Chromium acid ester

Chromic acid oxidation of saturated hydrocarbons starts with hydrogen abstraction to give a caged radical pair.113,114 The collapse of the latter leads to a chromium(IV) ester, which hydrolyzes to the product tertiary alcohol. The postulation of the caged pair was necessary to explain the high degree of retention in oxidation of (+)-3-methylheptane 113... 

What effect does co-ordination of the amino acid derivative 3.1 have upon the rate of hydrolysis The rates of the hydrolysis reactions depicted in Fig. 3-8 are only slightly more rapid than those of the free amino acid esters, and, in general, the rates of reactions involving monodentate TV-bonded ligands very closely resemble those for acid-catalysed hydration. This monodentate bonding mode is only exhibited with non-labile ions such as cobalt(m) or chromium(m) and is relatively rare even then. 

A further example shows that one may add more amino acids to an amino acid ester -earbene complex by employing the customary methods of peptide chemistry (64). Starting from pentacarbonyl[GlyOMe (phenyl) -earbene]chromium(0) and using the. V-hydroxysuccinimide/dicyclohexyl-carbodiimide) (NHS/DCCD) method, we succeeded in synthesizing the sequence 14 to 17 of human proinsulin C-peptide (Scheme 3). 

Fig. 17.10. Mechanism of the Cr(VI) oxidation of alcohols to carbonyl compounds. The oxidation proceeds via the chromium(VI) acid ester A ("chromic acid ester") and yields chromium(IV) acid. The chromium(IV) acid may either disproportionate in an "inorganic" reaction or oxidize the alcohol to the hydroxy-substituted radical B. This radical is subsequently oxidized to the carbonyl compound by Cr(VI), which is reduced to Cr(V) acid in the process. This Cr(V) acid also is able to oxidize the alcohol to the carbonyl compound while it is undergoing reduction to a Cr(III) compound.

Fatty alcohols are obtained by direct hydrogenation of fatty acids or by hydrogenation of fatty acid esters. Typically, this is performed over copper catalysts at elevated temperature (170°C-270°C) and pressure (40-300 bar hydrogen) [26], By this route, completely saturated fatty alcohols are produced. In the past, unsaturated fatty alcohols were produced via hydrolysis of whale oil (a natural wax occurring in whale blubber) or by reduction of waxes with sodium (Bouveault-Blanc reduction). Today, they can be obtained by selective hydrogenation at even higher temperatures (250°C-280°C), but lower pressure up to 25 bar over metal oxides (zinc oxide, chromium oxide, iron oxide, or cadmium oxide) or partially deactivated copper chromite catalysts [26],... 

Steps 1 and 2 constitute an oxidation by the ionic pathway by Cr(VI), and steps 6 and 7 a similar oxidation by Cr(V), which is produced by an electron-transfer process. Either Cr(VI) (step 3) or Cr(IV) (step 4) [Cr(IV) is produced in step 2] may abstract a hydrogen and the resulting acyl radical is converted to carboxylic acid in step 5. Thus, chromium in three oxidation states is instrumental in oxidizing aldehydes. Still another possible process has been proposed in which the chromic acid ester decomposes as follows ... 

In the proposed vapor phase processes for organic acid synthesis, carbon monoxide is passed with the vaporized aliphatic alcohol over catalysts similar in nature to those employed in the pressure synthesis of higher alcohols from hydrogen-carbon monoxide mixtures. Pressures on the order of 200 atmospheres are employed. Temperatures of about 200° to 300° C. are preferred but it is necessary to use somewhat higher ones in order to obtain sufficient reaction. Mixtures of the oxides of zinc and chromium or copper, promoted with alkali or alkaline earth oxides, are suitable catalysts for the formation of carbon-carbon linkages.97 Catalysts composed of an alkali, chromium, and molybdenum have been claimed for the synthesis of mixtures of higher alcohols, aldehydes, acids, esters, etc., from carbon monoxide and vaporized aliphatic alcohols as methanol, ethanol, etc., at temperatures of about 420° C. and a pressure of 200 atmospheres.98... 

