The chemical behavior of alkoxo-oxovanadium clusters lies between that of metal oxides and metal alkoxides. [1,2] These compounds are important as precursors for the synthesis of new polyoxometalates and as reagents for sol-gel processes and chemical vapor deposition (CVD) methods for the generation of metal oxides.[3] The interest in this substance class for basic research as well as for technical applications is attributed to vanadium's facile and often reversible change in its oxidation state.[4] By varying the alkoxo ligands and the V IV /V V ratio in such clusters, it is possible to control the properties and the composition of the metal oxides obtained by their thermal decomposition, which is of particular importance, for example, for the development of heterogeneous catalysts based on vanadium oxides.It [2b] or by complementary ligands such as squarate, [7] oxalate, acetylacetonate, or phenylphosphonate [2b] as well as metal [8] and organometallic [9] complexes. Recently we synthesized a series of dodecamethoxohep-which, while differing in their V IV /V V ratios (n = 3, 4, 5, 6), remain identical in their structural composition. Noteworthy is the fact that these compounds were not only observed as intermediates by electrochemical and spectroscopical methods, but have also been isolated and characterized by X-ray structural analysis.Besides vanadium oxides and vanadates, [1] polyoxovanadates containing large organic cations, [10] and thus soluble in organic solvents, as well as orthovanadic acid esters VO(OR) 3 of primary alcohols have also proven to be suitable precursors for the synthesis of alkoxo-oxovanadium compounds. [7,11] The uncharged coumpound of mixed valency [V 8À (M = Nb, Ta), [12] yet unknown for vanadium, by replacing the 12 m 2 -bridging oxygen atoms with methoxo groups. In this cluster, four vanadium atoms are present in the oxidation state + IV and the remaining two in the oxidation state + v. As could be expected from the strong molar mass peak in the mass spectrum (see Experimental Section), 1 can be sublimed under reduced pressure without noticeable decomposition. This makes 1 an interesting candidate for CVD experiments. During the thermal decomposition of the compound in the absence of oxygen, continuous mass loss occurs beginning at 200 8C and ending with the formation of V 2 O 3 at approximately 380 8C (mass loss (%): found: 43.0; calcd: 43.1).The cyclic voltammogram [13] of 1 (Figure 2) turns out to be a challenge for synthetic chemists because it reveals a series of electronic transitions assigned to hexametalate clusters(4Àn) of identical composition and structure, yet having different V IV /V V ratios. Starting from 1 (n = 4) and using suitable reducing and oxidizing agents, it was indeed possible to synthesize the singly (n = 5) and doubly (n = 6) reduced clusters in compounds 3 b, 3 a, 2 a, and 2 b respectively, as well as the singly [14] For the preparation of the singly reduced cluster in 3 b tetrabutylammonium tetrabutylborate was used, which acts as a mild reducing a...

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Addition of lithiated methoxyallene to pivalonitrile afforded after aqueous workup the expected iminoallene 1 in excellent yield. Treatment of this intermediate with silver nitrate accomplished the desired cyclization to the electron-rich pyrrole derivative 2 in moderate yield. Surprisingly, trifluoroacetic acid converted iminoallene 1 to a mixture of enamide 3 and trifluoromethyl-substituted pyridinol 4 (together with its tautomer 5). A plausible mechanism proposed for this intriguing transformation involves addition of trifluoroacetate to the central allene carbon atom of an allenyl iminium intermediate as crucial step. Enamide 3 is converted to pyridinol 4 by an intramolecular aldol-type process. A practical direct synthesis of trifluoromethyl-substituted pyridinols 4, 10, 11, and 12 starting from typical nitriles and methoxyallene was established. Pyridinol 10 shows an interesting crystal packing with three molecules in the elementary cell and a remarkable helical supramolecular arrangement. Trifluoromethyl-substituted pyridinol 4 was converted to the corresponding pyridyl nonaflate 13, which is an excellent precursor for palladium-catalyzed reactions leading to pyridine derivatives 14-16 in good to excellent yields. The new synthesis of trifluoromethyl-substituted pyridines disclosed here demonstrates a novel reactivity pattern of lithiated methoxyallene which is incorporated into the products as the unusual tripolar synthon B.

A series of 4-pyridyl nonaflates was coupled with several terminal alkynes to efficiently provide new 4-alkynyl-substituted 3-alkoxypyridine derivatives. Apt conditions were developed for their conversion into furo[2,3-c]pyridines. Sonogashira reactions of 4-alkoxy-substituted 3-pyridyl nonaflates allowed an access to regioisomeric furo[3,2-c]pyridines. For both types of alkynyl-substituted alkoxypyridines an alternative method for cyclization employing iodine monochloride furnished iodinated furo[2,3-c]-or furo[3,2-c]pyrid-