Ring contractions 1,2-diazocines

Benzodiazocine and dibenzo[Z>,/][ 1,4]diazocine-diones (220) and (218) are obtained by condensation of diethyl succinate and phthalate, respectively, with o-phenylenediamine (69JOC2138). Contrary to an early report, the use of phthaloyl chloride leads to other products and not the tricyclic diazocine. The dibenzo derivative is also formed in the oxidation of diazabiphenylene (219) (78T495). Ring contraction of (218) occurs readily on heating or on treatment with PCI5 or with base. 

Lithium aluminum hydride reduction of 6,7-diphenyldibenzo[e,r/J[l, 4)diazocine (42) affords a ring-contracted product (43) (59JOC306). 

Saunders and Sprake cyclized the amino ester and acid 14 and obtained the dibenzo[h,/]-l,4-diazocine 15. Dehydrosulfonation of 15 (R = alkyl) afforded ring-contracted products (see Section IV,C)(72JCS(P1)1964). Cycli-zation of the diamino acid 16 using DCC and triethylamine resulted in formation of 1,4-diazocinone 17. This compound was also prepared by a ring-expansion route (see Section II,B,4) (74JOC1710). 

An interesting ring-contraction route to a 1,4-diazocine was reported by Koch and Dessy, who electrochemically reduced the 12-membered cyclic diaza compound 59 and obtained a phenanthrodibenzodiazocine 60 as a minor product (the major product being an azepinodiazepine) (82JOC4452). 

Photolysis of triphenyl-l,2,3-triazine 17z in diethylamine (DEA) and triethylamine (TEA) as solvents gives diphenylethyne as the main product and the diazocine 73, its valence isomer l,2,4,5,6,8-hexaphenyl-3,7-diazabicy-clo[4.2.0]octa-2,4,7-triene 74, and a product of reductive ring contraction of 17z, namely the 3,4,5-triphenylpyrazole 75, as minor products (Equation 10). To generate 75, the amine has to act as a sacrificial donor on the excited triazine and it is proposed that, after a series of consecutive electron and proton transfers (totalling four reduction equivalents), contraction of the six- to the five-membered ring occurs with release of ammonia <2002TF11>. 

Whilst most ring transformations in the synthesis of 1,3-diazepines involve ring-expansion reactions, there are isolated reports of ring contraction. Thus irradiation of the bis-imine (70) affords the diazocine (71) which, after meso-dl transformation, is converted into the bis-l,3-diazepine (72) <95LA657>. The product is identical to that obtained by the electrochemical reduction of (70) <82JOC4452>. 

Beyond the few simple substitution reactions summarized below, the dominant chemistry reported for the fully-unsaturated diazocines (15) and (16) involves their thermal and photochemical ring contraction. Both the thermal and photochemical reactions appear to proceed via intermediate formation of bicyclo[4.2.0]octatriene valence tautomers. However, the product distribution differs considerably between the thermal and photochemical reactions. Thus, thermolysis of (15) in dode-cane solution at 140°C affords benzene (43.2%) and pyridine (56.8%) at 175°C, benzene becomes the major product (55.7% vs. 44.3% pyridine) (Scheme 3). Pyridazine, however, is not formed. In contrast, photolysis in tetrahydrofuran at >300 nm affords only benzene neither pyridine nor pyridazine are detected <79JOCl264>. 

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