Another applieation of photogenerated metal coordinated ketenes is based on the addition of protie nucleophiles and has been exploited in the synthesis of amino aeids and peptides. [66] As usual, the reactive intermediate is generated by photolysis of an aminoearbene complex 46, which may be a-alkylated in a previous step. The oxazolidine auxiliary applied successfully inducing asymmetry in the P-laetam formation, allowed an enantioselective synthesis of amino aeids. Sinee both enantiomers of the auxiliary may be obtained from the corresponding phenyl glycine enantiomer, natural (5) and non-natural R) amino acid esters 47 are accessible via this route (Scheme 25). A recent review on synthetical applications of chromium carbene photochemistry has been published, [li]... 

Many fillers are covered with a coating to improve their processing qualities. This measure reduces the attractant forces between the particles and improves their dispersibility in the PVC. The coating materials used in most cases for fillers based on carbonates or oxides are fatty acids and fatty acid esters, and silanes, titanates and chromium complexes for fillers based on silicates and hydroxides. The amounts of coating substances added in relation to the amount of filler used may be as high as 3 % by mass. 

Cupric oxide-zinc carbonate-chromium oxide Alcohols from carboxylic acid esters Continuous process s. 18, 103... 

Photolysis of a number of chromium carbene complexes enriched in 3c at the carbene atom and the carbonyl groups results in products containing two adjacent isotopiCally labelled carbon atoms. Products include 1,2-i>is-enriched amino acid esters (particularly useful as biochemical and biophysical probes), a dipeptide, a cyclobutenone, a P-lactam, and a dioxocyclam.372... 

No compound such as silicon chromate has been reported, but evidently silicon can be linked through oxygen to hexavalent chromium. A chromic acid ester of a silanol group was made by Schmidt and Schmidbaur (64), who prepared the tri-... 

Aminolysis of methoxy carbene metal pentacarbonyl complexes of the chromium group yields amino carbene compounds. If the aminolysis is performed with amino acid esters, amino carbene derivatives of amino acids are obtained [162,163]. The metal carbene is relatively stable under a variety of conditions but can be removed with TFA. It may therefore be regarded as an amino protecting group in peptide synthesis. The metal amino carbene amino acid may be activated and coupled to other amino acid esters. Following this idea, Weiss and Fischer prepared various dipeptides, the tripeptide (OC)5Cr(Ph)-Ala-Ala-Ala-OMe 69 [162] and the tetrapeptide (OC)5Cr(Ph)-Gly-Gly-Pro-Gly-OMe 70 (Scheme 5.35) [163]. Treatment with cone. TFA at 20 °C for 10 min. removes the metal carbene group and furnishes the free peptide esters along with Cr(CO)g. Removal of the metal carbene is also possible with 80% acetic acid (80 °C, 30 min.), which leaves Boc... 

Another example of the photoassisted substrate conversion due to a short-lived intermediate in the ground state is shown in figure 6. Chromic acid esters form chromium(V)/alkoxy radical pairs within the photochemical primary reaction. In the presence of such iron(III), cobalt(III), or copper(II) complexes which are able to interact coordinatively or by second sphere interactions with chromium(V) within the radical pair cage reoxidation to chromate(VI) occurs under simultaneous reduction of the metal complexes to corresponding iron(II), cobalt(II), and copper(I) species, respectively. Unfortunately, the efficiency of this photoassisted reaction is limited by... 

Scrimin, P., Tecilla, P., Tonellato, U. MetaUomiceUes as catalysts of the hydrolysis of carboxylic and phosphoric acid esters. J. Org. Chem. 1991, 56(1), 161-166. Mondal, S.K., Das, M., Kar, D., Das, A.K. MiceUar effect on the reaction of chromium(VI) oxidation of formaldehyde in the presence and absence of picoUnic acid in aqueous acidic media a kinetic study. Indian J. Chem. Sect. A 2001, 40A(4), 352-360. 

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