Chromium Experimental Procedures

Experimental Procedure 2.1.1. Preparation of a Chromium Carbene Complex from Chromium Hexaearbonyl [Methoxy(methyl)carbene]pentacarbonylchro-mium [37, p 130]... 

Experimental Procedure 2.2.1. Photolysis of a Chromium Carbene Complex 2-Benzyl-4-benzyloxy-4-methyl-2,3,4,4a,7,7a-hexahydro-li/-cyclopenta[c]pyri-din-3-one [294]... 

Experimental Procedure 2.2.2. Cyclopropanation with a Chromium Carbene Complex Diethyl n-a/i5-3-Methoxy-3-phenylcyclopropane-l,2-dicart>oxylate [325]. 

Experimental Procedure 2.2.3. Benzannulation with a Chromium Phenylcar-hene Complex [ 1 -4 4a,8a-Ti -2-(r rr-Butyl)-4-methoxy-1 -naphthol]tricarbonyl-chromium [37, p 140]... 

Photolysis or thermolysis of heteroatom-substituted chromium carbene complexes can lead to the formation of ketene-like intermediates (cf. Sections 2.2.3 and 2.2.5). The reaction of these intermediates with tertiary amines can yield ammonium ylides, which can undergo Stevens rearrangement [294,365,366] (see also Entry 6, Table 2.14 and Experimental Procedure 2.2.1). This reaction sequence has been used to prepare pyrrolidones and other nitrogen-containing heterocycles. Examples of such reactions are given in Figure 2.31 and Table 2.21. 

Experimental Procedure 2.2.8. [4 + 3] Cycloaddition of a Chromium Vinylcarbene Complex to a 1-Azadiene rranj-i-(2-Furyl)-2-inethoxy-5-methyl-4,5-dihydro-3H-azepine [390]... 

Experimental Procedure 2.2.4. Benzannulation with a Chromium Furylcarbene... 

Experimental procedure. The explosions of the hydrogen mixtures at an initial pressure equal to atmospheric were carried out in a cylinder 330 mm long of 8 liters volume welded from a sheet of stainless chromium-nickel steel. The mixture fired by a spark plug screwed in the top. Before the experiment 100 cm3 of water was poured into the cylinder so that the explosive mixture contained 2% water vapor. 

Recently a fairly inexpensive way of high-temperature experimentation has been found to investigate refractory sulfides and related multicomponent systems up to temperatures of nearly 2000 °C using resistance furnaces. These techniques are discussed below and applied to some sulfide systems, in particular of those metals which belong to the VI-B group. The binary systems chromium-sulfur, molybdenum-sulfur, tungsten-sulfur, as well as some other ternary and quaternary systems and their reactions are reviewed and completed within the limits of the new experimental procedure. 

The present overview deals with the application of Fischer chromium carbene complexes in the benzannulation reaction for the preparation of highly substituted aromatic compounds. Before focussing on specific arenes (Section 8.5), details of the mechanism are given (Section 8.2), and the scope and limitations of the reaction are defined (Section 8.3). A short description of the experimental procedure is given thereafter (Section 8.4). Finally, the contribution deals with the application of the chromium carbene benzannulation to natural compounds and molecules with biological activity (Section 8.6). 

Chromium-based oxidants are probably the most widely used of all oxidizing agents. Over the years they have been continually developed and modified to overcome the typical problems that occur during oxidation and to accept wider ranges of substrates with improved selectivities. They have been accepted readily by synthesis chemists since they are easy to handle and are often off the shelf reagents . However, they are not without their problems worit-up can be problematical overoxidation can occur, and, at all times, removal of the product from toxic chromium contaminants is a concern, especially with respect to large scale preparations. In an attempt to circumvent these problems the trend has been to develop the use of catalytic and/or supported reagents. Hiis review is concerned for the most part with the ai lica-tions and limitations of more recent chromium(VI) oxidants. Several other comprehensive reviews have appeared in this area and should be consulted for more detailed descriptions of older methods, chro-mium(V) oxidants, mechanism of oxidation and for typical experimental procedures. 

One other notable method has been used in the preparation of mixed transition metal molybdates, amongst many other oxide systems. This novel method(TT) involves preparation of the mixed metal oxides via an amorphous precursor such as a citrate salt of the appropriate metals, and then thermal decomposition of the complex to yield the resulting mixed oxides. The experimental procedures are described in four French patents(78-81), giving details of many different preparations including a proposed M0O3 rich, chromium doped iron molybdate, prepared as a possible selective oxidation catalyst. 

Typical Experimental Procedure -BuLi (1.4 mmol, 0.88 mL in THE) was added to a solution of TMS acetylene (1.4mmol in 2mL of THE) at -78°C under an N2 atmosphere. After 5 minutes a solution of chromium complex (1.1 mmol) in THE (3mL) was added dropwise. The yellow mixture was stirred for 1 hour at -78°C and then quenched with saturated aqueous NH4CI. Workup followed by chromatography furnished the products in high yields (70 to 90%). The compounds in CH2CI2 were exposed to sunlight for decomplexation to generate the chiral compounds. 

The experimental procedure to prepare molybdenum nitride was similar to that for preparing chromium nitride. During microwave processing of the fluidized molybdenum powders, yellow and orange streaks appeared after an induction time of about... 

Another common approach consists of the comparison between the experimental rate constants and theoretical values calculated by the procedure developed by Marcus (1956), Marcus and Sutin (1985) as well as Hush (1958). This classical procedure is used widely. Premsingh et al. (2004) gave the relevant references and described a detailed procedure to analyze the ion-radical reaction between anilines and chromium (V) complexes of azomethyne derivatives. Lepage et al. (2003) studied transformation of para-substituted thioanisoles to corresponding methylarylsulfoxides... 

Chromium, iron and selenium in foodstuffs from animal sources have been analyzed after closed vessel microwave digestion by collision cell ICP-MS.29 The limits of quantification (LOQ) of the analytical procedure were estimated under optimized experimental conditions with 0.025, 0.086 and 0.041 mg kg for Cr, Fe and Se, respectively. The results obtained for the three elements in nine different certified reference materials were, in all cases, in good agreement with the certified values.29... 

Chromium(II) salts are extremely sensitive and when affected by the least traces of air are oxidized to form chro-mium(III) salts. Because of these circumstances research and preparation of such compounds is very troublesome. The difficulties arise both in the investigation of the product s purity and absolute dryness during preparation, and in the removal of the final product from the protective conditions of the reaction field. The methods of obtaining chromium (II) salts employed at present are based on a rather complicated apparatus with indirect control. Thus preparation procedure has to be fixed a priori. Our method, described below, has been adapted particularly to prepare extremely sensitive compounds such as the chro-mium(II) salts. Using our apparatus the experimenter is able to intervene at any time and to handle the contents of the box in the same manner as he would on laboratory tables. 

There are two important experimental factors that must be accounted for if we are to be successful in running 15N experiments. The 15N nucleus tends to relax very slowly Tj s of greater than 80 seconds have been measured. Thus, either long pulse delays must be incorporated into our pulse sequence or, alternatively, we could provide another route for spin relaxation. A common procedure is to add a catalytic amount of chromium (III) acetylacetonate, a paramagnetic substance, whose unpaired electrons efficiently stimulate transfer of spin. In cases where Tt s are not known (and not intended to be measured), pulse delays and pulse angles must be considered carefully because the signal from one (or more) 15N resonance can accrue too slowly or be missed altogether. 

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