Styrene, 4-methoxy cycloaddition reactions

The same authors also reported the first example of a Lewis acid-catalyzed [2 -t 2] cycloaddition between styrenes and naphthoquinones [56], The reactions of both methoxy and acetoxynaphthoquinones with styrene proceeded regio- and stereospeci-fically in CH2CI2 at -78 °C under the influence of Me2AlCl (Sch. 33). 

In contrast to the preferred meta mode of intramolecular photoaddition of 5-phenyl-l-pentenes, where the alkene and benzene groups are separated by three atoms, irradiation of the styrene (64) yields a single stereoisomer of the ortho adduct (65). In (64), not only are the reacting units separated by 4 atoms, but also it is the styrene rather than the benzene which is excited. Comparable photoreactivity is seen for phenanthrene-styrene systems such as (66) which yield 2+2 adducts (67) along with products derived from competing Paterno-Btichi reaction of the ester carbonyl with the alkene side chain. The photochemical cycloaddition also proceeds in an intermolecular fashion between the ester of 9-phenanthrene carboxylic acid and para-methoxy-0-methylstyrene. The mechanism of this reaction is shown to involve addition of the styrene to the singlet excited state of the phenanthrene derivative. °... 

Electronic effects transmitted through conjugated systems are scarce. Table 17(d), corresponding to p-values around —0.7 for reactions of 1,1-diarylethylenes and of p-substituted styrenes with diphenylketene . In the 1,2-cycloaddition of diphenylketene to benzalaniline, a p-methoxy substituent on either phenyl group of benzalaniline increases the rate by a factor of 2.3, corresponding to p = — 1.4. 

As always, stereochemistry has proven to be a crucial indicator of mechanism. Many examples of highly stereospecific SET reactions have been found. An example is the Diels-Alder reaction of the l,2-di(aryloxy)-ethylenes shown below. Mixing the dienophile with cyclopentadiene and the very convenient SET reagent tris(p-bromophenyl)aminiiim (1 ) gives the cycloaddition adducts with high stereospecificity (first two examples below). The observation of several cases like this led many to conclude that the SET reactions really were concerted, pericyclic processes. However, more recent work has found clear exceptions. The deuterated 4-methoxy-styrene shown adds to cyclopentadiene under the same conditions with extensive loss of stereochemistry (third example). These systems are more complicated than conventional pericyclic reactions. 

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