Arenes intermolecular cycloaddition with

Excellent regioselectivity and stereoselectivity has been achieved in each photocycloaddition mode [45 48], Regiochemistry and stereochemistry in the meta process is decided by the orientation of the addends in the exciplex and by stabilization of biradical intermediates having a change transfer (CT) character (6) by the substituents on the arene. Intermolecular meta cycloaddition of arenes with cycloalkenes proceeds with endo selectivity (7) (Scheme 5). In the ortho-process, selectivities can be controlled mainly by the substituents on the reactants. 

The next selectivity issue, exo/endo preferences, can be predicted for both the ortho and meta modes of cycloaddition on the basis of secondly orbital interactions (FMO treatment) and by electrostatic considerations involving polarized species (54) and (27). In general, intermolecular reactions with simple al-kenes proceed with endo selectivity. Heteroatom-substituted or polarized alkenes (equation 11) give exolendo mixtures, whose composition can be explained by electrostatic considerations. Intramolecular cycloadditions of simple alkenes and arenes joined by a three-atom tether generally proceed with high exo selectivity due in part to orbital alignment effects. In all cases, alkene geometry is preserved, except for sterically encumbered alkenes, in which case excitation transfer from the arene to the alkene can occur. 

Arenes from enynes and diynes. A novel synthesis of alkynylarenes by the intermolecular cycloaddition of enynes and diynes has been developed. With enynes alone the products are 2,6-disubstituted styrenes which are formed by cyclodimerization. Note that benzynes undergo trimerization to give triphenylenes in situ, e.g., by treatment of o-trimethylsilylphenyl triflate with CsF in the presence of (PhjPl.Pd. ... 

Arene-alkene photocycloaddition reactions have been reviewed in detail. This review includes a tabular survey, synthetic applications and exhaustive bibliography. Many of the reactions considered in this review were however performed to determine answers to mechanistic questions, therefore they are not necessarily optimized and chemical yields are not indicated. Tables 2 and 3 bring together, from the references cited in the review , those intermolecular photochemical meta cycloadditions which are principally synthetic methods. More concise general treatments of this reaction have since been published, as have other papers mainly concerned with its mechanistic aspects. """ ... 

Niobium catalyzes an intermolecular [2 + 2 + 2] cycloaddition of alkynes and nitriles, which forms 2,3,6-trisubstituted pyridines (Scheme 20) (13JOC7771). The catalytic mixture involves low-valent niobium from NbCls, Zn, and an alkoxysilane. In addition to the desired 2,3,6-pyridines, a small amount of substituted arene was isolated in each reaction. Benzoni-triles with a variety of substituents underwent cycloaddition smoothly. Moreover, benzylnitriles also reacted smoothly however, the reaction was sluggish wnth aliphatic nitriles. 

The Lewis acid-mediated [4+2] cycloaddition of conjugated enynes with alkynes affords arenes however, it is limited to the intramolecular cycloaddition [44]. The use of transition metal complexes realizes the catalytic intermolecular variants. The first example was reported in the paUadium(O)-catalyzed homo-[4+2] cycloaddition of monosubstituted conjugated enynes giving substituted styrenes (Scheme 21.38) [45]. Importantly, this reaction proceeded with definite regioselectivity. 

The fully intermolecular variant of this concept has also been realized. A variety of propiolates were coupled with allenes to afford a regioselective aromatic cycloadduct (Scheme 2.12) [9]. A symmetric ester bearing an alkyne was coupled successfully with butyl allene to afford arene 46 in high yields. Unsymmetrical internal alkynes (i.e., methyl- and phenyl-substituted propiolates) were also coupled with butyl-, cyclohexyl-, and phenyl-substituted allenes in high yields (47 to 50, Scheme 2.12). The current scope of the reaction is confined to the use of activated alkyne substrates in this cycloaddition. 

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