Naphthalene, Diels-Alder with

A few routes to new silenes, usually involving flash vacuum pyrolysis at high temperatures, have been reported. Silenes were proposed as the result of the thermal expulsion of trimethylmethoxysilane, or a similar volatile fragment, from the starting material but frequently, proof that the silenes proposed to account for the observed products were in fact formed was not provided.116 119 The other thermal route employed was the retro-Diels-Alder regeneration of a silene from an adduct with an aromatic compound—often a 9,10-anthracene or 1,4-naphthalene adduct or, in some cases, a 1,4-benzene adduct, as illustrated in Eq. (19).120... 

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). 

Diarylmethylenecyclopropa[6]naphthalenes 14, unlike their benzene parent counterparts which give cycloaddition reactions at the cyclopropene bridge bond [10a], react on the exo double bond in Diels-Alder cycloadditions (see Sect. 2.1.1) [10b]. The reactions of 14 with the highly electron-deficient acetylenic(phenyl)iodonium triflate 584 give products 586a and 587, which are believed to derive from unstable primary [2 + 2] cycloadducts 585 (Scheme 82) [10b],... 

Generally, benzene and naphthalene derivatives show only little reactivity as dienes in Diels-Alder synthesis, contrary to anthracene and the higher acene derivatives which are frequently used as dienes. Exceptions are the reactions of benzene and naphthalene derivatives with highly reactive dienophiles such as dicyanoacetylene (DCA), which... 

Here again the high reactivity is due to the gain of aromatic stabilization in the adduct. Polycyclic aromatics are moderately reactive as the diene component in Diels-Alder reactions. Anthracene forms adducts with a number of dienophiles. The addition occurs at the centre ring. The naphthalene ring system is much less reactive. 

When the cyclic acetylene 260 is generated from a suitable precursor, it undergoes an isomerization reaction spontaneously generating naphthalene (263) and benzo-fulvene (264) as the finally isolable products. Very likely the process begins with a retro-Diels-Alder reaction to the [3]cumulene 261, which in a cascade reaction via the semicydic allene 262 rearranges to 263 and 264 [111]. 

Intramolecular Diels—Alder reactions without prior 1,4-addition of oxygen (cf. previous section) have similarly been postulated for a number of [2.2]paracyclophane analogs. When [2](2,5)furano[2](l,4)naphthalen-ophane (42) is heated in excess dimethyl acetylenedicarboxylate at 100 °C, a polycyclic compound of structure 134 is formed. The mechanism of formation of 134 is most probably as follows 101> the furan moiety reacts as active diene component in an intermolecular Diels—Alder reaction to give 135. This is followed by further intramolecular 1,4-addition with the unsubstituted naphthalene ring as diene component to give the product 133, which has been isolated. 

With one exception, naphthalen-l,4-imines with a double bond between C-2 and C-3 are not known to dissociate thermally by either possible retro-Diels-Alder pathway (the reverse of reactions described in Section III, A, 1 and 2), and the enthalpy requirements for the formation of a benzyne or an acylic acetylene are doubtless unfavorable. However, the mass spectra of compounds 93-99 reveal one important fragmentation of the molecular ions to be loss of dimethyl acetylene-dicarboxylate, and another fragmentation pathway involves the formation of nitrilium ions MeC=NR and PhC=NR from 93-95 and 96-99, respectively. ... 

The reactivity of the produced complexes was also examined [30a,b]. Since the benzopyranylidene complex 106 has an electron-deficient diene moiety due to the strong electron-withdrawing nature of W(CO)5 group, 106 is expected to undergo inverse electron-demand Diels-Alder reaction with electron-rich alkenes. In fact, naphthalenes 116 variously substituted at the 1-, 2-, and 3-positions were prepared by the reaction of benzopyranylidene complexes 106 and typical electron-rich alkenes such as vinyl ethers, ketene acetals, and enamines through the Diels-Alder adducts 115, which simultaneously eliminated W(CO)6 and an alcohol or an amine at rt (Scheme 5.35). 

Aromatic compounds can participate in both [2+2] and [4+2] photocycloaddition reactions with uracil derivatives to give either benzocyclobutane or ethenoquinazoline (barrelene) derivatives, which can then undergo a number of subsequent photochemical reactions. The products obtained are dependent upon the reaction conditions, and thus the photocycloaddition reaction between naphthalenes 470 and l,3-dimethyl-5-fluorouracil 471 in cyclohexane gave 4a-fluoro-5,10-ethenobenzo[/]quinazolines 472 as products as a result of a [4+2] photocycloaddition (photo-Diels-Alder) reaction <2002TL3113, 2003H(61)377>. 

The isopropylation of anthracene gave similar results to that of naphthalene.84 The selectivities for 2-isopropylanthracene (2-IPA) and 2,6-diisopropylanthra-cene (2,6-DIP A) over HM(25) were as high as 91% and 47%, respectively. On the other hand, the selectivities over HY were as low as 59% and 8% for both products. The Diels-Alder reaction of anthracene with propylene occurred at higher temperatures to yield large amounts of the adducts such as 11 -methyl-9, lO-dihydro-etano-anthracene. However, their formation was prevented by the addition of a small quantity of water. 

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... 

