Para cycloaddition

Figure 6 Distances between the benzene carbon atoms involved in ortho, meta, or para-cycloaddition.

A variety of four-membered ring compounds can be obtained with photochemical reactions of aromatic compounds, mainly with the [2 + 2] (ortho) photocycloaddition of alkenes. In the case of aromatic compounds of the benzene type, this reaction is often in competition with the [3 + 2] (meta) cycloaddition, and less frequently with the [4 + 2] (para) cycloaddition (Scheme 5.7) [38-40]. When the aromatic reaction partner is electronically excited, both reactions can occur at the 7t7t singlet state, but only the [2 + 2] addition can also proceed at the %% triplet state. Such competition was also discussed in the context of redox potentials of the reaction partners [17]. Most frequently, it is the electron-active substituents on the aromatic partner and the alkene which direct the reactivity. The [2 + 2] photocycloaddition is strongly favored when electron-withdrawing substituents are present in the substrates. In such a reaction, crotononitrile 34 was added to anisole 33 (Scheme 5.8, reaction 15) [41 ], and only one regioisomer (35) was obtained in good yield. In this transformation, the... 

Three types of cycloaddition products are generally obtained (Sch. 1). While [2+2] (ortho) and [2+3] (meta) cycloaddition are frequently described, the [2+4] (para or photo-Diels-Alder reaction) pathway is rarely observed in benzene ring systems. With naphthalene systems however, the para cycloaddition occurs more frequently [6,8]. The photo-Diels-Alder reaction and other photocyclization reactions are also observed with anthracene derivatives and higher condensed aromatic compounds. However, these reaction are not treated in this chapter since they are caused by the particular photophysical and photochemical properties of these compounds [6,9]. 

Interactions leading to para cycloaddition (S -i- ethylene LUMO) are only weakly stabUized. Para cycloadditions are also disfavored by the distance-determined (2.79 A) poor overlrq> between interacting orbitals of the arene with those of the alkene. From this analysis it is predicted that an ortho mode of cycloaddition will be favored for an arene AS"" transition while ortho but preferentially meta cycloaddition will be stabilized in the SA " transition. In addition to symmetry considerations, the effectiveness of the... 

The photoreactions of acyclic and cyclic alkenes with benzene depend on the ionization potential of the alkene relative to that of benzene, In general meta cycloaddition to give tricyclo[3.3.0.0 ]oct-3-ene derivatives occurs to some degree in every case and often predominates. ortho Cycloaddition tends mostly to occur with alkenes of lower ionization potential than benzene and gives bicyclo[4.2.0]octa-2,4-diene derivatives para cycloaddition gives bi-cyclo[2.2.2]octa-2,5-diene derivatives to a smaller extent. 

D. Dopp provides an example of the para cycloaddition which, apparently, is observed with dienes or as a secondary process of preformed ortho cycloadducts. Here the reactions involve the triplet state of a-acetylnaphthalene finally leading to formal ketene cycloadducts in a [4+2] mode. 

Synthetic applications of [4+2] photocycloadditions have not been extensively developed. It has been observed, however, that intramolecular photocycloaddition of arenes and allenes proceeds preferentially via the [4+2] reaction mode. This process appears to be reasonably general across a diverse range of allenes attached to aromatic aldehydes and ketones with a variety of tethers. Thns, the protected aniline 36 undergoes para-cycloaddition to give the intriguing bridged polycyclic product 37 (Scheme 15.13) [34]. Mechanistic details of this and related transformations have not yet been reported. 

Photoirradiation of benzene and its derivatives with alkenes give ortho-, meta-, para-cycloaddition products. In most cases, either meta- or orr/io-adducts are obtained as major products [5]. Bryce-Smith and Gilbert suggested a prefulvene type diradical intermediate mechanism (Path A) for the weta-adduct [6], whereas Morrison and Srinivasan groups [7, 8] proposed the exciplex mechanism (Path B) for the formation of these adducts (Scheme 9.1). The exciplex intermediates undergo photo-induced electron transfer processes between donor (D) and acceptor (A) to produce radical ion pairs as intermediates, stabilized by coulombic interactions to give adducts [9]. 

There are large number of cycloaddition reactions of benzene and its derivatives. Correlation diagram can be constructed to predict them which can be represented by examples of ortho-, meta- and para-cycloadditions of benzene and ethylene or benzene and butadiene. Ortho-, meta- and para-additions give different products. 

