Of cycloalkenes

Stereoselective cis-dihydroxylation of the more hindered side of cycloalkenes is achieved with silver(I) or copper(II) acetates and iodine in wet acetic acid (Woodward gly-colization J.B. Siddall, 1966 L. Mangoni, 1973 R. Criegee, 1979) or with thallium(III) acetate via organothallium intermediates (E. Glotter, 1976). In these reactions the intermediate dioxolenium cation is supposed to be opened hydrolytically, not by Sn2 reaction. 

Since then, the metathesis reaction has been extended to other types of alkenes, viz. substituted alkenes, dienes and polyenes, and to alkynes. Of special interest is the metathesis of cycloalkenes. This gives rise to a ring enlargement resulting in macrocyclic compounds and eventually poly-... 

Dall Asta and Motroni (44, 57) provided direct experimental evidence for the transalkylidenation mechanism in the case of cycloalkenes. With a catalyst system consisting of WOCI4, C2H6A1C12, and benzoyl peroxide they prepared a random copolymer of cyclooctene and cyclopentene, the cyclo-pentene double bond being labeled with 14C. The distribution of the radioactivity in the copolymer formed will depend on the site of ring opening. 

For the metathesis of cycloalkenes the scheme of Calderon can be depicted as follows ... 

Fig. 4. Kinetic model of Calderon applied to the ring-opening polymerization of cycloalkenes.

The preferred kinetic model for the metathesis of acyclic alkenes is a Langmuir type model, with a rate-determining reaction between two adsorbed (complexed) molecules. For the metathesis of cycloalkenes, the kinetic model of Calderon as depicted in Fig. 4 agrees well with the experimental results. A scheme involving carbene complexes (Fig. 5) is less likely, which is consistent with the conclusion drawn from mechanistic considerations (Section III). However, Calderon s model might also fit the experimental data in the case of acyclic alkenes. If, for instance, the concentration of the dialkene complex is independent of the concentration of free alkene, the reaction will be first order with respect to the alkene. This has in fact been observed (Section IV.C.2) but, within certain limits, a first-order relationship can also be obtained from many hyperbolic models. Moreover, it seems unreasonable to assume that one single kinetic model could represent the experimental results of all systems under consideration. Clearly, further experimental work is needed to arrive at more definite conclusions. Especially, it is necessary to investigate whether conclusions derived for a particular system are valid for all catalyst systems. 

Formation of mixtures of products in these reactions can be attributed largely to the properties of the acetate group. The reactions of a number of cycloalkenes with thallium(III) salts have been investigated in some detail and the results obtained have served both to elucidate the stereochemistry of oxythallation and to underline the important role assumed by the anion of the metal salt in these oxidations. The most unambiguous evidence as to the stereochemistry of oxythallation comes from studies by Winstein on the oxythallation of norbornene (VII) and norbornadiene (VIII) with thal-lium(III) acetate in chloroform, in which the adducts (IX) and (X) could be precipitated from the reaction mixture by addition of pentane 128) (Scheme 11). Both by chemical means and by analogy with the oxymercuration... 

Contact of cycloalkenes with a mixture of dinitrogen tetraoxide and excess oxygen at temperatures of 0°C or below produces nitroperoxonitrates of the general formula-CHN02-CH(OON02)—which appear to be unstable at temperatures above 0°C, owing to the presence of the peroxonitrate group. 

The synthesis of cyclic nitroalkenes via nitration of cycloalkenes is summarized in Table 2.2. Acyclic nitroalkenes are more readily prepared via the Henry reaction than by nitration of alkenes (see Section 3.2.1). 

The rings of cycloalkenes containing five carbon atoms or fewer exist only in the cis form. 

Streitwieser and Boerth studied the kinetic acidities of cycloalkenes with lithium cyclo-hexylamide (LiCHA) in cyclohexylamine for comparison with those of benzene and toluene66. The relative rates of deprotonation and the corresponding equilibrium pK values are tabulated in Table 12. These proton transfer transition states are stabilized by conjugation of the reacting C—H bond with the double bond. 

By analogy with the intramolecular insertion of phenylthiocarbenes, the reaction of (oo-oxido)diazoalkanes 93 resulted in the formation of cycloalkenes 94.38 However, the reaction was proven to proceed not via a carbene route b but a nitrene route a as shown in Scheme 26. The nitrene route is supported by the formation of heterocyclic products 98 and 99.39 This insertion reaction was used in the cydization step to the cyclopentene ring formation of isocarbacycline 97.40... 

Thioacetals having a terminal or 1,2-disubstituted double bond are transformed into the corresponding cycloalkenes upon treatment with titanocene(II) species first at room temperature and then at elevated temperature. On the other hand, only a small amount of cycloalkene is produced under the same reaction conditions from thioacetals having a tri-substituted double bond (Table 14.1) [29,30],... 

Arylation of cycloalkenes.1 Aryl halides undergo Heck coupling with cy-cloalkenes in the presence of a palladium catalyst. The reaction involves addition of an arylpalladium intermediate to the double bond followed by elimination of a palladium hydride. 

The reaction of cycloalkene oxides with EH should be a priori mechanistically similar to that of acyclic analogues, except that the presence of a cycle may impose characteristic stereochemical constraints on the products. A look at Table 10.1, however, indicates that cycloalkene oxides are poorer substrates for EH than their acyclic analogues. 

Synthetic Applications of Non-Heteroatom-Substituted Carbene Complexes Table 3.18. Formation of cycloalkenes by ring-closing metathesis. 

IV. Conformational Analysis and the Geometry of the Pertinent Transition States in the Hydrogenation of Cycloalkenes... 

Although the transition state for the exchange reaction may be described as the critical complex for the conversion of the half-hydrogenated state to either a jr-complexed olefin or an eclipsed vicinal diadsorbed alkane, the stereochemistry of hydrogenation of cycloalkenes on platinum at low pressures can be understood if the transition state has a virtually saturated structure. 

Hussey et al. (109) have recently reported measurements of the kinetics of hydrogenation in the liquid phase of a large number of cycloalkenes, both individually and competitively. The data permit the kind of analysis... 

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