Cycloaddition to Arenes

Cycloaddition to arenes. Benzene reacts with the 2-methoxyallyl cation (a), formed from 2-methoxyallyl bromide by reaction with silver trifluoroace-tate, to give bicyclo[3.2.2]nona-6,8-diene-3-one (1) at room temperature (6% yield). Adducts are also obtained from p-xylene (3.5% yield) and toluene. The... 

Adam, W., Peters, E. M., Peters, K., Prein, M., and von Schnering, H. G., Diastereoselective photooxygenation of chiral naphthyl alcohols the hydroxy group directing effect in singlet oxygen [4+2] cycloaddition to arenes, /. Am. Chem. Soc., 117, 6686,1995. 

The methodology is useful for a variety of synthetic purposes. The cycloadditions are not subject to steric hindrance. Thus diyne cycloadditions to 2,5-disubstituted furans or pyrroles, followed by elimination of the oxygen or nitrogen bridges, provides an excellent, short route to peri-substituted arenes, as in the following examples 4 6 8... 

The aziridinofullerenes and azafulleroids obtained by Mattay et al. were subjected to further reactions to yield fullerene adducts of the second generation. As a model compound 4-methyl-l,2,4-triazoline-3,5-dione (NMTAD) 187, which is one of the most reactive dienophiles for Diels-Alder reactions, was selected. Sheridan et al. studied the photoinduced [4 + 2] cycloadditions of NMTAD to arenes [295], With alkenes, however, NMTAD reacts in photoinduced cycloadditions in a [2 + 2] fashion under formation of diazetidines [296], Both the sulfonyl-azafulleroids (185) and aziridinofullerene (186) were irradiated at 420 nm to give both the same [2 + 2] cycloadduct 186 (Scheme 71) [291], This indicates that at a certain stage rearrangements must be involved. 

Other partners like alkynes in Rh- or 1,3-dienes in Pd-catalyzed reaction led to arenes 5 [8] or vinyl alkylidene cyclohexenes 6 [9]. Since these [4+2] cycloadditions take place between two electronically quite similar partners, a direct Diels-Alder reaction is not feasible. 

The first photochemical [3 + 2] cycloadditions of arenes with aikenes were reported by two groups in 1966 [76, 77]. This is one of three types of cycloaddition reaction ortho, meta, and para) of arenes with aikenes, and is referred to as the meta-cydoaddition reaction. There are many combinations of arenes and aikenes and, indeed, a large number of reports and reviews have been made on the regiochemistry and stereochemistry [78]. Because this reaction can be used easily to construct a tricyclo[3.3.0.02,8]oct-3-ene framework, the organic syntheses of many natural compounds using this method have been attempted [79], in which the photochemical [3 + 2] cydoaddition plays a key role. 

The role of the zwitterion intermediate 164 and its rearrangement in the photocyclization of aromatic thioethers 163 to arene dihydro thiophene derivatives 165 is supported [81] by its independent trapping (via 1,3-dipolar cycloaddition) with biphenyl maleimide to obtain 166 in high yields (Scheme 8.47). 

At the singlet excited state, ortho and meta photocycloadditions are often competitive processes and physicochemical investigations were carried out to rationalize the modes of cycloaddition of arenes with alkenes. In the context of the study of photochemical electron transfer reactions, it has been proposed that the difference of the redox potentials of the reaction partners might play an important role in this competition [10]. Such a discussion involves the intervention of an exciplex as intermediate. The Rehm-Weller equation [11] was used to quantify the relationship. When an electron transfer process is strongly endergonic (AG>1.5eV), the meta cycloaddition should be favored. When such a process is less endergonic (1 < AG< 1.5 eY), the ortho addition dominate [12]. This means that the... 

In contrast to the major developments in the use of [3C + 2C] and [SC + 2C] cycloadditions based on the arene-alkene meta cycloaddition, relatively less is known about the use of the meta cycloaddition as a means to achieve [3C + 4C] and [SC -i- 4C] connections. Nevertheless, these processes have great potential, provided that the factors determining their selectivity could be elucidated. For example, benzene and diene (176) (equation 16) undergo cycloaddition to furnish a [3C + 4C] (or [SC + 4C]) cycloadduct (177) and a [4C + 4C] cycloadduct (178). Cleavage of the former adduct as in the aforementioned cases would provide access to seven- or nine-membered rings. While most dienes react with arenes to give [4C + 4C] cycloadducts, the range of dienes and arenes studied thus far is too limited to rule out the development of [3C -i- 4C] and [SC -i- 4C] cycloadditions as useful synthetic processes. 

The photoreactions of anthracene in the presence of trans trans hexa-2,4-diene have been re-examined. The products from the reaction are the anthracene dimer, the two isomers (88) and (89) of [4+4]cycloaddition to the 9,10-positions of the arene, and a [2+4] adduct (90) as well as several unidentified minor adducts. Dimerization of the arene is reduced by the diene rather than enhanced, and a Diels-Alder reaction of the strained trans... 

Miscellaneous. Acylation of unprotected amino acids, desilylation of TBS-ethers, alkylation of ketone enolates with (iso)/quinolinium salts, Wolff rearrangement, bromination of active arenes with NBS, and dichloroketene formation from Zn and CI3CCOCI (for cycloaddition to alkynes) have all been promoted with ultrasound. 

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. 

---