Trimethylsilyl-substituted

The emphasis in the approaches to boron nitride [10043-11 -5] BN, precursors has been concentrated on cycHc compounds. There have been recent reports of trimethylsilyl-substituted aminoboranes being evaluated as B—N precursors. These are linear borylamines containing up to four boron atoms. Compounds were also synthesized with free —NH2 groups amenable to condensation with either dihaloboranes or dihaloborazines (65) and offering suitable monomers for linear B—N polymer synthesis and borazine-ring-linking appHcations. 

With the use of trimethylsilyl-substituted starting materials after Denmark et al., the disadvantageous formation of a mixture of isomers can be avoided. The vinyl silane derivatives react by loss of the TMS group in the last step ... 

The addition of lithium acetylides can also be carried out enantioselectively in the presence of 22-24 ]vjucieophiiic addition of the unsubstituted lithium acetylide led to the alkynyl alcohol with lower enantioselectivity than the addition ofsilyl-substituted acetylides. The trimethylsilyl substituted acetylides gave the best results. 

The preparation of cyclopentadienes with up to four trimethylsilyl groups can be performed easily on a large scale starting with monomeric cyclopentadiene by repeated metalation with n-butyllithium and treating the resulting anion with chlorotrimethylsilane [84], Any complication caused by formation of regioisomers does not occur, since all trimethylsilyl-substituted cyclopentadienes are fluxional by virtue of proto- and silatropic shifts [85], Upon deprotonation with n-butyllithium the thermodynamically most favorable anion is formed selectively (Eqs. 20, 21). Thus, metalation of bis(trimethylsilyl)cyclopentadiene 74, which exists preferentially as the 5,5-isomer, selectively affords the 1,3-substituted anion 75. Similarly, tris(trimethylsilyl)cyclopentadiene 76, which is found to be mainly as the 2,5,5-isomer, affords the 1,2,4-substituted anion 77. 

Several 4-alkoxy-2,3-diphenyl- and one 4-methoxy-2,3-bis(trimethylsilyl)-substituted cyclopentadienones have been isolated as reasonably stable compounds, see Herndon JW, Patel PP (1997) Tetrahedron Lett 38 59... 

Scheme 2.19. Carbonyl Olefination Using Trimethylsilyl-Substituted Organo-... 

The reaction of diorganozincs bearing bulky trimethylsilyl-substituted methyl groups and methyl- or phenyllithium in the presence of the 1,3,5-trimethyl-1,3,5-triazacyclohexane (TAGH), Scheme 53, afforded the corresponding lithium zincates as poorly soluble ion pairs.124... 

The acid-catalysed ring-closure of divinyl ketones to cyclopentenones (equation 6), the Nazarov reaction6-8, represents a conrotatory electrocyclization of 4jr-cyclopentadienyl cations. The conrotatory course of the reaction was confirmed for the case of the dicyclo-hexenyl ketone 7, which yielded solely the tricyclic ketone 8 on treatment with phosphoric acid (equation 7)3b. Cycloalkanocyclopentenones 10 with c/s-fused rings are obtained from the trimethylsilyl-substituted ketones 9 (n = 1, 2 or 3) and iron(III) chloride and... 

Similar results for the replacement of halogen on an olefinic linkage by phosphorus have been accomplished using dialkyl phosphites with palladium(O) catalysts.4179 Another reaction involving replacement of a vinylic halide by phosphorus utilizes palladium catalysis with a trimethylsilyl-substituted phosphine (Figure 6.19).80... 

Eq. 2.8. Regioselective formation of a zircona-cyclopentadiene using a trimethylsilyl-substituted acetylene. 

Insertion of phenyl, trimethylsilyl, and nitrile-stabilized metalated epoxides into zircona-cyclcs gives the product 160, generally in good yield (Scheme 3.37). With trimethylsilyl-substituted epoxides, the insertion/elimination has been shown to be stereospecific, whereas with nitrile-substituted epoxides it is not, presumably due to isomerization of the lithiated epoxide prior to insertion [86]. With lithiated trimethylsilyl-substituted epoxides, up to 25 % of a double insertion product, e. g. 161, is formed in the reaction with zirconacyclopentanes. Surprisingly, the ratio of mono- to bis-inserted products is little affected by the quantity of the carbenoid used. In the case of insertion of trimethylsilyl-substituted epoxides into zirconacydopentenes, no double insertion product is formed, but product 162, derived from elimination of Me3SiO , is formed to an extent of up to 26%. 

It is also possible to employ trimethylsilyl-substituted propargylic trichlorosilanes in electrophilic substitution reactions leading to allenylsilanes (Eq. 9.45) [50]. The trichlorosilanes can be prepared by SN2 or SN2 displacement of allenic or propargylic bromides by a trichlorosilyl copper reagent. The overall process, starting from an enantioenriched propargylic bromide of unknown enantiopurity, afforded a racemic allenylsilane (Eq. 9.46)... 

Allenyltrimethylsilanes add to ethyl glyoxalate in the presence of a chiral pybox scandium triflate catalyst to afford highly enantioenriched homopropargylic alcohols or dihydrofurans, depending on the nature of the silyl substituent (Tables 9.39 and 9.40) [62]. The trimethylsilyl-substituted silanes give rise to the alcohol products whereas the bulkier t-butyldiphenylsilyl (DPS)-substituted silanes yield only the [3 + 2] cycloadducts. A bidentate complex of the glyoxalate with the scandium metal center in which the aldehyde carbonyl adopts an axial orientation accounts for the observed facial preference ofboth additions. 

Scheme 20.27 Schmittel cyclization reactions of tert-butyl- and trimethylsilyl-substituted enyne-allenes.

When trimethylsilyl substituted diyne 607 was reacted with methyl vinyl ketone, the reaction proceeded with complete regioselectivity and without aromatization to afford 608 with 56% yield (equation 174). The regioselectivity observed was considered to result from a metallacyclopentene intermediate which was built up of the nickel atom, the double bond of methyl vinyl ketone and the less substituted triple bond of 607. 

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