Silyl radical ionization potential

Thermochemical information about neutral species can also be obtained from measurements of ions. Indeed, accurate bond dissociation energies for neutral molecules have been obtained from gas-phase ion chemistry techniques. In this section, we will summarize both the negative-ion and hydride-affinity cycles involving silicon hydrides (RsSiH) which are connected to electron affinity (EA) and ionization potential (IP) of silyl radicals, respectively [22-24]. 

Table 2.6 Gas-phase basicities of silanes and adiabatic ionization potential of silyl radicals ...

Yokoyama and coworkers investigated the discolouration of titanylphthalocyanine 60, and related porphyrins by silyl radicals derived from methylphenylpolysilane by molecular orbital methods60. Calculated values of AN, the index derived from the hard and soft acids and bases concept was determined from HF/3-21G calculated ionization potentials, electron affinities, and HOMO and LUMO orbital energies of the systems of interest. These studies predict that PhMe2Si should donate an electron readily to 60 to form 60 (equation 18). This prediction is of direct relevance to observations that irradiation of titanylphthalocyanine-coated methylphenylpolysilane films leads to discolouration of the film60. 

Quite obviously, direct measurement of the oxidation potentials of the intermediate, highly unstable enols poses a severe experimental problem. From work by Kresge, Capon and others [58-60] it is known that under appropriate conditions even enols of aliphatic ketones can be prepared with lifetimes in the order of minutes. However, no electrochemical data on these systems are available so far. Even for arylacetones, where the enol content is in the order of 10 %, direct measurement of the oxidation potential is not known to date. Two approaches have been applied to estimate their oxidation potentials [183]. First, using a linear correlation [184] between the experimental oxidation potentials of benzyl radicals [185,186] and their AMI calculated ionization potentials (IPs) the Fiyj of the four enols 86a-86d were derived from the calculated IPs. Secondly, oxidation potentials of silyl enol ethers were used to... 

Desilylative coupling of cinnamyltrimethylsilane results in 3,6-diphenyl-1,5-hexadiene as shown in Scheme 2.60. The cross-coupling reaction of silyl enol ethers and allylic silanes proceeds chemoselectively to give Y,8-unsaturated ketones, in which the oxovanadium(V) oxidatively desilylates the more readily oxidizable organosilicon compound [126], Their redox potentials determine whether they will act as a radical generator or acceptor. These redox potentials can be predicted from calculated ionization potentials. VO(OR)Cl2 is a versatile oxidant, which can induce chemoselective coupling via the oxidative desilylation of a variety of organosilicon compounds under controlled conditions, as shown in Scheme 2.61. 

---