Antarafacial attack

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. 

In a very small number of reactions, the twisted geometry of one of the components makes it possible for antarafacial attack to take place on one of the components. In these rare cases, since the opposite lobe of the antarafacial component is being used, the An+ 2 rule is broken, and An electrons becomes the favoured total. Heptafulvalene (2) has a twisted structure (Figure 7.9) and reacts antarafacially with tetracyanoethene to give the adduct 3 (reaction 7.4). 

These are rare. We have already met a 16-electron system in reaction (7.4), where the bicyclic geometry of the 14-electron system holds the ends of its n system close together and at an angle which facilitates antarafacial attack by the substituted ethene molecule. 

Figure 7.15 Impossibility of antarafacial attack in ethene dimerization...

According to the Woodward-Hoflfmann rules, concerted thermal [2+2] cycloadditions are symmetry-forbidden, but should proceed via supra-antarafacial attack of the reactants. [2+2] cycloadditions of ketenes and related reactive intermediates generated in situ proceed by a stepwise mechanism. " Photochemical [2+2] cycloadditions are symmetry-allowed. Asymmetric [2+2] cycloadditions leading to 4-membered heterocycles, e.g. Staudinger reactions or Patemo-BUchi reactions, have been extensively studied in the past. 

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 simple carbocation intermediate of Equation 10-1 does not account for formation of the antarafacial-addition product. The results with SN1 reactions (Section 8-6) and the atomic-orbital representation (see Section 6-4E) predict that the bonds to the positively charged carbon atom of a carbocation should lie in a plane. Therefore, in the second step of addition of bromine to cyclo-alkenes, bromide ion could attack either side of the planar positive carbon to give a mixture of cis- and trans-1,2-dibromocyclohexanes. Nonetheless, antarafacial addition occurs exclusively ... 

An SN2-type of attack of bromide ion, or other nucleophile, at carbon on the side opposite to the bridging group then results in formation of the antarafacial-addition product ... 

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). 

Chiral allylsilanes react antarafacially the incoming electrophile attacks the double bond on the surface opposite to the silyl group (Scheme 13) and the major piquets are syn, in this case even from the (Z)-allylsilane. The overall anti stereospecific reaction (anti to the extent of 90 10) in favor of product (32) having the substituents on the carbon chain syn to the extent of about 80 20 is still found with the (Z, )-silane (31) in an Se2 reaction (Scheme 14), even though the new stereocenters are five and six... 

Pericyclic reactions in which p-orbitals at the ends of the ir-component of each system overlap and form the new a-bonds on the opposite surface are called antarafacial cycloaddition. In actual practice, straightforward antar-afacial attack is rare because it is sterically difficult for one 7u-system to suffer this type of attack by another 7r-system, and needs at least one usually long and flexible unsaturated system. 

For many electrophiles, under ionic conditions (particularly when E has non-bonded electrons [Table 6.1, examples 6 and 8-13]), a so-called onium" ion (Figure 6.6, E = E ) is apparently formed. This ion is distinct from the initial n-complex since here a bond within the electrophile has broken and the onium ion bears a positive charge. However, as with the n-complex, the ion apparently has one face blocked by the electrophile and so subsequent attack by a nucleophile preferentially occurs from the opposite face. The net result is antarafacial (or anti- or trans-) addition across what was the carbon-carbon double bond. For example, consider the case of addition of bromine (Br2) (Table 6.1, example 8) in carbon tetrachloride (tetrachloromethane, CClt) solution to the isomers (Z)- and ( )-2-butene [(Z)- and ( )-CH3CH=CHCH3] (Scheme 6.16). 

The reaction mechanism and the stereochemical diversity of the addition of water to disilene has been studied at the MP2/6-311-h+G level. Two pathways are feasible leading to syn and anti-addition. The syn addition proceeds via nucleophilic attack by water oxygen with a barrier of ca. 12 kJ mol k anti-Addition proceeds via intramolecular electrophilic attack by water hydrogen in a weakly bound disilene/water complex with antarafacial approach, in accordance with the Woodward-Hoffmann rules, and leads to an activation barrier of ca. 22 kJ mol ... 

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