[84673-88-1] · C6H13BrSi · 3-Bromo-1-trimethylsilyl-1-propene · (MW 193.16)
(preparation of functionalized vinylsilanes1)
Physical Data: bp 88-90 °C/63 mmHg.
Solubility: insol H2O; sol most common organic solvents.
Form Supplied in: colorless liquid.
Analysis of Reagent Purity: GLC, NMR.
Preparative Methods: bromination (Phosphorus(III) Bromide, 0 °C;2,3 N-Bromosuccinimide-Dimethyl Sulfide4) of 3-(trimethylsilyl)allyl alcohol, which in turn can be prepared by Sodium Bis(2-methoxyethoxy)aluminum Hydride reduction of 3-trimethylsilylpropargyl alcohol.5 Allylic bromination of Allyltrimethylsilane gives a mixture of the title compound and its regioisomers, which is also useful for some applications.6
Purification: distillation under reduced pressure.
Handling, Storage, and Precautions: stable in air, and should be kept in a tightly capped bottle.
The allylic bromide and congeners serve as excellent alkylating agents of enolate anions, providing a useful synthetic route to a variety of substituted vinylsilanes. The lithium enolate of 2-methylcyclohexanone reacts with the trimethylsilyl allylic iodide in excellent yield, and subsequent oxidation of the vinylsilane group smoothly gives a 1,5-diketone (eq 1).1
The SN2 regioselectivity of the nucleophilic displacement of the bromide group is due to the steric bulk of the silyl group, and is general not only for lithium enolates but for other enolates,7 lithioenamines, cuprates, and enamines (eq 2).8
The vinylsilanes thus obtained have been used as substrates of polyene cyclization (eq 2),4,9 and intramolecular acylation (eq 3);2 the vinylsilane moiety was found to be more electrophilic than the simple allyl group.
Reduction of the bromide with a low-valent metal generates a nucleophilic allylating reagent, which undergoes highly SE2´-selective allylation reactions. Thus the reaction of a regioisomeric mixture of the bromide with Chromium(II) Chloride in the presence of an aldehyde proceeds via a trimethylsilyl allylic chromium reagent with excellent regio- and stereoselectivity to give a b-hydroxysilane (eq 4).6 Similarly, allylic lead and zinc reagents prepared in situ undergo regio- and chemoselective addition to carbonyl compounds10 or imines.11
The reaction of the silyl allylic bromide with Triphenylphosphine readily provides an allylic phosphonium salt, which upon treatment with n-Butyllithium and an appropriate carbonyl compound (e.g. acetone, cyclohexanone, fluorenone) gives 1-trimethylsilylbutadiene derivatives (eq 5).3
Tokyo Institute of Technology, Japan