Anthracenes 4 + 3 cycloadditions

Few isothiazoles undergo simple cycloaddition reactions. 4-Nitroisothiazoles add to alkynes (see Section 4.17.7.4). With 5-thiones (84) and dimethyl acetylenedicarboxylate, addition to both sulfur atoms leads to 1,3-dithioles (85) (77SST(4)339, 80H(14)785, 81H(16)156, 81H(16)595). Isothiazol-3-one 1-oxide and the corresponding 1,1-dioxide give normal adducts with cyclopentadiene and anthracene (80MI41700), and saccharin forms simple 1 1 or 1 2 adducts with dimethyl acetylenedicarboxylate (72IJC(B)881). 

Bis(trifluoromethyl)-l,l-dicyanoethylene is a very reactive dienophile. It undergoes facile and high-yield [2+4] cycloadditions with 1,3-dienes, cyclopen-tadiene, and anthracene [707] (equation 86). It is reactive enough in a Diels-Alder reaction with styrene [702] (equation 86). 

For a recent discussion on the stereochemical aspects of the Diels-Alder reaction with vinyl sulphoxides see References 662, 663. It should be pointed out that vinyl sulphoxides can be considered in [2 + 4]-cycloadditions as acetylene synthons since the sulphinyl moiety may be removed from the product by sulphenic acid elimination. Paquette and coworkers took advantage of this fact in the synthesis of properly substituted anthracenes 562664, (equation 360). 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

Luche and coworkers [34] investigated the mechanistic aspects of Diels-Alder reactions of anthracene with either 1,4-benzoquinone or maleic anhydride. The cycloaddition of anthracene with maleic anhydride in DCM is slow under US irradiation in the presence or absence of 5% tris (p-bromophenyl) aminium hexachloroantimonate (the classical Bauld monoelectronic oxidant, TBPA), whereas the Diels Alder reaction of 1,4-benzoquinone with anthracene in DCM under US irradiation at 80 °C is slow in the absence of 5 % TBPA but proceeds very quickly and with high yield at 25 °C in the presence of TBPA. This last cycloaddition is also strongly accelerated when carried out under stirring solely at 0°C with 1% FeCh. The US-promoted Diels Alder reaction in the presence of TBPA has been justified by hypothesizing a mechanism via radical-cation of diene, which is operative if the electronic affinity of dienophile is not too weak. 

The mechanism of cycloaddition reaction of maleic anhydride with anthracene promoted by US irradiation has been the subject of many controversies [32, 37]. Recent work of Da Cunha and Garrigues [35] shows that the reaction proceeds in toluene solution in the 60 85 °C temperature range in 6 3 h. 

The single-electron transfer from one excited component to the other component acceptor, as the critical step prior to cycloaddition of photo-induced Diels Alder reactions, has been demonstrated [43] for the reaction of anthracene with maleic anhydride and various maleimides carried out in chloroform under irradiation by a medium-pressure mercury lamp (500 W). The (singlet) excited anthracene ( AN ), generated by the actinic light, is quenched by dienophile... 

Rideout and Breslow first reported [2a] the kinetic data for the accelerating effect of water, for the Diels Alder reactions of cyclopentadiene with methyl vinyl ketone and acrylonitrile and the cycloaddition of anthracene-9-carbinol with N-ethylmaleimide, giving impetus to research in this area (Table 6.1). The reaction in water is 28 to 740 times faster than in the apolar hydrocarbon isooctane. By adding lithium chloride (salting-out agent) the reaction rate increases 2.5 times further, while the presence of guanidinium chloride decreases it. The authors suggested that this exceptional effect of water is the result of a combination of two factors the polarity of the medium and the... 

An example of a /zctcro-Diels-Alder reaction in SC-CO2 is the cycloaddition of anthracene with 4-phenyl-1,2,4-triazoline-3,5-dione, carried out at 40 °C and at a pressures between 75 and 216 bar [86]. The rate constant increases with decreasing pressure and the highest reactivity was observed at the critical pressure. The value of the rate constant at the critical pressure was higher than that observed in liquid CHCI3 and MeCN at the same temperature. At higher pressures, the rate is slower than that in the polar solvents, which reflects the apolar nature of SC-CO2 as a solvent. 

The study of the cycloaddition behavior of l,l-dichloro-2-neopentylsilene, C Si =CHCH2Bu (2) [3], reveals the high polarity of the Si=C bond and a strong electrophilicity. The [4+2] cycloaddition reactions with anthracene (3), cyclopentadiene (4) and fulvenes (5) proceed as expected surprising, however, the Diels-Alder reactions with dienes are of lower activity, like naphthalene (6) and furans (7). 

Kochi and co-workers engineered heteromolecular charge-transfer crystals of a tricyclic dithiin 34 stacked alternately with anthracene, which can undergo spontaneous Diels-Alder cycloaddition to give a novel artificial crystal system <2001JA87, 2001JA4951>. 

