Acyclic mechanism

An acyclic mechanism is possible as well. It is much slower than the cyclic mechanism, but is operative when the cyclic intermediate can not form ... 

In a certain sense, the simplest class of reaction mechanism is that whose bipartite graphs do not contain cycles, i.e. are acyclic. The dynamic behaviour of the corresponding reactions is always extremely simple [7]. An example for such a mechanism can be Ax - A2 - A3 - . . . - A [see Fig. 2(a)]. The contribution of acyclic mechanisms to the kinetics of catalytic reactions is not of importance. The mechanisms of catalytic reactions always contain cycles and these cycles are oriented, the directions of all the arrows being matched [the end of the ith arrow is the beginning of the... 

Shorter reviews have already appeared. Shafizadeh discussed the factors which affect rates of hydrolysis, in terms of the acyclic mechanism which... 

Obviously, few of the results presented thus far can be taken as unequivocal evidence for the cyclic mechanism (see equation 1). However, there is little, if any, evidence for the acyclic mechanism (equation 2) (see, however. Ref. 1), and it is generally agreed that the acid-catalyzed hydrolysis of glycopyranosides proceeds through a cyclic, carbonium-ion intermediate. The following discussions will, therefore, be based on the first mechanism. 

The cyclic and the acyclic mechanisms differ fundamentally. In the cyclic mechanism, the acid catalyst attacks the anomeric oxygen atom, and electrons move from the anomeric carbon atom to the... 

The chelate rule will only be applicable if addition via the chelate is faster than addition by the acyclic mechanism i.e, kc > ka in Scheme 4.2). Because the chelate is rigid, it is often considerably more stereoselective as well. However, the relative rate issue is independent of the stereoselectivities of the two processes. For example, chelation can be used to control regiochemistry selective reduction of a diester is achieved by preferential chelation to a 5-membered ring over a 6-membered ring by magnesium bromide (Scheme 4.3, [40]). 

Figure 4.14. Applications of oxathianes linalool [53], dimethyl acetylcitramalate [54], mevalolactone [56], malyngolide [55], and the mosquito oviposition attractant [39]. For the latter, the C-5 stereocenter was formed by a chelate-controlled reduction while the C-6 position could be produced as either epimer by a chelate or acyclic mechanism, depending on the reducing agent.

Sawaki, Y., and C. S. Foote Acyclic Mechanism in the Cleavage of Benzils with Alkaline Hydrogen Peroxide. J. Amer. Chem. Soc. 101, 6292 (1979). 

Examples of simple bipartite graphs for irreversible reactions (A) acyclic mechanism and (B) cyclic... 

In the analysis of bipartite graphs, the concept of cycles is crucial. The simplest class of reaction mechanisms is that with bipartite graphs that do not contain cycles (see Fig. 3.12A). These reaction mechanisms are called acyclic mechanisms and can be represented in general form as ... 

---