Anthracene retro-Diels-Alder reaction

Starting from 27, cyclo-Cig was prepared in the gas phase by laser flash heating and the neutral product, formed by stepwise elimination of three anthracene molecules in retro-Diels-Alder reactions, was detected by resonant two-photon-ionization time-of-flight mass spectrometry [23]. However, all attempts to prepare macroscopic quantities of the cyclocarbon by flash vacuum pyrolysis using solvent-assisted sublimation [50] only afforded anthracene and polymeric material. 

Most forms of carbon interact strongly with microwaves. When irradiated at 2.45 GHz, amorphous carbon and graphite in powdered form rapidly reach ca. 1000 °C within 1 min of irradiation. An example of a solvent-free Diels-Alder reaction performed on a graphite support is shown in Scheme 4.5. Here, diethyl fuma-rate and anthracene adsorbed on graphite reacted within 1 min of microwave irradiation under open-vessel conditions to provide the corresponding cycloadduct in 92% yield [14]. The maximum temperature recorded by an IR-pyrometer was 370 °C. In other cases, it was necessary to reduce the microwave power and therefore the reaction temperature in order to avoid retro-Diels-Alder reactions [13]. 

Similarly, the use of a higher input power in retro-Diels-Alder reactions of anthracene derivatives has been reported to afford complete reaction in 3-5 min [36], This method is an alternative to the use of flash thermolysis. The use of graphite is a prerequisite for obtaining high temperatures in a short time. 

Heating the anthracene monoadduct in toluene affords the component molecules [1,8], which demonstrates a facile retro-Diels-Alder reaction. A thermal gravimetric analysis (TGA) of solid Cgo(anthracene) shows a cleavage of the anthracene moiety... 

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. 

These examples already prove that the potential of such reactions for the synthesis of stable fuUerene derivatives is restricted due to the facile cycloreversion to the starting materials. Nevertheless, cycloreversion can also be useful. Reversibility of dimefhylanthracene addition was utilized for the selective synthesis of Ti -symme-trical hexakisadducts (see Chapter 10) [12]. In another example, a dendritic polyamidoamine-addend was reversibly attached to via an anthracene anchor (Figure 4.1) [14, 15]. The dendrofullerene, which is soluble in polar solvents, can be obtained in 70% yield and the retro-Diels-Alder reaction at 45 °C proceeds with a conversion rate of more than 90%. 

The stereospecificity of methanol addition to neopentylsilenes has been investigated by Jones and Bates68. The mild thermal retro-Diels-Alder reaction (at ca 200 °C) of E and Z anthracene [4 + 2] cycloadducts 110 liberates stereospecifically the corresponding silenes 111, which are trapped by methanol. The ratio of the diastereomeric products 396a/396b coincides with the E/Z ratio of the precursors 110 (equation 117). In photochemical reactions of similar silene precursors, alcohols were used also to probe the decomposition mechanism69. 

Retro-Diels-Alder reactions giving thiocarbonyl compounds are favored when simultaneously a comparatively stable diene is formed1. This is the case with anthracene and cyclopentadiene Diels-Alder adducts 81 and 82 which, upon heating, afford a wide array of thioaldehydes and thioketones (equation 84). These adducts are stable at room temperature and have become a convenient way of storing very reactive thiocarbonyl compounds. Cyclopentadiene is the cheapest and most reactive diene for use in Diels-Alder reactions. Also, strain in the bridged cycloadducts facilitates retro-Diels-Alder cleavage224. 

Nitrosyl cyanide (109) has been briefly examined as a dienophile by Kirby et This intermediate can be generated from nitrosyl chloride and silver cyanide (equation 42) and trapped with 9,10-diinethyl-anthracene to yield adduct (110). As with acyl nitroso compounds, this adduct can be used as a stable source of (109) via a retro Diels-Alder reaction. 

Anodier selenoaldehyde preparation has been develqied by Kirby and Trethewey (equation 107). Dienophile (199) can be trapped directly by 1,3-dienes or by anthracene to give adduct (200). Retro Diels-Alder reaction of (200) in the presence of another diene affords a new cycloadduct. 

Formation by a concerted cycloaddition reaction A phenantlirene tnolecuie would be formed in two ways as below tlie [4 + 2] cycloaddition of biphenyl with two benzene molecules followed by a retro Diels-Alder reaction and by dehydrogenation the 14 + 2] cycloaddition of naphthalene witli benzene followed by the retro Diels-Alder reaction and dehydrogenation. An anthracene molecule would be formed by the [4 + 2] cycloaddition of naphtlialene with benzene followed by retro Diels-Alder reaction and by dehydrogenation. In these cases, tlie yield relation would be phenanthrene anthracene, because biphenyl is produced more abundantly than is naphtlialene during shock reaction. This statistical consideration fits the experimental results. 

Compound 2 was chosen as a direct precursor to cyclo-Ci since it should lose three anthracene molecules in a retro-Diels-Alder reaction under thermal conditions (Scheme 13-1). The synthesis of 2 (Scheme 13-2) started with the Diels-Alder reaction of anthracene and tra 5-dichloroethene [10], followed by dehydrochlorination and subsequently bromination to 10. The latter conversion was best achieved by simply adding elemental bromine to a solution of the vinyl anion formed with n-BuLi [11]. Palladium-catalyzed alkynylation of 10 with trimethylsilylacetylene in n-butylamine followed by deprotection with aqueous KOH in MeOH gave the diethynyl derivative 11 as very unstable crystals, which in one case exploded spontaneously. 

Two chemical syntheses of stereospecifically labeled serines (91, 92) relied on the stereospecific reduction of the acetylene 76 followed by anti addition of MeOBr (91) or HOBr (92) to the product 77. The second of these syntheses (92) is shown in Scheme 21. It involved catalytic reduction of the anthracene adducts of the propiolates 76, Ha = H, and 76 with hydrogen and deuterium, respectively, followed by a retro Diels-Alder reaction. The products were the (Z)-isomer 77, Ha = H, and the (f)-isomer 7, Hg = H, respectively. Reaction of these with Af-bromosuccinimide in sulfuric acid gave three parts of the bromohydrins 78 together with one part of the alternative regioisomers. 

Wang GW, Chen Z-X, Murata Y, Komatsu K. [60]Fullerene adducts with 9-substituted anthracenes mechanochemical preparation and retro Diels-Alder reaction. Tetrahedron 2005 61 4851-6. 

---