Aryl silanes

Reaction of aryl silanes with fluoride Addition of silanes to alkenes... 

Reactions of silicon hydrides with m-(R3P)2PtCl2 (R = Bu, Ph) have been widely investigated. Alkyl- and aryl-silanes underwent metathetical H/Cl exchange [Eq, (18) (R = Et, Ph, PhCHj, C.H.j)] (55). 

Figure 4.3 Correlation between UV peak intensity (e), full width at half maximum (FWHM), and a of various poly(dialkylsilane)s and poly(alkyl(aryl)silane)s in THF at 30°C.

It is considered that, if ideal, optically active poly(alkyl(aryl)silane) homopolymer and copolymer systems could be obtained which had stiffer main-chain structures with longer persistence lengths, it should be possible to clarify the relationship between the gabs value and the chiral molar composition. The magnitude of the chirality of the polyisocyanates allowed precise correlations with the cooperativity models.18q In the theory of the cooperative helical order in polyisocyanates, the polymers are characterized by the chiral order parameter M, which is the fraction of the main chain twisting in one helical sense minus the fraction of the main chain twisting in the opposing sense. This order parameter is equal to the optical activity normalized by the value for an entirely one-handed helical polymer. The theory predicts... 

Careful studies by C. Eabom have shown that electrophilic aromatic substitution of silicon is faster than substitution of hydrogen. Thus a silicon in an aromatic ring directs substitution with hardly any rearrangement. This technique is particularly useful for preparation of specifically deuterated arenes as protolysis (deuterolysis) or aryl silanes is rapid. 

Reductive Coupling Products from Aryl Silanes... 

Comparison of all reported cleavages gives the apparent ordering H > Ph (or aryl) > alkyl in terms of tendency for groups to be cleaved from silicon, with the exception that at least one aryl substituent is retained on silicon in the anion VI. It is clear that reduction of aryl silanes leads to significant weakening of Si—R bonds that have appropriate symmetry for hyperconjugative interactions with the reduced n systems, a point that has been cited as evidence for the importance of n — type interactions in unsaturated silanes (86). 

Under certain conditions, aroyl chlorides are converted to arylsilanes by the reaction with disilanes. The oxidative addition of aroyl chloride and decarbonylation are followed by transmetallation and reductive elimination to give aryl silanes. Neat trimellitic anhydride acid chloride (377) reacts with dichlorotetramethyldisilane (376) at 145 °C to generate 378, which affords 4-chlorodimethylsilylphthalic anhydride (379) by reductive elimination. Finally it was converted to 380 and used for polyimide formation [185], Biphenyltetracarboxylic anhydride 381 is obtained at a higher... 

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). 

Tab, 1. Generation of arenes upon cleavage of support-bound aryl silanes and arylgermanium compounds. PS cross-linked polystyrene (PS) PS with spacer. 

Aryl tellurium trichlorides are easily converted to symmetrical or unsymmetrical diaryl tellurium dihalides via reactions with organometallic compounds (see p. 549). Trimethyl(aryl)silanes transfer only the aryl group to tellurium1. 

Aryl silanes undergo ipso substitution with electrophiles... 

Exactly the same sort of mechanism accounts for the reactions of aryl silanes with electrophiles under Friedel-Crafts conditions. Instead of the usual rules governing ortho, meta, and para substitution using the directing effects of the substituents, there is just one rule the silyl group is replaced by the electrophile at the same atom on the ring—this is known as ipso substitution. Actually, this selectivity comes from the same principles as those used for ordinary aromatic substitution (Chapter 22) the electrophile reacts to produce the most stable cation—in this case (3 to silicon. Cleavage of the weakened C-Si bond by any nucleophile leads directly to the ipso product. 

This reactivity of aryl silanes is used to convert the stable phenyl dimethylsilyl group into a more reactive form for conversion into an alcohol by the silyl Baeyer-Villiger reaction described above. Overall this makes the phenyl dimethylsilyl group a bulky masked equivalent for a hydroxyl group. This is useful because the silane will survive reaction conditions that the 1. Br2... 

Vinyl and aryl silanes react with electrophiles at the same (ipso or a) atom occupied by silicon. Allyl silanes react at the end of the alkene furthest from silicon (y). In both case a (J-silyl cation is an intermediate. 

Protection of a catechol as its diphenylmethylene acetal also featured in a synthesis of the Galanthamine alkaloids.182 Here, the deprotection reaction was accomplished using trifluoroacetic acid — conditions that also effected protonolysis of the aryl silane [Scheme 3.98]. 

The HjSi derivatives (H3Si)4C and (H3Si)2CMe2 can be prepared by treating the aryl silanes with acid, followed by reduction with LAH [122]. 

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