Yorke E. Rhodes

Reactive silica: VII. Chemisorption and pyrolysis procedures in a flow system

Abstract The vacuum pyrolysis of surface SiOCH 3 groups leads to the formation of SiH 2 and silanol groups which disappear on degassing. The silica then becomes remarkably active. To check if the reaction was unique to SiOCH 3 , a variety of reagents were tested using a flow system and monitoring the reactions by infrared spectroscopy. Most alcohols and simple esters became chemisorbed as silyl alkyl ethers. In general, with the exception of benzyl alcohol, a reagent was effective in producing SiH 2 if, upon chemisorption, methoxy groups were formed directly or if the adsorbed species itself contained methoxy groups. The results suggest that the initial reaction of a reagent with surface silanol s involved a mechanism in which an initial hydrogen bonding led to the formation of a surface-stabilized ion pair which could then readily suffer a direct displacement; the postulated mechanism can account for the observed order of chemisorption. The pyrolysis of alkoxy groups is thought to involve the homolytic cleavage of the OC bond to generate a siloxy radical which would then react further with desorbed material and/or with neighboring alkoxy groups. With methoxy, the SiH 2 and silanols were formed because the siloxy radical was too far from neighboring methoxy groups for interaction to occur. With alkoxy groups other than methoxy, however, the siloxy radical was within bonding distance of the alkoxy, and direct interaction was possible, surface silanols being generated.

Acetolysis and formolysis of (R)-and (S)-1-deuterio-2-cyclopropylethyl p-toluenesulfonates

Abstract Acetolysis and formolysis of the enantiomers of 1-deuterio-2-cyclopropylethyl tosylate, 3 -OTs and 4 -OTs, led to 17–18% retention (cyclopropyl participation) and 82–83% inversion (nucleophilic solvent assistance) of configuration in the 2-cyclopropylethyl product.

Resolution of cyclopentanol methylene proton signals with Eu(fod)3

Abstract Complete resolution of the five different protons of cyclopentanol has been achieved with the use of the lanthanide shift reagent Eu(fod) 3 for the first time, which permits stereochemical differentiation of the cis - and trans -2,5- and 3,4-protons for mechanistic studies of elimination, solvolytic, and other reactions of the cyclopentyl system. The separated peaks are assigned (in order of increasing downfield shift) as: H 3 - and H 4 - trans , H 2 - and H 5 -trans, H 3 - and H 4 - cis , H 2 - and H 5 - cis , and H 1 . An example of application of the technique to mechanistic study is presented in the formolysis of deuterated cyclopentanol.