Synthesis and Valence Isomerization of 2, 7-Dihydro-2, 2, 7, 7-tetramethylpyrene

Abstract

Synthesis and Valence Isomerization of 2 , ? -D i h y d r 0 -2 , 2 , 1 , 7 -t e t r a m 1The potential to study the photochemical and thermal valence isomerization shown in Scheme 1 has been our prime incentive to synthesize the cross-conjugated polyene 2.7-dlhydropyrene. We have chosen the blocked and sterically shielded derivative 2,7-dihydro-2,2,7,7-tetramethylpyrene (1) as a model compound to protect the system against polymerization, tautomerization, disproportionation, and oxidation/l/. Briv.-l)oz. Dr. J. Wirz, Dip1.-Chem. E. Hasler, Dr. H. Angliker, Dip1.-Chem. J. Ackermann, Physikalisch-Chemisches Institut der Wniversitat Basel, Klingelbergatrasse 80, CH-4056 Basel (Switzerland). ** This work is part of project nr. 2.616-0.80 of the Swiss National Science Foundation. Due to the geometrical constraints, the ring opening of a cyclopropane fused to the peri-positions of naphthalene can proceed only in a disrotatory manner; the valence isomerizations (L)* (2,syn) and (?,anti) therefore classify as intramolecular n 6 s + n6s and rr6a + s i o n s , respectively. They are thus "forbidden" in the sense of the Woodward-Hoffmann rules /2/ and are likely to proceed via a reactive biradlcal intermediate (2) with the n-topology Of 1,8-naphthoquinodimethane / 3 / . cycloadditions/rever--Scheme 1: ( 1 ) (3) -1430 -(2,syn) (2,ontilThe preparation of (1) was achieved in five steps starting from naphthalene-l,4,5,8rtetracarboxylic acid dianhydride ( : I by the reaction sequence shown in scheme 2 (preparative procedures are given below). To date, we have obtained the tetraketone ( 2 ) in poor yields only (5% relative to (q)), whereas the three-step reduction of (2) to (1) proceeded smoothly (overall yields > 50%) without isomer separation of the intermediate products (0, RIOH and Br, resp.). The dihydropyrene (1) is isolable as a yellow, crystalline solid (dec. 180°1 and is stable in aerated solutions when light is rigorously excluded. The NMR chemical shifts of the vinyl hydrogens substantiate the expected polyene character of and provide no evidence for the presence of an 'aromatic" ring current in the central naphthalene moiety.