Suprafacial attack

Suprafacial attack of me ethene molecule on anotlier (left) is not permitted by the Woodward-Hoffmann id the alternative antarafacial mode of attack is sterically unfavourable. Suprafacial attack is however permitted Diels-Alder reaction between butadiene and ethene (right). 

The transition state in suprafacial attack is designated as of Huckel type in which no sign inversion of the cycle has taken place. The other type of migration involves one sign inversion. This is called mobius type inversion. The Huckel type of inversion occurs when the total number of electrons is 2, 6,. .., (4n + 2). This is also called aromatic transition state. In mobius type the participating electrons is 4, 8,. .. i.e. An. 

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 . ... 

All the other kinds of pericyclic cycloaddition discussed so for, not just Diels-Alder reactions, are also suprafacial on both components. Thus, 1,3-dipolar cycloadditions involve suprafacial attack on the dipolarophile, as in... 

Note that this substituent is absent in (71) and hence the radical mechanism would be discouraged in favor of a concerted reverse cycloaddition. The final product (69) could then arise by two-step cycloaddition of (76) to another mole of (67). The above suggested mechanism has the advantage of avoiding symmetry problems associated with the simpler mechanism (73—<-79— 69). If the latter mechanism is concerted according to the FMO theory, the overlapping lobes of the HOMO orbital of one reactant and the LUMO orbital of the other reactant should have the same phase. The coefficients of the HOMOs and LUMOSs of (67) and (78) reveal a mismatch for suprafacial attack and therefore the reaction is disallowed. Similarly, suprafacial extrusion of N2 from (73) is disallowed. 

The cycloadditions of (Ti -allyl)Fp complexes to alkenes proceed with high chemoselectivity. Only highly activated alkenes such as methylenemalonates, benzylidenemalononitrile, TCNE or I,2-di-cyano-4,5-dichloroquinone will participate in the reaction. With the less electrophilic a,p-unsaturated systems, cycloaddition can only be effected with Lewis acid activation. Thus the cycloadduct (6) is formed in reasonable yield (as a mixture of stereoisomers) from (3) and cyclohexenone using freshly sublimed AlBn (equation 4). The same reaction gives only a 8% yield with Aids. The exclusive formation of a ci.r-hydrindanone system can be explained by suprafacial attack of the enone on the allyl unit, which is then followed by another suprafacial ring closure of the dipolar intermediate, affording the thermodynamically preferi cyclic system (7a) and (7b). ... 

Two ethene molecules do not react thermally to give cyclobutane (reaction 7.2). This 4n system has the wrong number of electrons for suprafacial attack to take place, and geometric reasons make it impossible for overlap to take place from a p-orbital lobe away from the direction of approach to give an antarafacial component (Figure 7.15). 

All the examples so far in this chapter have been thermal reactions. Many reactions which do not go thermally, such as (7.2), can be made to take place photochemically. If the alkenes are disubstituted, the stereochemistry of the products is consistent with entirely suprafacial attack, as shown in reaction (7.18). 

The lanthanide catalyzed hetero Diels Alder reaction is a promissing route to many heterocycles where various stereoselectivities could be achieved by structural changes of the catalyst (57,58). The activation of the aldehyde by coordination can be envisaged as the key step of the process, which is followed by a suprafacial attack of the diene with an endo approach of the aldehyde hydrogen atom. 

In the case of concerted suprafacial attack, which is of our concern here, the assigning tables are extremely simple and reduce to identity. The transformation of the product molecular orbitals from the primed to unprimed basis thus consists simply in omitting the primes in eqs. (38). On the basis of this transformation, the overlap determinant can be straightforwardly calculated and its zero value (eq. (39)) suggests, in keeping with the prediction of Woodward-Hofftnaim rules, that the reaction should be forbidden. 

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. 

If, instead of electrocyclization, electrophilic attack of the closer upper edge of the phenyl group by the carbene carbon atom occurs, a zwitterionic intermediate might result, which upon 1,4-elimination of (COljW would yield a 1-methoxy-1,3-cyclopentadiene. Suprafacial hydrogen migration would finally lead to the formation of the observed diastereomer. 

The highly selective addition was rationalized by a chelation control as shown in equation 8. Tetracoordinate organolithium can form complexes 16a and 16b with the chiral oxazoline. The addition, which can be viewed as a suprafacial 1,5-sigmatropic rearrangement, proceeds from 16b where the C—Li bond is parallel to the 7T-orbital of the naphthalene ring. The attack of an electrophile on the azaenolate 17 formed by the... 

Attack by the carbene on a suitably placed aryl ring (Scheme 58152) gives an intermediate 164, which undergoes a thermally allowed suprafacial [l,5]-hydrogen shift to give the indene 165.151,152 The intramolecular nature of the hydride shift was established by lack of exchange with deuterium in the solvent.152... 

