Cascade processes

Comparison of electrolytic dcsilvering versus cascade processing... 

A typical cascade process. A fast atom or ion collides with surface molecules, sharing its momentum and causing the struck molecules to move faster. The resulting fast-moving particles then strike others, setting up a cascade of collisions until all the initial momentum has been redistributed. The dots ( ) indicate collision points, tons or atoms (o) leave the surface. 

In a cascade process, one incident electron (e ) collides with a neutral atom ((S)) to produce a second electron and an ion ( ). Now there are two electrons and one ion. These two electrons collide with another neutral atom to produce four electrons and three ions. This process continues rapidly and — after about 20 successive sets of collisions — there are millions of electrons and ions. (The mean free path between collisions is very small at atmospheric pressures.) A typical atmospheric-pressure plasma will contain 10 each of electrons and ions per milliliter. Some ions and electrons are lost by recombination to reform neutral atoms, with emission of light. 

In the synthesis of the squalenoid glabrescol (72 originally attributed structure), containing five adjacent (all cis) THF rings, the necessary precursor of the polyepoxide cascade, the pentaepoxide 71, was achieved by epoxidation of each of the trisubstituted double bonds of the known (R)-2,3-dihydroxy-2,3-dihydrosqualene (70) by the Shi epoxidation approach (Scheme 8.18) [34]. Treatment of 71 with CSA at 0 °C and subsequent purification by column chromatography provided the pure polycyclic ether 72 by a cascade process reasonably initiated by the free secondary alcohol functionality [35a]. 

A lifetimes comparison for each excited state (Table 8) shows that they are smaller for the A n state except for v =0. The vibrational decays occur by means of cascade processes. Rotational effects (v =l - 2 i =l - 2) appear to be even more intense than for the corresponding transition bands and in either case these effects disappear for j =4. For... 

A lanthanide-mediated, sequential hydroamination/C-C cyclization reaction served to prepare the benzo[ ]quino-lizine derivative 358 from precursor 357, using a Nd species as a catalyst (Equation 12). This cascade process proceeded in good yield and with high diastereoselectivity <2003T10581>. 

The reaction of compound 364 with a radical initiator gave a mixture of diastereomeric benzo[tf]quinolizidines 365 and 366 through the radical cascade process summarized in Scheme 83 <1999TL1149>. 

A palladium-catalysed carbometallation-alkyne cross coupling cascade process has been reported for the stereo- and regio-controlled synthesis of dibenzoxepines with substituted exocyclic alkene functionality <06OL1685>. 

In Eqs. (II. 1)—(II.4) we have assumed that there is only one system oscillator. In the case where there exists more than one oscillator mode, in addition to the processes of vibrational relaxation directly into the heat bath, there are the so-called cascade processes in which the highest-frequency system mode relaxes into the lower-frequency system modes with the excess energy relaxed into the heat bath. These cascade processes can often be very fast. The master equations of these complicated vibrational relaxation processes can be derived in a straightforward manner. 

A cascade process involving a Ni-catalysed coupling, carbonylation, cyanation and heterocyclisation results in the rapid, efficient conversion of propynyl halides and alcohols into 5-cyanopyran-2-ones in aqueous conditions (Scheme 37) <00JCS(P1)1493>. 

In certain cases, the C-H activation/Cope rearrangement is so favorable that double C-H functionalization can occur as illustrated in Equation (38). The product 33 was formed in 99% ee with excellent control of stereochemistry at four centers due to the involvement of a cascade process rather than a direct C-H insertion. 

It is worth noting that product 443 was not obtained without silicon assistance, which means that the reaction is in fact initiated by an Si-[Rh] species (Scheme 112). Fused tricyclic benzenes such as 443 are formed exclusively using an exactly stoichiometric amount of silane. On the other hand, 2 equiv. of silane preferably lead to the formation of silylated benzenes such as 447 through a hydrosilyIation-carbocyclization-/ -hydride elimination cascade process. 

An allene moiety can serve as a nucleophile vis-a-vis a 7r-allylpalladium species generated from an allylic acetate moiety in substrates such as 495 (Scheme 124). The cyclization involving these two moieties generates another 7r-allyl intermediate, and the stage is set for the subsequent carbonylative cascade process as demonstrated by the transformation of 495 to 496.402... 

Very recently, Ma has reported a rhodium-catalyzed route to 18,19-norsteroid skeletons from bis-allenes, involving a cyclometallation-carbometallation-reductive elimination-Diels-Alder reaction cascade process.410... 

Fig. 13. Continuous indirect oxidation of toluene and toluene derivatives using a cascade process. Pilotstate [140]...

Catalytic methods, chemo- as well as bio-catalysis, are of vital importance in the conversion of natural products into derivatives (semi-synthesis). In chemo-catalysis conventional catalysts, such as mineral acids, are being replaced by recyclable solid catalysts. Further progress is also expected in cascade processes in which synthesis steps are combined to one pot methods. 

Special attention is given to the integration of biocatalysis with chemocatalysis, i.e., the combined use of enzymatic with homogeneous and/or heterogeneous catalysis in cascade conversions. The complementary strength of these forms of catalysis offers novel opportunities for multi-step conversions in concert for the production of speciality chemicals and food ingredients. In particular, multi-catalytic process options for the conversion of renewable feedstock into chemicals will be discussed on the basis of several carbohydrate cascade processes that are beneficial for the environment. 

Effective and simple immobilization of enzymes can be obtained by the cross-linking of enzyme aggregates, so-called CLEAs [55]. In this way, essentially any enzyme, including crude preparations, can be transformed into a heterogeneous type of material, insoluble in both water and organic solvents, that is stable and recyclable with high retention of the enzyme s original activity [56], These enzyme preparations are, therefore, of special value for both bio-bio and bio-chemo cascade processes. 

As an extension of the Heck reaction, Pd-catalyzed hydroarylation of alkynes and alkenes continnes to attract high level of research interest in simple couphng processes and in cyclization reactions. The use of this type of transformation as part of a domino reaction will be of increasing interest. The research in the field of domino reactions is attracting considerable attention in synthetic organic chemistry since it enables the rapid assembly of complex molecirles in one-pot processes. Very elegant examples of palladium-catalyzed cascade processes where a single catalytic cycle entails several sequential bond transformations have been recently reported [la, b, 2a, b, c]. 

Because many of the reactions involving heteroatom-substituted carbene complexes are cascade processes involving several different reaction types, they are... 

They stimnlate the proliferation of B-cells that are already responding to the antigen to increase the number of effector cells and hence the prodnction of antibodies that is, they stimulate the cellular cascade process. 

In each of the tandem iminium ion/enamine cascade processes described above, the enamine is trapped in an intramolecular fashion. The ability to perform the trapping seQuence in an intermolecular manner would allow for the one—pot introduction of three points of diversity. IVIacNlillan has realised this goal and described a series of secondary amine catalysed conjugate addition—enamine trapping sequences with oc P Unsaturated aldehydes using tryptophan derived imidazolidinone 115 to give the products in near perfect enantiomeric excess (Scheme 47) [178]. 

Although condensation reactions nsnally resnlt in achiral products they represent important additional reactivity of the active imininm ion which must be considered. Design of condensation reactions into cascade processes will provide further intriguing catalytic seqnences. 

Dipoles can also be generated from rearrangements that take place after the formation of an initial rhodium carbenoid product ]40, 70, 71]. One example of this type of transmutation, also known as a dipole cascade process, involves the formation of an azomethine ylide via the initial formation of a carbonyl ylide [72]. This process was... 

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