Sterically Crowded Ring Systems. Preparation and X-ray Crystal Structure of 9,10,11,11,13,13-Hexamethyl-12-oxo-9,10-propanoanthracene
Giguere, RJ; Hoffmann, HMR; Hursthouse, MB; Trotter, J
9,10,11,11,13,13-Hexamethyl-12-oxo-9,10-propanoanthracene (2a) and 9,10-dimethoxy-ll,ll,13,13-tetramethyl- 12-oxo-9,10-propanoanthracene (2b) have been obtained by reductive intermolecular cyclodehalogenation of 2,4-dibromo-2,4-dimethyl-3-pentanone with zinc-copper couple/chlorotrimethylsilane in the presence of the requisite 9,10-disubstituted anthracenes (la,b). The cycloadducts were characterized spectroscopically (1H NMR, 13C NMR, IR, mass spectra), the IR carbonyl band of 2a appearing at 1677 cm-1 in KBr and at 1681 cm-1 in CCI4. Compound 2a was also characterized by single-crystal structure analysis. The crystals are monoclinic, space group P2U with a = 9.737 (2) Ã, b = 9.651 (2) A. c = 9.741 (2) A. β = 103.06 (2)°, and Z = 2. The structure was solved by direct methods and refined to a final R of 0.043 (Rw = 0.048) for 1306 out of 1554 measured data satisfying the condition I > 1.5 Ï(I). This X-ray analysis of 2a shows the clash of the six methyl groups which forces bond angle deformations, nonbonded repulsions, and eclipsing strains. The molecule adopts an approximate structure, with the five carbon atoms C(10), C(ll), C(12), C(13), and C(9) defining the bridge, being almost coplanar, and having an internal carbonyl angle of 130°. The preparations of 2a and 2b are the first examples for 4 + 3 â 7 cyclizations in which two diquaternary carbon-carbon bonds are made. The principle of constructing crowded carbon-carbon bonds by combining nucleophilic but weakly basic tt systems and allyl cations or similar SNl-like alkylating agents seems general. © 1981, American Chemical Society. All rights reserved.