Bond-making mechanisms

In thermally accelerated or high-pressure-promoted allylation using crotylstan-nane, the geometry of product reflects the 2-butenyl moiety of starting staimanes, and E and Z isomers furnish anti and syn isomers, respectively (Scheme 12.1) [9, 13]. This stereochemistry can be explained by invoking a six-membered cychc transition state and a concerted bond-breaking and bond-making mechanism. 

A mechanistic study of the reaction of ruthenium ammine complexes with NO was reported almost four decades earlier. Although [Ru(NH3)6] is very inert, an acidic solution of this complex ion was observed to undergo a rapid reaction in the presence of NO to form [Ru(NH3)s(NO)]. [164] An electrophilic substitution process was invoked, with NO considered to function as an electrophile and the product proposed to be Ru -NO". The conclusion from a subsequent study was that a bond making mechanism operates for this... 

The mechanism of the haloform reaction begins with a halogenation via the eno late The electron attracting effect of an a halogen increases the acidity of the protons on the carbon to which it is bonded making each subsequent halogenation at that car bon faster than the preceding one... 

Hi) Reaction selectivity substituent effects). This property can be used as a diagnostic criterion of mechanism since a bimolecular attack is expected to be sensitive to the structure of the substrate to an extent depending on the role of the bond-making step. The importance of the... 

Honk et al. concluded that this FMO model imply increased asynchronicity in the bond-making processes, and if first-order effects (electrostatic interactions) were also considered, a two-step mechanisms, with cationic intermediates become possible in some cases. It was stated that the model proposed here shows that the phenomena generally observed on catalysis can be explained by the concerted mechanism, and allows predictions of the effect of Lewis acid on the rates, regioselectivity, and stereoselectivity of all concerted cycloadditions, including those of ketenes, 1,3-dipoles, and Diels-Alder reactions with inverse electron-demand [2],... 

Active Figure 18.1 The mechanism of the Claisen rearrangement. The C—0 bond-breaking and C—C bond-making occur simultaneously. Sign in afwww.thomsonedu.com to see a simulation based on this figure and to take a short quiz. 

Some quantities associated with the rates and mechanism of a reaction are determined. They include the reaction rate under given conditions, the rate constant, and the activation enthalpy. Others are deduced reasonably directly from experimental data, such as the transition state composition and the nature of the rate-controlling step. Still others are inferred, on grounds whose soundness depends on the circumstances. Here we find certain features of the transition state, such as its polarity, its stereochemical arrangement of atoms, and the extent to which bond breaking and bond making have progressed. 

The Sn2 mechanism as shown above involves the simultaneous movement of three pairs of electrons. However, Bordwell and co-workers contended that there is no evidence requiring that this bond making and bond breaking be in fact concerted, and that a true Sn2 mechanism is a myth. There is evidence both for and against " this proposal. There is also a review of the Sn reaction. ... 

For Sn2 reactions, no such simple correlations are found. In this mechanism bond breaking is about as important as bond making in the ratedetermining step, and substituents have an effect on both processes, often in... 

On the basis of the preceding discussion, the systematic trend in AV observed for water exchange on [M(H20)6]2+ may be rationalized through a More-O Ferral type of diagram (4, 7) as shown in Fig. 4. The bond-making and bond-breaking contributions to AV are plotted on the two axes, which are scaled to AVt for a D mechanism being... 

The molecularity refers to the number of species (molecules, ions, etc.) that are undergoing bond-breaking and/or bond-making in one step of the reaction, usually in the rate-limiting step. It is important to realise that the molecularity is not an experimentally determined quantity, and has significance only in the light of the particular mechanism chosen for the reaction it is an integral part of the mechanistic interpretation of the reaction and is susceptible to re-... 

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