By far the most important property of benzo[c] furans is their capacity to act as 471-components in cycloaddition reactions. Whereas the reactions described before 1969 were almost always of the Diels-Alder type, more recent investigations have shown that they can also participate in [7 4 + 714]-and [714 + TCgj-addition (Section IV,C). In this chapter Diels-Alder reactions will be discussed. Benzo[c]furans have been used for two main purposes. First, Diels-Alder adducts with olefinic compounds can conveniently be dehydrated to naphthalene derivatives or higher condensed hydrocarbons not easily accessible by other methods second, benzo[c]furans are excellent... 

Activated alkynes have been shown to react in a Diels-Alder style with phthalans to furnish polysubstituted naphthalenes (80CJC2573). 1,1-Diethoxyphthalan (41) was prepared by alkylation of phthalide (40) with triethyloxonium fluoroborate, followed by treatment... 

The intramolecular Diels-Alder addition of a benzyne to a furan has been exploited in a synthesis of mansonone E (81TL4877). The benzyne (211), generated from the anthranilic acid (210), yields the adduct (212 86%) which is easily converted into the naphthalene (213 Scheme 79). A similar addition was achieved by generating the benzyne by treating an o -dibromobenzene with butyllithium. 

The first successful Diels-Alder addition of thiophene appears to have been with tetrafluorobenzyne (66CC143). The adduct apparently loses sulfur, resulting in the isolation of the tetrafluoronaphthalene in 40% yield (Scheme 78). Other thiophene derivatives (69T25) and fluorinated benzynes <71JCS(C)604) have been similarly reacted. Subsequently it was found (80H 14)647) that unsubstituted benzyne, generated from diphenyliodonium-2-carboxylate, could be trapped with thiophene to form naphthalene in 33% yield. The earlier failure to add thiophene to benzyne generated by other methods must have been due to intervention of side reactions. Thiophyne also has been similarly trapped to yield benzo[A]thiophene (see Section 3.14.2.2). 

In Chapter 13 we encountered the Diels-Alder reaction, which involves addition of a reactive alkene (dienophile) to the 1,4 positions of a conjugated diene. Neither benzene nor naphthalene reacts significantly with dienophiles on simple heating, but anthracene does react. Cycloaddition occurs between the 9,10 positions ... 

Tetradehydrodianthracene undergoes 4 + 2-cycloaddition with electron-deficient dienes such as 1,2,4,5-tetrazines.263 The Diels-Alder reactions of [3.3 ortho-anthracenophanes witli A-(p-nitro, chloro, or methoxy-substituted phenyl)malehnides yield approximately equal quantities of inside and outside adducts.264 The photooxidation of bulky water-soluble 1,4-disubstituted naphthalenes with singlet oxygen yields both the expected 1,4- and the unexpected 5,8-endoperoxides.265... 

Deoxygenation of naphthalene-1,4-endoxides. A new method for conversion of these Diels-Alder adducts of benzynes with furanes to naphthalenes consists in reduction with sodium borohydride in trifluoroacetic acid.5 Excess acid is used when the substrate bears methyl groups at the bridgeheads. Substrates lacking such groups tend to undergo acid-catalyzed rearrangement to naphthols, but are reduced satisfactorily with THF as solvent and a limited amount of acid. 

Tsuge and co-workers - building on work by Bailey and Blomquist [5] - have already made extensive use of the preparative potential of these so-called diene transmissive Diels-Alder additions (DTDA additions) [8]. Clearly, the dienophile does not have to be identical in the two stages of the reaction, which increases the preparative potential of these double cycloadditions considerably. The adducts of type 22 can be further processed in various ways, for example by dehydrogenation to give 1,2,6,7-tetrasubstituted naphthalene derivatives as will be discussed below. Consecutive reactions of this type with their excellent atom economy are of considerable interest particularly in view of the current efforts to increase the efficiency of organic transformations. 

Mukaiyama reported that 4—10 mol% of Ph2Sn=S and AgC104 catalyzed the Diels-Alder reaction between but-3-ene-2-one (183) or (E)-1 -phenylbut-2-en-1 -one (182) and unactivated dienes 184-186 to afford the products 187-192 in 57-95% yield (Scheme 2.48).80 The reaction with cyclic dienes 185 and 186 was highly endo-selective (dr 99 1). Naphthalene-1,4-dione also participated in the reaction with similar yields. 

This reaction is conducted on a scale of >200,000 tonnes/annum. More recently, another elegant application of electrochemistry on a multi-thousand-tonne scale has been HydroQuebec s investment in a cerium (IV)-mediated oxidation of naphthalene to naphthaquinone, a process licensed from W.R. Grace.87 HydroQuebec then uses the naphthaquinone in a Diels-Alder reaction with butadiene to produce anthraquinone ... 

Three types of cycloaddition products are generally obtained (Sch. 1). While [2+2] (ortho) and [2+3] (meta) cycloaddition are frequently described, the [2+4] (para or photo-Diels-Alder reaction) pathway is rarely observed in benzene ring systems. With naphthalene systems however, the para cycloaddition occurs more frequently [6,8]. The photo-Diels-Alder reaction and other photocyclization reactions are also observed with anthracene derivatives and higher condensed aromatic compounds. However, these reaction are not treated in this chapter since they are caused by the particular photophysical and photochemical properties of these compounds [6,9]. 

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