For example, the ene reaction proceeds stereoselectively in a suprafacial manner specifically, oxygen attack and hydrogen removal take place on the same side of the double bond. As a result, neither racemization of optically active com-pounds nor cis-trans isomerization occurs. Diradical or dipolar intermediates are not consistent with these observations. The lack of a Markovnikov-type directing effect also rules out dipolar intermediates.360 399 Isotope effects observed in the reaction of different deuterium-labeled 2,3-dimethyl-2-butenes indicated that... 

The preliminary rules, thermal and photochemical, given on p.16, need now to be qualified—they apply only to cycloadditions that are suprafacial on both components. Nevertheless, almost all pericyclic cycloadditions are suprafacial on both components. It is physically difficult for one conjugated system to suffer antarafacial attack from another, since it implies that one or another of the components can reach round from one surface to the other 2,85. Only if at least one of the components has a long conjugated system can it twist enough to make this even remotely reasonable. Straightforward antarafacial attack in cycloadditions is therefore very rare indeed. Keep in mind, however, that these rules only apply to pericyclic cycloadditions— there are other kinds of cycloaddition, in which the two bonds are formed one at a time, and to which none of these rules applies. 

All the other cycloadditions, such as the [4+2] cycloadditions of allyl cations and anions, and the [8+2] and [6+4] cycloadditions of longer conjugated systems, have also been found to be suprafacial on both components, wherever it has been possible to test them. Thus the trans phenyl groups on the cyclopentene 2.65 show that the two new bonds were formed suprafacially on the rrans-stilbene. The tricyclic adducts 2.61, 2.77, 2.79, and 2.83, and the tetracyclic adduct 2.82, show that both components in each case have reacted suprafacially, although only suprafacial reactions are possible in cases like these, since the products from antarafacial attack on either component would have been prohibitively strained. Nevertheless, the fact that they have undergone cycloaddition is important, for it is the failure of thermal [2+2], [4+4] and [6+6], and photochemical [4+2], [8+2] and [6+4] pericyclic cycloadditions to take place, even when all-suprafacial options are open to them, that is significant. 

The chemical properties of 2,3-dihydro-1,4-diazepines are dominated by the presence of the diene portion of the molecules, either by electrophilic attack at the 1, 4 or 6 positions, or nucleophilic attack at positions 5 or 7. There have previously been no observations of the involvement of the saturated part of the molecule. It has now been demonstrated, however, that in vacuum flash pyrolysis there is a suprafacial 1,5 hydrogen shift of a proton from position 7 to position 3 . This has been confirmed by deutero-labelling studies when 7-deutero-2,3-dihydro-methyl-1,4-diazepine is converted into the 3-deutero-isomer. When the study is repeated with the bicyclic t/.s-2,3-cyclohexano derivative the /ram-isomer is isolated, confirminf a suprafacial shift mechanism... 

Several reactions in organometaUic chemistry also appear to contravene the rule, but which can be explained in a somewhat similar way. Hydrometallation [5.45, see (Section 5.1.3.4) page 162], carbometallation, metallo-metallation, and olefin metathesis reactions are all stereospecifically suprafacial [2 + 2] additions to an alkene or alkyne, for which the all-suprafacial pathway is forbidden. Hydroboration, for example, begins with electrophilic attack by the boron atom, but it is not fully stepwise, because electron-donating substituents on the alkene do not speed up the reaction as much as they do when alkenes are attacked by electrophiles. Nevertheless, the reaction is stereospecifically syn—there must be some hydride delivery more or less concerted with the electrophilic attack. The empty p orbital on the boron is the electrophilic site and the s orbital of the hydrogen atom is the nucleophilic site. These orbitals are orthogonal, and so the addition 6.126 is not pericyclic. 

Similarly, the silyl enol ether of (+ )-dihydrocarvone gives exclusively the m-enantiomer with the sulfonamino moiety in the 3-position, the result of exclusive axial attack of the sclcno reagent and subsequent suprafacial rearrangement105. 

A 5-endo reaction takes place when 4-methyl-4-dcccnal is treated with tin(IV) chloride. After ring closure by nucleophilic attack of the olefin on the polarized carbonyl group, the resulting tertiary cation is stabilized by a sequence of two consecutive suprafacial [1,2] hydride shifts and ra . -2,3-dialkylated cyclopentanone 1 is formed stereoselectively2Q. 

When treating dimethyl-1,5-nonadicne with A -bromosuccinimide the bromonium ion attacks the nonhalogenated. electron-rich double bond. After Markovnikov ring closure the resulting cation is stabilized by a [1,2] suprafacial hydrogen shift, and dibromo-m-hydrindanone 1 is formed stereoselectively32. 

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