Quinones, 11 Synthesis of 3,7‐Di‐tert‐butyl‐9,10‐dimethyl‐2,6‐anthraquinone

Abstract

A 2,6-anthraquinone only stabilized by alkyl groups, 3.7-di-terrbutyl-9,10-dimethyl-2,6-anthraquinone (7), has been synthesized for the first time. The positions of the alkyl shielding groups has been determined on the basis of simple MO considerations.Extended anthraquinones, i.e. anthraquinones with their carbonyl groups in different rings, are not known2'. The reason is the high reactivity towards nucleophiles such as water and the high tendency to undergo Diels-Alder type dimerisations if s-cis diene partial structures are present3). A possibility to avoid this difficulty is the introduction of substituents, which reduce the reactivity by (a) lowering the ground state energy e.g. by resonance, thus enhancing the thermodynamic stability or (b) by shielding the molecule from attack sterically, thus enhancing the kinetic stability.Attempts to isolate the 9,lO-diphenyl derivative of 2,6-anthraquinone failed4'. Only 3,7-dihydroxy-9,10-dimethyl-2,6-anthraquinone has been synthesized? The hydroxy groups should belong to the group (a) and the methyl groups predominantly to the group (b) substituents. But substituents of group (a) as OH should influence the x-electron system considerably. Therefore we tried to synthesize a 2,6-anthraquinone stabilized only by alkyl groups, which belong mainly to the group (b) substituents. They should not influence the x-electron system of the quinone markedly.2,6-Anthraquinone possesses no s-cis diene partial structure, therefore there is no danger of Diels-Alder dimerisation. The positions which are especially liable for attack of water may be predicted by P M O calculations. For the estimation of relative rates of the attack of a nucleophilic reagent to various but similar substrates Fukui proposed the equation6)$" is a reactivity index, called superdelocalisability, representing binding interactions between the reactants. The greater the negative value of S!"' the faster the reaction with the nucleophile should be. ti represents the energy of the ith unoccupied MO of the substrate (with i = 1, 2, 3 ,,,). c!'' is the A 0 coefficient at the attacked atom T of the substrate in the ifh MO, a is the HOMO energy of the nucleophile (in this case water) and / I the resonance integral. But as our calculations showed3) about 90% of the value of the reac- Ein nur durch Alkylgruppen stabilisiertes 2,6-Anthrachinon, 3,7-Di-tert-butyl-9.10-dimethyl-2,6-anthrachinon (7), konnte erstmals synthetisiert werden. Die Stellung der Alkyl-Schutzgruppen wurde auf Grund einfacher MO-Betrachtungen bestimmt. tivity index $"' have their origin in the interaction of the HOMO of the nucleophile with the LUMO of the substrate. Therefore it should be sufficient to look at the LUMO coefficients of the substrate in order to determine the positions of the attack of water qualitatively. The Table shows some LUMO-A0 coefficients of 2,6-anthraquinone as calculated by MNDO. Water should attack preferently the equivalent positions 1 and 5 and with similar rates the meso positions 9 and 10, the carbon atoms with the grea...

New Persistent Radicals: Synthesis and Electronic Spin Structure of 2,5-Di-tert-butyl-6- Oxophenalenoxyl Derivatives

“…The radical precursors, 2,5-di- tert -butyl-6-hydroxyphenalenone derivatives 3 and 4 , were efficiently synthesized from commercially available 2,7-dimethoxynaphthalene ( 5 ) in nine steps (Scheme 1) . Although the tert -butylation of 5 by the conventional method ( t -BuCl, AlCl 3 ) failed, the reaction of 5 with t -BuOH in CF 3 COOH in the presence of concentrated H 2 SO 4 successfully gave di- tert -butylated product 6 in quantitative yield, which was formylated by Vilsmeier−Haack reaction. Reaction of the aldehyde 7 with lithium enolates generated from tert -butyl acetate or benzyl tert -butylacetate with lithium diisopropylamide (LDA) gave the aldol products 8 or 9 as a mixture of diastereomers, respectively.…”