[78108-48-2] · C5H11LiOSi · (Z)-2-(Trimethylsilyloxy)vinyllithium · (MW 122.19)
(vinylogation of aldehydes and ketones under mild conditions; preparation of b-methoxy aldehydes and b-silyl silyl enol ethers)
Solubility: sol Et2O, THF.
Preparative Methods: bromine-lithium exchange starting from (Z)-2-Bromo-1-(trimethylsilyloxy)ethylene in dry Et2O (0.15 M) and t-Butyllithium at -70 °C under inert atmosphere.1 One equivalent of t-BuLi is sufficient if (Z)-(trimethylsilyloxy)vinyllithium is condensed with reactive electrophilic reagents which are added 10-20 min after the end of the addition of t-BuLi.1,2 In other cases, two equivalents of t-BuLi are needed, the second equivalent is used to destroy t-BuBr formed during the exchange3 and the electrophilic reagents are added after the end of addition of t-BuLi.
Handling, Storage, and Precautions: when prepared with 2 equiv of t-BuLi the reagent is stable in Et2O over 20 h at -70 °C;1 from -60 °C it begins to decompose with formation of Acetylene.4
(Z)-(Trimethylsilyloxy)vinyllithium (1) has essentially been used for a one-pot vinylogation of carbonyl compounds. Condensation of (1) with aldehydes and ketones (2) occurs readily at low temperature (-70 °C); then treatment of the reaction mixture with an acidic solution in mild conditions (1 N Hydrochloric Acid, 0 °C to rt) produces the a,b-unsaturated aldehydes (3), without double bond migration. The (E) isomers are obtained alone (from aldehydic compounds) or very predominantly (from ketonic compounds). The intermediate adducts, g-hydroxy enol ethers (4), can be isolated by slightly basic mild hydrolysis (eq 1).1
As condensation with a,b-unsaturated compounds occurs exclusively in a 1,2-fashion,1,2 this reagent has been used in terpene synthesis. A synthesis of retinal (3b) in three steps from b-ionone (2a) (via b-ionylideneacetaldehyde and C18 ketone) was reported using reagent (1) for the first and third steps (eq 2).5
For the step (2a) -> (3a), reagent (1) competes favorably with other classical vinylogation reagents: Formylmethylenetriphenylphosphorane (0%);6 lithium cyclohexylvinylamide (2 steps, 34%);7 lithio salt of 5,6-Dihydro-2,4,4,6-tetramethyl-1,3(4H)-oxazine (3 steps, 54%);8 lithium t-butyl-2-trimethylsilylvinylamide (1 step, 88%)9 (see Duhamel et al.2 for other comparative studies).
Thus (Z)-(trimethylsilyloxy)vinyllithium (1) is an excellent reagent for the preparation of building blocks such as (4), (5a), and (5b) (eq 3), which are useful for short convergent syntheses of terpenoid compounds (eqs 4 and 5).10
Condensation of acetylacetaldehyde dimethyl acetal with reagent (1) leads to the d-aldolacetal (3c) in 90% yield, which has been transformed by classical methods into bromoacetal (4) and bromo enol ethers (5a,5b) (eq 3).10
For the first time a one-step synthesis of retinal (3b) from b-ionone (2a) was reported, using reagents (5a) and (5b) (eq 4),10 whereas two-step syntheses of dehydrocitral from acetone (60%), pseudo-retinal from pseudo-ionone (61%), and retinal from b-ionone (55%) were described using reagent (4).10
A short synthesis of phytol (6) from 6-methyl-5-hepten-2-one was described using reagents (1) and (4). Condensation of 6-methyl-5-hepten-2-one with (4) yielded the hydroxy acetal (7), which was converted in one pot into pseudo-phytone (8). Catalytic hydrogenation of (8) gave phytone, which was transformed into phytol (6) after condensation with (1) followed by reduction (eq 5).11
The adduct of (1) with carbonyl compounds can be trapped by methyl fluorosulfonate, leading to g-methoxy enol ethers (9) precursors of b-methoxy aldehydes (10) (eq 6).12
Depending on the experimental conditions, reaction of (1) with t-Butyldimethylsilyl Trifluoromethanesulfonate leads either to the expected b-trimethysilyl enol ether (11), or to its isomer (12) formed by a 1,3-migration of the trimethylsilyl group from the oxygen atom to the carbon atom (1 -> 13). Reaction with t-Butyldimethylchlorosilane always gives (12).13 Trimethylsilyl enol ether (11) is easily hydrolyzed to 2-(t-butyldimethylsilyl)acetaldehyde (14) (eq 7).14
The study of the 1H NMR spectra of (1) in Et2O-d10 at -70 °C is in agreement with the proposed structure; the isomeric enolate (13) is not normally detected (eq 7), but in THF at -70 °C it slowly forms.4
University of Rouen, Mont-Saint-Aignan, France