[1450-14-2] · C6H18Si2 · Hexamethyldisilane · (MW 146.42)
(silylating or reducing reagent in combination with a Pd catalyst or a nucleophile)
Physical Data: bp 112-114 °C; d 0.729 g cm-3.
Solubility: sol common organic solvents; insol H2O.
Form Supplied in: oil; commercially available.
Analysis of Reagent Purity: 1H NMR (CDCl3-Me4Si) d 0.045 (s).
Preparative Methods: by standard synthesis.1
Purification: grossly impure sample (25% impurities) is purified by repeated spinning band distillation. This lowered the impurity level to 500 ppm. The main impurity is identified as 1-hydroxypentamethyldisilane.
Handling, Storage, and Precautions: stable to H2O and air; flammable liquid; irritant. Readily oxidized by oxidants such as halogen and peroxide. Use in a fume hood.
Treatment of hexamethyldisilane (1) with an alkyllithium, metal alcoholate, or fluoride ion generates the trimethylsilyl anion (2) (eq 1).2 Reaction of (2) with aryl halides gives trimethylsilylarenes in 63-80% yields along with reduced products (ArH, 4-27%).3 Tetra-n-butylammonium Fluoride (TBAF) catalyzed reaction of Me6Si2 with butadienes gives 1,4-disilyl-2-butenes (15-81% yield) with high (E) selectivity.4
Aliphatic aldehydes react with Me6Si2/TBAF in HMPA to afford 1-trimethylsilyl-1-alkanols (3), whereas aryl aldehydes give pinacols (4) under the same conditions (eq 2).5
The trimethylsilyl anion adds to a,b-unsaturated ketones in a 1,4-manner to give b-silyl ketones. This reaction has been applied to the synthesis of carbacyclin.6
a-Diketones (5) react with Me6Si2 in the presence of PdCl2(PMe3)2 or Pt2(dba)3-P(OCH2)3CEt to give the corresponding 1,2-bis(trimethylsilyloxy)ethylenes (6) (eq 3), while a-keto esters (7) provide pinacol 2,3-bis(trimethylsilyl) ethers (8) (eq 4).7 The regio- and stereoselectivities are moderate (62:38-87:13).
Allyl alcohols are converted into allylsilanes (11) with n-Butyllithium and Me6Si2 (eq 5). Here, the in situ generated lithium allyl alcoholates (9) first react with Me6Si2 to produce trimethylsilyllithium and trimethylsilyl allyl ethers (10), which subsequently react in an SN2´ manner to give the products. Similarly, the lithium alcoholates generated from a,b-unsaturated aldehydes (or ketones) and an alkyllithium, or alternatively from ketones and vinyllithium, can be converted into the corresponding allylsilanes.8
In the presence of Tetrakis(triphenylphosphine)palladium(0), vinyl halides (12) are converted to the corresponding vinylsilanes (13) with the aid of Me6Si2 and Tris(dimethylamino)sulfonium Difluorotrimethylsilicate (TASF) in 53-92% yields with high chemoselectivity and stereospecificity (eq 6).9 Similarly, aryl-, benzyl-, and allylsilanes are obtained from the corresponding halides in 57-100, 8-48, and 52-93% yield, respectively.10 Allyl esters also are transformed by Pd-catalyzed reaction of Me6Si2 into allylsilanes.11
Intermolecular disilylation of 1-alkynes (14) are promoted using Palladium(II) Acetate-1,1,3,3-Tetramethylbutyl Isocyanide at reflux12 or Pd(dba)2-P(OCH2)3CEt at 120 °C13 to give (Z)-1,2-bis(trimethylsilyl)alkenes (15) in yields up to 91% (eq 7). Internal alkynes are unreactive. The latter catalyst system allows 1-alkynes to insert between the Si-Si bonds of poly[p-(disilanylene)phenylene] and poly(dimethylsilylene).
Enolizable ketones (16) are readily transformed to silyl vinyl ethers (17) by the action of disilane and a catalytic amount of Sodium in HMPA (eq 8).14 The regioselectivity of the silyl vinyl ether formation depends on the structure of the ketones.
Electron-deficient aromatic acid chlorides (ArW-COCl; 18) are converted into biphenyls (19) by the dichlorobis(triphenylphosphine)palladium(II) catalyzed reaction of Me6Si2 in mesitylene at 165 °C (eq 9).15 Arylsilanes are produced as byproducts, but these are switched to major products with (ClMe2Si)2.16 In contrast, aromatic acid chlorides (ArD-COCl; 20) bearing an electron-donating group give acylsilanes (21) upon catalysis with Bis(benzonitrile)dichloropalladium(II)-Triphenylphosphine (eq 10).17
A halogen substituent on the carbon next to a carbonyl or cyano group is reduced with Me6Si2 in the presence of Pd(PPh3)4 catalyst (eq 11). In the case of a-halo ketones, oxy-p-allyl(trimethylsilyl)palladium intermediates are proposed.18
A mixture of Me6Si2, aryl isocyanide (24), a catalytic amount of Palladium(II) Acetate and toluene is heated to reflux to give disilylformaldehyde imines (25) (eq 12). This reaction is applied to the insertion of the isocyanide unit into all the Si-Si linkages of (tetradecamethyl)hexasilane (26) to give oligomeric silylimines (27) (eq 13).
Alkyl bromides (29) are obtained in high yields by the reaction of the corresponding alcohols (28) with Me6Si2 and Pyridinium Hydrobromide Perbromide (eq 14).20 The substitution reaction is very fast for tertiary, allylic, and benzylic alcohols, but very slow for primary and secondary alcohols. Inversion of configuration is observed with secondary alcohols.
Me6Si2 in combination with a catalytic amount of Iodine converts p-quinones (30) into 1,4-bis(trimethylsiloxy)arenes (31) in almost quantitative yields (eq 15).21 The products are a protected form of quinones, which are regenerated by oxidation with Pyridinium Chlorochromate (PCC).22
The reaction of gem-dichloroalkanes (32) with Me6Si2 proceeds smoothly in the presence of a catalytic amount of Pd(PPh3)4 to give dimerized alkenes (33) in fairly good yields (eq 16). The (E/Z) ratio of the products is moderate. R1-CCl2-SiMe3 also reacts with Me6Si2 in the presence of Pd catalyst to yield (E)-R1(Me3Si)C=CR1(SiMe3) quantitatively.
Tamejiro Hiyama & Manabu Kuroboshi
Tokyo Institute of Technology, Yokohama, Japan