends) come together to give suprafacial addition. Two examples follow, which are drawn to emphasize how suprafacial... 

In practice, the adduct with the endo2 configuration usually is the major product. As a general rule, Diels-Alder additions tend to proceed to favor that orientation that corresponds to having the diene double bonds and the unsaturated substituents of the dienophile closest to one another. This means that addition by Equation 13-3 is more favorable than by Equation 13-4, but the degree of endo-exo stereospecificity is not as high as the degree of stereospecificity of suprafacial addition to the diene and dienophile. 

Carbenes are much more reactive toward carbon-carbon double bonds than toward single bonds. Without doubt the most useful feature of a elimination is that it provides a practical route to cyclopropanes and cyclopropenes by [2 + 1 ] cycloaddition of carbenes to double or triple bonds. These additions are stereospecific suprafacial additions if they involve singlet carbenes, but can give mixtures with triplet carbenes ... 

The most important method of preparation involves oxidation, or epoxidation, of an alkene with a peroxycarboxylic acid, RC03H. This reaction achieves suprafacial addition of oxygen across the double bond, and is a type of electrophilic addition to alkenes (see Exercise 15-53) ... 

Write a mechanism for the reaction of frans-2-butene with trifluoroperoxo-ethanoic acid to give trans-2,3-dimethyloxacyclopropane that is consistent with the fact that the reaction is first order in each participant and gives suprafacial addition. 

This reaction has been described previously (Section 13-3 A) and is an example of a [4 + 2] cycloaddition. Such reactions occur thermally (by simply heating the reactants) and appear to be entirely concerted. By this we mean that the reactants are converted to products in one step, without involving the formation of reaction intermediates. The principal evidence for the concertedness of [ 4 + 2] cycloadditions is the fact that they are highly stereospecific and involve suprafacial addition of both components. The configuration of substituents in the diene and the dienophile is retained in the,adduct ... 

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. 

The [2+2] Mechanism Already in 1977 Sharpless proposed a stepwise [2+2] mechanism for the osmylation of olefins in analogy to related oxidative processes with d°-metals such as alkene oxidations with CrO,Cl2 [23, 24], Metallaoxetanes [25] were suggested to be formed by suprafacial addition of the oxygens to the olefinic double bond. In the case of osmylation the intermediate osmaoxetane would be derived from an olefm-osmium(VIII) complex that subsequently would rearrange to the stable osmium(VI) ester. 

In the asymmetric aminohydroxylation (AA) an olefin is converted into a vicinal amino alcohol by means of an osmium(VIII)-mediated suprafacial addition of a nitrogen and an oxygen atom to the double bond. Like the AD, the AA has been developed by modifying an originally stoichiometric, achiral version. Although the first aminohydroxylations were reported in 1976 [70], the asymmetric catalytic protocol is still underdevelopment [71]. 

The Woodward-Hoffmann rules predict high activation energies for the suprafacial-suprafacial addition of two carbon-carbon double bonds, which can be lowered, however, by polar effects. [2 + 2] Photocycloadditions are common and usually involve diradical intermediates e.g., photoexcited ketones react with a variety of unsaturated systems (Scheme 1). Both the singlet and triplet (n, 7t ) excited states of the ketones will form oxetanes with electron-rich alkenes. With electron-deficient alkenes only the singlet states give oxetanes. Diradicals are the immediate precursors to the oxetanes in all cases, but the diradicals are formed by different mechanisms, depending on the availability of electrons in the two components. 

An anomalous cycloaddition is the insertion of a carbene into an alkene. Some cheletropic reactions are straightforwardly allowed pericyclic reactions, which we can illustrate with the drawing 6.127 for the suprafacial addition of sulfur dioxide to a diene, and with the drawing 6.128 for the 8-electron antarafacial addition of sulfur dioxide to a triene. The problem comes with the insertion of a carbene into a double bond, which is well known to be stereospecifically suprafacial on the alkene with singlet electrophilic carbenes [see (Section 4.6.2) page 149]. This is clearly a forbidden pericyclic reaction if it takes place in the sense 6.129. 

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 ". ... 

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