[13683-41-5] · C5H11BrSi · (1-Bromovinyl)trimethylsilane · (MW 179.13)
(undergoes lithium-halogen exchange in the presence of alkyllithium;2 used as a dienophile in [4 + 2] cycloaddition reactions;3 used in palladium-catalyzed reactions;4 substrate for cyclopropanation5)
Alternate Names: 1-(trimethylsilyl)vinyl bromide; 1-bromo-1-(trimethylsilyl)ethylene.
Physical Data: bp 124 °C/745 mmHg; d 1.156 g cm-3.
Solubility: completely miscible with THF and Et2O.
Form Supplied in: clear liquid.
Analysis of Reagent Purity: 1H and 13C NMR can be used.
Preparative Methods: prepared, in good yield, from the reaction of Vinyltrimethylsilane with Bromine at low temperature followed by dehydrohalogenation in the presence of an amine base.1 Alternative syntheses and reagents, i.e. (1-bromovinyl)triphenylsilane and (1-bromovinyl)triethylsilane, are also known.2a
Handling, Storage, and Precautions: store in a dry area.
1-Trimethylsilylvinyl metal species are mostly used as acyl anion equivalents and as hindered vinyl anion substitutes. 1-(Trimethylsilyl)vinyllithium is easily accessible from (1-bromovinyl)trimethylsilane by treatment with n-Butyllithium at -78 °C in ether (eq 1).2a Other solvents and alkyllithiums can also be used for the metal-halogen exchange.2b The use of (1-chlorovinyl)trimethylsilane or the direct reaction of (1-bromovinyl)trimethylsilane with lithium metal are not proper methods for generating the lithium species.2c
The corresponding Grignard reagent is made by reaction of Magnesium with (1-bromovinyl)trimethylsilane in THF (eq 2).2d It has the same general uses as the lithium analog (1-(Trimethylsilyl)vinyllithium). Furthermore, it has been shown to react with allylic acetates, in the presence of palladium catalysts, to give the corresponding dienes in moderate to low enantiomeric excess when the catalyst was chiral (eq 3).6 Generally, Grignard reagents will add to the carbonyl of the acetate group in preference to allylic substitution. The authors described 1-(trimethylsilyl)vinylmagnesium bromide as a soft stabilized anion to explain this peculiar reactivity.
This Grignard reagent has also been reacted with Carbon Dioxide2f and the cuprate of this Grignard reagent adds to alkynyl compounds and provides a practical synthesis of 1,3-butadienes (eq 4).2g
A number of substituted vinyltrimethylsilanes react with dienes and nitrile oxides in Diels-Alder6 and 1,3-dipolar cycloadditions.3 The trimethylsilyl group has a deactivating effect. This reaction with Benzonitrile Oxide gives 3-phenyl-5-trimethylsilylisoxazole (eq 5).
Numerous nucleophiles react with (1-bromovinyl)trimethylsilane in the presence of palladium complexes. The bromine has been substituted by phenylthio,4a vinyl,4b and aryl4c groups. This approach gives reasonable yields of the desired products. However, the substitution reactions sometimes lack regiospecificity. For example, a mixture of regioisomers was obtained in eq 6. Two mechanisms have been proposed for the formation of the b-substituted product. One involves an elimination step to give Trimethylsilylacetylene as an intermediate, which then undergoes catalyzed additions with the nucleophile at either the a- or b-positions.4a The other mechanism involves the formation of a pentacoordinated palladium intermediate, leading to the formation of isomeric products.4c
Dichlorocarbene adds to vinyltrimethylsilane, and the adduct is a source of 1-chlorocyclopropene, a reactive dienophile for Diels-Alder reactions.5b Dichlorocarbene also reacts with (1-bromovinyl)trimethylsilane5a to form (1-bromo-2-dichlorocyclopropyl)trimethylsilane. b-Elimination in the presence of Tetra-n-butylammonium Fluoride gives 1-bromo-2-chlorocyclopropene, another reactive dienophile (eq 7).
Denis Labrecque & Tak-Hang Chan
McGill University, Montreal, Quebec, Canada