[51666-94-5] · C5H11LiSi · 1-(Trimethylsilyl)vinyllithium · (MW 106.19)
Alternate Names: 1-lithio-1-(trimethylsilyl)ethylene; a-(trimethylsilyl)vinyllithium.
Solubility: sol THF, ether.
Preparative Method: prepared in situ under an inert atmosphere by lithium-halogen exchange of (1-Bromovinyl)trimethylsilane with t-Butyllithium.
Handling, Storage, and Precautions: solutions are inflammable; it is critical to preclude contact with air and moisture.
a-(Trimethylsilyl)vinyllithium (1) adds to ketones and aldehydes. The adducts do not undergo the expected Peterson elimination reaction when treated with Sodium Hydride or Potassium Hydride, but the corresponding allene can still be obtained if the alcohol is converted to the chloride followed by fluoride-catalyzed b-elimination (eq 1).5a,b Other uses have also been made of the allyl chlorides made in this manner: they react with cuprates,5c or they can be oxidized to the epoxides to serve as precursors to allene oxides.5d-l
Under certain conditions (KH in HMPA), products from the homo-Brook rearrangement are observed (eq 2).6 This is therefore a good method of desilylation of b-hydroxyvinylsilanes to the corresponding vinyl alcohols. Other conditions for desilylation were found to give better yields (eq 3).7 Since a-(trimethylsilyl)vinyllithium was found to add to chiral aldehydes with better diastereoselectivity than Vinyllithium, the overall sequence is an improvement for chelation controlled 1,2-asymmetric induction.7a In the sequence of eq 3, diastereoselectivity varied as the protecting group RŽ was changed; it was 20:1 for RŽ-benzyl. This illustrates the usefulness of a-(trimethylsilyl)vinyllithium in stereoselective polyol synthesis.7
The alcohol adducts from a-(trimethylsilyl)vinyllithium and aldehydes have found many uses in organic synthesis. The ketones obtained by oxidation (eq 4) are especially good Michael acceptors and have been used in a modified Robinson annulation reaction for the construction of cyclohexenones.8 Cyclopropanation leads to cyclopropylsilane adducts, which can be converted into a variety of cyclopentenes (eq 5).9 Halogenation followed by stannylation gives synthons (eq 6) useful for making substituted vinylsilanes through radical reactions.10
Rearrangements of alcohols bearing an epoxide functionality give ketones that can be stereoselectively reduced. The new stereocenter formed is of opposite configuration when the trimethylsilyl group
R is present on the vinyl moiety (eq 7).11b,c Thus a-(trimethylsilyl)vinyllithium can be viewed as a large vinyl anion equivalent (e.g. eq 3) which complements vinylmagnesium and lithium chemistry.11 These reactions were elegantly used in the total syntheses of avenaciolide, isoavenaciolide,11b,c and (+)- and (-)-eldanolide.11f
a-(Trimethylsilyl)vinyllithium undergoes Michael addition to enones, in good yields, in the presence of copper salts (eq 8).2 The vinyl group can be oxidized to the epoxide, which is then converted to the acyl group.2,12 The overall transformation is the addition of an acyl anion equivalent. a-(Trimethylsilyl)vinyllithium compares favorably with other acyl anion equivalents, particularly with the vinyl ether analogs which do not add well to hindered enones.13
a-(Trimethylsilyl)vinyllithium dimerizes in the presence of Copper(I) Iodide. This reaction was first observed as a side product during the formation of organocuprates,3a then developed for the synthesis of 2,3-bis(trimethylsilyl)buta-1,3-diene (eq 9).3b
a-(Trimethylsilyl)vinyllithium reacts with ethylene oxide to give an intermediate which has been used to set up an impressive eight-membered ring cyclization for the synthesis of (-)-laurenyne (eq 10).4a Here, vinyltrimethylsilane was preferred over the simple vinyl moiety because of its greater reactivity.
The reaction of this anion with carbon monoxide leads to the formation of an interesting cyclopropyl product (eq 11).4c Other electrophiles, for example selenium cyanide,4d Iodine,4f chlorosilanes,4g and imines,4h react with a-(trimethylsilyl)vinyllithium to give the corresponding adducts.
Other types of 1-lithio-1-vinylsilanes have been made, mainly through the lithium-halogen exchange reaction described before. For example, one or all of the methyl groups on silicon can be substituted by phenyls. The compounds (2)4c and (3)14 have properties similar to a-(trimethylsilyl)vinyllithium.
Monosubstitution in the 2-position on the vinyl moiety has been extensively exemplified in the literature. The stereochemistry of the double bond implies two possible types of reagents. For example, the 2-(Z)-monosubstituted (4,15 511d) and the 2-(E)-monosubstituted (6,16a 716b) 1-lithiovinylsilanes have been treated with carbonyl and alkyl electrophiles.
The stereochemistry of the 1-lithiovinylsilanes is easily lost, as exemplified in eq 12 where (9), prepared by a transmetalation reaction, subsequently equilibrates to a mixture containing (9) and (10).15,16b
One other related reagent includes the species (11),17 obtained from silylcupration of the corresponding lithium acetylide. This is yet another way to obtain 1-lithiovinylsilanes substituted in the b-position.
See also Lithium (Trimethylsilyl)acetylide, (E)-2-(Trimethylsilyl)vinyllithium, and Vinyllithium.
Denis Labrecque & Tak-Hang Chan
McGill University, Montreal, Quebec, Canada