Reaction mechanisms heterolytic

The reaction chemistry of simple organic molecules in supercritical (SC) water can be described by heterolytic (ionic) mechanisms when the ion product 1 of the SC water exceeds 10" and by homolytic (free radical) mechanisms when <<10 1 . For example, in SC water with Kw>10-11 ethanol undergoes rapid dehydration to ethylene in the presence of dilute Arrhenius acids, such as 0.01M sulfuric acid and 1.0M acetic acid. Similarly, 1,3 dioxolane undergoes very rapid and selective hydration in SC water, producing ethylene glycol and formaldehyde without catalysts. In SC methanol the decomposition of 1,3 dioxolane yields 2 methoxyethanol, il lustrating the role of the solvent medium in the heterolytic reaction mechanism. Under conditions where K klO"11 the dehydration of ethanol to ethylene is not catalyzed by Arrhenius acids. Instead, the decomposition products include a variety of hydrocarbons and carbon oxides. 

As mentioned at the end of Chapter 1, an understanding of heterolytic reaction mechanisms must be accompanied by an understanding of the properties of organic acids and bases. Through this understanding, an ability to predict the reactive species in organic reactions and the reactive sites in organic molecules will evolve. Therefore, this chapter focuses on the properties of acids, dissociation constants, and the relative acidities observed for protons in different environments. 

This is an example of a cation-tt cyclization. Note that unlike the previously described heterolytic reaction mechanisms, this reaction is influenced by a positive charge. Also, please note that this reaction shares some characteristics with... 

Szele and Zollinger (1978 b) have found that homolytic dediazoniation is favored by an increase in the nucleophilicity of the solvent and by an increase in the elec-trophilicity of the P-nitrogen atom of the arenediazonium ion. In Table 8-2 are listed the products of dediazoniation in various solvents that have been investigated in detail. Products obtained from heterolytic and homolytic intermediates are denoted by C (cationic) and R (radical) respectively for three typical substituted benzenediazonium salts and the unsubstituted salt. A borderline case is dediazoniation in DMSO, where the 4-nitrobenzenediazonium ion follows a homolytic mechanism, but the benzenediazonium ion decomposes heterolytically, as shown by product analyses by Kuokkanen (1989) the homolytic process has an activation volume AF = + (6.4 0.4) xlO-3 m-1, whereas for the heterolytic reaction AF = +(10.4 0.4) x 10 3 m-1. Both values are similar to the corresponding activation volumes found earlier in methanol (Kuokkanen, 1984) and in water (Ishida et al., 1970). 

Figure 4. Reaction Mechanism for N-Acetoxy Arylamines (V). Ac, acetyl RSCH3 methionine RNH2, N2-guanine-nucleosides, -nucleotides, or -nucleic acids RCH, C8-guanine-nucleo-sides, -nucleotides, or -nucleic acids. Pathways and heterolytic cleavages a and b are discussed in the text. Dashed arrows indicate proposed pathways.

The stoichiometry of this reaction is usually close to unity [6-9]. Thus, cumyl hydroperoxide oxidizes triphenyl phosphite in the stoichiometry A[ROOH]/A[Ph3P] from 1.02 1 to 1.07 1, depending on the proportion between the reactants [6], The reaction proceeds as bimolecular. The oxidation of phosphite by hydroperoxide proceeds mainly as a heterolytic reaction (as follows from conservation of the optical activity of reaction products [5,11]). Oxidation is faster in more polar solvents, as evident from the comparison of k values for benzene and chlorobenzene. Heterolysis can occur via two alternative mechanisms... 

A slightly different catalytic system was reported by Lau and collaborators [14], These authors used Ru(t 5 T l-C5H4-(CH2)3NMe2)(dppm) (dppm = bis(diphenylphosphino)methane) as a catalyst and proposed a modified reaction mechanism where H2 undergoes heterolytic activation between the Ru center and the NMe2 group in the ligand. 

The structural similarity of the catalytic domains of the enzymes of the AAH family, together with the identical reaction that they catalyze (i.e., hydroxylation of aromatic substrates) and the common dependency on BH4 and 02 (Section I), suggests that the mechanisms by which these enzymes operate are similar. It is assumed that the general AAH reaction mechanism follows a two-step reaction route in which a high-valent iron-oxo (FeIV=0) complex is formed in the first step, and that this intermediate is responsible for the hydroxylation of the aromatic amino acid substrate in the second step (15,26-28,50). The first step starts with 02 binding and activation and proceeds via a Fe-0-0-BH4 bridge and a subsequent heterolytic cleavage of the... 

The hypothetical reaction mechanism shown in Fig. 10 is a variation of Scheme (3), and is consistent with the redox and pH dependence of the EPR-detectable nickel species (65). Hydrogen is known to undergo heterolytic cleavage (81) it is proposed that this is an intramolecular reaction, leading to the formation of a nickel (II) hydride and a protonated base in the enzyme (Step 1 in Fig. 10). The Ni-C species is postulated to be a protonated Ni(I) species. An alternative formulation for this state would be a dihydrogen complex, Ni(III) H2, as suggested by Crabtree (104). Ultimately the exact mechanism can only be determined by kinetic measurements. 

Cationic and anionic mechanisms involve species that have either positive or negative charges, respectively (heterolytic reactions). An example of a reaction that proceeds via a cationic mechanism is the hydrolysis of t-butyl bromide (Scheme 2). 

The acid-base catalysis is carried out in BRC, with H+ or 2H+ ion transfer. Poltorak notes [99] that when H+ and e or H+ and H are transferred, the reaction mechanisms relate to H+-dependent redox type. BRC with electron transfer describes heterolytic oxidative processes. 

The significance of this work is its identification of SC water as a medium which supports and enhances aqueous phase chemistry ordinarily observed at much lower temperatures. Fundamental studies of the reaction chemistry of biopolymer related model compounds described in this paper offer insights into the details of reaction mechanisms, and facilitate the choice of reaction conditions which enhance the yields of valuable products. Chemical reaction engineering in supercritical solvents, based on the ability to choose between heterolytic and homolytic reaction mechanisms with foreknowledge of results, holds much promise as a new means to improve our utilization of the vast biopolymer resource. 

Similar to homolytic mechanisms, the heterolytic reactions can be divided into three groups (i) reactions with hydroperoxides, (ii) activation of molecular oxygen, and (iii) direct reaction of metal complexes with substrate. 

When drawing arrows to illustrate movement of electrons, it is important to remember that electrons form the bonds that join atoms. The following represent heterolytic-type reaction mechanisms ... 

The following represents a heterolytic-type reaction mechanism ... 

In search of model systems for iron hydrogenases, Sellmann et al. (67) investigated the interaction of I e(hdt)2 2 with H+, H2, and H . Formation of H2 was observed in the reaction with H+. The reaction mechanism was proposed to follow a step-wise protonation, forming a thiol-hydride complex and H2 is proposed to form via heterolytic elimination from the metal hydride species (Scheme 6). Theoretical calculations suggest that concerted H2 elimination from a dithiol species is thermally forbidden (67). 

---