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10 TT electron heteromonocycle is isolated

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Two chemists at Syracuse University, Syracuse, N.Y., have synthesized the first isolable parent IOTT electron het­eromonocycle—IV- carbethoxy- 1-azacy-clonona-2,4,6,8-tetraene (iV-carbeth-oxyazonin). Dr. A. G. Anastassiou and John Gebrian believe that this may be the first known uncharged IOTT mon-ocycle. Their preliminary observa­tions concerning the facile thermal and photochemical structural reorganiza­tion of this compound indicate that it probably possesses little, if any, aro­matic stability.

Interest in IOTT electron systems has been extensive in the past few years. Hiickel molecular orbital theory pre­dicts aromatic stability for these sys­tems by virtue of their closed-shell con­figurations (4n + 2, n = 2) . The IOTT electron compounds are a synthetic challenge, however, since medium-sized rings (such as the nine-mem-bered rings considered by the Syra­cuse group) are expected to oppose the aromatic system in several ways. If the ring were of a planar, all cis geometry, the bond angles would be severely strained resulting in unfavor­able energetics. If some of the bonds were trans, nonbonded atoms would interfere with each other in the middle of the ring (Mislow's nonbonded re­pulsions ).

Recently, the nine-membered carbo-cycle—cyclononatetraenyl anion—was

synthesized in an all-cis configuration and was found to possess aromatic properties. This suggested to Dr. Anastassiou that the iso-77-electronic, iso-atomic heteromonocycle might also possess enough electron stabilization to compensate for the energetic disad­vantages due to angle strain, resulting in a compound with aromatic proper­ties.

Isolation. Although Dr. Anastassiou had postulated some time ago that N-carbethoxyazonin might be an inter­mediate in the thermal decomposition of N-carbethoxy-9-azabicyclo [6.1.0]-nona-2,4,6-triene into N-carbethoxy-4-azabicyclo[5.2.0]nona-2,5,8-triene, he was unable to obtain the heteromono­cycle. In this new approach, he was not only able to synthesize the IOTT compound but also to isolate it and study several of its properties.

Specifically, the Syracuse scientists irradiate the bicyclic aziridine, N-car-bethoxy-9-azabicyclo [6.1.0] nona-2,4,6-triene, briefly at 0° C. in an ether solu­tion with benzophenone as a sensitizer [/. Am. Chem. Soc, 91, 4011 (1969)] . Using the Woodward-Hoffman orbital symmetry rules they predict that rup­ture of the internal C—C bond of the electronically excited aziridine would result in an all-cis A/-carbethoxyazonin (disrotatory opening). They find, in fact, that 40% of the reaction mixture is the azonin; the remainder is starting material. The nine-membered hetero­monocycle is isolated by column chro­matography of the crude photolysate at about - 1 5 ° C. NMR and UV spec-

Controlled photolysis yields 10 - electron heteromonocycle

N c o o c ^ H g

N-carbethoxy-9-azabicyclo [6.1.0] nona-2f4,6-triene

Characterization of the tetraene:

hv(o°),Sens

hv(o°) V ^ :Ncooc2H5

N-carbethoxy-1-azacyclonona-2,4,6,8-tetraene

^ % :iMCc?oc2H5

3 5 - 4 0 ; VRh

Ncooc2H,

N-carbethoxy-8,9-dihydroindole N-carbethoxyazacyclononane

48 C&EN SEPT. 15, 1969

Staufifer

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inn s mm tral studies are used to help them identify it. Ultraviolet absorption, found at Amax = 220 nm (c = 9000), and A = 270 nm (€ = 2200), is simi­lar to the absorption displayed by cy-clooctatetraene—a nonplanar molecule.

Proof. Dr. Anastassiou and Mr. Gebrian have also found chemical proof for the structure of the nine-membered tetraene. Low-temperature catalytic hydrogenation of IV-carbeth-oxyazonin yields N-carbethoxyazacy-clononane, which is identified by direct comparison with an authentic sample of the azacyclononane.

In regard to the thermal stability of the N-carbethoxyazonin, Dr. Anastas­siou explains that it is only moderately stable at 0° C ; upon standing for four hours at 35-40° C , the ZV-carbethoxy-azonin yields a single product which is characterized by NMR as IV-carbeth-oxy-8,9-dihydroindole. They then cat-alytically hydrogenate the dihydroin-dole at low temperatures to get a fully hydrogenated urethane, N-car-bethoxy-cis-octahydroindole, identical in all respects to an authentic sample synthesized from ds-8,9-octahydroin-dole and ethyl chloroformate.

Dr. Anastassiou adds that direct photolysis at 0° C. of the azonin yields the starting aziridine in large yields.

Nonplanar. Noting that the N-car-bethoxyazonin is definitely less ther­mally stable than the cyclononatetra-enyl anion with which it is isoelec-tronic, Dr. Anastassiou adds that it is slightly more stable than the all-ds-1,3,5,7-cyclononatetraene, which has been recently synthesized by his group at Syracuse. He emphasizes that the heteromonocycle does not appear to have sufficient pi stabilization to offset the angle strain which is inherent in a perfectly planar arrangement. This is probably due to the fact that the nitrogen atom does not contribute its unshared electrons to the pi electron system to the same extent as the car­bon atom in the carbocyclic anion.

In addition, scale Dreiding models indicate a nonplanar arrangement of the nine atoms, with a twofold rotating axis of symmetry containing the hetero-atom and bisecting the remote C—C bond. Furthermore, both NMR and UV data are fully in agreement with a nonplanar arrangement.

Dr. Anastassiou stresses that all bond relocations described for the car-bethoxyazonin occur consistent with "frontier" orbital symmetry rules.

The group at Syracuse is pres­ently looking at the physical and chemical properties of N-carbethoxy-azonin. It is also studying chemical reactions of the compound, such as substitution reactions and metal com-plexing, in an attempt to gain more in­formation about the aromatic or non-aromatic character of the compound.

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SEPT. 15, 1969 C&EN 49

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