Chromium-Based Initiators

Only few reports exist on the successful use of chromium carbenes for ROMP. These few examples comprise the ROMP of 2,3-dihydrofuran catalyzed by Cr(=CPh2)(CO)5 [44] as well as the ROMP of 2,3,4,5-tetrahydrooxypin-2-yl acetate [45]. 

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An interesting consequence of the fast formation of the chromic ester is that, sometimes, chromium-based oxidants counter-intuitively are able to oxidize quicker alcohols possessing a greater steric hindrance, as the initially fonned chromic ester releases greater tension on evolving to a carbonyl. Thus, axial alcohols are oxidized quicker than equatorial ones with chromic acid.6 The reverse—a somehow expected behavior—is observed, for example in oxidations with activated DMSO.7... 

As a proof of concept, the initial hypothesis has been illustrated along the text through two different applications focused on the recovery of heavy metals and mineral acids from industrial effluents (1) the recovery of zinc from spent chromium-based passivation baths and SPSs using the EPT and (2) the recovery of hydrochloric acid from SPSs by means of membrane diffusion dialysis. From the analysis previously undertaken, it is concluded that the process design needs the kinetic analysis of the separation and recovery... 

Once the chromate intermediate is formed, pyridine (or any other base) initiates an E2 reaction to give an aldehyde (an oxidized alcohol) and H0Cr02 (a reduced chromium species) (Fig. 16.68). 

Unlike Ziegler-type catalysts, based mostly on titanium, where an aluminum alkyl cocatalyst is responsible for the formation of a Ti-alkyl bond and initiation of the ethylene polymerization process, chromium-based catalysts do not require such a cocatalyst. 

The first successful glass fiber treatment found was a chromium complex of methyl-methacrylate. The initial versions of feis product were the first coupling agents and reduced the strength losses under humid conditions to about 40%. This chromium-based chemistry was improved and later commercialized by DuPont and is still in use wife glass fiber to some extent today. 

The synthesis of verrucarin J (40) by Esmond, Fraser-Reid, and Jarvis (42) involved an oxidative ring closure of trichoverrin B (71) as shown in Scheme 34. This two-step oxidation, initiated by the chromium-based oxidant PDC, compares favorably with standard macrolactonizations as a 50% yield of verrucarin J (40) can be realized. Presumably, the hemiacetal (285) is the crucial intermediate for the second oxidation step. The ability to form the macrocycle without isomerization of the olefin bonds is a significant feature of this reaction. 

A series of nickel—chromium—iron alloys based on the soHd solution Inconel 600 alloy (see Table 4) was developed, initially depending on aluminum ... 

The process consists of pre-etching, etching, etch neutralization, catalyst appHcation, catalyst activation, and plating. Most commercial appHcations, except REl/EMl shielding, use the initial copper or nickel deposit as a base for subsequent electrolytic plating of electrolytic copper, nickel, or chromium. The exact types and thicknesses of metal used are determined by part usage, eg, automotive exterior, decorative, plumbing, and others (24). 

The kinetics of aquation of a number of azidochromium(III) complexes have been investigated.303,655 Compared with other acidochromium(III) complexes, the chromium-azide bonds in these species seem remarkably stable to thermal substitution. Hence in the base hydrolysis of [CrN3(NH3)s]2+ a pathway involving initial loss of NH3 concurs with the usual base hydrolysis pathway involving loss of Nj. The aquation of azidochromium(III) complexes is H+-assisted with protonation of the azido ligand accounting for the enhanced reactivity. 

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