Chromium complex compounds with pyridine

Use of chromium(VI) reagents for oxidative transformation is compromised by their inherent toxicity, involved preparation of the various complex forms (with pyridine or acetic acid), and cumbersome work-up procedures. Chromium trioxide (CrOa) immobilized on premoistened alumina enables efficient oxidation of benzyl alcohols to carbonyl compounds by simple mixing with different substrates... 

A better reagent for the limited oxidation of primary alcohols to aldehydes is pyridinium chlorochromate (PCC), a complex of chromium trioxide with pyridine and HC1. PCC oxidizes most primary alcohols to aldehydes in excellent yields. Unlike most other oxidants, PCC is soluble in nonpolar solvents such as dichloromethane (CH2C12), which is an excellent solvent for most organic compounds. PCC can also serve as a mild reagent for oxidizing secondary alcohols to ketones. 

The chromium carbonyl linkers 1.40 (98) and 1.41 (99) were prepared from commercial triphenylphospine resin and respectively from pre-formed p-arene chromium carbenes and Fischer chromium amino carbenes. Their SP elaboration is followed by cleavage with pyridine at reflux for 2 h (1.40) and with iodine in DCM for 1 h at rt (1.41) both linkers produce the desired compounds in good yields. A similar cobalt carbonyl linker 1.42 (100) was prepared as a mixmre of mono- (1.42a) and bis- (1.42b) phosphine complex, either from pre-formed alkyne complexes on triphenylphosphine resin or by direct alkyne loading on the bisphosphine cobalt complex traceless cleavage was obtained after SP transformations by aerial oxidation (DCM, O2, hp, 72 h, rt) and modified alkynes were released with good yields and... 

The reactions of the phosphabenzene system [124] confirm these conclusions. Phosphabenzenes have low basicity towards hard acids. They are not protonated by CF3CO2H nor alkylated by trialkyloxonium salts. However, soft acids attack at phosphorus. For instance, 2,4,6-triphenyl-phosphabenzene forms compounds 4 with the hexacarbonyl derivatives of Cr, W and Mo in which the phosphorus coordinates to the metal, possibly with metal-P back-donation. The complexes 4 rearrange photochemically or thermally affording the 67i-heteroarene complexes 5. Although 2,4,6-triphenyl-pyridine is protonated on nitrogen, it undergoes complex formation with chromium hexacarbonyl exclusively on the phenyl moieties yielding the ri -arene complexes 6 [125]. 

The condensation of metal vapors with hexafluorobenzene and benzene gives unstable arene complexes of V, Cr, Mn, Fe, Co, Ni, and Pd, for instance, [Cr(C6H6)(C5F6)]. The condensation of vapors of chromium or iron with the mixture of vapors of benzene and PF3 or hexafluorobenzene and PF3 leads to the formation, with low yields, of the derivatives [Cr(C6F6)(PF3)3], [Cr(PhH)(PF3)3], and [Fe(PhH)(PF3)2]. Heteroleptic compounds of chromium, for example, [ (CgF ) (Pp3)3], are stable in contrast to [Cr(C6F6)2] which is unstable and explosive. The condensation method was also utilized for the preparation of hexahapto pyridine complexes of chromium. 

CHROMIUM TRIOXIDE-PYRIDINE COMPLEX, preparation in situ, 55, 84 Chrysene, 58,15, 16 fzans-Cinnamaldehyde, 57, 85 Cinnamaldehyde dimethylacetal, 57, 84 Cinnamyl alcohol, 56,105 58, 9 2-Cinnamylthio-2-thiazoline, 56, 82 Citric acid, 58,43 Citronellal, 58, 107, 112 Cleavage of methyl ethers with iodotri-methylsilane, 59, 35 Cobalt(II) acetylacetonate, 57, 13 Conjugate addition of aryl aldehydes, 59, 53 Copper (I) bromide, 58, 52, 54, 56 59,123 COPPER CATALYZED ARYLATION OF /3-DlCARBONYL COMPOUNDS, 58, 52 Copper (I) chloride, 57, 34 Copper (II) chloride, 56, 10 Copper(I) iodide, 55, 105, 123, 124 Copper(I) oxide, 59, 206 Copper(ll) oxide, 56, 10 Copper salts of carboxylic acids, 59, 127 Copper(l) thiophenoxide, 55, 123 59, 210 Copper(l) trifluoromethanesulfonate, 59, 202... 

B. General Oxidation Procedure for Alcohols. A sufficient quantity of a 5% solution of dipyridine chromium (VI) oxide (Note 1) in anhydrous dichloromethane (Note 7) is prepared to provide a sixfold molar ratio of complex to alcohol. This excess is usually required for complete oxidation to the aldehyde. The freshly prepared, pure complex dissolves completely in dichloromethane at 25° at 5% concentration to give a deep red solution, but solutions usually contain small amounts of brown, insoluble material when prepared from crude complex (Note 8). The alcohol, either pure or as a solution in anhydrous methylene chloride, is added to the red solution in one portion with stirring at room temperature or lower. The oxidation of unhindered primary (and secondary) alcohols proceeds to completion within 5 minutes to 15 minutes at 25° with deposition of brownish-black, polymeric, reduced chromium-pyridine products (Note 9). When deposition of reduced chromium compounds is complete (monitoring the reaction by gas chromatography or thin-layer chromatography analysis is helpful), the supernatant liquid is decanted from the (usually tarry) precipitate and the precipitate is rinsed thoroughly with dichloromethane (Note 10). 

Potassium Tetrathiocyanato-dipyridino-chromium, [Cr py2 (SCN)4]K.2H,0, is prepared by heating potassium ehromithiocyanate with anhydrous pyridine. The melt is reerystallised from a little wTater, and on cooling a stable red powder separates. The thiocyanato-groups are all within the complex, and the salt in solution therefore gives no coloration with ferric chloride. It is decomposed by chlorine in presence of cold water with formation of tetraquo-dipyridino-chromic chloride. If crystallised in the dry state from pyridine it forms transparent red crystals of the addition compound, [Cr py2(SCN)JK.4py, which quickly effloresce in air. 

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