Cyclopentadiene (6) reacted with 105 at 0°C to give 108 (entry 1). At 100°C, butadiene (12) afforded 109 (entry 2). No [2 + 2] cycloadduct was formed in either reaction. Perfluoromethylenecyclopropane (105) failed to react with cis,cis- or cis,trans-2,4-hexadiene at 100 °C, although 110 was readily formed from trans,frans-2,4-hexadiene (106) under these conditions [29] (entry 3). Anthracene (107) added to 105 at 100 °C. The dienophilicity of 105 is exceptional when compared with the reactivity of simple fluoroolefins, such as perfluoro-isobutylene, which require 150 and 200 °C to undergo cycloaddition to cyclopentadiene [30] and anthracene, respectively. 

Among the dienes known as weakly reactive are anthracene (1), metacrolein di-methylhydrazone (2) and 3,6-diphenyl-l,2,4,5-tetrazine (3). DA cycloadditions with these dienes require long reaction times under classical heating conditions (Tab. 7.1). 

Methods have been described that involve microwave-assisted graphite-supported dry media for the cycloaddition of anthracene, 1-azadienes and 1,2,4,5-tetrazines with several C-C dienophiles and carbonyl compounds in hetero-Diels-Alder reactions [35], This technique leads to a shortening of reaction times, a situation that enables work to be undertaken at ambient pressure in an open reactor to avoid the formation of unwanted compounds by thermal decomposition of reagents or products. 

The thermal Diels-Alder reactions of anthracene with electron-poor olefinic acceptors such as tetracyanoethylene, maleic anhydride, maleimides, etc. have been studied extensively. It is noteworthy that these reactions are often accelerated in the presence of light. Since photoinduced [4 + 2] cycloadditions are symmetry-forbidden according to the Woodward-Hoffman rules, an electron-transfer mechanism has been suggested to reconcile experiment and theory.212 For example, photocycloaddition of anthracene to maleic anhydride and various maleimides occurs in high yield (> 90%) under conditions in which the thermal reaction is completely suppressed (equation 75). 

The site selectivity in the Diels-Alder reactions of 19 and 20 with anthracene is especially noteworthy. The cycloaddition of 19 takes place at... 

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. 

Use has been made of the C-N cleavage in the conversion of the bicyclic tertiary amines, derived from the 4tc + 2tc cycloaddition of pyrroles and isoindoles with benzynes, into aromatic systems, e.g. naphthalen-l,4-imines and anthracen-9,10-imines yield naphthalenes and anthracenes with the extrusion of the nitrogen bridge [24] in yields which are higher than those obtained by standard oxidation procedures. 

Kochi and co-workers studied photoinduced Diels-Alder cycloadditions via direct photoexcitation of anthracene as a diene with maleic anhydride and various maleimides as dienophiles. Here, fluorescence-quenching experiments, time-resolved absorption measurements, and the effect of solvent polarity provide striking evidence for an ion-radical pair to be the decisive intermediate [83],... 

This first example of the participation of benzyne in a polar cycloaddition gains added importance from two ring-oi>ening reactions which Fields et al. have carried out on the adducts (26). The first is simply thermolysis to afford 9-(2-pyridyl)-anthracene derivatives (27), while the second involves thermolysis of the reduction product of 26, affording anthracene derivatives (28) in excellent yields. 

Maprotiline Maprotiline, iV-methyl-9,10-ethanoanthracen-9(10H)-propylamine (7.1.22), is synthesized by a 4-1-2 cycloaddition reaction of 9-(3-methylaminopropyl)anthracene with ethylene [39 1]. 

Four different sorts of double bonds are accessible for a [4+2]-cycloaddition. Anthracene reacts with CggF jg in refluxing toluene mainly by addition to two of the four possible positions, giving the adducts 15 and 16 (Scheme 9.9) [69]. Seemingly, the least sterically hindered position is attacked. Furthermore, the less stable CjoFjg-anthracene adduct 16 can rearrange to the sterically more stable isomer 15. 

A large number of electron-rich polycyclic aromatic systems including anthracenes, azaanthracenes and larger homologues arenes undergo [4 - - 2]-cycloaddition with singlet... 

Thiete sulfones show an irregular behavior pattern when involved in cycloaddition reactions. With 1,3-dienes, dienamines, enamines, ynamines, diazoalkenes, cyclopropadiene, and its substitution products, furan, and anthracene, the addition proceeds in the normal fashion. With certain Diels-Alder reagents such as tetraphenylcyclopentadienone (tetracycloneX however, the cyclic sulfones react anomalously. The Diels-Alder adducts undergo decomposition with SO 2 and CO extrusion to a seven-membered ring, the tetraphenylcycloheptatriene 223. Bicyclic octadienone is produced as well (Eq. 62). The mechanism of this unusual reaction is proposed by... 

---