A reactions via

With alkali cyanides, a reaction via a SN2-mechanism takes place the alkyl halide is attacked by cyanide with the more nucleophilic carbon center rather than the nitrogen center, and the alkylnitrile is formed. In contrast, with silver cyanide the reaction proceeds by a SnI-mechanism, and an isonitrile is formed, since the carbenium intermediate reacts preferentially with the more electronegative center of the cyanide—i.e. the nitrogen (Kornblum s rule, HSAB concept). ... 

The first two examples clearly indicate that slight modifications (02 to N2 atmosphere, unsubstituted to 4-nitrosubstituted benzenediazonium ion) can change a heterolytic reaction into a homolytic one. The changeover from heterolytic to homolytic dediazoniation will be discussed in more detail in Section 8.7 of this chapter. The ratio of products in the third example suggests, as indicated in Scheme 8-2, that a DN + AN substitution (A), a reaction via an aryne (B), a o-substitution of the type described by Kovacic and Gormisk (1966) (D), and a... 

Owing to their strong bond on Ru(OOOl), mixed COa 0.55 V, the shift of the equilibrium between water and adsorbed OHad/Oad towards the latter increases the density of the respective species in the intermixed adlayer, which increases the repulsions between the adsorbed species and hence leads to more weakly bound OHad/Oad and COad species. These latter species are less stable against COOHad or CO2 formation, because of the reduced reaction barrier ( Brpnsted-Polanyi-Evans relation [Bronstedt, 1928]), and can support a reaction via (14.9) or (14.12), respectively, at low rates. (Note that the total density of the adlayer does not need to remain constant, although also this is possible.)... 

The terminal splitting is a reaction via the 2C complexes which, when belonging to group (ii), are preferentially bound to the valleys. 

The three routes are not always kinetically distinguishable. Silversmith and Smith (1958), for example, mentioned that the second-order kinetics in the reaction of l,l-diphenyl-2-fluoroethylene with ethoxide ion fits a reaction via a carbanionic intermediate, or the formation of a fluoroether, if the latter is either formed rapidly and decomposed slowly (kikzKk-i + kz ki), or if it formed in a rate-determining step (k1k2l(k-l + k2Xki). 

Upon changing the a-substituent from p-anisyl to phenyl the decrease in amount of ionic products was not accompanied by a detectable increase in the amount of reduction products. The authors propose two alternative mechanisms (a) reaction via an excited vinyl bromide which fragments directly to ion pair or radical pair or (b) reaction via an excited vinyl bromide which fragments directly to an ion pair or undergoes electron transfer to give vinyl bromide radical anion, which then fragments. More work is required on these alternatives to establish a clear choice. 

Photooxidation suitable for the epoxidation of aromatic olefins also occurs with a-diketones (benzil, biacetyl), benzophenone, benzoin, and a-ketoacids, Isotopic mechanistic studies point to a reaction via a biradical. Photooxidation with an a-diketone or a-ketoacid recently has been interpreted in terms of a photochemical a-cleavage leading to an acylperoxy radical, which can effectively transfer an oxygen atom to olefins. Vinylallenes have similarly been photooxidized in the presence of biacetyl. ... 

Figure 3.16 shows the correlation diagram for the peiicyclic four-center reaction via a Hiickel-type transition complex as mentioned above. It contains two crossing correlation lines (72 - und 72-), which are indicative of a thermaUy forbidden transition (2). Examples for such a reaction via an antiaromatic four-membered Htickel ring are the dimerization of ethylene and the disrotatory ring opening of cyclobutene (41), both of which occur only photo-chemically but not thermaUy. 

In contrast, the observation of a CIDNP effect clearly indicates a reaction via radical pairs. In such a situation, however, the question is often raised—in particular by reviewers less familiar with CIDNP— whether that pathway via radicals is the only pathway, or if it is only a minor side reaction that is detected nevertheless because of the signal amplification by the polarizations while the greater part of the reaction proceeds via non-radical routes. Much of this worry is due to a misconception as to the size of the signal enhancements, which were indeed very large in the early days of CIDNP but are much smaller on modem high-field instmments (e.g. at 300 MHz, the Boltzmann population differences are five times larger than at 60 MHz but the polarizations are smaller by a factor of about /5, so the enhancements are lower by an order of magnitude). Yet, the objection is partly valid because CIDNP would indeed not capture any parallel non-radical route. That issue can be addressed in the same way as in classical mechanistic chemistry When a reaction is believed to involve X as a key intermediate, one prepares X by an independent route if X so obtained decays to the same products and with the same yields as before, the reaction does occur exclusively via X. 

