Stereochemistry can indicate neighbouring group participation

How do we know that neighbouring group participation is taking place? Well, the first bit of evidence is the increase in rate. The neighbouring groups become involved only if they can increase the rate of the substitution reaction—otherwise the mechanism will just follow the ordinary SN₂ pathway. But more important information comes from reactions where stereo chemistry is involved, and one of these is the last of the four examples at the start of the chapter. Here it is again in more detail. Not only does the fi rst of these reactions go faster than the second—its stereochemical course is different too.

Although one starting material has syn and the other anti stereochemistry, the products have the same (anti) stereochemistry one substitution goes with retention and one goes with inversion. Again, neighbouring group participation is the reason. To explain this, we should first draw the six-membered rings in their real conformation. For the anti compound, both substituents can be equatorial. However, not much can happen in this conformation—but, if we allow the ring to flip, you can see immediately that the acetate substituent is ideally placed to participate in the departure of the tosylate group.

What results is an entirely symmetrical intermediate—the positive charge on one of the oxygens is, of course, delocalized over both of them. The intramolecular SN₂ reaction takes place with inversion, as required by the orbitals, so now the junction of the two rings is cis. The next step is attack of acetic acid on the intermediate. This is another SN2 reaction, which also proceeds with inversion and gives back a trans product.

Overall, we have retention of stereochemistry. As you know, SN2 reactions go with inversion and SN1 reactions with loss of stereochemical information, so this result is possible only if we have two sequential SN2 reactions taking place—in other words neighbouring group participation. Why, then, does the other diastereoisomer react with inversion of stereochemistry? Well, try drawing the mechanism for intramolecular displacement of the tosyl group. Whether you put the tosylate or the acetate group equatorial doesn’t matter; there is no way in which the acetate oxygen’s lone pair can reach the σ* orbital of the tosylate C–O bond.

Neighbouring group participation is impossible, and substitution goes simply by inter molecular displacement of OTs by AcOH. Just one SN2 step means overall inversion of configuration, and no participation means a slower reaction.

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مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct