Rate constants Group 14 hydrides

This review focuses on the kinetics of reactions of the silicon, germanium, and tin hydrides with radicals. In the past two decades, progress in determining the absolute kinetics of radical reactions in general has been rapid. The quantitation of kinetics of radical reactions involving the Group 14 metal hydrides in condensed phase has been particularly noteworthy, progressing from a few absolute rate constants available before 1980 to a considerable body of data we summarize here. 

Scheme 1 illustrates the design of an experiment that could be used to determine the rate constant for H-atom abstraction from a group 14 hydride. Radical A- reacts with the hydride to give product A-H. In competition with this reaction, radical A- gives radical B- in a unimolecular or bimolecular reaction with a known rate constant, and product radical B- also reacts with the hydride, giving B-H. The rate constant for reaction of A- with the metal hydride can be determined from the product distribution, the known rate constant for conversion of A- to B-, and the concentrations... 

For the primary and secondary a-alkoxy radicals 24 and 29, the rate constants for reaction with Bu3SnH are about an order of magnitude smaller than those for reactions of the tin hydride with alkyl radicals, whereas for the secondary a-ester radical 30 and a-amide radicals 28 and 31, the tin hydride reaction rate constants are similar to those of alkyl radicals. Because the reductions in C-H BDE due to alkoxy, ester, and amide groups are comparable, the exothermicities of the H-atom transfer reactions will be similar for these types of radicals and cannot be the major factor resulting in the difference in rates. Alternatively, some polarization in the transition states for the H-atom transfer reactions would explain the kinetic results. The electron-rich tin hydride reacts more rapidly with the electron-deficient a-ester and a-amide radicals than with the electron-rich a-alkoxy radicals. 

We have focused on experimental values in this work, but it is important to note two areas of group 14 hydride kinetics that are not discussed but in which we expect to see considerable progress in the near future. One is the kinetics of reactions of radicals centered on atoms of the third row and above, and the other is computational estimates of rate constants. 

The kinetic data reported in this chapter have been determined either by direct measurements, using for example kinetic EPR spectroscopy and laser flash photolysis techniques or by competitive kinetics like the radical clock methodology (see below). The method for each given rate constant will be indicated as well as the solvent used. An extensive compilation of the kinetics of reaction of Group 14 hydrides (RsSiH, RsGeH and RsSnH) with radicals is available [1]. 

Figure 4.12. Second-order rate constants for reactions of hydrogen atom donors with various radical types at ambient temperature. Data sources group 14 (IV A) hydrides (15) aminyl radicals (69) amidyl radicals (70) alkyl radials with group 16 (VI A) hydrides (71) acyl radical with PhSeH (72).

C. Chatgihaloglu and M. Newcomb, Hydrogen donor abilities of the group 14 hydrides, Adv. Organometal Chem. 1999, 44, 67 (rate constants for radical reactions with Group 14 (IV A) metal hydrides). 

The rate constant of HAT from 1 has been determined as (3.4 1.0) x 104 M-1 s-1 at 28 °C by using a cyclobutyl carbinyl radical as clock. Also, the log A term of the Arrhenius equation is normal for a second-order HAT and thus the entropic demand of the NHC boranes is similar to that of group 14 metal hydrides. From the rate constant a BDE of about 88 kcal mol 1 for 2 was estimated by applying an Evans-Polanyi relationship. This value is somewhat higher than the calculated value of 80 kcal mol-1. 

Transfer of hydrogen to carbonyl groups differs from analogous transfer to unsaturated hydrocarbons primarily due to the greater likelihood for involvement of free ionic or ion-pair intermediates in the former reaction. Linstead and coworkers (36) have shown that transfer from dihydroaromatics to quinones is best explained by a rate limiting step involving hydride ion transfer. The applicability of this mechanism to other systems is presently unclear (40). For example, under appropriate conditions quinones can generate free radicals and form adducts (37). Pseudo-first order rate constants for... 

Hydrides based on aluminum have proved most versatile and selective for the reduction of esters (RCO2R ) to aldehydes (RCHO), but it should be noted that the nature of R and R (aliphatic, aromatic, branched, electron-withdrawing or electron-donating, steric size) is important. Most of the reactions described here have been carried out on simple methyl or ethyl esters. It is frequently found that aliphatic esters give much better yields than do aromatic esters. Reductions are carried out at very low temperatures such that esters are still reactive to the hydride but any aldehyde produced is not. Alternatively, the intermediate formed by addition of metal hydride to the ester (15 Scheme 5) may be sufficiently stable that no significant amount of aldehyde is formed. Thus, if the rate constant 1 > k2 then little aldehyde will be released, and no aldehyde formed even if 3 > /(. It also seems likely that the difference in the Lewis basicity of esters and aldehydes affects the complex of rate constants shown as 1, ki and h, and hence affects the relative reactivity of these two functional groups towards the different hydride reagents. 

Some of the new theoretical relations, the cross-relation between the rates of a cross-reaction of two difierent redox species with those of the two relevant selfexchange reactions, were later adapted to non-electron transfer reactions involving simultaneous bond rupture and formation of a new bond (atom, ion, or group transfer reactions). The theory had to be modified, but relations such as the crossrelation or the effect of driving force (—AG°) on the reaction rate constant were again obtained in the theory, in a somewhat modified form. For example, apart from some proton or hydride transfers under special circumstances, there is no predicted inverted effect. Experimental confirmation of the cross-relation followed, and an inverted effect has only been reported for an H+ transfer in some nonpolar solvents. The various results provide an interesting example of how ideas obtained for a simple, but analyzable, process can prompt related, yet different, ideas for a formalism for more complicated processes. 

---