Suprafacial interactions

In the transition state a boat like structure appears and there will be a suprafacial cis addition to the termini of the n bond. The ene reaction does not have a symmetrical transition state and it is a thermally allowed concerted reaction. Its transition state involves a suprafacial interaction of six electrons (4 from the k bonds and two from the o bond) So it is a Huckel system and transition state is aromatic. In the terminatlogy of Woodward and Hoffmann it can be regarded as o2s + n2s + 7t2s reaction. 

Recall from the previous chapter (Section 11.3, p. 581) that ground-state allowed electrocyclic processes are disrotatory (suprafacial, interaction diagram 33) for 4tz + 2 and conrotatory (antarafacial, with one phase inversion, inter-... 

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]. 

Additional experiments were performed with the corresponding lithi-um/TMEDA complexes (S)- and (R)-255. Most of the reactions take the same sense of stereospecificity, independent from the ligands at lithiiun. An exception is the triisopropoxytitanation, it proceeds with retention of configuration with the TMEDA complexes whereas the sparteine complexes react with inversion [ 169,168]. Similar to results discussed in the benzyl section (Sect. 3), the interaction of the isopropoxy residue with the hthium cation may determine the reaction course. It seems that in the presence of the bulky (-)-sparteine as a Hgand,such a suprafacial interaction does not contribute significantly. For the metal exchange with tris(diethylamino)titanium chloride inversion was observed, too [169]. 

For TT systems and lone pairs the distinction is simple suprafadal interactions involve the same face of the system, while antarafacial interactions are on opposite faces. Although we have only examined tt systems thus far, we will examine pericyclic reactions below that involve a bonds. Therefore, we need a similar definition for these kinds of bonds. For a bonds, the distinction is less obvious, but is consistent with the other systems. With suprafa-cial interactions, the two loops are drawn to either the inner lobes or outer lobes, while with an antarafacial interaction one loop is to an inner and one is to an outer lobe (see Figure 15.9 for this to make sense). Note that a suprafacial interaction at a cr bond involving two sp hy-... 

Thermal )6-elimination reactions of acetates, benzoates, xanthates, sulfoxides, selenoxides, and N-oxides are also group transfer reactions. All these elimination reactions are yn-stereospecific and proceed through a cyclic six membered—or five membered—ring transition state of 6e process by intramolecular transfer of hydrogen atom, where all the participating orbitals have suprafacial interactions. These reactions are fundamentally retro-group transfer reactions. 

The conrotatory motion involves an antarafacial interaction between the termini, and disrotation involves a suprafacial interaction between these centres. There are two distinct possibilities for each mode of rotation, and therefore four possible products in aU. In many cases, however, the inherent mmetry of the system may not allow such distinction to be made. Even in cases where the two forms of one mode are distinguishable, it does not necessarily follow that both will occur the geometry of the system and the steric factors can be decisive. For example, the conrotatory ting-opening of trans-3,4-dimethylcyclobut-l-ene should, in principle, yield a 1 1 mixture of trans, /nms-hexa-2,4-diene and as, cis-hexa-2,4-diene (Equation 3.17). However, the inward conrotation of the two methyl groups is unfavourable because of the rapid increase in steric compression between these two substituents, and the 78... 

Greek letter w for this purpose. Such to orbitals can interact in a suprafacial or antarafacial sense, as shown in Fig. 3.17. Thus a suprafacial interaction on the occupied -orbital of Xyz is designated 2 (where 2 indicates the presence of two electrons), whereas an antarafacial interaction on the unoccupied p-orbital is denoted by On this basis the electrocyclic conversion of the cyclopropyl cation into the allyl cation (Equation 3.19) is defined as a [ 2g + ] process if it occurs by the disrotatory mode indi-... 

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... 

The transition state for such processes is represented as two interacting allyl fragments. When the process is suprafacial in both groups, an aromatic transition state results, and the process is thermally allowed. Usually, a chairlike transition state is involved, but a boatlike conformation is also possible. 

For a successful cycloaddition to take place, the terminal tt lobes of the two reactants must have the correct symmetry for bonding to occur. This can happen in either of two ways, called supra facial and antara facial. Suprafacial cycloaddjtions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on the same face of the other reactant. Antarafacial cycloadditions take place when a bonding interaction occurs between lobes on the same face of one reactant and lobes on opposite faces of the other reactant (Figure 30.8). 

How can we predict whether a given cycloaddition reaction will occur with suprafacial or with antarafacial geometry According to frontier orbital theory, a cycloaddition reaction takes place when a bonding interaction occurs between the HOMO of one reactant and the LUMO of the other. An intuitive explanation of this rule is to imagine that one reactant donates electrons to the other. As with elec-trocyclic reactions, it s the electrons in the HOMO of the first reactant that are least tightly held and most likely to be donated. But when the second reactant accepts those electrons, they must go into a vacant, unoccupied orbital—the LUMO. 

Fora [4 + 2 -7r-electron cycloaddition (Diels-Aldei reaction), let s arbitrarily select the diene LUMO and the alkene HOMO. The symmetries of the two ground-slate orbitals are such that bonding of the terminal lobes can occur with suprafacial geometry (Figure 30.9), so the Diels-Alder reaction takes place readily under thermal conditions. Note that, as with electrocyclic reactions, we need be concerned only with the terminal lobes. For purposes of prediction, interactions among the interior lobes need not be considered. 

Figure 30.9 Interaction of diene LUMO and alkene HOMO in a suprafacial 14 - 2] cycloaddition reaction (Diels-Alder reaction).