Partially hydrogenated pyrrolizines are easily obtained in high yield from a reaction of imides with cyclopropylphosphonium salt (195). Pyrrolizine ester (197) was obtained from succinimide, in a reaction via (196) phthalimide gave the benzopyrrolizinone (198) <80TL4585,81TL719>. Compound (199) was obtained from succinimide with triphenylcyclopropylphosphonium tetrafluoroborate... 

As mentioned in the introduction, it is believed that oxidation of substrates by cytochrome P450 enzymes is mediated by a ferro-oxo species called Cpd I [5-6]. Details of the so-called rebound mechanism could be rationalized by quantum chemistry only recently [13-14]. Based on all available data, it seems reasonable to assume that the route via a doublet state of Cpd I is more plausible than a reaction via a quartet state. Figure 4 shows Fukui functions calculated for a model system. 

Mercaptoethanol reacts with trimethylsilyl enol ethers to give oxathiolans of the parent ketones. The anodic dimerization of enol ethers to give ketals of 1,4-dicar-bonyl compounds, has been studied. The electrochemical reaction is first order with respect to enol ether, and zero order with respect to the solvent, methanol. These data, together with the observed tail-to-tail coupling, indicate a reaction via radical intermediates. ... 

In marked contrast to the reductive elimination of all l, acyl and aryl-azolium salts, examples of mechanistically well-understood reductive eliminations of 2-haloazolium and azolium [i.e. C-H reductive elimination) salts from transition metal centres are much rarer, despite the ubiquity of transition metal-NHC complexes containing either coordinated halides or hydrides (or both). This may be due, in part, to the ease with which a 2-haloazolium or azolium salt can oxidatively add to a low valent transition metal centre, meaning that even if a 2-haloazolium or azolium salt were formed during a reaction via reductive elimination, a further oxidative addition reaction can rapidly re-form the starting complex. Furthermore, when an azolium salt is the decomposition product of a reaction, it is often not possible to discern whether this has been formed by a genuine NHC-hydride reductive elimination, or... 

Figure 16-1 Monitoring a reaction via a change in color intensity.

Several mechanisms will be examined in an effort to account for the results just outlined. The 1-phenylethyl compounds will be used as examples since most of our results have been obtained with these compounds where comparisons have been made with analogs, e.g. Table 6, the results have been similar. A reaction via firee diazoalkanes is clearly ruled out. Some type of close complex between the diazoalkane and carboxylic acid involving a rotation of the partners about one another is conceivable (equation 130), although we know of no H... 

However, it has to be noted that the inhibition is much lower at somewhat higher temperatures [31, 35, 36]. Moreover, Marko et aL showed acylcobalt tetracarbonyls to be present in the reaction mixture of technical 0x0 reactions. These acylcobalt tetracarbonyls react to aldehydes in the further course of the reaction. This could proceed via the acylcobalt tricarbonyls in the equilibrium of equation (5 a). However, a reaction via (6 a) cannot completely be excluded from the data available so far. 

Acetylenes generally react with carbenes to form cyclopropenes. Whether this is a reaction via a singlet carbene or a two-step process via a biradical cannot be determined from the product, as is the case in the trapping with olefins. Isolation of indenes (116) from the reaction of acetylenes with di-phenylcarbene generated photochemically from the diazo-compound, however, was considered to be evidence for the intermediacy of a triplet carbene. 

The cleavages of 3 bonds all proceed in an adiabatic reaction in a higher excited singlet state. A reaction via the lowest triplet state yields only minor products. Simultaneously,... 

In contrast to many studies on cycloaromatization via transition metal-vinylidene complexes as key reactive intermediates, only one example of such a reaction via transition metal-allenylidene complexes has been reported to date. In 2008, Yada et al. reported the formation of substituted fiirans 78 from 3-butyne-l,2-diols 77 in the presence of a catalytic amount of thiolate-bridged diruthenium complex (Scheme 21.33) [45]. This methodology was also applied to the formation of a substituted pyrrole 80 from l-amino-2-butyn-2-ol 79. It is noteworthy that thiolate-bridged diruthenium complexes worked as effective catalysts toward cyclization involving both ruthenium-allenylidene and ruthenium-vinylidene complexes as key reactive intermediates. 

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