Addition and Abstraction

In reductive elimination or migratory insertion, ligand transformations occur within the coordination sphere of the metal. In contrast, we now look at outer sphere processes in which direct attack of an external reagent can take place on a ligand without prior binding of the reagent to the metal. 

The Organometallic Chemistry of the Transition Metals, Sixth Edition. Robert H. Crabtree. 

For an electrophile, the situation is different. As a Oe reagent, an electrophile does not increase the electron count of the metal whether it attacks the metal or the ligand. Attack at the metal is thus always a possible alternative pathway even for an 18e complex except for cf complexes, which have no lone pairs on the metal. Of course, large electrophiles, such as Ph3C+, may still have steric problems that prevent attack at the metal. This lack of selectivity has made electrophilic attack less useful. 

---

The hydrogen abstraction addition ratio is generally greater in reactions of heteroatom-centered radicals than it is with carbon-centered radicals. One factor is the relative strengths of the bonds being formed and broken in the two reactions (Table 1.6). The difference in exothermicity (A) between abstraction and addition reactions is much greater for heteroatom-centered radicals than it is for carbon-centered radicals. For example, for an alkoxy as opposed to an alkyl radical, abstraction is favored over addition by ca 30 kJ mol"1. The extent to which this is reflected in the rates of addition and abstraction will, however, depend on the particular substrate and the other influences discussed above. 

Gaffney, J.S., Levine, S.Z. (1979) Predicting gas phase organic molecule reaction rates using linear free-energy correlations. I. 0(3P) and OH addition and abstraction reactions. Int. J. Chem. Kinet. 11, 1197-1209. 

The chemical details of the reaction sequence following HO radical addition and abstraction from olefins should be explored fully— ... 

Allylic hydroperoxides are primary products in the autoxidation of - olefins, and lack of definite information on their reactivity and chemical behavior has hampered efforts to understand olefin oxidation mechanisms (2). This deficiency is most strongly felt in determining the relative rates of addition and abstraction mechanisms for acyclic olefins since assignment of secondary reaction products to the correct primary source is required. Whereas generalizations about the effect of structure on the course of hydroperoxide decompositions are helpful, most questions can be answered better by directly isolating the hydroperoxides involved and observing the products formed by decomposition of the pure compounds. 

Conversely, at the lower temperatures, the rate constant for H-abstraction is small while, at the same time, the rate of adduct decomposition is lowered. As a result, at the lower temperatures (right side of Fig. 6.11), adduct formation predominates and a negative temperature dependence, as well as a dependence on pressure is observed for the overall rate constant. In the intermediate region, both addition and abstraction are occurring at significant rates, leading to the curved OH decay plots in Fig. 6.10 and the discontinuities in the Arrhenius plots of Fig. 6.11. 

Gaffney, J. S., and S. Z. Levine, Predicting Gas-Phase Organic Molecule Reaction Rates Using Linear Free Energy Correlations. I. OC P) and OH Addition and Abstraction Reactions, Int. J. Chem. Kinet., 11, 1197-1209 (1979). 

A second example is isomerization of isopentenyl diphosphate to dimethylallyl diphosphate (Eq. 13-56) 304-307 The stereochemistry has been investigated using the 3H-labeled compound shown in Eq. 13-56. The pro-R proton is lost from C-2 and a proton is added to the re face at C-4. When the reaction was carried out in 2H20 a chiral methyl group was produced as shown.304 A concerted proton addition and abstraction is also possible, the observed trans stereochemistry being expected for such a mechanism. However, the... 

The significant changes imposed on the dithioaromatic ligands and complexes upon sulfur addition are illustrated in the structure of the Ni(p-/-PrPhDtaXp-(-PrPhDtaS) complex (Fig. 48) (Table XXII), determined by Fackler et al. (233, 257). The same workers explored the sulfur addition and abstraction reaction in depth (232) (see also Section IV). The rates and mechanisms of substitution reactions of square planar nickel(II) 1,1 -dithiolate complexes (502) is discussed in Section IV. 

By this technique these authors have determined the rate constants and collision yields for a number of simple olefins, substituted olefins, and some aromatic hydrocarbons. For a number of years these determinations represented the only extensive set of rate constants of hydrogen atom reactions with olefins. The technique did not differentiate between addition and abstraction by hydrogen atoms from the olefins and the rates were the sum of the two. 

Insertion, addition and abstraction reactions of free silicon atoms can lead to the formation of silylenes (equation 9)1-5. Silylene formation for reaction, spectroscopic and... 

In their modeling study of DMS oxidation, Yin, et al. consider both addition and abstraction channels to produce SO2 and MSA (2). They do not consider other energetically accessible channels such as... 

