Cycloadditions chloroprene

In the thermal dimerization of chloroprene 1 (Table 4, entry 1) the activation volumes for two [4 + 2] cycloadditions leading to 2 and 3 were found to be smaller (more negative) than those of the third [4 + 2] and the [2 + 2] cycloadditions leading to 4, 5 and 6, respectively. Stewart50 explained these results in terms of concerted Diels-Alder... 

Chloroprene undergoes three different [4+2]-cycloadditions with itself, proceeding as parallel reactions. One of these [4+2]-cycloadditions does not occur in a stereoselective fashion with respect to the dienophile. These cycloadditions are dimerizations that yield compounds A-C in Figure 15.16. Chloroprene plays two roles in these [4+2]-cycloadditions it serves as diene and also as dienophile. In addition, small amounts of chloroprene dimerize (in a multi-step process ) to give a [2+2]-cycloadduct D and to give a [4+4]-cycloadduct E (Figure 15.16). 

Fig. 15.16. Thermal cycloadditions of chloroprene (2-chlorobutadiene) I product distribution.

Fig. 15.17. Thermal cycloadditions of chloroprene II -evidence of and explanation for the two-step formation of a [4+2]-adduct.

Chloroprene undergoes three different [4+2]-cycloadditions with itself, proceeding as parallel reactions. One of these [4+2]-cycloadditions does not occur in a stereoselective fashion with respect to the dienophile. These cycloadditions are dimerizations... 

The dimerization of chloroprene leading to the [4+2]-cycloadduct C (Figure 12.16) definitely is a multistep process. This has been demonstrated by analysis of the stereochemistry of a [4+2]-cycloaddition that led to the dideutero analogs of this cycloadduct (Figure 12.17). Instead of chloroprene, a monodeuterated chloroprene (trans-[D]-chloroprene) was dimerized. This monodeuterated chloroprene of course also... 

In fact, the cycloaddition of butadiene to ethylene, as well as cycloadditions of similar non-polar dienes to non-polar alkenes seem experimentally to be cases where concerted and stepwise (biradical or biradicaloid) mechanisms compete. We have recently discussed a number of cases, such as the dimerization of butadiene, piperylene, and chloroprene, the cycloadditions of butadiene or methylated dienes to halogenated alkenes, and others, where non-stereospecificity and competitive formation of [2 + 2] adducts indicate that mechanisms involving diradical intermediates compete with concerted mechanisms10). Alternatively, one could claim, with Firestone, that these reactions, both [4 + 2] and [2 + 2], involve diradical intermediates1 In our opinion, it is possible to believe that a concerted component can coexist with the diradical one , and that both mechanisms can occur in the very same vessel 1 ). Bartlett s experiments on diene-haloalkene cycloadditions have also been interpreted in this way12). 

Levchenko and Balon" have examined the regiochemistry of cycloaddition of several symmetrical bis(aryl)sulfonyl sulfur diimides with ( )-piperylene and isoprene which afforded the C-3 and C-5 substituted adducts, respectively (Scheme l-III). These results were confirmed by Wucherpfennig and Kresze, who also found that chloroprene gave the 5-substituted product. ... 

In the thermal dimerization of chloroprene (1), the activation volumes of two [4 + 2] cycloadditions leading to 2 and 3 were found to be smaller than those of the third [4 + 2] and the [2 + 2] cycloadditions leading to 4, 5, and 6, respectively. Steward [26] explained these results in terms of concerted Diels-Alder reactions competing with stepwise [2 + 2] cycloadditions. According to its larger (less negative) activation volume, the third Diels-Alder adduct 4 should also be formed in a non-concerted process. Similarly it can be concluded from the pressure dependence of the dimerization of 1,3-cyclohexadiene (7) that the endo-Diels-Alder dimer 8 and the [6 + 4]-ene product 9 are formed concertedly while the exo-Diels-Alder adduct 10 and the [2 + 2] cyclodimers 11 and 12 arise via diradical intermediates. 

According to the activation volume data the Diels-Alder dimerization of 1,3-butadiene [39] and o-quinodimethane (Table 2.5, entries (3) and (4), respectively) fall into the same class of concerted processes as those discussed for 1 and 7, while the Diels-Alder dimerization of hexamethylbis(methylene)cyclopentane seems to occur in stepwise fashion. According to the activation volume data summarized in Table 2.6 only the Diels-Alder reaction of 1,3-butadiene with a-acetoxyacrylonitrile seems to proceed concertedly while all other Diels-Alder and homo-Diels-Alder adducts are probably formed in stepwise processes comparable to the corresponding competitive [2 + 2] cycloadditions. Stereochemical investigations of the chloroprene and 1,3-butadiene dimerization using specifically deuterated derivatives confirm the conclusions drawn from activation volume data. In the dimerization of ( )-l-deuteriochloroprene (17) the diastereomeric Diels-Alder adducts ISa-Dz and... 

The pressure dependence of the dimerization of chloroprene (127) has been investigated. The results support the view that one of the vinylcyclohexenes, (128), is a product of a biradical mechanism, like the divinylcyclobutanes, and not a product of a concerted cycloaddition, like (129) and (130). 

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