Fullerenes 2 + 4 -photocycloaddition

Among the reactions applied in the synthesis of fullerene derivatives cycloaddition reactions such as [2 + 1]-, [2 + 2]-, [3 + 2] and [4 + 2] cycloadditions play a dominant role. In these reactions ring-fused fullerene derivatives are obtained, at least with incorporation of heteroatoms such as oxygen, nitrogen, or silicon. In this section photochemical reactions leading to cycloalkyl ring-fused fullerene adducts will be presented. Photocycloaddition reactions leading to C6o-fused heterocycles will be discussed later. 

A very versatile method of formation of fullerene adducts is the [2 + 2] photocycloaddition. The preparation and isolation of well characterized [2 + 2]... 

The photochemical addition of cyclic 1,3-diones such as dimedone, 1,3-cylohexandione 62, or their respective silyl enol ethers leads to the formation of two fused furanylfullerenes, (1) achiral 63 and (2) chiral 64 [244], The latter having an unusual bis-[6,5] closed structure. In the initial step of this reaction, [2 + 2] photocycloaddition across a [6,6] bond to form cyclobutanols or the corresponding TMS ethers is involved (Scheme 26). Oxidation with 02 yields in the formation of the radical 65a. Cleavage to 66a followed by cyclization gives furanyl radical 67a. H abstraction by 102 or a peroxy radical finally leads to product 63. In competition, formation of fullerene triplets by absorption of a... 

Orfanopoulos et al. studied the photochemical reaction of alkenes, aryalkenes, dienes dienones, and acyclic enones with [60]fullerene to obtain various substituted cyclobutylfullerenes [240,241,243,247], For example, the photocycloaddition of cis- and Irans-1 -(p-mcthoxyphenyl)-1 -propenc 68 to C6o gives only the trans [2 + 2] adducts (Scheme 27), thus the reaction is stereospecific for the most thermodynamically stable cycloadduct. A possible mechanism includes the formation of a common dipolar or biradical intermediate between 3C o and the arylalkene. Subsequent fast rotation of the aryl moiety around the former double bond leads exclusively to the trans-69 [2 + 2] adduct. Irradiation of this product, yielded 90% trans-68,10% cis-68 and cycloreversion products. Thus, a concerted mechanism can be excluded because the photocycloreversion is expected to give the trans-68 as the only product. These results can be explained by the formation of a common dipolar or diradical intermediate. Similarly, cycloreversion products from C6o and tetraalkoxyethylene... 

As mentioned previously, photocycloaddition reactions are a useful method for obtaining fullerene derivatives fused directly to heterocycles. Such compounds have attracted much interest because they might have interesting properties, e.g., amino acid fullerene derivatives as biologically active compounds or pyrrolidino-fullerenes as key precursors for a great number of fullerenes donor-acceptor bridged dyads and triads. Fullerenes functionalized with silicon compounds are of great interest in materials science. The synthesis of these compounds will be described in this section. 

By using the hypersensitive molecular mechanistic probe 2-(2-methoxy-3-phenylcy-clopropyl)-5-methylhexa-2,4-diene in the 2 + 2-photocycloaddition of [60]fullerene, it was shown that the reaction proceeds via a biradical and not a dipolar intermediate.6 Zirconium-induced cyclodimerization of heteroaryl-substituted alkynes produces tetrasubstituted cyclobutenes with high regio- and stereo-selectivity.7 The ruthenium-... 

Figure 1.44. The different stereoisomers obtained by [2 + 2] photocycloaddition of 3-methyl-cyclohex-2-enone to Cgo (left, the newly formed stereogenic centers are marked by an asterisk), and illustration of a sector mle devised for the correlation of the absolute configuration of a 6-6 closed 1,2-adduct of Cgo to the CD band around 430 nm (right). According to the sign ((+) or (-)) of the sector in which an addend moiety (R1, R2,R3, or R4) is located, it is postulated to add a positive or negative contribution to the Cotton effect associated with the ca. 430 nm UV/Vis absorption of the fullerene chromophore.

Additions to Cyclohexenones and Related Systems - Caldwell and his coworkers have studied the photochemical addition of 1,1-diphenylethene to 4,4-dimethylcyclohex-2-enone. The products from this reaction, carried out in cyclohexane, are shown in Scheme 1. Although other evidence (see reference 9b above and references cited therein) has suggested that an exciplex is not a key interaction in such (2+2)-photocycloaddition reactions Caldwell et al. conclude from their detailed study of this system that a triplet exciplex is involved. Schuster and his coworkers have reported that a variety of cyclic enones (17) -(19) add photochemically to fullerene. The yields of the adducts vary but with some of the less heavily substituted enones the yields can be reasonable as shown by the data under the appropriate structure. Suginome et aO report the synthesis of the cycloadducts (20) by the photochemical addition of various ethenes to the enone (21). The adduct (22) was also synthesized by photochemical addition of methoxycyclohexene to the enone (20). 

The (photo)chemical and physical properties of fullerenes, spherical carbon molecules, are an important topic in current research, especially in nanotechnology. The molecule with 60 carbons, C(,o. having properties of an electron-deficient alkene, undergoes for example [2 + 2] photocycloaddition similar to aromatic moieties. The absorption spectrum of Cprincipal maxima (/max = 211, 256, 328 nm) and a weak band at /. 540nm.xl3 Very weak fluorescence from the lowest excited... 

In the early 1990s, the derivatization of fullerenes was based on thermal reactions, e.g., the Diels-Alder reaction of Cso with substituted dienes leading to cyclohexyl-fused fullerene derivatives, and only a few examples were known, where a fullerene adduct was obtained by photochemical reactions. The pioneering work in the field of photochemical derivatization of fullerenes was done by Wilson, Schuster, and colleagues who, for example, reported the [2 + 2]-photocycloaddition of enones to CgQ. Nowadays, more and more fullerene derivatives are synthesized by photochemical reaction pathways. 

In general, by [2 -1- 1]-photocycloaddition reactions methanofullerenes, fullerene epoxide, and azirid-inofullerenes as the corresponding oxygen and nitrogen analogues with a [6,6]-closed structure are obtained as shown in Figure 28.5. 

FIGURE 28.5 Fullerene derivatives obtained by [2+1 ] -photocycloaddition methanofiillerene, fullerene epoxide, and aziridinofullerene. 

In contrast, Nishimura et al. obtained stable monoadducts 62a-g possessing a hydroxy group suitable for further derivatization from the [2 -1- 4]-photocycloaddition of the corresponding carbonyl compounds 60a-g and [60]fullerene (Scheme 26). The adducts 62 were found to adopt one or both of two conformers A and E, which possess pseudoaxial and pseudoequatorial hydroxy groups, respectively (see Table 28.4). Two conformers existed for 62a, c, d, and f, while 62b, e, and g exclusively adopted E, A, and E, respectively. The comformer ratios remarkably depend on the bulkiness of the substituents attached to the aromatic nucleus and the cyclohexene ring. 

The synthesis of fullerene epoxides via [2 -I- l]-photoq cloaddition of singlet oxygen has already been discussed (in the section [2 -1- l]-Photocycloaddition ). Alternatively, fullerene oxides can also be obtained by thermolysis or photolysis of the [6,6]-closed ozone adduct of Qq and C70, namely 60 3 75 and C70O3 77.Thermolysis of QqOj yields the fullerene epoxide with a [6,6]-closed structure. In contrast, photolysis resulted in the formation of the [5,6]-open oxidoannulene 76 with an ether structure (Scheme 32). 

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