Aryl silylation

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. ... 

Aminosilanes, 87-9 Artemisia ketone, 33 Aryl silyl ethers, 41 Arylsilanes, 39-43 Arylsulphonylhydrazones, 12 Aryltrimethylsilanes, 39, 42,43 Azetidin-2-ones, 70 Azetidin-2-one 4-carboxylic acids, 92... 

Dienols with 2 equiv. of 1 gave the corresponding bimetallic alkoxide organozir-conocene complexes however, protolysis allowed recovery of the alcohol functionality (Scheme 8-29) [107]. Alcohols can also be easily converted to ethers. Alkyl, aryl, silyl [85,112,183, 210] and THP [17, 153, 211, 219] ethers are stable under hydrozirconation conditions side products were observed only with the trimethylsUyl group [220, 221]. 

The most fundamental issues of the structures of heavier group 14 element-centered anionic derivatives R3EM (R = alkyl, aryl, silyl E = Si, Ge, Sn, Pb M = alkali or alkaline earth metals) turned out to be the questions of their aggregation states (monomeric, dimeric, or oligomeric), nature of the E-M bond (covalent or ionic), and configuration of the anionic centers E (tetrahedral, pyramidal, or planar). The most important experimental techniques that are widely used to clarify these questions are NMR spectroscopy and X-ray diffraction analysis. 

The cleavage of benzyl ethers using hydrobromic acid is promoted by tetra-n-butylammonium bromide [38]. Selective cleavage of aryl silyl ethers can be effected in the presence of aliphatic silyl ethers using solid sodium hydroxide with tetra-n-butyl-ammonium hydrogen sulphate [39]. 

The additions of alkyl or aryl silyl chlorides to 1 -ethoxycarbonyl- 1H-azepine in HMPA solution in the presence of magnesium proceed via anion radicals to yield trans adducts... 

This survey is confined to an overview of group 13 amides containing M-NR2 (R = H, alkyl, aryl, silyl, etc.) groups derivatives where nitrogen is part of a delocalized or polydentate ligand, are not generally covered. However, as in the 1980 book, iminome-tallanes (metal imides) of formula (RMNR ) are treated because of their close relationship to amides and also because of the relevance of the lower derivatives (n= 1, 2 or 3) for possible M N multiple bonding. 

Aryl silyl ethers are cleaved by a variety of reagents. Treatment of alkoxysilanes with electrophilic reagents like the boron trifluoride-diethyl ether complex results in the cleavage of the Si-O bond to form mono-, di- and trifluorosilanes.22... 

Spectroscopic studies have also been devoted to the question of ion-pairing phenomena in silyl anions. It was found that in arylated silyl anions the 13C NMR as well as the 7Li NMR chemical shifts are only slightly influenced by the polarity of the solvent36,51. This is in clear contrast to the NMR-spectroscopic behavior of aryl-substituted carbanions, which show a marked solvent dependence. This was interpreted in terms of a significant covalent interaction between silicon and lithium in metalated silanes36. Further evidence for a significant covalent nature of the Si—Li bond arises from the observation of a scalar... 

The historical sketch outlines the class of lanthanide amides this article is to deal with and which is further manifested in Scheme 1. Organometallic amides which can be classified as dialkyl (-aryl, -silyl) amides and amides derived from unsaturated heterocyclic ligands are seen with respect to N-unsubstituted (primary, inorganic) amides. The consideration of more classic coordination compounds like acid amides or sulfonamides, often ascribed as wet chemistry , is excluded. The historical data also demonstrate the relatively late start of lanthanide amide chemistry reflecting the late industrial establishment of the lanthanide elements (separation, purification, etc.) [9], However, lanthanide amides are still the youngest class in conjunction with the most important pillars of organometallic lanthanide chemistry, namely the alkyls/cyclopentadienyls (LnCp3, 1954, [10]) and the alkoxides (Ce(OR)4 1956 [11a] La(OR)3 , 1958 [lib]). Indeed most of the work has been conducted in the last ten years. 

More recently, Aldridge further exploited this approach for the synthesis of [p-BMcs [(q S-CsI Is)Fc(CO)2 2] (10) [Eq. (5)] which represents a rare example of a structurally authentic complex with an unsupported borylene ligand B-R (R = alkyl, aryl, silyl). Interestingly, the formation of 10 is not accompanied by CO liberation although rather harsh conditions had to be applied.97... 

The remaining stannylene with weak intramolecular stabilization by a Lewis base is the bis[2,6-(A(At-dimethylamino)phenyl]stannylene A-lN/ NbCgFLjjSn (Ar Sn) (29)33. Two Me3N groups show close Sn - N contacts of 259.9 and 266.3 pm, the proposed lone-pair being directed to the space above and below the C—Sn—C plane where the empty p-orbital of Sn is assumed (Figure 4). Similar interactions are found in the related heteroleptic aryl(silyl)stannylene 42 mentioned in Section II. A.2.b37. 

Surprisingly, in compound 31 two relatively short Pb Si contacts to adjacent Me3Si groups are observed49 which are only about 20% longer than the covalent Pb(II)—Si bonds in heteroleptic aryl(silyl)plumbylenes. 

On the other hand, the cyclization reaction of a vinyllithium onto an acetylenic unit provides an efficient route to five- and six-membered bis-exocyclic 1,3-dienes, which react stereoselectively with a wide range of dienophiles157. The 5-exo carbolithiation reaction of vinyllithiums 369, derived from the corresponding vinyl bromides, is syw-stereospecific giving, after hydrolysis, the /(-isomer of five-membered outer-ring dienes 370 and tolerates aryl-, silyl- or alkyl-substituents at the distal acetylenic carbon (Scheme 97). However, the alkyl-substituted alkynes are far more resistant to rearrangement than the aryl- or silyl-substrates and the addition of TMEDA and longer reaction times are needed for the latter... 

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