Insertion mechanism reactivities

Filatov, M., Shaik, S., 1998b, Theoretical Investigation of Two-State-Reactivity Pathways of H-H Activation by FeO+ Addition-Elimination, Rebound , and Oxene-Insertion Mechanisms , J. Phys. Chem. A, 102, 3835. 

Since the two-spin state forms can lead to different products, the products obtained will be a mixture that reflects the initial fractionation of the reaction between the two-spin states. The fractionation in turn is a reflection of the interplay and the probability of cross-over between the two-spin states (8). Thus, the two-state reactivity paradigm resolves the dilemma of whether a radical recombination or a direct insertion mechanism governs cytochrome P450-catalyzed hydroxylation actually they are both involved and the degree to which either is expressed depends upon the specific substrate hydroxylated and the specific enzyme. 

ROP of p-lactones is highly prone to numerous side reactions, such as transester-fication, chain-transfer or multiple hydrogen transfer reactions (proton or hydride). Specifically, the latter often causes unwanted functionalities such as crotonate and results in loss over molecular weight control. Above all, backbiting decreases chain length, yielding macrocyclic structures. All these undesired influences are dependent on the reaction conditions such as applied initiator or catalyst, temperature, solvent, or concentration. The easiest way to suppress these side reactions is the coordination of the reactive group to a Lewis acid in conjunction with mild conditions [71]. p-BL can be polymerized cationically and enzymatically but, due to the mentioned facts, the coordinative insertion mechanism is the most favorable. Whereas cationic and enzymatic mechanisms share common mechanistic characteristics, the latter method offers not only the possibility to influence... 

Recently, trans insertion of hexafluorobutyne into one of the M—H bonds in some metallocene hydrides, Cp2MH , was studied in some detail (47). Experiments carried out in the presence of various radical-sensitive reagents such as TV-phenyl-a-naphthylamine suggested that a free radical mechanism was unlikely. A stepwise ionic mechanism, involving a zwitter-ionic intermediate, Cp2(H2)M+—C(CF3)==CCF3, is improbable, since (i) the stereochemistry and the apparent rate are not influenced by the polarity of the solvents, (ii) no deuterium is incorporated in the reaction in EtOD, and (iii) the trend in reactivity (Mo > W) does not reflect the trend in v-basicity or M—C bond stability (W > Mo). An essentially concerted trans-insertion mechanism is inferred, which is supported inter alia by the low kinetic deuterium isotope effect (kH/k0 = 1). 

Because of the marked differences in reactivity of I, II, and III on palladium, Gault and co-workers prefer the metal-olefin/metal-alkyl insertion mechanism outlined above for this metal in contrast to platinum. It may be noted, incidentally, that the results described for reaction of I, II, and III on platinum would be consistent with the simplest possible mechanism of ring closure, namely through aco-diadsorption ... 

The internal stress in plasma polymer films is generally expansive, i.e., the force to expand the film is strained by external compressive stress. According to the concept presented by Yasuda et al. [1], the internal stress in a plasma polymer stems on the fundamental growth mechanisms of plasma polymer formation. A plasma polymer is formed by consecutive insertion of reactive species, which can be viewed as a wedging process. The internal stress is related to how frequently the insertion occurs as well as on the size of inserting species. The both factors are dependent on the operational factors of plasma polymerization. 

The most thoroughly studied mechanism of protein protease inhibitors is that of the standard mechanism (or Canonical or Laskowski mechanism) inhibitors of serine proteases (1) (Fig. 2). Standard mechanism inhibitors include the Kazal, Kunitz, and Bowman-Birk family of inhibitors and bind in a lock-and-key fashion. Ah standard mechanism inhibitors insert a reactive loop into the active site of the protease, which is complementary to the substrate specificity of the target protease and binds in an extended fi-sheet with the enzyme in a substrate-like manner. WhUe bound to the protease, the scissile bond of standard mechaiusm inhibitors is hydrolyzed very slowly, but products are not released and the amide bond is re-ligated. The standard mechanism is an efficient way to inhibit serine proteases, and it is thus used by many structurally... 

In contrast to this cr-allyl insertion mechanism, the butenyl group can also react with the butadiene from the rf coordination. This r-allyl insertion mechanism, which is also described in Scheme 3, was introduced more recently [36] to relate the cis-trans selectivity to the reactivity of the butenyl group in its anti or syn configuration [23, 26] and bas been established by quantum chemical calculations using the density functional theory [37]. 

If the 1,4-polymerization is realized by the (T-allyl insertion mechanism, then the butenyl group in the anti and in the syn structure should be practically equally reactive and the cis-trans selectivity can be determined by the different mode of the butadiene coordination in the butenyl-lanthanide complex. 

During polymerization with Cr /silica in the solution process, the polymer is formed, and remains, in hydrocarbon solution. In this situation, the macromer insertion mechanism of Scheme 24A seems intuitive and plausible, because the terminated chain is free and mobile in solution and can be considered to be just another (albeit large) reactive comonomer. However, in the slurry and fluidized-bed processes, the polymer chains are not in solution, but instead they are "frozen out" or immobilized in a solid-phase immediately as they are formed. Therefore, this conventional mechanism of macromer incorporation is intuitively less likely, because it is not clear how the vinyl end-groups have access to the active sites. This issue is seldom considered in the literature. One possible explanation is that the active sites are embedded in the amorphous phase of the polymer, and that chain ends, being excluded from crystallites, are therefore concentrated into this same phase, where they do have some degree of limited motion (Scheme 24B). 

Further evidence for a biphilic insertion mechanism is derived from a study of the reaction of the bicyclic phosphoramidites (115) and (116) with diethyl peroxide to form (117) and (118), respectively. The strained 1-phosphabi-cyclo[3.3.0]octane (115) reacts much faster than its l-phosphabicyclo[4.4.0]-decane analogue (116), but the opposite reactivity is found towards diphenyl disulphide. At room temperature (116) produced (120) within seconds, presumably via an intermediate phosphonium salt, whereas (115) gave (119) [S 3ip = —19 p.p.m.] after 1 h at 58 °C. As mentioned earlier, the latter appears to be the only phosphorane reported so far with two exocyclic P-S bonds. 

---