Cleavage type

Another mode of cleavage [type (c)] was found when unsymmetrical 3,4-disubstituted furoxans were subjected to thermolysis. Thus, 3-methyl-furoxan-4-carboxylic acid was found to afford the a-oximinonitrile oxide with concurrent... 

Many compounds of types not otherwise available are accessible through application of the cleavage type of oxidation to the glycosidic derivatives of the sugars. Periodic arid oxidation is applicable to glycosides 9 >91 of various groups of sugars, as shown by the... 

Fragment analysis is based on the RECAP algorithm, published in 1998 [55], This retrosynthetic combinatorial analysis starts with a collection of active molecules and then fragments these molecules using any set of retrosynthetic reactions (Figure 8.12). For example, Cisapride is cleaved into four fragments based on three different bond cleavage types. 

Radiationless Processes. The primary processes of 2-pentanone are similar to those of the other ketones. The dominant pathway for the depopulation of the first excited singlet state at lower energies is the S —intersystem crossing. Both a-cleavage (type I) and Norrish type II processes can occur from the singlet state and from the triplet state. The intersystem crossing yield has only been measured in solution (see ref. 247) With 313-nm excitation, a value of isC = 0.81 was measured by... 

Homolytic oc-cleavages (Type I reactions to produce a radical pair) involving the n, re states of ketones, have been studied extensively in homogeneous solutions 9c>, but only recently have ketones been employed to investigate this important class of photoreactions in micellar environments. For ease of discussion, we can classify reactions of this type in terms of the resulting products ... 

Another way to obtain a library of fragments is to decompose a known database of drug- or lead-like molecules by /j gMifo-retrosynthetic rules like RECAP (Retrosynthetic Combinatorial Analysis Procedure).The RECAP bond cleavage procedure consists of eleven bond cleavage types shown in Figure 7.3. This approach has several advantages ... 

Crack-opening mechanisms have been proposed that simply relate to the effect of environment and local alloy composition on the atom-to-atom bond strength at the crack tip. Reduction in this bond strength has been attributed to stress-induced changes in alloy composition as just described and to adsorption of atoms from the environment. Since dislocation movement is not considered in the mechanism, breaking bonds in the plane of the crack propagation leads to a cleavage-type rupture (Ref 159). 

SCHEME 13.9 Homogeneous synthesis of graft copolymers by a-cleavage type photoinitiation. 

Cleavage between C and Cg is referred to as Type-II photoelimimtion to distinguish it from a-cleavage. Type-II photoeliminations are observed for both aryl and allyl ketones. Studies aimed at establishing the identity of the reactive excited state... 

C) giving cyclobutanols, and elimination (E), giving acetophenone and olefins. The a,a-alkyl substituted derivatives, 101, undergo a-cleavage (type I process) as well, forming a radical pair which can either disproportionate... 

For example, the acyl-aryloxo bond cleavage (type b) is shown by the reaction of Ni(cod)2 with phenyl propionate in the presence of PPhs (Scheme 3.34) or 2,2 -bipyridine [65]. The reaction products are ethylene, phenol, and (car-bonyl)nickel complex. Formation of these products is conveniently understood by initial oxidative addition of EtC(0)-0Ph followed by decarbonylation, )S-hydrogen elimination and reductive elimination, though (acyl)(aryloxo)nickel(II) intermediate is not isolated. However, such an intermediate is isolated by the selective insertion of CO into the (alkyl)(aryloxo)nickel (or palladium) complexes, which smoothly affords esters by reductive elimination promoted by electron deficient olehns. The results suggest that the oxidative addition involving C-0 bond cleavage is essentially reversible. 

Radical polymerization is induced by photoinitiators, cleaverage type, and hydrogen abstraction type. The merits of radical polymerization are its higher cure rate and availability of a variety of materials. Most acrylate and methacrylate monomers are applied for radical polymerization. Figure 4.7 shows typical polymerization of a monomer. Mono-, bi-, and multifunctional vinyl monomers are used for polymerization. Figure 4.8 shows typical initiators for photopolymerization. Benzophenone is a hydrogen abstract type initiator. 2,2-Diethoxy-l,2-diphenylethane-l-one is a cleavage type initiator. 

N. A. Kahn and E. A. Imbembo, "A Method of Evaluating Transition from Shear to Cleavage Type Failure in Ship Plate and Its Correlation with Large Scale Plate Tests," The Welding Journal, Vol.27. No. 4, p, 169-S (April, 1958). 

Those that undergo intramolecular bond cleavage, known as homo-lytic fragmentation type or Type 1 photoinitiators (a-cleavage type). Examples are benzoin ethers, substituted acetophenone derivatives, acyloxime esters, benzil ketals, and cyclic benzoin and benzils. Representative examples are shown in Figure 5.15. 

Copolymers of the phenyl isopropenyl ketone and styrene undergo P-cleavage (Type II Norrish process) according to reaction ... 

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