Cycloalkenes, formation

It can be concluded that the metathesis reaction of acyclic alkenes and cycloalkenes proceeds via the rupture and formation of carbon-carbon double bonds, i.e. that the metathesis of alkenes is a true transalkylidenation reaction. 

More research efforts have focused on the ring-closing enyne metathesis, which usually [176] provides conjugated vinyl cycloalkenes (cf Fig. 2a, exo mode) useful for further manipulation, but also allows tandem metathesis processes for the formation of polycyclic compounds. 

Almost all evidence indicates that the transition state must be planar. Deviations from planarity as in 17-3 (see p. 1322) are not found here, and indeed this is why six-membered heterocyclic nitrogen compounds do not react. Because of the stereoselectivity of this reaction and the lack of rearrangement of the products, it is useful for the formation of trans cycloalkenes (eight-membered and higher). 

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... 

When large quantities of oxygen are introduced into N204/alkene or cycloalkene mixtures, detonations occur at a temperature starting at 0 C. These detonations have been explained by the formation of peroxynitrates and nitro-peroxynitrates, which are unstable ... 

Scheme 2.18 gives some representative olefination reactions of phosphonate anions. Entry 1 represents a typical preparative procedure. Entry 2 involves formation of a 2,4-dienoate ester using an a, 3-unsaturated aldehyde. Diethyl benzylphosphonate can be used in the Wadsworth-Emmons reaction, as illustrated by Entry 3. Entries 4 to 6 show other anion-stabilizing groups. Intramolecular reactions can be used to prepare cycloalkenes.264... 

Likewise it is possible to differentiate between substituted and unsubstituted alicycles using inclusion formation with 47 and 48 only the unbranched hydrocarbons are accommodated into the crystal lattices of 47 and 48 (e.g. separation of cyclohexane from methylcyclohexane, or of cyclopentane from methylcyclopentane). This holds also for cycloalkenes (cf. cyclohexene/methylcyclohexene), but not for benzene and its derivatives. Yet, in the latter case no arbitrary number of substituents (methyl groups) and nor any position of the attached substituents at the aromatic nucleus is tolerated on inclusion formation with 46, 47, and 48, dependent on the host molecule (Tables 7 and 8). This opens interesting separation procedures for analytical purposes, for instance the distinction between benzene and toluene or in the field of the isomeric xylenes. 

Simple criss-cross cycloadditions described so far are in fact limited to aromatic aldazines and cyclic or fluorinated ketazines. Other examples are rather rare, including the products of intramolecular criss-cross cycloaddition. The criss-cross cycloadditions of hexafluoroacetone azine are probably the best studied reaction of this type. It has been observed that with azomethine imides 291 derived from hexafluoroacetone azine 290 and C(5)-C(7) cycloalkenes < 1975J(P 1)1902, 1979T389>, a rearrangement to 177-3-pyrazolines 292 competes with the criss-cross adduct 293 formation (Scheme 39). 

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... 

Particularly interesting is the reaction of enynes with catalytic amounts of carbene complexes (Figure 3.50). If the chain-length between olefin and alkyne enables the formation of a five-membered or larger ring, then RCM can lead to the formation of vinyl-substituted cycloalkenes [866] or heterocycles. Examples of such reactions are given in Tables 3.18-3.20. It should, though, be taken into account that this reaction can also proceed by non-carbene-mediated pathways. Also Fischer-type carbene complexes and other complexes [867] can catalyze enyne cyclizations [267]. Trost [868] proposed that palladium-catalyzed enyne cyclizations proceed via metallacyclopentenes, which upon reductive elimination yield an intermediate cyclobutene. Also a Lewis acid-catalyzed, intramolecular [2 + 2] cycloaddition of, e.g., acceptor-substituted alkynes to an alkene to yield a cyclobutene can be considered as a possible mechanism of enyne cyclization. 

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

One special case of cross metathesis is ring-opening cross metathesis. When strained, cyclic alkenes (but not cyclopropenes [818]) are treated with a catalytically active carbene complex in the presence of an alkene, no ROMP but only the formation of monomeric cross-metathesis product is observed [818,937], The reaction, which works best with terminal alkenes, must be interrupted when the strained cycloalkene is consumed, to avoid further equilibration. As illustrated by the examples in Table 3.22, high yields and regioselectivities can be achieved with this interesting methodology. 

Hatakeyama, S., Ohno, M., Weng, J., Takagi, H., and Akimoto, H. Mechanism for the formation of gaseous and particulate products from ozone-cycloalkene reactions in air. Environ. Scl. Technol., 21(l) 52-57, 1987. 

When a CH2CI2 solution of 119a is stirred in the presence of 10 mol % of Ic under ethylene gas (1 atm) at room temperamre, pyrrolidine derivative 120a is obtained in high yield. Various cycloalkene-ynes 119b-c are examined, and pyrrolidine derivative 120b-c is formed in each case. Formally, the double bonds of cycloalkene and ethylene are cleaved, and each methylidene part of ethylene is combined with the cycloalkene and alkyne carbons, respectively, and bond formation between the double and triple bonds occurs to give pyrrolidine derivative 120 ... 

ROMP is generally an irreversible process, and relief of ring strain is the driving force for the forward reaction. Reversibility (with the formation of macrocyclic oligomers) is possible for less strained cycloalkenes like cyclooctene when the lifetimes for propagating chains are high. 

---