Allyltrimethylsilane1

[762-72-1]  · C6H14Si  · Allyltrimethylsilane  · (MW 114.26)

(carbon nucleophile for the introduction of allyl groups by Lewis acid-catalyzed or fluoride ion-catalyzed reaction with acid chlorides, aldehydes, ketones, iminium ions, enones, and similar carbon electrophiles)

Physical Data: bp 85-86 °C, d 0.717 g cm-3.

Solubility: freely sol all organic solvents.

Form Supplied in: colorless liquid. Methods for the synthesis of allylsilanes in general have been reviewed.3

Analysis of Reagent Purity: d 5.74 (1H, ddt, J 16.9, 10.2 and 8), 4.81 (1H, dd, J 16.9 and 2), 4.79 (1H, dd, J 10.2 and 2), 1.49 (2H, d, J 8) and -0.003 (9H, s).2

Handling, Storage, and Precautions: inflammable; the vapor is irritating to the skin, eyes, and mucous membranes.

As a Carbon Nucleophile in Lewis Acid-Catalyzed Reactions.

Allyltrimethylsilane is an alkene some 105 times more nucleophilic than propene, as judged by its reactions with diarylmethyl cations.4 It reacts with a variety of cationic carbon electrophiles, usually prepared by coordination of a Lewis acid to a functional group, but also by chemical or electrochemical oxidation,5 or by irradiation in the presence of 9,10-dicyanoanthracene.6 The electrophile attacks the terminal alkenic carbon to give an intermediate cation, and the silyl group is lost to create a double bond at the other terminus. Among the more straightforward electrophiles are acid chlorides (eq 1),7 aldehydes and ketones (eq 2),8 their acetals (eq 3),9 and the related alkoxyalkyl halides (eq 4),10 iminium ions, and acyliminium ions (eqs 5 and 6),11,12 and tertiary and allylic or benzylic alkyl halides (eqs 7 and 8).13,14

The reaction with acetals does not always need the acetal itself to be synthesized: it can be made in the same flask by mixing the allylsilane, the aldehyde, the silyl ether of the alcohol, and a catalytic amount of an acid such as Diphenylboryl Trifluoromethanesulfonate,15 Trimethylsilyl Trifluoromethanesulfonate,16 or Fluorosulfuric Acid.17

The further reaction of the double bond in the first-formed product is an occasional complication, as in the formation of tetrahydropyrans from aldehydes when Aluminum Chloride is the Lewis acid (eq 9),18 and of piperidines from primary amines and formaldehyde (eq 10).11

a,b-Unsaturated esters, aldehydes, ketones, and nitriles generally react in Michael fashion in what is called a Sakurai reaction (eq 11).19 The intermediate silyl enol ether may be treated with a second electrophile to set up two C-C bonds in one operation (eq 12).20 Occasionally the silyl group is not lost from the intermediate cation but migrates instead to give a cyclopentannulation byproduct (eq 13).21

a,b-Unsaturated nitro compounds initially give nitronic acid derivatives, which can be reduced directly with Titanium(III) Chloride to give the corresponding ketone (eq 14).22

Some electrophiles require a separate activation step, as in the stereospecific reaction of alkenes with benzenesulfenyl chloride (eq 15).23 The intermediate that actually reacts with the allylsilane is presumably the episulfonium ion. There is a corresponding reaction of epioxonium ions derived from 2-bromoethyl ethers.24 Intramolecular hydrosilylation of an ester generates an acetal, which reacts with allyltrimethylsilane in the usual way and with high stereocontrol to make an anti 1,3-diol derivative (eq 16).25

The reactions with a- or b-oxygenated aldehydes can be made to give high levels of stereocontrol in either sense by choosing a chelating or nonchelating Lewis acid (eq 17).26

Sugar acetals (eq 18)27 and glycal 3-acetates (eq 19)28 and related compounds react stereoselectively in favor of axial attack at the anomeric carbon.

The choice of Lewis acid in all these reactions is often important in getting the best results, one Lewis acid being much the best in several cases. Most are used in molar amounts, but some, especially Iodotrimethylsilane, Trimethylsilyl Trifluoromethanesulfonate, and the more powerful trifluoromethanesulfoxonium tetrakis(trifluoromethanesulfonyl)boronate29 [TfOH2+B(OTf)4-], have the advantage that they can be used in catalytic quantities. In addition to the Lewis acids illustrated, the following less obvious Lewis acids have also been used with one or more of the electrophiles: Triphenylmethyl Perchlorate,30 titanocene ditriflate,31 Lithium Perchlorate,32 Tin(IV) Chloride in the presence of either Tin(II) Trifluoromethanesulfonate33 or Zinc Chloride,34 Antimony(V) Chloride,35 Chlorotrimethylsilane combined with indium chloride,36 Ethylaluminum Dichloride,37 and Diphenylboryl Trifluoromethanesulfonate.38 Among carbon electrophiles not illustrated are quinones,39 cyclopropanedicarboxylic esters,40 oxetanes,41 nitriles,42 dithioacetals,43 diselenoacetals,44 the intermediate sulfur-stabilized cation from a Pummerer rearrangement,45 and propargyl ethers with46 or without47 octacarbonyldicobalt complexation.

As a Carbon Nucleophile in Uncatalyzed Reactions.

Some electrophiles do not need Lewis acids, being already cationic and electrophilic enough to react with allyltrimethylsilane. Examples are the dithianyl cation (eq 20),48 the tricarbonyl(cyclohexadienyl)iron cation (eq 21),49 (p-allyl) tetracarbonyliron cations,50 and Chlorosulfonyl Isocyanate (CSI).51 Other reagents react directly by cycloaddition, but need further steps to achieve an overall electrophilic substitution, as in the reactions with nitrones (eq 22).52

Although heteroatom (N-, P-, O-, S-, Se-, and halogen-based) electrophiles react with allylsilanes, the products with allyltrimethylsilane itself are usually too simple for this to be an important synthetic method. An exception perhaps is the reaction with Palladium(II) Chloride, which gives the p-allylpalladium chloride cation.53

As a Carbon Nucleophile in Fluoride Ion-Catalyzed Reactions.

The reactions with aldehydes, ketones (eq 23),54 and a,b-unsaturated esters (eq 24)55 can also be catalyzed by fluoride ion, usually introduced as Tetra-n-butylammonium Fluoride (TBAF), or other silicophilic ions such as alkoxide. These reactions produce silyl ether intermediates, which are usually hydrolyzed before workup. The stereochemistry of attack on chiral ketones can sometimes be different for the Lewis acid- and fluoride ion-catalyzed reactions.56 In addition some electrophiles only react in the fluoride-catalyzed reactions, as with the addition to trinitrobenzene giving an allyl Meisenheimer complex.57

Other Reactions.

Allyltrimethylsilane reacts with some highly electrophilic alkenes, carbonyl compounds, azo compounds, and singlet oxygen to a greater or lesser extent in ene reactions that do not involve the loss of the silyl group, and hence give vinylsilanes in a solvent-dependent reaction (eq 25).58

Hydroalumination (and hydroboration) take place regioselectively to place the aluminum (or boron) atom at the terminus, creating a 3-trimethylsilylpropyl nucleophile.59 Radicals attack allyltrimethylsilane at the terminus, and the intermediate radical reacts further without the trimethylsilyl group being expelled, as the corresponding germanium, tin, and lead groups are.60 Treatment with strong bases gives trimethylsilylallyl-metal compounds (see Trimethylsilylallyllithium and Allyltrimethylsilylzinc Chloride).


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Ian Fleming

Cambridge University, UK



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