Two procedures for the preparation of trimethylsilyl enol ethers from aldehydes and ketones are described. Reaction of the ketone with chlorotrimethylsilane and triethylamine in dimethylformamide solution usually affords an equilibrium mixture of the trimethylsilyl enol ethers. Successive reaction of the ketone with lithium diisopropylamide and with chlorotrimethyhilane in l12-dimethoxyethane solution normally produces a mixture in which the less highly substituted enol ether (from kinetically controlled enolate formation) is the principal product. A number of representative structurally and stereochemically isomeric trimethylsilyl enol ethers have been characterized (see Table 11) and their physical properties have been studied. (1968). (5) For descriptions of the preparation of silyl enol ethers, see (a) C. R. Krager and E. G . Rochow, J . Orwnometal. Chem., 1, 476 (1964); (b) J. I. Baukov, G . S. Burlachenko, and I. F. Lutsenko, ibid., 8, 478 (1965); (c) R. J. Fessenden and J. S. Feesenden, J . Ow. Chem., 82, 3535 (1967); (d) R. Bourhis and E. Frainnet, Bull. Soe. Chim. Fr., 3552 (1967); (e) U. Schmidt and M . Schwochan, Tetrahedron Lett., 4491 (1967); (f) G. Stork and P. F. Hudrlik, J . Amer. Chem. Soc., 90, 4462, 4464 (1968). (6) For examples of the rearrangement of 2-silyl ketones to enol ethers, see (a) A. G . Brook, D . M. MacRae, and W. W.Limburg, J . Amer. Chem. Soc., 89, 5493 (1967); (b) I. F. Lutsenko, Yu. I. Baukov, 0. V. Dudukina, and E. N. Kramarova, J . Organometal. Chem., 11, 35 (1968). (8) The use of trimethylsilyl chloride as a scavenger for alkoxides has been used to advantage in the acyloin condensation. See (a) J. J. Bloomfield, Tetrahedron Lett., 587, 591 (1968); (b) G. E. Gream and 9. Worthley, ibid., 3318 (1868).
Coupling reactions of lithium diphenylcuprate and appropriate aryl halides have been developed to provide efficient syntheses of 1,8-diphenylnaphthalene (Id), 1-iodo-8-phenylnaphthalene (3), 1-phenyl-9,lO-anthraquinone (13), and 1,8-dipheny1-9,lO-anthraquinone (15). Appropriate transformations of the anthraquinones 13 and 15 yielded l&diphenylanthracene (22), 1,9-diphenylanthracene (25), and lJ8,9-triphenylanthracene (26). The spectroscopic properties of all these phenylated anthracenes and naphthalenes are consistent with the existence of these molecules in conformations with the phenyl rings parallel to one another and perpendicular to the plane of the anthracene or naphthalene ring.Earlier publicationss-8 have described preparative routes to anthracene and naphthalene derivatives which contain at least two aryl substituents at adjacent peri positions. Particularly in the naphthalene series, it has been common to obtain these substances by constructing alicyclic intermediates with the necessary carbon skeleton. Aromatization has then been accomplished by a combination of dehydration and/or dehydrogenation steps. These synthetic pathways have provided a sufficient variety of 1,8-diarylnaphthalene derivatives to show (uv, nmr, and dipole moment measurements) ab,c that in solution these molecules exist primarily in the conformation illustrated in structure 1 with the aryl rings approximately parallel to one another and approximately perpendicular to the plane of the naphthalene nucleus. I n derivatives (e.g., la)with meta-substituted aryl rings, the energy barrier to rotation of the substituted ring (AG* = 16 kcal/mol at 25") is sufficiently low that it is not practical to separate cis and trans isomers of structures such as l b or IC at room temperature. A complete X-ray crystallographic analysis of the parent hydrocarbon, 1,8-diphenylnaphthalene (ld),g has shown this molecule to have the dimensions and packing pattern in the crystal (1)
mal conductivity or heat of combustion and applied to correct the observed areas.The relative retention times of materials analyzed are as follows: low-boiling compounds (analyzed on 12 ft X 0.25 in.Abstract: Lithium dialkylor diarylcuprates (R2CuLi) react with aryl iodides in ether solution by competing metal-halogen exchange and coupling reactions. Using appropriate reagents, high yields of the coupled product R-Ar can be obtained by allowing the metal-halogen exchange to proceed to completion in the presence of an excess of RCuLi, and oxidizing the resulting mixture of organometallic species present in solution with nitrobenzene or oxygen. This reaction sequence works well with lithium di-n-alkyl-and diarykuprdtes; it fails with disec-alkyl-and di-t-alkylcuprates. Although most alkyllithium reagents react with aryl iodides only by metalhalogen exchange, methyllithium, uncomplexed with copper, smoothly converts aryl iodides to arylmethanes. Coupling of lithium dialkyl-and diarylcuprates with a/kyI halides appears to take place without significant metalhalogen exchange. The reaction of lithium diphenylcuprate with (-)-(R)-2-bromobutane occurs with the predominant inrersion of configuration (84-92% stereoselectivity) expected of an S~Z-like displacement. Coupling reactions involving an n-alkyl halide as one reaction partner proceed in high yield; those involving r-alkyl halides fail. Lithium di-n-alkyl-, di-sec-alkyl-, and di-t-alkylcopper reagents all couple with n-alkyl halides. A number of preparations for representative copper(1) ate complexes are described, and the sensitivity of the coupling reactions to the method of preparation of the ate complex, to the nature of the solvent, and to the presence of lithium halides, trialkylphosphines, and dialkyl sulfides in solution are discussed.(8) Normant has reported that use of hexamethylphosphoramide as solvent for the reaction of alkyl halides with Grignard reagents leads to synthetically useful yields of coupled products; however, the generality of this solvent effect has not yet been demonstrated: cf.
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