Lewis acid solutions in an oxygen donor-containing solvent ^ or solvent mixture The invention relates to Lewis acid; solutions in asymmetrically substituted ethers or in solvent mixtures containing asymmetrically substituted ethers and hydrocarbons, preparation of the solutions according to the invention, and use of the solutions in inorganic, organic, and organometallic synthesis. The invention further relates to solutions of halide compounds of elements of groups 8, 12, and 13 of the periodic table of the elements in asymmetrically substituted ethers, or in mixtures of asymmetrically substituted ethers and hydrocarbons. Lewis acids find numerous applications in the area of chemical synthesis. By adding Lewis acids for example in Diels-Alder reactions, radical-mediated reactions, Friedel-Crafts alkylations or acylations, or aldol reactions, the yield of a synthesis reaction may be increased, and the regio-, enantio-, or diastereoselectivity of the corresponding reaction may optionally be controlled (H. Yamamoto, Lewis Acids in Organic Synthesis, Wiley-VCH, 2000, Volumes 1 and 2, and citations therein). In addition, Lewis acids find applications in the preparation of organometallic compounds, elemental hydrogen compounds (EHX), elemental hydrogen halide compounds, metal element hydrides, and complexed metal hydrides, for example in the preparation of organocopper or organozinc compounds, alane (AIH3), chloroalanes (AIHXCI(3_X)), zinc borohydride, or lithium aluminum hydride (P. Knochel, P. Jones in Organozinc Reagents (Editors: L. M. Harwood, C. J. Moody), Oxford University Press Inc., New York, 1999, and citations therein; C. Elschenbroich, A. Salzer, Organometallchemie [Organometallic Chemistry], Teubner, 1993, 3rd edition; A. J. Downs, C. R. Pulham, Chem. Soc. Rev. 1994, 175; A. E. Finholt, A. C. Bond, Jr., H. I. Schlesinger, J. Am. Chem. Soc. 1947, 69, 1199). Solutions of Lewis acids have the disadvantage that they decompose upon contact with trace amounts of water, so that the starting materials should have an extremely low water content in order to maximize the content of Lewis acids in the solutions. Halogen-containing Lewis acids are also': frequently used as raw materials for preparing organosubstituted or also chiral Lewis acids. Another field of application is C-C coupling reactions which are catalyzed by Lewis acids (M. Nakamura, S. Ito, K. Matsuo, E. Nakamura, Synlett. 2005, 11, 1794;