Diversion Secondary Isotope Effects

The authors found just such an effect in the presumably concerted H- 2]-cycloaddition of allene to hexachlorocyclopentadiene, but not in its [ 2 H- 2]-cycloaddition to acrylonitrile. There, athough no isotope effect was observed on the rate of reaction, there was a substantial direct isotope effect ( h d) on product formation protium was preferentially incorporated in the ring. The authors therefore concluded that the reaction takes place in two steps. 

According to Fig. 6.7, the rate-limiting step is bonding C2 of the olefin to the central C atom of allene to form a biradical this is followed by competitive closure of Ci to either Cj or C3. On the face of it, the direction of the isotope ef- 

The latter is something of an overstatement, in view of the fact that the rates of addition of diphenylketene (DPK) to various olefins span a range of seven powers of ten [35]  

The initial orientation chosen is the favorable one for formation of the most stable biradical according to Postulate 3 of Section 6.3.1.1. 

A similar set of experiments was carried out by Baldwin and Kapecki [50] on the cycloaddition of diphenylketene (DPK) to styrene. 

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The discussion in this chapter has ranged well outside the main theme of the book. In addition to the writer s early involvement with secondary isotope effects, which can serve as partial extenuation, the mechanism of [2-f2]-cycloaddition has sufficient intrinsic interest to justify the digression. Orbital symmetry conservation plays but a small part in its mechanistic analysis, but it is a crucial one. Fig. 6.2 applies strictly only to the cyclodimerization of ethylene, or to an olefin symmetrically tetrasubstituted by substituents that do not add to the essential number of electrons involved in the reaction. Nevertheless, the principal conclusion drawn from it, that the initial plane-rectangular interaction of the two tt systems leads to formation of a bond between diagonally situated atoms, is remarkably robust. It can be applied to a variety of reactions with different electronic and steric requirements, provided that the specifics of each reacting system are kept firmly in mind. The wealth of diverse, superficially contradictory, experimental results cannot be fit into a consistent logical framework without it. 

This chapter constitutes an attempt to frame observed secondary isotope effects in one coherent scheme. A self-consistent interpretation of many diverse effects in terms of a relatively small number of factors can hardly be ad hoc it is, however, necessarily post hoc. The value of any mechanistic criterion residues in its predictive power. Therefore, only when an observed secondary isotope effect is able to call an accepted mechanism into question, and reinvestigation of the reaction by independent means then confirms that judgment, will it be possible to say that secondary isotope effects have come of age. 

Chondrites are the oldest and most primitive rocks in the solar system. They are hosts for interstellar grains that predate solar system formation. Most chondrites have experienced a complex history, which includes primary formation processes and secondary processes that inclnde thermal metamorphism and aqneons alteration. It is generally very difficult to distinguish between the effects of primary and secondary processes on the basis of isotope composition. Chondrites display a wide diversity of isotopic compositions including large variations in oxygen isotopes. 

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