Aryl-silicon bond

The acid cleavage of the aryl— silicon bond (desilylation), which provides a measure of the reactivity of the aromatic carbon of the bond, has been applied to 2- and 3-thienyl trimethylsilane, It was found that the 2-isomer reacted only 43.5 times faster than the 3-isomer and 5000 times faster than the phenyl compound at 50,2°C in acetic acid containing aqueous sulfuric acid. The results so far are consistent with the relative reactivities of thiophene upon detritia-tion if a linear free-energy relationship between the substituent effect in detritiation and desilylation is assumed, as the p-methyl group activates about 240 (200-300) times in detritiation with aqueous sulfuric acid and about 18 times in desilylation. A direct experimental comparison of the difference between benzene and thiophene in detritiation has not been carried out, but it may be mentioned that even in 80.7% sulfuric acid, benzene is detritiated about 600 times slower than 2-tritiothiophene. The aforementioned consideration makes it probable that under similar conditions the ratio of the rates of detritiation of thiophene and benzene is larger than in the desilylation. A still larger difference in reactivity between the 2-position of thiophene and benzene has been found for acetoxymercuration which... 

Finally, oxidative cleavage of the remaining aryl-silicon bond with lead tetrakis(trifluoroacetate), [Pb(OCOCF3)4]19, furnishes ( )-estrone [( )-1 ] in nearly quantitative yield. 

The dicyclopentadienyl metal compounds undergo Friedel-Crafts alkylation and acylation, sulfonation, metalation, arylation, and formyla-tion in the case of ferrocene, dicyclopentadienyl ruthenium, and dicyclopentadienyl osmium, whereas the others are unstable to such reactions ( ). Competition experiments (128) gave the order of electrophilic reactivity as ferrocene > ruthenocene > osmocene and the reverse for nucleophilic substitution of the first two by n-butyl lithium. A similar rate sequence applies to the acid-catalysed cleavage of the cyclopentadienyl silicon bonds in trimethylsilylferrocene and related compounds (129), a process known to occur by electrophilic substitution for aryl-silicon bonds (130). 

Since an insertion reaction into alkyl or aryl silicon bonds seems to be difficult, a radical mechanism usually takes place in the pyrolysis of such compounds. On the other hand, at Si—H, Si—Hal, Si—OR and Si—N bonds, a silylene mechanism occurs because the insertion reaction has only a small activation energy. 

Replacements of one or two chlorine atoms and hydrogen by fluorine were observed in various organosilicon compounds in reactions with xenon difluoride99,135, while aryl-silicon bond cleavage was observed in the presence of potassium fluoride136,137 (Scheme 57). 

In addition, the cleavage of aryl-silicon bonds by neutral reagents delineates even more the vast differences between the fields of organosilicon and carbon chemistry. 

Solid-phase synthesis was initialed with acylation of 170 onto (aminomethyl)polystyrene resin using DMAP and DIEA to give 171. A synthetic sequence analogous to the one described in Scheme 4.1.7 afforded 1,4-benzodiazepines 172 linked to the solid support. The aryl-silicon bond was then cleaved in the final step with anhydrous HF, delivering benzodiazepines 173 in good yields (Scheme 4.2.2). 

It is well known that the aryl-silicon bonds are susceptible to cleavage by electrophilic reagents. Deans and Eaborn reported that nitration of l,4-bis(trimethyl-silyl)benzene by fumic nitric acid resulted in the substitution of one trimethylsilyl... 

Aryl—silicon bonds are more reactive. Acid cleavage of Et SiPh proceeds 10000 times as fast as proton exchange in benzene. R3Si behaves as an electronreleasing group which stabilizes the positively charged Wheland intermediate. 

Very few examples were described in this class and the formation of the aryl-silicon bond was obtained analogously to the alkylation of aryl nitriles presented in Section 4.3.3 by ipso-substitution reaction induced by an initial electron transfer process. Thus, tetracyanobenzene in the presence of hexamethyldisilane was in part silylated... 

The transformation of aryl-silicon bonds to aryl-fluorides has received relatively little attention. This is due in part to the fact organosilanes are not... 

Acetyl hypofluorite is very effective m the fluorination of the aryl-metal (Hg, Ge, or Si) bond, but yields are frequently low. With aryl silicon compounds some competition exists for replacement of an aromatic hydrogen [5i, 52, 55, 54] (equations 25-27). Fluoroxytrifluoromethane fluorinates p-methoxypheny 1 mercuric acetate to givep-fluoroanisole in 86% yield [52]... 

