Anthracenes benzynes

The benzyne adducts prepared from A -methylpyrrole (and A -methylisoindole) are deaminated conveniently by dichlorocarbene generated under phase-transfer conditions (81JOC1025 to give a convenient route to substituted naphthalenes (134) (and anthracenes) (Scheme 49). 

When benzyne is generated in the presence of potential dienes, additions at the highly strained triple bond occur. Among the types of compounds that give Diels-Alder addition products are furans, cyclopentadienones, and anthracene. 

Because of their strong aromatic character, benzene and naphthalene are very unreactive as dienes however anthracene 19 reacts with highly reactive dienophiles, such as dehydrobenzene (benzyne) 20 ... 

In the context of this section it is important that Ruchardt and Tan (1970 a) found that (solid) benzenediazonium fluoroborate gave benzyne adducts with potassium acetate in the presence of aryne trapping agents such as tetracyclone or anthracene. This is, however, not the case if water is present (Cadogan, 1971). As a consequence of these observations, Cadogan et al. (1971) simplified the formation of arynes from diazonium ions by converting aniline or its substitution products into arynes in a... 

Dehydrobenzene or benzyne 158 can be trapped by all manner of species. 1,2-Dehydro-o-carborane 159 has been shown to undergo many of the same reactions as its two-dimensional relative, 1,2-dehydrobenzene. Although dehydroaromatic molecules can be formed in a variety of ways, synthetic pathways to 1,2-dehydro-o-carborane are quite limited. An effective procedure reported so far78 first forms the dianion by deprotonation of o-carborane with 2 equiv. of butyllithium. Precipitated dilithium carborane is then treated with 1 equiv. of bromine at 0°C to form the soluble bromo anion 160. Thermolysis of 160 with anthracene, furan, and thiophene as substrates leads to the adducts 161-164.79 80 1,2-Dehydro-o-carborane reacts with norbomadiene to give both homo 2+4 and 2+2 addition, leading to three products 165-167, in a 7 1 ratio79. An acyclic diene, 2,3-dimethyl-... 

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. 

Benzyne reacts with 185 to give 17% of the ring-opened 1 1 adduct (203) and 9% of anthracene. The formation of 203 is explained by a... 

The formation of anthracene in reactions of 185 and 186 with benzyne, which was unexplained by Wittig et aZ., possibly is due to an alternative reaction of the intermediate zwitterion (202) with another molecule of benzjme or with a benzyne precursor. Benzyne reacted with the isoindole (206) to give the tetramethyltriptycene (208) and, in a separate run using excess of the benzyne precursor, W-benzylcarbazole. The latter product would appear to be made up of the iV-benzyl group from an intermediate anthracen-9,10-imine (207) and two molecules of benzyne. Mass spectral evidence also implicated the adduct 207, and the formation of 208 was attributed to benzyne-induced deamination of 207 to 1,4,9,10-tetramethylanthracene, which was trapped by further addition of benzyne across the 9- and 10-positions. 

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. 

The discovery that benzyne (Scheme 30) adds to acridizinium ion to produce an azoniatrip-tycene (34) which, on reduction with sodium borohydride followed by hydrolysis, affords anthracene, offers a new route to the synthesis of related hydrocarbons (71JOC3002). This, the Fields anthracene synthesis, has already found application in the preparation of 1,4,5,8,9-pentamethylanthracene (75TL4639). 

In 1972 Berry and co-worker detected 3,4-pyridyne (101) by MS. Trapping experiments also provided evidence for the existence of this intermediate, although the chemistry of 101 differs considerably from that of o-benzyne. Thus, neither anthracene nor dimethylfulvene form Diels-Alder adducts with 101. Nam and Leroi were able to generate 101 in nitrogen matrices at 13 K and characterized it by IR spectroscopy. Irradiation of 3,4-pyridinedicarboxylic anhydride (103) with 1 > 340 nm results in formation of 101, which upon short wavelength photolysis (k > 210 nm) fragments to buta-l,3-diyne (104) and HCN, and to acetylene (105) and cyanoacetylene (106, Scheme 16.24). The assignment of an intense... 

Aromatic compounds can also behave as dienes.858 Benzene is very unreactive toward dienophiles very few dienophiles (one of them is benzyne) have been reported to give Diels-Alder adducts with it.859 Naphthalene and phenanthrene are also quite resistant, though naphthalene has given Diels-Alder addition at high pressures.860 However, anthracene and other compounds with at least three linear benzene rings give Diels-Alder reactions readily. The interesting compound triptycene can be prepared by a Diels-Alder reaction between benzyne and anthracene 861... 

The presence of benzyne has been more directly demonstrated by trapping experiments. For example, the generation of benzyne in the presence of anthracene gives the Diels-Alder adduct, triptycene (Equation 7.94).187... 

The benzyne functions as a dienophile towards reactive diene systems. The reactivity of the 9,10-positions in anthracene is well known (Diels-Alder reaction, Section 7.6), and addition of benzyne to 9-bromoanthracene yields the interesting cage-ring alkyl halide 9-bromotriptycene (9-bromo-9,10-o-benzenoanthracene). The reaction is incomplete and some unreacted 9-bromoanthracene remains in the crude reaction products, but may be removed by virtue of its ready conversion into a maleic anhydride adduct in a further Diels-Alder-type reaction. 

The AuCl-catalysed 4 + 2-cycloaddition of benzyne with o-alkynyl(oxo)benzenes produced anthracene derivatives having a ketone in the 9-position, in good to high yields under mild conditions.118 Hypervalent iodine compounds, [5-acyl-2-(trimethyl-silyl)]iodonium triflates, readily yielded acylbenzynes which could be trapped with furan.119 Both DMAD and benzyne reacted with borabenzene to yield substituted borabarrelenes and borabenzobarrelenes, respectively.120... 

Triptycene is the Diels-Alder cycloaddition product from the reaction of benzyne with anthracene (compound A). Although anthracene is aromatic, it is able to undergo cycloaddition at the center ring with a dienophile because the adduct retains the stabilization energy of two benzene rings. 

The widely differing product distributions frorn the two anhydrides reflect the difference in structure and reactivity of the two naphthalynes. 2-3-Naphthalyne (14) behaves like benzyne, giving anthracene by 1,2-and 1,4-addition, and phenylnaphthalene by insertion, as well as some binaphthylene (15) by dimerization ... 

As benzothiophene was a product of both the reaction of benzyne with thiophene and the pyrolysis of thiophene alone (Fields and Meyerson, 1966d), we investigated the reaction of benzyne with benzothiophene. Pyrolysis of a mixture of phthalic anhydride and benzothiophene gave the products shown in Table 11. Anthracene and dibenzothiophene probably arose via 1,2-, and phenanthrene via 1,4-addition to the thiophene ring ... 

The reaction of anthracene with benzyne gives a product, C2oH 4, that shows only four peaks in its l3C-NMR spectrum. Show the structure of this product and explain its formation. 

Benzyne may not look like a good dienophile but it is an unstable electrophilic molecule so it must have a low-energy LUMO (71 of the triple bond). If benzyne is generated in the presence of a diene, efficient Diels-Alder reactions take place. Anthracene gives a specially interesting product... 

It is difficult to draw this mechanism convincingly. The two flat molecules approach each other in orthogonal planes, so that the oribitals of the localized k bond of benzyne bond with the p orbitals on the central ring of anthracene. 

The dienic character of the central ring is illustrated by the reaction of anthracene with dienophiles in Diels-Alder reactions. For example, c -butenedioic anhydride (maleic anhydride) reacts readily when benzyne is generated in the presence of anthracene, triptycene (19) is produced. 

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