[13411-42-2] · C7H16S2Si · 2-Trimethylsilyl-1,3-dithiane · (MW 192.46)
(precursor of dithioketene acetals1 which are good substrates for both cationic2 and anionic3 cyclization processes; the anion reacts with unsaturated ketones and aldehydes to give vinyl dithioketene acetals that can be used as dienes4 or for the preparation of substituted a,b-unsaturated alkyl ketones;5 alkylation of the anion provides a general synthesis of acylsilanes6,7)
Physical Data: bp 54-55 °C/0.17 mmHg.
Solubility: insol H2O; sol organic solvents.
Form Supplied in: commercially available.
Preparative Method: prepared by alkylation of 2-Lithio-1,3-dithiane with Chlorotrimethylsilane (eq 1).1
Handling, Storage, and Precautions: use in a fume hood.
This reagent (1) is generated from the title compound by treatment with n-Butyllithium at -78 °C (eq 2)1 and is the species utilized in many of the following transformations.
Anion (1) reacts with aldehydes and ketones to provide the corresponding thioketene acetals;1 aryl and unsaturated aldehydes and ketones are good substrates for this reaction (eqs 3 and 4). Enolizable alkyl ketones also react to provide thioketene acetals (eq 5). Alternative methods for the preparation of these cyclic thioketene acetals involve the use of phosphonate derivatives,8 mixed zinc-titanium organometallic reagents,9 and N,N-dimethyl thioamides.10 The phosphonate reagents are more nucleophilic than (1) and are superior when competitive deprotonation is a problem.
A variation of this method has also been developed for cases in which reactions with (1) give poor results (eq 6). Thus the carbonyl compound is treated with 2-Lithio-1,3-dithiane followed by TMSCl to generate the silyl ether. Subsequent addition of a second equivalent of n-butyllithium effects alkenation, affording the thioketene acetal in good yield.11
The thioketene acetals derived from the reaction of anion (1) have found several applications in cyclization methodology. The thioketene acetals can be used as either electrophiles (eq 7)2 or nucleophiles (eq 8)3 in a cyclization process which depends on the experimental conditions.
Substituted trimethylsilyldithianes can be converted into nucleophiles via fluoride ion desilylation and used in anionic cyclization strategies.12 This anion not only undergoes condensations with carbonyls (eq 9), but it is also an effective Michael donor (eq 10).
The reaction of anion (1) with various a,b-unsaturated aldehydes and ketones occurs in a 1,2-fashion and provides vinyl thioketene acetals that are acceptable dienes for Diels-Alder reactions (eq 11);4 dienes of this type are difficult to access using other protocols. This method is also convenient for the introduction of a protected carbonyl group.
Treatment of acrolein and cyclohexenone with 2-lithio-2-trimethylsilyl-1,3-dithiane provides the corresponding unsaturated dithioketene acetals (eq 12).5a Subsequent exposure to n-butyllithium followed by MeI produces protected forms of a,b-unsaturated ketones in almost quantitative yield; anion addition occurs distal to the dithiane and alkylation takes place at the dithiane stabilized anion (eqs 13 and 14).5b Another interesting application is the use of suitably substituted 2-aryl-2-trimethylsilyl-1,3-dithianes as precursors for o-quinodimethanes (eq 15).13
The reaction of anion (1) with alkyl halides generates a functionalized dithiane which, when hydrolyzed under mild conditions, provides the corresponding acylsilane (eq 16).2 More highly substituted acylsilanes have been accessed by initial formation of the dithiane from the aldehyde, followed by deprotonation and silylation with TMSCl (eq 17). Acylsilanes have been used as sterically hindered aldehyde equivalents for regiocontrol in addition reactions (eq 18)15 and also as precursors for silyl-substituted vinyl triflates (eq 19).16
There are several alternative methods for the formation of acylsilanes; these include the reaction of silyl cuprates with acid halides,17 palladium-assisted coupling of acyl halides with Hexamethyldisilane,18 the reaction of thiopyridyl esters with Tris(trimethylsilyl)aluminum in the presence of CuI salts,19 and the hydroboration of trimethylsilyl-substituted alkynes.20 While all of these procedures are complementary, the method involving the title compound provides an easy, inexpensive route to acylsilanes.
John W. Benbow
Lehigh University, Bethlehem, PA, USA