Cycloaddition-ortho mechanism

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. °... 

TiCU also operates through an aldol reaction mechanism. Unlike BF3 OEt2, however, TiCU can form an ordered Zimmerman-Traxler transition state and is also able to chelate a-alkoxy aldehydes. This forces the production of syn pyrones from the reaction with aldehydes that contain groups that are able to chelate to the metal aldehydes that cannot coordinate give trans aldol products. Primary [4 + 2] cycloaddition products (i.e. vinylogous ortho esters) have not been isolated in the TiCU-catalyzed reactions. 

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]. 

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