Trimethylsilyl vinylketene

Kevin J. Moriarty Rkdne-Poulenc Rarer, Collegeville, PA, USA 

Solubility sol CH2CI2, CHCI3, CCI4, toluene, benzene, hexane, most organic solvents. 

Form Supplied in yellow-green liquid not commercially available. 

Preparative Methods conveniently prepared as outlined in eq 1. Treatment of l-(trimethylsilyl)propyne (2) with 1.1 equiv of diisobutylaluminum hydride (25 °C, 21 h) and 1.1 equiv of methyllithium (0 °C, 0.5 h) in ether-hexane, followed by reaction of the resulting vinylalanate with anhydrous carbon dioxide, yields (Z)-2-(trimethylsilyl)-2-butenoic acid (3) in 68% yield. Exposure of the potassium salt of this acid (4) to 1.1 equiv of oxalyl chloride in pentane containing a catalytic amount of iVW-dimethylfonnamide (0-25 °C, 1.5 h) then produced a mixture of the acid chloride (5) and its geometric isomer, which was dehydrohalogenated without further purification. A solution of (5) in pentane was added dropwise over 1-2 h to a solution of 0.9 equiv of triethylamine in pentane 

Handling, Storage, and Precautions purified vinylketenes can be stored under nitrogen in solution at 0 °C without appreciable decomposition for 1-2 weeks. 

---

DOTZ, K.H., B. TRENKLE, U. SCHUBERT. 1981. Addition to ynamines to vinylketenes. Angew. Chem. 93 296. DANHEISER, R.L., H. SARD. 1980. (Trimethylsilyl) vinylketene. A stable vinylketene and reactive enophile in [4 2]cycloadditions. J. Org. Chem. 

Reactive Enophile in [4 + 2] Cycloadditions. Vinylketenes are not effective as dienes in Diels-Alder reactions because they undergo only [2 + 2] cycloaddition with alkenes, as predicted by frontier molecular orbital theory. However, silylketenes exhibit dramatically different properties from those found for most ketenes. (Trimethylsilyl)vinylketene (1) is a relatively stable isolable compound which does not enter into typical [2 + 2] cy do additions with electron-rich alkenes. Instead, (1) participates in Diels-Alder reactions with a variety of alkenic and alkynic dienophiles. The directing effect of the carhonyl group dominates in controlling the regiochemical course of cycloadditions using this diene. For example, reaction of (1) with methyl propiolate produced methyl 3-(trimethylsilyl)sahcylate with the expected regiochemical orientation. ProtodesUylation of this adduct with trifluoroacetic acid in chloroform (25 °C, 24 h) afforded methyl salicylate in 78% yield (eq 2). 

Trimethylsilyl)vinylketen (20), prepared in four steps from l-(trimethyl-silyl)propyne in 40—60% yield, participates in Diels-Alder reactions as a reactive diene, generating a-(trimethylsilyl)cyclohexenones directly from olefinic dienophiles (Scheme 18). °... 

Trimethylsilyl)vinylketene (95) is stable and a reactive enophile in [4 + 2]cycloadditions. Trimethylsilylallenes have been used (Scheme 18) in the stereoselective synthesis of rra/t5-a,/3-unsaturated carbonyl compounds they have also been used to prepare the acetylenes (96) and (97) from ethylenic acylcyanides and carbonyl compounds/ respectively. In the presence of KF-18-crown-6, benzoyl trimethylsilanes (PhCOSiMea) behave as acyl anion equivalents in their reaction with organic halides to give ketones, ... 

In a similar study, Park has shown125 that cyclic dienes other than cyclo-pentadiene will also undergo a [4 + 2] cycloaddition reaction with the (trimethylsilyl)alkynylcarbene complex (184.b). When 184.b was reacted with 1,3-cyclohexadiene in THF, the diene complex 191 was isolated along with the vinylketene complex 192, which was prone to hydrolysis during column chromatography as expected.123 When 192 was stirred with silica gel and water in hexane, an almost quantitative conversion to the aldehyde complex 193 was observed. 

Vinylketene thioaeetais. 2-Lithio-2-trimethylsilyl-l,3-dithiane (1) also reacts with 3,/8-unsalurated aldehydes and ketones exclusively with the carbonyl group to give vinylketene thioaeetais (2). These undergo Dicls-Alder cycloaddition, which Carey and... 

Carbonylation of the chromium carbene complex (CO)5Cr=C(OCH3)Ph at 150bar pressure in the presence of l-vinyl-2-pyrrolidone resulted in Cr(CO)6 and organic products that were rationalized by the intermediate formation of methoxyphenylk-etene [33]. The formation of a free vinylketene and vinylketene chromium complexes were found in the reaction of (CO)5Cr=C(OCH3)Ph and bis(trimethylsilyl)acetylene at 50 °C (reaction 8.19) [34-36]. 

---