The question of C- vs. O-silylation of ketenes: Electrophilic triethylsilylation of diphenylketene

Abstract

Electrophilic triethylsilylation of diphenylketene leads to exclusive C-silylation giving the diphenyl(triethylsilyl)acetyl cation in the solution phase even though density functional theory calculations at the B3LYP͞6-311؉G* level indicate that the O-silylation of diphenylketene is preferred over C-silylation by 5.4 kcal͞mol in the gas phase. On the other hand, in the case of the parent ketene, similar density functional theory calculations show that C-silylation is preferred over O-silylation by 8.2 kcal͞mol.K etenes are unusual alkenes as well as carbonyl compounds that are reactive toward both electrophiles and nucleophiles. From 13 C NMR chemical shift studies it has been shown that ketenes are highly polarized molecules with the C ␤ and oxygen atoms bearing a substantial negative charge and C ␣ bearing the positive charge (ref. 1; for a review, see ref.2). Electrophiles attack at C ␤ or oxygen centers from above or below the plane of the ketenes, and nucleophiles attack at C ␣ in plane. Protonation of ketenes has been the subject of numerous experimental and theoretical studies. Theoretical studies (3)(4)(5)(6)(7)(8) show that the protonation at C ␤ of ketene 1a (to form acylium ion 2a in Eq. 1) is more favorable than protonation at oxygen (to form Oprotonated ion) by 43 kcal͞mol. In solution, the site of proton addition to ketenes has been mostly determined from kinetic studies of acid-catalyzed hydration (9-13). We have previously shown that protonation of diphenylketene and of di-tertbutylketene in Magic Acid solution at low temperature resulted in the formation of corresponding acylium ions 2b and 2c (Eq. 1), as indicated by the 13 C NMR signals for CO ϩ at ␦ 13 C 154.7 and 154.1, respectively (14).Long lived trialkylsilyl cations are still elusive in the condensed phase due to their kinetic instability. The in situ generation of trialkylsilyl cations in solution, by hydride transfer͞abstraction from organohydrosilanes by the triphenylmethyl (trityl) cation (so-called ''Corey hydride transfer'') (15), has been adopted for the preparation of a number of silylated onium ions (16-21). However, electrophilic silylation of ketenes has not yet been reported. Thus far, silylation of ketenes employing silicon compounds has been known to produce only silyl enol ethers, formed by nucleophilic addition of a leaving group, such as CN Ϫ or N 3 Ϫ , at C ␣ followed by O-silylation (22, 23). Herein, we report a theoretical and experimental study of electrophilic silylation of ketenes in solution. In principle, the incipient silyl cation may react with the C ␤ of ketene to give ␤-silylacylium ion or with oxygen to give silylcarboxonium ion due to strong Si-O bond formation (Eq. 2). Experimental MethodsThe preparation of ␤-silyl carbenium ion 5 was as follows. Diphenylketene (0.13 mmol) and triethylsilane (25 mg, 0.22 mmol) in dry CD 2 Cl 2 (0.25 ml) were loaded into a dry, 5-mm NMR tube in a glove box under an argon atmosphere. The NMR tube was taken out of the glove box and then cooled by using dry ice͞acetone to Ϫ78°...

Unexplored volatile organic compound emitted from petrochemical facilities: implications for ozone production and atmospheric chemistry

“…Ketene has a proton affinity of 196 kCal mol −1 , which is much higher than that of water (166 kCal mol −1 ), and therefore, it is possible to detect protonated ketene using PTR-TOF-MS. The direct protonation of ketene can result in three different structural conformations, and the most stable structure of protonated ketene is the acylium ion (Vogt et al, 1978;Nobes et al, 1983;Leung-Toung et al, 1989;Prakash et al, 2005), which is detected at m/z 43.018 using PTR-TOF-MS. Therefore, even though we have detected the same acylium ion (at m/z 43.018) that can be formed from direct protonation of ketene and the fragmentation of vinyl acetate, methyl acrylate, and 2,3-butanedione, our results suggest that the origin of the acylium ion detected during our field campaign is the protonation of directly emitted ketene and not the fragmentation of vinyl acetate.…”

Electrophilic Formylation of Arenes by Silylium Ion Mediated Activation of Carbon Monoxide

“…Adducts of silylium ions and ketones [22] or ketenes [23] have already been studied by NMR spectroscopy while the corresponding aldehyde-derived silylcarboxonium ions are still unexplored. Therefore,a ll products were fully charac- Table S1 in the Supporting Information).…”

Substituted ketenes offer exceptional carbon bases in gas phase: Computational study by density functional theory method

“…[ 25 ] The experimental and theoretical investigations of ketene protonation in gas phase have been the research topic of a few scientific groups. [ 26–31 ]…”