A-Trimethylsilyl-substituents

The presence of activating substituent on the carbocyclic ring can, of course, affect the position of substitution. For example, Entries 4 and 5 in Table 14.1 reflect such orientational effects. Entry 6 involves using the ipso-directing effect of a trimethylsilyl substituent to achieve 4-acetylation. 

The ability to promote /S elimination and the electron-donor capacity of the /3-metalloid substituents can be exploited in a very useful way in synthetic chemistry. Vinylstannanes and vinylsilanes react readily with electrophiles. The resulting intermediates then undergo elimination of the stannyl or silyl substituent, so that the net effect is replacement of the stannyl or silyl group by the electrophile. An example is the replacement of a trimethylsilyl substituent by an acetyl group by reaction with acetyl chloride. 

Metalated epoxides are a special class of a-alkoxy organometallic reagent. Unstabilized oxiranyl anions, however, tend to undergo a-elimination. On the other hand, attempts to metalate simple unfunctionalized epoxides may lead to nucleophilic ring opening. The anion-stabilizing capability of a trimethylsilyl substituent overcomes these problems. Epoxysilanes 22 were... 

It must be noted that the saturated ring system, the 1 -chloro-A -phosphirane was synthesized [84] a cyclic phospheniiun cation was postulated which is stable towards ring opening [85]. A corresponding cation could not be isolated, due to a methanid shift from a trimethylsilyl substituent. 

Thermolysis of 44 produced products derived from the Myers-Saito cyclization reaction. However, when 43 having a trimethylsilyl substituent at the acetylenic terminus was subjected to heating in the presence of 1,4-CHD at 70 °C for 3 h, the 1H-cyclobut[a]indene 46 was produced. A reaction mechanism involving an initial Schmittel cyclization to generate the benzofulvene biradical 45 followed by an intramolecular radical-radical coupling was proposed to account for the formation of the formal [2 + 2]-cycloaddition product 46. 

A-Trimethylsilyl substituents are readily removed by treatment with aqueous acid. ... 

The addition of unstabilized a-nitrile carbanions to a,3-unsaturated carbonyl acceptors affords predominantly 1,2-addition products, 56 while lithiated acetonitrile derivatives having a-alkoxy, a-aromatic, a-dialkylamino, a-phenylselenyl, a-phenylthio or a-trimethylsilyl substituents afford 1,4-adducts. However, some of these are acyl anion equivalents (Section 1.2.2.3.2) so this discussion is limited to a-stabi-lized nitriles in which the nitrile function is retained after removal of the activating group. Notable examples are trimethylsilylacetonitrile (208),157 phenylthioacetonitriles (209),158a b phenylselenylacetoni-... 

Block et al.194 examined the effects of trimethylsilyl substitution on the first vertical ionization potentials by photoelectron and Penning ionization electron spectroscopy studies of a range of cyclic and noncyclic sulfides and ethers. It was shown that substitution of oxirane 218 with a trimethylsilyl substituent as in 219 lowered the ionization potential by 0.90 eV (20.8 kcal/mol), while similar substitution of dimethyl ether 220 in 221 lowered the ionization potential by 0.64 eV (14.8 kcal/mol). By comparison, the effects of silyl substitution on sulfur lone-pair ionization potentials was found to be smaller thus the ionization potential of dimethyl sulfide 222 is lowered by 0.37 eV upon trimethylsilyl substitution in 223, and the trimethylsilyl-substituted thiirane 225 is lowered by 0.59 eV relative to thiirane 224. The raising of the energy of the sulfur lone-pair electrons in the thiirane 225 is also apparent from its UV spectrum, where there is a bathochromic shift in the absorption maximum compared to the parent 224. 

Jamison, following the precedent of Hudrlik, has used a trimethylsilyl substituent on a polyepoxide that enables both a highly stereoselective epoxidation process, but also directs intramolecular attack (Equation 13). Conveniently, the silyl substituents are cleaved under the reaction conditions <2003OL2339, 2006JA1056>. 

The position of a trimethylsilyl substituent in phenols 33 determines great yield differences in both thiocarbamate formation and rearrangement [57, 58]. The required TMS-substituted phenols, which were prepared from the respective bromophenols, were tested under standard conditions (Scheme 19). The yield of the rearrangement decreased as the substituent moved from para to ortho. 

It is harder to make a straightforward choice between routes (2) and (3), since they have been applied to the preparation of different compounds. The azastannole routes are short and efficient. They are limited to A -/-butyl and A -trimethylsilyl substituents, although the Fu procedure, outlined in Scheme 8, shows that many different B substituents are available. The RCM procedures appear to offer a wider choice of both B and N substituents. One drawback of the RCM procedures is that it is necessary to prepare the precursor 5-vinyl, A -allyl boramine. Clearly, the RCM procedures are excellent for preparing fused ring compounds as shown in Scheme 18. 

Recently, Paquette and coworkers reported on the stereochemical consequences of having a trimethylsilyl substituent at the radical site. The Hunsdiecker reaction, as well as the Cristol-Firth modification thereof, on ( —)-(R)-l-trimethylsilyl-2,2-diphenyl-cyclopropanecarboxylic acid (71) resulted in racemic ( )-l-bromo-l-trimethylsilyl-2,2-diphenylcyclopropane (72). The trimethylsilyl group, bulky as it is, could not slow down the inversion frequency of the cyclopropyl a radical sufficiently to prevent complete racemization. More to the point, recentESR studies have demonstrated that the radical intermediate is planar, or nearly so. 

Substituents capable of resonance stabilization of a cyclopropyl cation can lead to the formation of substitution products with a conserved three-membered ring vide supra). /S-Cation stabilization by a trimethylsilyl substituent can also inhibit silver(I)-assisted ring opening of several 7,7-dihalo-l-(trimethylsilyl)bicyclo[4.1.0]heptanes, e.g. 16, but not the analogous bi-cyclo[3.1.0]hexanes and -[b.l.OJnonanes. ... 

In additions to alkynylsilanes, the Mg is attached exclusively to the carbon bonded to Si. Additions also are observed to dienes, alkenols ° , alkynols, and to dienes and alkynols having a trimethylsilyl substituent attached to one of the multiply bonded carbons. 

Organolithium reagents bearing a-trimethylsilyl substituents react with aldehydes and ketones to produce olefins in a manner similar to the Wittig reaction/ The presence of the silicon substituent may facilitate formation of the anionic carbon... 

An example is the replacement of a trimethylsilyl substituent by acetyl by reaction with acetyl chloride. 

L. Lockwood. 1. Effects of a Trimethylsilyl Substituent on the Photochemical Rearrangement... 

The simplest substituted thiophenes (Scheme 14.3) with a trimethylsilyl substituent at the 3- or 2-position (9 and 10) and the 2,5 or 2,4-bis (trimethylsilyl) thiophene (11 and 12) have been synthesized in high yields upon reaction of the corresponding Grignard or lithium reagents with trimethylchlorosilane [6-12]. 

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