Eight-Membered Heterocycled

7//-Dibenzo[d,g][l,5]telluroazocines 65 are the only representatives of Te, N-containing eight-membered heterocycles known thus far. Treatment of bis(2-bromomethylphenyl)tellurium dibromide 66 with methylamine followed by reduction of the reaction mixture with sodium sulfide gives, instead of the expected 

A -methyl-5//,7//-dibenzo[d,.g][l,5]telluroazocine 65, its thiooxide 67a (X=S, R= Me) (95JA6388). The same type of reaction with benzylamine and sodium selenide gives A-benzyl-5//,7//-dibenzo[d,g][l,5]telluroazocine selenoxide 67b (X = Se, R = CH2Ph). Reaction of the tellurium dibromide 66 with sodium sulfide carried out in the absence of the primary amines affords the eight-member Te,S-containing heterocycle 5//,7//-dibenzo[d,g][l,5]telluroazocine (92CL151). 

The stmcture of the compound 67b was determined by X-ray (95JA6388). The length of the Te=Se bond (2.445 A) is 0.1 A shorter that in bis(2-ethylcarboxy-phenyltellurenyl) selenide (81CSC1353), which points to a greater double bond character of this bond in the former compound. Selenoxide 67b has a boat conformation. The length of the transannular Te- N bond (2.620 A) is appreciably shorter than the corresponding van der Waals contact (3.70 A). The intramolecular coordination Te- N bond thus formed contributes to the stabilization of the boat conformation of the compounds 67. 

On treatment with an equivalent amount of (Et2N)3P, thiotelluroxide 67a is reduced to give telluroazocine 65 in almost quantitative yield, which can be converted into telluroxide 68 by the usual procedures for a diorganyl telluride to telluroxide transformation. It is worth noting that when treated with Lawesson reagent, the telluroxide 68 forms thiotelluroxide 67a, whereas diaryl telluroxides are reduced by this reagent to diaryl tellurides. 

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Two moles of diphenylacetylene insert into the benzyl methyl sulfide complex 481 to afford the eight-membered heterocycle 482[440j. The cinnolinium Salt 483 is prepared by the insertion of alkynes into the azobenzene com-plex[44l]. 

This section deals mainly with nitrogen-containing, eight-membered heterocycles that are able to form a fully conjugated n-electron system, including both 8 n ( pyrrole type ) and iOn ( pyridine type ) systems, i.e. the free lone pair electrons of the heteroatom incorporated, or not incorporated, in conjugation. 

With cyano groups as substituents the diazasemibullvalene cannot be isolated as an intermediate, but reacts directly to provide the eight-membered heterocycle.45... 

Insertion of a C=0 or C=S group between the two amino functions of a 1,5-diamino compound can be used to produce eight-membered heterocycles. Higher membered heterocylces are also obtained on this way ... 

Depending on the relative amount of reagents, three types of products may be obtained in the reaction of hydroxymethylphosphines with amines acyclic aminomethylphosphines, 1,3,5-diazaphosphorinanes, and 1,5,3,7-diazadiphosphacyclooctanes. Indeed, the interaction of phosphorus-containing diols with 1 mol of amine gives eight-membered heterocycles. The initial assumption about the formation of 1,3-azaphosphetidines... 

In the case of the most reactive compounds, substitution at the carbon atom of diselenonium and ditelluronium dications is also a possible pathway. For example, formation of diselenide 117 from selenoxide 115 was explained by demethylation of intermediate dication 116 with trifluoroacetate anion.126 Dealkylation of salt 118, which is stable up to —20°C, leads to formation of nitrilium salt 119. The latter is transformed to amide 120 upon hydrolysis.64 Dealkylation of intermediate diselenonium dication 122 was suggested as the key step in the oxidative synthesis of 1,2,4-diselenazolidines 123 from eight-membered heterocycles 121 (Scheme 46).127... 

The betainic imidazo[l,2- 7][l,2,4]triazinium-olate 107 was found to react as a 1,3-dipole in 1,3-dipolar cycloaddition with ynamines to yield a bridged skeletone 108 <1999T13703> as shown in Scheme 15. This cycloadduct 108 underwent subsequent rearrangement upon heating, and resulted in formation of a fused eight-membered heterocycle 109. With acetylenes other than ynamines, the transformation was found to proceed slowly and in bad yields. The fact that ynamines were used successfully, as well as theoretical considerations (cf. Section 11.17.2) in this chapter, indicated that these Diels-Alder reactions are of inverse electron demand. 

Fig. 9 Five-, seven-, and eight-membered heterocyclic carbene species 55-58...

The addition of other bifunctional nucleophiles also allowed the preparation, in moderate to high yields, of the five-, seven-, and eight-membered heterocyclic carbene species 55-58 (Fig. 9) which, in some cases, were formed along with minor amounts of other derivatives [63]. Related 1,2,3-diheterocyclizations involving (ethoxy)allenylidene complexes have been described, ethanol instead of HNMe2 being released in this case [62],... 

Diazocines are eight-membered heterocycles containing two nitrogen atoms. The A-nitro and A-nitroso derivatives of 1,5-diazocines are energetic materials with potential for use in high-energy propellants. 

Eight-membered heterocyclic systems embrace a very broad variety of compounds ranging from cyclic ethers and imines to highly labile analogs of cyclooctatetraene, and the properties and chemical behavior of these compounds of course depend entirely on the extent and location of unsaturation. For this reason, preparative methods and reactions are treated together for each ring system and oxidation level. 

Most of the highly unsaturated monocyclic eight-membered heterocycles contain one or two nitrogen atoms and have been obtained by bond reorganization processes from strained bicyclic or polycyclic precursors. Although several of the less substituted compounds without stabilizing substituents are highly labile substances, 1,4-dihydro-1,4-diazocines qualify as dihetera[8]annulenes and display distinct aromatic properties. 

One of the earliest authentic eight-membered heterocyclic compounds to be described was 6,7-diphenyldibenzo[e,g][l,4]diazocine (212) prepared by Tauber in 1892 by condensation of 2,2 -diaminobiphenyl and benzil. The diazocine ring is opened hydrolytically under vigorous conditions and is reduced to the 5,6,7,8-tetrahydro compound with sodium amal-... 

Several other sulfur eight-membered heterocycles also exist in boat-chair conformations. 5-Thiacanone (441) has a conformation similar to its oxygen analog (430) and shows barriers to pseudorotation and ring inversion of 28 and 34 kJ mol-1, respectively (80JOC1224). The... 

The bridged biphenyl (472) (67ZN(B)1296> has a rigid twist-boat-chair conformation in the crystal (72CC985) and is an eight-membered heterocycle with two torsional constraints in a 1,3 relationship. It has been resolved by chromatography on cellulose acetate, and AG for racemization is 120 kJ mol-1 (67M1323). 

The planarity or non-planarity of aromatic or potentially aromatic eight-membered heterocycles has already been discussed in Section 5.19.4, and will therefore not be further elaborated upon in this section. 

Reactions which insert an O or NH group next to a carbonyl can be used to form heterocycles (Scheme 23). The Schmidt reaction or the Beckmann rearrangement can accomplish this for nitrogen, the Baeyer-Villiger oxidation does it for oxygen. For example, cyclohexanone is converted in this way into 2-azepinone and into 2-oxepinone cycloheptanone yields the corresponding eight-membered heterocycles. 

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