New Reactions of Contraction of the o-Quinone Ring with the Formation of Derivatives of 2-(2-Indolyl)-cyclopenta[b]pyrrole-3,4-diones and Pyrindino[1,2-a]indoles: A Combined Experimental and Density Functional Theory Investigation

Abstract

Derivatives of 2-(2-indolyl)-cyclopenta­[b]­pyrrole-3,4-diones and pyrindino­[1,2-a]­indoles were synthesized by a new reaction of contraction of the o-quinone ring, and their structures were investigated by X-ray crystallography and nuclear magnetic resonance spectroscopy. The mechanisms of the reactions were suggested based on density functional theory calculations of the critical parts of the potential energy surfaces.

Reaction of quinaldine with 4,6-di(tert-butyl)-3-nitro-1,2-benzoquinone. Dependence of the outcome on the reaction conditions and a deeper insight into the mechanism

“…In the reaction of the o -quinone ring contraction, proton transfer occurs only from the hydroxyl group, as shown for transition state A’ , and is not accompanied by proton transfer from the heteroatom. This reaction pathway can be fully realized in the case of a highly basic heterocyclic nitrogen atom, as we have proved by the example of 2,3,3-trimethylindoline [ 9 ]. It should be mentioned that the reaction of o -quinone ring contraction competes with the expansion reaction only at high temperatures and, in this case, it is not realized at all at room temperature or moderate temperature (60–70 °C).…”

“…* Calculated energy barrier of the main stage of the reaction of 2-methylquinoline with 1,2-benzoquinone [ 17 ]. ** Calculated energy barrier of the main stage of the reaction of 2,3,3-trimethylindoline with 1,2-benzoquinone [ 9 ]. …”

“…This reaction proceeds under more harsh conditions, which is confirmed by higher yields of 8 when boiling 1 and 2 in acetic acid ( Table 1 , method E). The detailed mechanism of the o -quinone ring contraction was studied using quantum chemical calculations on the example of the interaction of 2,3,3-trimethylindolines with quinone 2 [ 9 ]. The structural 2- tert -butylcyclopent-2-enone fragment in product 8 can be formed by contraction of the o -quinoline ring of nitroquinone 2 .…”

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Scheme 4 The proposed mechanism for the formation of compound 8 . ** Calculated energy barrier of the main stage of the reaction of 2,3,3-trimethylindoline with 1,2-benzoquinone [ 9 ].

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“…The reactivity of 4,6-di ( tert -butyl)-3-nitro-1,2-benzoquinone in reactions with 2-methylenactive nitrogen heterocycles differs significantly and can result in the development of other reaction pathways competing with the o -quinone ring expansion. These processes are accompanied by the formation not only of the corresponding 2-hetaryl-4-nitro-1,3-tropolones as the main products, but also of other heterocycles [ 9 , 10 ], which can also act as ligand systems to obtain metal complex compounds with high biological activity [ [11] , [12] , [13] , [14] , [15] , [16] ].…”