Diels-Alder reactions solid-state

The low solubility of fullerene (Ceo) in common organic solvents such as THE, MeCN and DCM interferes with its functionalization, which is a key step for its synthetic applications. Solid state photochemistry is a powerful strategy for overcoming this difficulty. Thus a 1 1 mixture of Cgo and 9-methylanthra-cene (Equation 4.10, R = Me) exposed to a high-pressure mercury lamp gives the adduct 72 (R = Me) with 68% conversion [51]. No 9-methylanthracene dimers were detected. Anthracene does not react with Ceo under these conditions this has been correlated to its ionization potential which is lower than that of the 9-methyl derivative. This suggests that the Diels-Alder reaction proceeds via photo-induced electron transfer from 9-methylanthracene to the triplet excited state of Ceo-... 

The photo-induced single and double Diels-Alder reactions between [60]fullerene and 9-methylanthracene (212) which gave 213 and 214 were performed in the solid state by Mikami and colleagues (equation 60)141. The Diels-Alder reaction was considered to proceed following a photo-induced electron transfer from 9-methylanthracene to fullerene. The higher ionization potential of anthracene should explain its inreactivity toward the cycloaddition reaction with [60]fullerene. 

Endo selectivity in some simple quantitative gas-solid Diels-Alder reactions has been secured [120]. However, there was not much motivation to repeat more well-known Diels-Alder reactions in the solid state. For example, maleic anhydride (381) and vapors of cyclohexadiene (382) give a quantitative yield... 

An even more pronounced retro-Diels-Alder reaction occurs by using 1,3-di-phenylisobenzofuran (DPIF), 9-methylanthracene or 9,10-dimethylanthracene as dienes [8, 10-12]. The monoadduct of DPIF cannot be isolated from the reaction mixture, while the monoadduct of the 9-methyl- or 9,10-dimethyl- derivatives of anthracene can be isolated at temperatures lower than room temperature [10]. Both anthracene derivatives decompose at room temperature, the adduct with one methyl group within hours, the adduct with two methyl groups within minutes. For DPIF and the anthracene compounds the retro-Diels-Alder reaction seems to be facilitated by steric repulsion due to the bulky groups. However, as shown by Wudl and coworkers [13], the cycloadduct of with isobenzofuran (Scheme 4.2), which was generated in situ from l,4-dihydro-l,4-epoxy-3-phenylisoquinoline, is stable in the solid state as well as in solution and shows no tendency to undergo cycloreversion. 

Irradiation of 4-(but-3-enylidene)-2-phenyl-5(477)-oxazolone 733 leads to the [4 + 2] adduct 734 derived from 733 acting as both a diene and a dienophile in a solid-state photo Diels-Alder reaction (Scheme 7.226). °... 

The thermal Diels-Alder reaction ([4 + 2] cycloaddition) is widespread in the synthesis of fullerene derivatives. In contrast, only a few examples of the photochemical Diels-Alder reaction in solution or in the solid state are known. The first example is described by Tomioka and coworkers [249], Irradiation of ketone 73 and C6o at 10°C with a high pressure mercury lamp through a Pyrex filter led to the formation of 61-hydroxy-61-phenyl-l,9-(methano[l,2]benzenomethano) fullerene 75 (Scheme 29). This compound is unusually unstable and yields the monoalky 1-1,2-dihydrofullerene 76 either by silica gel chromatography or upon heating. 

The first papers dealing with the use of optically active sulfinyl nitroalkenes were published by Fuji et al. in 1991 [71,72]. These papers describe the reaction of Danishefsky s diene with compound 70 (the dienophile containing a six-membered ring does not react) to afford a 1 1 mixture of two compounds (endo-71 and exo-71) resulting from desulfinylation of the endo t) and exo(t) adducts. The optical purity of these compounds ee > 95 %) indicates a complete 7r-facial selectivity in the cycloaddition. X-ray diffraction studies of dienophile 70 showed the s-trans conformation of the sulfinyl oxygen (like that depicted in Scheme 38) in the solid state. By assuming steric approach control for the Diels-Alder reaction, the stereochemistry of endo-71 and exo-71 suggests that such a con-... 

The above-mentioned examples belong to the captivating field of covalent reversible polymers. This field includes examples like reversible polymerization of cyclic oligomers from poly (ethylene terephthalate) [62], thermo-reversible polymers based on Diels-Alder and retro-Diels-Alder reaction (see, for example [63-69]), reversible photo-polymerization in the solid state of molecules... 

Hersh and co-workers reported that catalysis for the Diels-Alder reactions of cy-clopentadiene with a, ff-unsaturated enones was induced by (Me3P)(CO)3(NO)W-FSbFs. In order to probe the mechanism of the Diels-Alder catalysis, a single-crystal X-ray diffraction study of [(Me3P)(CO)3(NO)(acrolein)W] SbF was carried out, and CT-type coordination was found to be present in the solid state [25]. This structure provides clear evidence for the preferred s-trans conformation of the a,//-unsaturated aldehyde unit, which would be expected by theoretical studies (Fig. 1-7). 

Fig- 12. Thennosetting mechanism of MSP based on solid-state NMR results, (a) Diels-Alder reaction between the Ph-C=C and the C=C and (b) the hydrosilylation reaction between the Si-H and the C=C. 

Klin dig s cationic iron(II) complex 39a, derived from tra s-l,2-cyclopentanedi-ol, is a stable, isolable brown solid that possesses sufficient Lewis acidity to catalyze Diels-Alder reactions between unsaturated aldehydes and dienes [95]. The highest selectivities and yields were realized using bromoacrolein as the dienophile (Scheme 32). Further inspection reveals that dienes less reactive than cy-clopentadiene give cycloadducts in higher yield and enantioselectivity, a characteristic that is even more impressive when one considers that the endo and exo transition states produce enantiomeric products for isoprene and 2,3-dimethyl-butadiene. Cyclohexadiene may be used in the reaction with bromoacrolein to afford the cycloadduct in 80% de and >99% ee. In the case of cyclopentadiene. 

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