Cycloaddition Cycloalkene

UV irradiation of hexafluorobenzene with indene or cycloalkenes gives high yields of 2+2 adducts, which undergo further intramolecular cycloaddition to form hexafluoropolycycloalkanes [754] (equation 38) Photolysis of fluormated deriva tives of vinylbenzenes afford benzocyclobutenes, whereas allyl benzenes yield Dewar benzene-type products [755]... 

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). 

Cycloaddition with nitrile oxides occur with compounds of practically any type with a C=C bond alkenes and cycloalkenes, their functional derivatives, dienes and trienes with isolated, conjugated or cumulated double bonds, some aromatic compounds, unsaturated and aromatic heterocycles, and fullerenes. The content of this subsection is classified according to the mentioned types of dipolarophiles. Problems of relative reactivities of dienophiles and dipoles, regio- and stereoselectivity of nitrile oxide cycloadditions were considered in detail by Jaeger and... 

Cycloalkene Derivatives Cyclopropenes readily interact with nitrile oxides. Reactions of a broad series of 3,3-disubstituted cyclopropenes with 4-substituted benzonitrile, methoxycarbonyl- and cyanoformonitrile oxides (229) as well as with di(isopropoxy)phosphorylformonitrile oxide (230) give 2-oxa-3-azabicyclo[3.1.0]hexene derivatives 62. Stereoselectivity of the cycloaddition is governed by both steric and polar factors. In particular, steric factors are supposed to prevail for 3-methyl-3-phenylcyclopropene affording 62 [R1 =... 

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. 

The inverse-electron-demand Diels-Alder reaction of 3,6-dichloro[l,2,4,5]tetrazine with alkenes and alkynes provides the synthesis of highly functionalized pyridazines. ° Also, the 4 + 2-cycloaddition reactions of the parent [l,2,4,5]tetrazine with donor-substituted alkynes, alkenes, donor-substituted and unsubstituted cycloalkenes, ketene acetals, and aminals have been investigated. ... 

If ROM-RCM of cycloalkene-yne 123, which has a substituent at the 2-position of cycloalkene, is carried out under ethylene gas, what compound is formed In this reaction, ruthenium carbene XIX would be formed via [2-1-2] cycloaddition of ruthenium methylidene carbene and alkyne as shown in Eq. (6.91). If XIX reacts with an olefin intramolecularly or ethylene, bicyclic compound 124 or triene 125... 

The most versatile method for the preparation of m-hexahydro-1,3-benzoxazines and the related cycloalkane e -fused dihydrooxazines is amidoalkylation via 1,4-polar cycloaddition of the amidomethyl ion 172 to a cycloalkene. The addition is stereospecifically cis, and in accordance with the Markownikov rule is generally regiospecific. 

The 1,4-cycloadditions gave the diexo adducts stereoselectively from norbornene, and cis adducts in the cycloalkene series. 

Similarly to the dihydrooxazinones (Section II,A,5), the title compounds can be prepared by the cycloaddition of thioacyl isocyanates to cycloalkenes containing an activated C = C bond. 

Face Selectivity of Nitrile Oxide Cycloadditions with Chiral Cycloalkenes and the Like... 

The cycloaddition reactions of isoquinolinium species produce fused isoquinoline products. The Af-ylide of 53, formed with base addition, couples with alkenes <99S51> or imines <99T7279> to afford tricyclic products, such as 54. Pyrrole-fused isoquinolines result from the reaction between mUnchnone imine intermediates and a,yff-ethylenic esters <99EJOC297>. N-Arylimides undergo 1,3-dipolar cycloaddition with strained frani-cyclooctenes, as opposed to common cycloalkenes, to tdford the pyrazolidine-fused ring system <99H(50)353>. 

Cycloaddition reactions of C=C double bonds are often facilitated by increased olefinic strain. For example, phenyl azide does not react with unstrained alkenes, but does react readily with the smaller tran -cycloalkenes. Many interesting cycloaUcenes are not sufficiently stable to be isolated, or even observed in solution. However, in many cases they can be trapped by reagents that lead to a 3+2] or [4+2] (Diels-Alder) reaction. 

Strained n/d/ -Bredt alkenes as well as -cycloalkenes will undergo [2 + 2] cycloadditions with alkenes. The bridgehead bicyclo[3.3.1]nonenones 16, produced as a transient species by the dehydrobromination of the corresponding bridgehead bromoketone, undergo cycloaddition with 1,1-dimethoxyethenc (15) to give the tricyclic ketones 17 in quantitative yields.7... 

On a similar note, a series of bridgehead alkenes and -cycloalkenes have been reported to undergo cycloadditions with l,l-dichloro-2,2-difluoroethene (12) to give the corresponding dichlorodifluorocyelobutanes.s... 

Other reactive cyclobutadienes including the parent derivative have been reported to undergo cycloadditions with cycloalkenes to give tricycloalkenes. In these instances both syn-and anti-isomers are obtained.11-15... 

The stereochemistry of ketene to alkcne cycloadditions is such that retention of the alkene configuration is observed. Furthermore, in cycloadditions with unsymmetrically substituted ketenes the larger of the two ketene substituents ends up as with respect to the adjacent alkene substituent (or eiulo in cycloalkene cycloadditions). This stereochemical outcome was originally attributed to the concerted [ff2a + n2a] nature of kctcnc to alkene cycloadditions,21 although more recent experimental and theoretical evidence indicate that these reactions are asynchronous and in some cases in which polarized double bonds are involved actual zwittcrions may be intermediates.9 1195 Also in certain cases the endo product in ketene to alkene cycloadditions may be the thermodynamic product from equilibration studies.22,23 Nevertheless, stereochemical control can be achieved in most such reactions as shown by the examples of 12,24 13,29 14,25 15,26 16,27 and 17.28... 

Table 5. Cycloaddition of Ketenes to Endocyclic Conjugated Cycloalkenes ...

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