Cycloreversions stereochemistry

Nitrilimines (621) are another class of 1,3-dipoles which provide a useful entry into the pyrazole ring. They are often generated by cycloreversion (79AG(E)72l) of tetrazoles, triazolopyridines and oxadiazolones (79JOC2957) (Scheme 55). Pyrazolines of known stereochemistry, pyrazoles and indazoles (Section 4.04.3.1.1(iii)) have all been prepared from nitrilimines. 

The reverse process is also useful in synthesis. The active components of the cycloreversion reaction are the two a bonds which will be broken and any systems to which both a bonds are allylic (i.e., at least one of m, n is an even number greater than 2). The stereochemistry of the reaction may also be specified in terms of the stereochemical mode of reaction of the active components of the reaction. 

Using the Woodward-Hoffmann rules, and realistic drawings of the transition structures, as in the preceding question, predict the stereochemistry of the double bonds produced in the following reactions, which involve stereospecific cycloreversions ... 

It has been suggested previously that the thermal cycloreversion of cyclohexene to ethylene plus buta-1,3-diene proceeds via a vinylcyclobutane intermediate and that, as a consequence, the stereochemistry of deuterium labels on the cyclohexene is not reflected in the deuterated ethenes obtained. This conclusion is supported by results of a study of the stereochemistry of thermal conversion of 1 -viny 1-2.3-r/.v-didcuteriocyclo-butane to butadiene and 1,2-dideuterioethylenes equal amounts of ( )-CHD=CHD and (Z)-CHD=CHD were formed.32... 

The [2 + 2] cycloreversion reaction proceeds stereospecifically with retention at carbon atoms, its rate obeys first-order kinetics and it depends on the stereochemistry and number of substituents at the ring carbon atoms. It is therefore concluded that the reaction mechanism is a concerted [2S + 2a] cycloreversion. 

However, only limited experimental evidence is available concerning the key step of the dimerization, i,e. the addition of the radical cation to the parent olefin. Does this addition occur stepwise or in concerted fashion Does the radical cation serve as a the diene component ([3 + 2]cycloaddition) or as dienophile ([4+ l]cy-cloaddition) The observed retention of dienophile stereochemistry and orbital symmetry arguments (Fig. 7) favor the [4 + l]cycloaddition type. Although it is difficult to distinguish the [3 + 2] from the [4 + l]addition type, a stepwise component for the cycloaddition and the complementary cycloreversion has been established in at least one system, viz., spiro[2.4]heptadiene. 

Cyclobutanes may be converted to alkenes thermally, the reverse of the [2 + 2] cycloaddition reaction. These retroaddition or cycloreversion reactions have important synthetic applications and offer further insights into the chemical behavior of the 1,4-diradical intermediates involved they may proceed to product alkenes or collapse to starting material with loss of stereochemistry. Both observations are readily accommodated by the diradical mechanism. Generation of 1,4-tetramethylene diradicals in other ways, such as from cyclic diazo precursors, results in formation of both alkenes and cyclobutanes, with stereochemical details consistent with kinetically competitive bond rotations before the diradical gives cyclobutanes or alkenes. From the tetraalkyl-substituted systems (5) and (6), cyclobutane products are formed with very high retention stereospecificity,while the diradicals generated from the azo precursors (7) and (8) lead to alkene and cyclobutane products with some loss of stereochemical definition. ... 

Intermolecular [4 + 2] cycloadditions exhibit strongly negative activation volumes and reaction volumes. High pressure, therefore, can be applied to accelerate Diels-Alder reactions and to shift the reaction equilibrium towards the cycloadducts. These effects are of particular advantage to (1) promote odierwise slow [4 + 2] cycloadditions involving heat or Lewis acid sensitive educts or products (2) suppress cycloreversion processes which are eidier thermodynamically favored or would interfere with a kinetically controlled stereochemistry. In view of a recent review (1985) only a few examples are presented here. 

Since a cycloreversion, the reverse of a cycloaddition, travels the same reaction path as the forward reaction, the considerations of stereochemistry and orbital symmetry that govern concerted cycloadditions are equally applicable to cycloreversions. The number of such reactions that have been studied in detail is not large, but there is sufficient information to establish that orbital-symmetry controls are indeed operating. The principles of orbital-symmetry conservation specify which processes can occur in concerted fashion and the stereochemical restrictions that are imposed by a concerted mechanism. We will first discuss some reactions that do occur by concerted mechanisms, and then turn to some of the elimination processes that involve discrete intermediates. 

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