C-H bond energies. Therefore, such a dramatic increase in the reactivity of OH towards DMSO compared to DMS would not be expected if an abstraction mechanism was operative in each case. It is possible that the presence of the O in DMSO or the higher oxidation state of sulfur is increasing the reactivity of the molecule toward an addition type of reaction. The reaction of the addition aduct, OH-DMSO, with Oj could lead to an enchancement of the effective rate constant. Such a mechanism has been envoked to explain the dependence of the rate constant for the reaction of OH with CS2 on tne partial pressure of O2 in the reaction system (26.271 The detection of both SO2 and DMSO2 as reaction products, as described below, indicates that both addition and abstraction reaction pathways are operative. 

We wish to stress again that for claritiy s sake the reaction intermediate has been indicated as a carbonium ion, but that a concerned reaction, in which the two steps of addition and abstraction occur simultaneously, is conceivable as well. 

Taylor et al. [104] investigated the reaction of hydroxyl radicals with acetaldehyde in a wide temperature range using a quantum RRK model to describe the competition between addition and abstraction. They conclude that different reaction mechanisms occur, depending on the temperature, and that OH addition followed by CH3 elimination is the dominant reaction pathway between 295 and 600 K. Moreover, they claimed that the H-atom elimination pathway is largely insignificant, except possibly at the lowest temperatures. 

There is little rate data available for addition and abstraction reactions involving higher alpha-olefins. Where such data are available, they have usually been obtained from lower temperature studies (12,13,14), However, the available information indicates that addition should be competitive with abstraction in the temperature range of this study. For example, Steacie s data for ethyl radical reactions with 1-hexene and 1-heptene indicate that at 525°C, the addition rate would be about seven-tenths of the abstraction rate. For dodecene, one would expect this ratio to decrease because of the increase in abstractable hydrogen, but addition should still be a significant pathway. For methyl radicals and H atoms, available data (13,14) indicate that addition is somewhat faster relative to abstraction. 

A reeent re-evaluation of the rate coefficient and the branching ratio has been made by Williams et al. (2001) using the pulsed laser photolysis-pulsed laser induced fluorescence (PLP-PLIF) teehnique. The effective rate coefficient for the reaction of OH -1- DMS and OH + DMS-db was determined as a function of O2 partial pressure at 600 Torr total pressure in N2/O2 mixtures the temperature was 240 K for DMS and 240, 261, and 298 K for DMS-db. This new work shows that at low temperatures the currently recommended expression underestimates both the effective rate coefficient for die reaction and also the branching ratio between addition and abstraction. For example, at 261 K a branching ratio of 3.6 was obtained as opposed to a value of 2.8 based on the work of Hynes et al. (1986). At 240 K the discrepancy increases between a measured value of 7.8 and a value of 3.9 using the extrapolated values from the 1986 work of Hynes et al. (the branching ratio is defined here as (kobs-kia)/kia). In addition, at 240 K the expression for Us in 1 atm air based on the work of Hynes et al. (1986) predicts a value which is a factor of 2 lower then the value measured at... 

Figure 3. Temperature dependence of the branching ratio between the addition and abstraction channel.

Figure 3 shows the temperature dependence of the branching ratio between the addition and abstraction channel for DMS + OH obtained using the data from this work, Hynes et al. 

Williams et al. (2001) and Atkinson et al. (2004). For die calculation of the branching ratio between addition and abstraction for this work, values for the abstraction rate coefficients were taken, for comparison piuposes, both from the expression reported by Hynes et al. (1986) and also from the new recommendations of Atkinson et al. (2004). 

The stereoselectivity in addition and abstraction reactions of cyclopentyl radicals has been reviewed recently3. It has been concluded that /f-substituents at the radical, as well as the alkene substituents, have a large influence on the selectivity, however only small solvent effects have been found. 

Although S2032- is a known product of the radiolysis of S(-II), no pathways that lead to its formation have been proposed. We included two possible pathways the reaction of HS2- with S032- (R14) and oxidation of HS2 by OH (R44) followed by successive addition and abstraction reactions (R45 and multistep R47). The rate constant of R14 is not expected to be higher than the rate constant for the reaction of S032- with N02 (R38) and was therefore set at 107 M"1 s"1. The rate constants for reactions R44-R47 are expected to be near the diffusion-controlled limit and were set at 109 M"1 s"1. Even if krl4 = 108 M"1 s"1 and kTii 17 = 5 X 109 M"1 s"1, these reactions cannot account for the observed formation of S2032-. However, the values of these rate constants are not critical for the overall rate of S(-II) oxidation. 

The main two reactions of OH radical are connected with addition and abstraction of an H+ atom. 

---