The ortho-para rule is explained by FMO theory on the basis of the orbital coefficients of the atoms forming the cr-bonds. The regiochemistry is determined by the overlap of the orbitals that have larger coefficients (larger lobes in Scheme 1.15). The greater the difference between the orbital coefficients of the two end atoms of diene and two atoms of dienophile, which form the two cr-bonds, the more regioselective the cycloaddition. 

Lewis-acid-catalyzed cycloadditions of dienophiles, such as a,/l-unsaturated carbonyl compounds, with open-chain carbon-dienes, are generally highly ortho-para regioselective because the oxygen complexation increases the difference of LUMO coefficients of the alkene moiety. 

The presence of two substituents at C-4 also strongly influences the regios-electivity as shown in the cycloaddition of dienone 13 with isoprene (2) (Equation 3.1). In violation of the para-rule for Diels-Alder reaction, only metfl-adduct was obtained [19,20]. 

Attempts to prepare the corresponding tetrameric para-PAM from 62 were unsuccessful. Dehydrobromination in furan afforded a symanti mixture of cycloadducts 65, that were subsequently transformed to the known dibenzodiyne (66). Formation of 65 is likely to arise from stepwise elimination/cycloaddition rather than to involve the intermediacy of the highly strained tetrameric PAM. 

Recently, a series of cycloadducts possessing unusual flipping modes have been isolated from the 1,3-dipolar cycloaddition of 3,4,5,6-tetrahydropyridine IV -oxide to piperidides of cinnamic acid and para-substituted cinnamic acids (791). 

As well as undergoing carbonyl addition, titanacyclopentadiene intermediates generated from two unsymmetrical acetylenes have been shown to react with ethynyl para-tolyl sul-fone to afford an aryltitanium compound of the structure shown in Scheme 9.6 [34], The reaction may proceed according to path a or path b, as shown in Scheme 9.7. In path a, the first step should be regioselective [4+2] cycloaddition of the titanacyclopentadiene with the sulfonylacetylene to afford the bicyclic titanacycle, at least in an equilibrium concen-... 

A [2.2]paracydophane was prepared by dimerization of p-quinodimethane 232, which was obtained by a [4+ 2]-cycloaddition reaction of bisallene with DMAD [184], This sequence represents one of the most general approaches to functionalized para-cyclophanes. 

Similar intramolecular cycloadditions are encountered where an ether linkage has been incorporated into the met a or para linking groups 123. In these cyclizations the better yields were obtained from the para-attached systems. The yields obtained are again dependent on the chain length of the separator and are indicated below the appropriate structures (124)57. Other hetero-atom-substituted cyclophanes (125) can be obtained by irradiation of the divinyl compounds (126)58,59. The use of tin and germanium derivatives... 

The yields of product isolated when para-substituted styrenes were allowed to react with the acridizinium ion (Table I) are not indicative of the rates of reaetion. In an experiment patterned after that used by Sauer and Wiest in the first demonstration of the existence of cycloaddition with inverse electron demand, it was shown that the relative rates of addition of para-substituted styrenes to the acridizinium nucleus were as follows CHgO, 4.3 CHg, 1.7 H, 1.0 NOg, 0.34 or in the order expected from the nucleophilicity of the styrenes. 

Allenylidenecomplexes 46 also react with thecarbon-carbon triple bond ofynamines to yield similar mononuclear cydobutenyhdene derivatives 48, although mixtures with the corresponding alkenyl-aminoallenylidene spedes 49 are formed (Scheme 2.20) [ 1 Oc]. The former isomer results from the addition of the C=C bond of y namines across the Co,=Cp unsaturation, while the latter is provided bythe formal [2 + 2] cycloaddition between C=C and Cp=Cy bonds and subsequent cycloreversion. In both processes, stepwise cyclization initiated by the addition of the nucleophilic R C=CNEt2 carbon at the Co, or Cy position, respectively, is proposed. Relative proportions of 49 with respect to 48 increase with the electron-releasing capacity of the para-substituents of the diarylallenylidene skeleton (NMe2 > OMe > Me > H). In contrast, the formation of 48 is favored when the reaction is carried out in low polarity solvents. 

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