One-step synchronous

The basic mechanism of fragmentation (equation 1) is a concerted one, with a contribution from each of the five centers to the transition state. Two-step carbonium ion or, very rarely, carbanion mechanisms mean in all cases that substitution, elimination or ring closure can take place rather than fragmentation. However, the one-step synchronous fragmentation is allowed only when the compound can adopt a conformation where the leaving group X as well as the electron pair of the electrofuge Y are antiperiplanar to the C(2)—C(3) bond (Scheme 5). ... 

Figure 3.19 Resolution of the concerted versus stepwise mechanism of excited-state double proton transfer in doubly hydrogen-bonded dimer of 7-azaindole [90, 91]. Schematic presentations of (a) the concerted, one-step, synchronous double proton transfer in 7-azaindole dimer and (b) the stepwise proton transfer following formation...

The mechanism of the D-A rxn is believed to be a one-step, concerted, non-synchronous process. 

More than one mechanism can account for the experimental observation of the Diels-Alder reaction.521,522,528 However, most thermal [4 + 2]-cycloadditions are symmetry-allowed, one-step concerted (but not necessarily synchronous) process with a highly ordered six-membered transition state.529 Two-step mechanisms with the involvement of biradical or zwitterion intermediates can also be operative.522,528... 

Various reaction mechanisms are known for ene reactions. Both single-step synchronous reaction and stepwise processes involving diradicals or zwitterionic transition states have been discussed. One of the three bonds broken in the course of the reaction is a a bond, which dictates a high activation energy relative to a Diels-Alder reaction (see Chapter 2). For this reason if the reaction is conducted thermally, temperatures above 100 C are required. However, the reactivity of the enophile can be increased by addition of a Lewis acid, permitting milder reaction conditions. The Lewis acid coordi-... 

In contrast to strongly solvent-dependent [2- -2]cycloaddition reactions, which proceed through a 1,4-dipolar zwitterionic intermediate by a two-step mechanism or through a dipolar activated complex by a one-step mechanism cf. Section 5.3.2, and Eqs. (5-33) to (5-35) [92, 94-107], [2- -2]cycloadditions are also known that exhibit concerted, nearly synchronous bond formation without significant charge separation on activation in the transition state. An example is given in Eq. (5-47). Since the rate constant for this diphenylketene/styrene addition is practically independent of solvent polarity [140], it can be classed as concerted. 

Stepwise PT and ET reactions occur along the edges of Fig. 17.1, while PCET includes the entire space within the square. The stepwise and PCET mechanisms (including HAT) are clearly distinct. The PCET mechanism is defined by a single transition state in which the proton and electron both transfer in one step, with no intermediate states populated along the reaction coordinate PCET is thus concerted but the electron and proton events can be asynchronous as opposed to their synchronous transfer for a HAT reaction. In a stepwise mechanism, an intermediate is formed and there are two distinct rate constants for the forward reaction and two separate transition states. Stepwise ET/PT or PT/ET can, in principle, be broken down and treated experimentally and theoretically as separate ET and PT events. Like any series of reactions, the rate-limiting rule applies. 

The average AV of -16 + 1 cm mol and the solvent independence of the process suggested that the reaction follows a nonpolar concerted, synchronous one-step mechanism. The observed pressure acceleration is very similar to that found for the insertion of dipropylcyanamide and l-(diethylamino)propene into the metal-carbene bond of pentacarbonyl(methoxyphenyl-carbene)chromium and -tungsten (shown in Scheme 1.2) for which varies between —17 and —25 cm mol ... 

A face-to-face reaction of two jr-orbitals to form a cyclobutane involves the formation of two new C—C a-bonds. The reaction may be imagined to occur under the preservation of symmetry, in this case C2v, i.e. concerted (one-step, no intermediates) and synchronous (both bonds are formed at the same rate). 

The conversion of 11 to 12 illustrates a reaction that involves direct conversion of starting material to product. In other words, there is only one chemical reaction, or one step, and the reaction is said to be synchronous. There are reactions in which an unstable product is formed, and that product undergoes a second chemical reaction to give a more stable product. In other words, the starting materials do not give the final product directly, but rather give a transient product prior to the formation of the final product. Such transient products are known as intermediates. 

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