In situ prepared activated carbons

Py X., Daguerre E., Menard D. Composites of expanded natural graphite and in situ prepared activated carbons. Carbon 2002 40 1255-65. 

The enantioselective 1,4-addition addition of organometaUic reagents to a,p-unsaturated carbonyl compounds, the so-called Michael reaction, provides a powerful method for the synthesis of optically active compounds by carbon-carbon bond formation [129]. Therefore, symmetrical and unsymmetrical MiniPHOS phosphines were used for in situ preparation of copper-catalysts, and employed in an optimization study on Cu(I)-catalyzed Michael reactions of di-ethylzinc to a, -unsaturated ketones (Scheme 31) [29,30]. In most cases, complete conversion and good enantioselectivity were obtained and no 1,2-addition product was detected, showing complete regioselectivity. Of interest, the enantioselectivity observed using Cu(I) directly in place of Cu(II) allowed enhanced enantioselectivity, implying that the chiral environment of the Cu(I) complex produced by in situ reduction of Cu(II) may be less selective than the one with preformed Cu(I). 

Since 1985, several thousands of publications have appeared on complexes that are active as catalysts in the addition of carbon monoxide in reactions such as carbonylation of alcohols, hydroformylation, isocyanate formation, polyketone formation, etc. It will therefore be impossible within the scope of this chapter to review all these reports. In many instances we will refer to recent review articles and discuss only the results of the last few years. Second, we will focus on those reports that have made use explicitly of coordination complexes, rather than in situ prepared catalysts. Work not containing identified complexes but related to publications discussing well-defined complexes is often mentioned by their reference only. Metal salts used as precursors on inorganic supports are often less well defined and most reports on these will not be mentioned. 

Dhas, N. A., Cohen, H., and Gedanken, A., In situ preparation of amorphous carbon-activated palladium nanoparticles. J. Phys Chem. B 101, 6834 (1997a). 

A large number of intermediate pathways arc possible when catalytic reactions interfere with the polymerization-dehydrogenation steps. A common scenario is the catalytic dehydrogenation of hydrocarbons on nickel surfaces followed by dissolution of the activated carbon atoms and exsolution of graphene layers after exceeding the solubility limit of carbon in nickel. Such processes have been observed experimentally [40] and used to explain the shapes of carbon filaments. In the most recent synthetic routes to nanotubes [41] the catalytic action of in situ-prepared iron metal particles was applied to create a catalyst for the dehydrogenation of cither ethylene or benzene. 

For complexes of type 10 (with a hydrogen at the carbene carbon) a synthesis was worked out in which a formamide is first reacted with K2[Cr(CO)j] followed by reaction with TMSCI [7]. This way, the non-racemic formamide 12 leads to the chirally modified amino carbene complex 13, which serves as starting material for the diastereoselective synthesis of various optically active yff-lactams [8]. An example is the (formal) total synthesis of 1-carbacephalothin 16, a carbon analog of the cephalosporins (Scheme 5) [8b]. In this case, the complex 13 is irradiated in the presence of in situ prepared imine 14 to afford the /(-lactam with high dia-stereoselectivity but only in modest yield. The product (15) could (in principle) be converted in to the target compound 16. 

Micro-calorimetric adsorption measurements require a proper in situ vacuum activation at a higher temperature than the adsorption process. A first pre-treatment under oxygen is performed in the calorimetric cell in order to eliminate the impurities present on the sample (essentially carbonates, nitrates, carbonaceous residues and water present from the preparation, calcination and exposure to atmosphere) and to avoid the partial reduction of the surface of an oxide that is easily reduced under vacuum. 

The initial biotransformation in a one-pot process, however, can also be used to prepare in situ an activated reagent which then reacts with an added substrate. Also not exactly fitting into the above-mentioned scheme of a one-pot two-step process, also here more than one synthetic step is carried out without a work-up in between. An elegant example in this area was reported by Novo Nordisk researchers, who converted in a first step acetic acid into acetic peracid through a catalytic reaction with a lipase and hydrogen peroxide, followed by a subsequent epoxidation of alkenes, for example, 46, with the in situ formed peracid [44]. By means of this method, a range of epoxides were prepared with yields up to >99%. A selected example is shown in Scheme 19.16. A related example was reported by Riisch gen. Klaas and Warwel [45], who started from dimethyl carbonate and hydrogen peroxide for in situ preparation of the needed peracid. 

Buchwald et al. have reported a number of bulky phosphine ligand PR 2R which give high activity in Suzuki— Miyaura catalysis for the synthesis of sterically hindered biaryls.The conditions used in the catalysis involved the in situ preparation of the Pd(0) catalyst from Pd2(dba)3 and the phosphine. Two ligands which proved superior to the others had in common the same polyaromatic R (R = Cy, Ph) as a difference from the rest. A Pd(0) complex could be prepared by mixing Pd2(dba)s and PR 2R in toluene, which has the X-ray structure 40 sketched in the upper reaction of Scheme 25. The key structural feature of the complex is the short distance of Pd to one double bond of the phenanthrene moiety (distances to the two carbons are 2.298 and 2.323 A) supporting an 77 -phenanthrene... 

The in situ prepared catalyst Ru(OAc)2(p-cymene) in the presence of the K2CO3 was almost as active as the isolated complex and was preferentially used [65]. This catalytic system was selected for the efficient preparation of a variety of polyheterocyclic compounds or tridentate ligands such as the tripodal tris-1,2,3-(heteroaryl)benzene [(Eq. 8)] [65]. The same type of diarylated derivatives and polyheterocyclic compounds can now be preferentially prepared in diethyl carbonate, a non-toxic solvent, instead of NMP even if the reaction is slower [71]. 

In situ preparation of amorphous carbon-activated palladium nanopartides. Journal of Physical Chemistry B, 101, 6834-8. 

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