In contrast with the thermal process, photochemical [2 + 2] cycloadditions me observed. Irradiation of an alkene with UV light excites an electron from i /, the ground-slate HOMO, to which becomes the excited-slate HOMO. Interaction between the excited-state HOMO of one alkene and the LUMO of the second alkene allows a photochemical [2 + 2j cycloaddition reaction to occur by a suprafacial pathway (Figure 30.10b). 

Figure 30.10 (a) Interaction of a ground-state HOMO and a ground-state LUMO in a potential [2 - 2] cycloaddition does not occur thermally because the antarafacial geometry is too strained, (b) Interaction of an excited-state HOMO and a ground-state LUMO in a photochemical [2 r 2] cycloaddition reaction is less strained, however, and occurs with suprafacial geometry. 

Cycloaddition reactions are those in which two unsaturated molecules add together to yield a cyclic product. For example, Diels-AJder reaction between a diene (four tt electrons) and a dienophile (two tt electrons) yields a cyclohexene. Cycloadditions can take place either by suprafacial or antarafacial pathways. Suprafacial cycloaddition involves interaction between lobes on the same face of one component and on the same face of the second component. Antarafacial cycloaddition involves interaction between lobes on the same face of one component ancl on opposite faces of the other component. The reaction course in a specific case can be found by looking at the symmetry of the HOMO of one component and the lowest unoccupied molecular orbital (LUMO) of the other component. 

According to frontier molecular orbital theory (FMO), the reactivity, regio-chemistry and stereochemistry of the Diels-Alder reaction are controlled by the suprafacial in phase interaction of the highest occupied molecular orbital (HOMO) of one component and the lowest unoccupied molecular orbital (LUMO) of the other. [17e, 41-43, 64] These orbitals are the closest in energy Scheme 1.14 illustrates the two dominant orbital interactions of a symmetry-allowed Diels-Alder cycloaddition. 

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.

The prediction that the suprafacial path is forbidden, and the antarafacial one allowed (8) stimulated many experiments. In particular, the thermal rearrangements of the molecules shown in Fig. 17 a have been studied in detail (26) here the constraints due to molecular architecture do not allow antarafacial paths, so that stereochemical mutations must take place to preserve orbital symmetry (Fig. 17b)- These mutations can also be controlled by the bulk of the substituents R and R, so that steric and symmetry factors interact in a most interesting way. 

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. 

The predominant interactions are all second-order. Interaction K( t ) -+0(ti ) stabilizes an all-suprafacial concerted cycloaddition,... 

The question of the identity of the reactive excited state was left open, although n-n excited state was considered to be more probable.140 The interaction diagram, Fig. 7, shows that a n-n state would have an additional stabilizing interaction Ri(tt) ->-R2(7r) with orbital coefficients in phase for an all-suprafacial concerted reaction. The dominant reaction would theoretically depend upon the relative placements of the several levels, and since no experimental information for maleic and fumarate esters is presently available, a clear choice cannot be made. It is interesting that the reactive excited state could be inferred if both stereochemistry and a good molecular diagram were available. 

From examination of Fig. 11, it is inferred that the zn-n state is less reactive, and a biradical mechanism should be the major reaction pathway. The degenerate stabilizing perturbation of the bonding levels is missing, and concerted pathways are not likely if stabilized only by much smaller secondary interactions. If the hi-n singlet state could be intercepted in some way, the all-suprafacial concerted mechanism would be favored [K(ji ) -0(jr )] relative to the suprafacial-antarafacial mechanism [O(tt) - -K( i )]. 

The above interaction is suprafacial with respect to one component and antarafacial with respect to the other and is therefore a k2s + n2a] process. A bonding interaction could occur between two pairs of lobes of the same sign, but the nearer alkene molecule would need to twist about its original n bond, and so it will make it geometrically inaccessible. 

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. 

Various geometries are possible for the transition state and they can be classified on whether each of the allyl systems interacts with lobes of the other system on the same side (suprafacially) or on opposite sides (antarafacially). Three transition states have been given. All have been classed on Huckels system, on the basis of aromatic transition state approach and so all are thermally allowed. The following picture gives the allowed transition state for thermal [3, 3] shifts. 

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 authors assume that the halides react with stereoinversion, whereas the tosylates prefer suprafacial attack due to binding interaction with the lithium ion in the transition state . A comparable dependence of the stereochemical course from the leaving group has been observed in other stereodefined benzyllithiums, too . The addition of 268 to A-methyl-benzylideneimine proceeds with only low yield . ... 

Cycloadditions of ketenes and alkenes have been shown to have synthetic utility for the preparation of cyclobutanones.101 The stereoselectivity of ketene-alkene cycloaddition can be analyzed in terms of the Woodward-Hoffmann rules.102 To be an allowed process, the [2n + 2n] cycloaddition must be suprafacial in one component and antarafacial in the other. An alternative description of the transition state is a [2ns + (2ns + 2ns)] addition.103 Figure 6.6 illustrates these transition states. The ketene, utilizing its low-lying LUMO, is the antarafacial component and interacts with the HOMO of the alkene. The stereoselectivity of ketene cycloadditions can be rationalized in terms of steric effects in this transition state. Minimization of interaction between the substituents R and R leads to a cyclobutanone in which these substituents are cis. This is the... 

Why are [4 + 2] and [2 + 2] cycloadditions different Simple molecular orbital theory provides an elegant explanation of this difference based on the An + 2 rule described in Section 21-9. To understand this, we need to look in more detail at how the p orbitals of the double bonds interact in concerted addition mechanisms by suprafacial overlap, as in 36 and 37 ... 

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