Bond direct attack

Alkenyl zirconium complexes derived from alkynes form C—C bonds when added to aHyUc palladium complexes. The stereochemistry differs from that found in reactions of corresponding carbanions with aHyl—Pd in a way that suggests the Cp2ZrRCl alkylates first at Pd, rather than by direct attack on the aUyl group (259). 

A carbonium ion, CHj, is formed by adding a hydrogen ion (H ) to a paraffin molecule (Equation 4-6). This is accomplished via direct attack of a proton from the catalyst Bronsted site. The resulting molecule will have a positive charge with 5 bonds to it. 

In the case of allyl peroxides (12 X= CH2, A=CH2, BO),1 1 1 intramolecular homolytic substitution on the 0-0 bond gives an epoxy end group as shown in Scheme 6.18 (1,3-Sn/ mechanism). The peroxides 52-59 are thermally stable under the conditions used to determine their chain transfer activity (Table 6.10). The transfer constants are more than two orders of magnitude higher than those for dialkyi peroxides such as di-f-butyl peroxide (Q=0.00023-0.0013) or di-isopropyl peroxide (C =0.0003) which are believed to give chain transfer by direct attack on the 0-0 bond.49 This is circumstantial evidence in favor of the addition-fragmentation mechanism. 

The validity of this statement is confirmed by the rates of IC1 additions (see Table 12). Because for these additions the formation of a cationic intermediate by direct attack of the electrophile on the double bond is rate determining, their order of rates is comparable to those of polymerizations. It is therefore understandable that the polymerization rates correlate much better with the reactivities of the monomers during an electrophilic addition of cationogenic agents (such as IC1) than with the relatively unspecific EDA complex formation. 

Note that it is necessary to open the epoxide (9) and substitute to get (10) - direct attack on a double bond would give the wrong isomer (11). Inversion of... 

For cinnamic acid at 9.6 °C, a = 0.107, b = 1.25 and k = 0.69 l.mole .sec E = 26.7 0.5 kcal.mole" and AS = 34.5 eu. Identification of products of oxidation of a number of acids indicates two concurrent mechanisms. Predominating is direct attack on the double bond to give, ultimately, cleavage products, e.g. benzaldehyde from cinnamic acid (some phenylacetaldehyde is also found, indicating oxidative decarboxylation to occur) and also acetophenone from 3-phenylcrotonic acid. 

The isotope effect in peroxyl radical reactions with C—H/C—D bonds of attacked hydrocarbon shows the direct hydrogen atom abstraction as the limiting step of this reaction [15], For example, the cumylperoxyl radical reacts with the C—D bond of a-deuterated cumene (PhMe2CD) ninefold slower than with the C—H bond (cumene, 303 K [118]). The second isotope effect (ratio fcp(PhMe2CH)//tp(Ph(CD2)2CH) is close to unity, i.e., 1.06 per C—D bond [118],... 

The mechanism of the reaction is understood to involve direct attack of the phosphorus nucleophile on the electrophilic substrate generating the new C-P bond, as shown in Equation 3.7.144145... 

The conclusions reached about proton transfer from phenylazoresorcinol monoanions are quite different from the behaviour which has been described for other hydrogen-bonded acids. For phenylazoresorcinol monoanions, it appears that direct attack by base on the hydrogen-bonded proton is an important process and can compete with two-step proton removal. For two-step proton transfer through an open form of the phenylazoresorcinol monoanion it is found that the rate of proton transfer from the open form is... 

There seems to be a direct attack of alkene at the oxometal, with C-0 bond formation (Figure 6.2). 

Some organometallic compounds and Lewis bases can also act as initiators. In such cases initiation occurs by a direct attack of these compounds on the double bond of the monomer molecule. 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

In the case of acetylenic amides, the carbonyl oxygen atom turned out to be nucleoplilic enough to directly attack the coordinated triple bond, owing to the conjugation with the amide moiety. Thus, cGa-dialkyl substituted 2-ynylamides smoothly underwent oxidative cyclization-alkoxycarbonylation to afford new oxazoline derivatives in good yields (Eq. 44) [102,113]. 

Removal of the proton from an intramolecular hydrogen bond by a base occurs in a two-step mechanism (a rapid equilibrium between H-bonded and non-H-bonded forms followed by base catalyzed proton removal from the non-H-bonded form) rather than by direct attack of the base on the intramolecularly hydrogen bonded species (6). 

Table II summarizes a temperature jump study (14) of the reaction of hydroxide ion with various intramolecularly hydrogen bonded malonic acid monoanions and points up the fact that, as the steric hindrance increases, a considerable strengthening in the hydrogen bond occurs with a concomitant slowing down of the rate at which the reaction proceeds (generalization number 4). At the time the authors did not foresee that it would be possible to distinguish between whether the hydrogen bond was broken directly by the attacking base or whether, in fact, there first had to be a collapse of the hydrogen bond into an open form of the anion that would subsequently react with the base. Thus, they simply postulated the former mechanism (direct attack). ...

Perlmutter-Hayman and Shinar (15, 16) have studied by temperature-jump the reactions of bases with different acid-base indicators having intramolecular hydrogen bonds. With Tropaeolin 0, direct attack of the base on the hydrogen bridge predominates according to their interpretation, whereas, for Alizarin Yellow G, the observed relaxation is ascribed chiefly to diffusion controlled reaction between the base and that part of the indicator present in the open form. Thus, data exist that lead one to doubt the generality of statement number 5. 

There are of course borderline cases when the reacting hydrocarbon is acidic (as in the case of 1-alkynes) a direct attack of the proton at the carbanion can be envisaged. It has been proposed that acyl metal complexes of the late transition metals may also react with dihydrogen according to a o-bond metathesis mechanism. However, for the late elements an alternative exists in the form of an oxidative addition reaction. This alternative does not exist for d° complexes such as Sc(III), Ti(IV), Ta(V), W(VI) etc. and in such cases o-bond metathesis is the most plausible mechanism. 

Eyring (Haslam et ai, 1965a Eyring and Haslam, 1966 Jensen et ai, 1966) preferred to explain the reduced rate in terms of a one-step mechanism in which the base directly attacked the hydrogen-bonded proton as in (24). 

The complex kinetic dependence on hydroxide-ion concentration was explained by the mechanism in (27). Proton removal from the phenylazo-resorcinol monoanion by the hydroxide ion to give the dianion occurs by two different routes. One route is first order with respect to the hydroxide ion with rate coefficients and and is assumed to consist of a direct attack by the hydroxide ion on the hydrogen-bonded proton. The other route leads to a complex dependence of the rate of approach to equilibrium on the hydroxide-ion concentration, and involves prior opening of the hydrogen bond (rate coefficients kj and followed by proton removal (rate coefficients and k j). Equation (28) is derived from the mechanism... 

It is worth mentioning that the photooxidation of porous silicon behaves differently [49]. Indeed, ETIR spectra show that there is a tremendous increase in vsi o, without a correspondingly large loss of vsi H peak intensity. The decrease of the vsi H band is offset by an increase in the vosi—h band, resulting in no net loss of hydride species on the surface during the course of the photooxidation reaction. These data apparently suggest that oxidation does not result in the removal of H atoms, implying that Si—Si bonds are attacked directly. 

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