Eaborn, C., Cleavages of aryl-silicon and related bonds by electrophiles, <7. Organomet. Chem. 100, 43 (1975). 

As a result of several decades of research it is now known that a polysilane of three or more contiguous silicon atoms is susceptible to reaction by one or more of several pathways when photolyzed, each associated with cleavage of a silicon-silicon bond. The two most common processes observed are the homolysis of a silicon-silicon bond to yield a pair of silyl radicals, and the elimination of a silicon atom from the chain in the form of a silylene. As discussed in Section VII, the use of trisilanes, particularly where the central silicon atom bears aryl groups, has become an important route for the preparation of a wide variety of diarylsilylenes, A Si , many of which have been captured in glasses at low temperature, or have been allowed to dimerize to disilenes by warming. 

Eaborn and co-workers28 have reported product isotope ratios for the cleavage of benzyl-silicon, benzyl-tin, and aryl-tin bonds using an equimolar mixture of MeOH and MeOD as the solvent. A free carbanion would not be expected to discriminate between MeOH and MeOD in the fast step (24), and hence28 the product isotope ratio kH/kD should be unity. The values of kJkD in Table 11 indicate that the carbanions are not entirely free, but that some degree of electrophilic attack by the solvent at the benzyl carbon atom takes place, as in (V). 

Treatment of aryl silane 10 with the iodonium source IC1 in an iododesilylation reaction yields compound 44, which proceeds by an ipso substitution mechanism.9 Activation towards electrophilic attack arises from stabilization of resonance form 45 by Si. Silicon is arranged / to the positive charge and the carbon-silicon bond can overlap with the empty Jt orbital (hyperconjugation). 

Pale and coworkers provided the first example of combined desilylation/coupling catalytic for silver. They found that 1-trimethylsilyl-l-alkynes in the presence of tetrakis(triphenylphosphine)palladium, a silver(I) salt, and an activator (potassium carbonate in methanol, or TBAF-3H20) in DMF coupled with vinyl triflates and aryl iodides to give enynes good yields (Scheme 1.66).143,144 Although silver salt was not necessary for reaction when TBAF-3H20 was used for activation of the carbon-silicon bond, a small to significant improvement was observed for all reported... 

Although H NMR spectra of aryllead triacetates can be recorded in CDCI3 solution in the presence of tetra-methylsilane, cleavage of the methyl-silicon bond takes place when tetramethylsilane is treated with an aryllead(rv) tricarboxylate in trifluoroacetic acid.164 When the aryl moiety is substituted with an electron-withdrawing group, quantitative conversion of TMS was observed to give aryl(methyl)lead dicarboxylates 111, (Equation (136)) that were even isolated in two instances, although in modest to moderate yields 55% yield for the 4-fluorophenyl compound and 23% yield for the 4-trifluoroacetamidophenyl compound. 

These higher chlorides of silicon hydrolyze in the same way as does the tetrachloride, and they likewise undergo similar reactions with Grignard reagents and with sodium alkyls or aryls.30 Under some conditions, however, the silicon-silicon bond is cleaved by sodium.31 The silicon-silicon chains are destroyed by the action of aqueous alkalies, liberating hydrogen at the rate of one molecule of H2 per silicon-silicon bond, just as are the higher silanes. 

The reasons why alkynes 14 and 19 having aryl or vinyl groups, respectively, insert into the zirconium-silicon bond of 3, whereas the alkyne 17 having alkyl groups inserts into the carbon-zirconium bond of 3, are still not clear. Presumably, electronic factors are important for the insertion reaction. 

Hi. Carbon-silicon bonds. Following the earlier reports mentioning the palladium-catalysed addition of organosilylstannanes to alkynes or isonitriles , Mori and coworkers realized tandem transmetallation-cyclization reactions with bifunctional halogeno triflates and Bu3SnSiMe3 18. The reactivity of 18 under palladium catalysis was used for the silylstannylation of alkenes or the synthesis of allylic silanes via a three-component (aryl iodide - - diene - - 18) coupling reaction. Recently, a similar... 

This chapter concentrates on those processes in which oxidative cleavage of a carbon-silicon bond results in production of the alkyl/aryl fragment as an alcohol/phenol. Other cleavage processes are dealt with, but more briefly. 

Platinum-silicon bonds may be made by the reaction of trans-[PtH(ClXPMe2Ph)2] with R3SiH in the presence of solvent, provided that R is an electron-withdrawing group such as Cl or aryl. For Cl3SiH reaction occurs at RT but for triarylsilanes at 80-90°C ... 

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