Silyl aluminums

Aryl and alkyl trimethylsilyl ethers can often be cleaved by refluxing in aqueous methanol, an advantage for acid- or base-sensitive substrates. The ethers are stable to Grignard and Wittig reactions and to reduction with lithium aluminum hydride at —15°. Aryl -butyldimethylsilyl ethers and other sterically more demanding silyl ethers require acid- or fluoride ion-catalyzed hydrolysis for removal. Increased steric bulk also improves their stability to a much harsher set of conditions. An excellent review of the selective deprotection of alkyl silyl ethers and aryl silyl ethers has been published. ... 

Transmetalation of 19 by treatment with two equivalents of diethylaluminum chloride generates the aluminum enolate species 23. The latter reacts with acetaldehyde to produce the stable aluminum aldolates 24 which do not undergo the Peterson elimination23. A protic quench then provides the a-silylated aldol adducts of tentative structures (2 R)-25 and (2 V)-25 with little diastereoselectivity. Other diastereomers are not observed. 

As an alternative to lithium enolates. silyl enolates or ketene acetals may be used in a complementary route to pentanedioates. The reaction requires Lewis acid catalysis, for example aluminum trifluoromethanesulfonate (modest diastereoselectivity with unsaturated esters)72 74 antimony(V) chloride/tin(II) trifluoromethanesulfonate (predominant formation of anti-adducts with the more reactive a,/5-unsaturated thioesters)75 montmorillonite clay (modest to good yields but poor diastereoselectivity with unsaturated esters)76 or high pressure77. 

Alkali metal derivatives of 2-(trimethylsilyl)aminopyridines can be further derivatized by insertion of 1,3-dicyclohexylcarbodiimide. Functionalized guani-dinates are formed in this reaction via a 1,3-silyl shift. Scheme 170 illustrates the reaction sequence as well as the preparation of an aluminum complex of the modified ligand, which exhibits pseudo jS-diketiminate binding of the metal center, thus exemplifying the coordinative versatility of this new multi-N-donor system. ... 

Benzene reacted with excess 3 in the presence of aluminum chloride at room temperature for 4 h to give peralkylated product," hexakis[2-(methyldichloro-sily I )ethyljbenzcne (4a) and other alkylated products pentakisl2-(mcthyldichloro-silyl)ethyl]benzenc (4b), tetrakis 2-(methyldichlorosilyl)ethyljbenzene (4c), tris 2-(methyldichlorosilyl)ethyl]benzene(4cl), and bis 2-(methyldichlorosilyl)ethylJ-benzene (4e) (Eq. (6)). The product distributions were plotted against mole ratios of 3/benzene in Fig. I. 

Similar reactivity is realized with 2-acetylthiophene using triethylsilane with aluminum chloride.259 Treatment of the ethylene glycol acetal of 2-thiophenecar-baldehyde with Et3SiH/TFA results in reduction of the ring and oxidation of the side chain to the silylated carboxylic acid (Eq. 119),260 whereas similar treatment of 2-thiophenecarbaldehyde gives 2-methyltetrahydrothiophene and 2-... 

Oxidative addition of the silyl species to nickel is followed by insertion of unsaturated substrates. Zero-valent nickel complexes, and complexes prepared by reducing nickel acetylacetonate with aluminum trialkyls or ethoxydialkyls, and in general Ziegler-Natta-type systems, are effective as catalysts (244, 260-262). Ni(CO)4 is specific for terminal attack of SiHCl3 on styrene (261). 

The use of reactive metal electrodes are also effective for the silylation of various organic halides and simple arenes [75]. For instance, Dunogues et al. reported that electrolysis of aryl chlorides in the presence of excess Me3SiCl in a one-compartment cell equipped with a sacrificial aluminum anode in 80 20 THF/HMPA gave the corresponding aryltrimethylsilanes (Scheme 36). When... 

Lithium aluminum tri-tert-bu-toxyhydride, 53, 53 Lithium amide, bis(trimethyl-silyl), 53, 66 Lithium amide, diisopropyl-,... 

Most of the more recently described allenic steroids bear an allene group at the 17-position, which was usually formed by an SN2 substitution [106] or reduction [86d] process of a suitable propargylic electrophile. Thus, reduction of the pro-pargylic ether 109 with lithium aluminum hydride followed by deprotection of the silyl ether resulted in the formation of the allenic steroid 110, which irreversibly inhibits the biosynthesis of the insect moulting hormone ecdysone (Scheme 18.35) [107]. 

AlRg groups. The latter assumption is favored by the experience that incomplete reaction of the silanol groups occurs always with trimethyl-silyl chloride. This is caused by steric hindrance (219, 220). Diisobutyl aluminum groups are at least as bulky as trimethylsilyl groups. Therefore, since one aluminum atom is bound for every silanol group, a smaller ratio than two of alkyl groups to aluminum must prevail. No carbon determinations have been made, unfortunately. 

The addition reaction of allyltriorganosilanes to 1-alkenes in the presence of anhydrous aluminum chloride as catalyst at room temperature gives regiospecific allylsilylated products, in which the silyl group adds to the terminal carbon and the allyl group adds to the inner carbon of the double bond [Eq. (3)]. Compared with the starting alkenes, the products of the allylsilylation reaction possess two additional carbon atoms in addition to a (triorganosilyl)methyl branch at the carbon (3 to the double bond. 

---