2-silyl-2/- biphenyl

This latter compound could also be aromatized to 4,4 -bis(eyclopropyl-(diphenyl)silyl)biphenyl with DDQ. 

In 1987 silylated biphenyl (101) was reacted with tribromoborane to suffer methyl abstraction and give the bromodimethylsilyl derivative (102) <87CB99l>. Heating (102) at 150°C causes cyclization to 5-dimethylsiladibenzoborole (103) (Scheme 17). Further reaction of (103) with tribromoborane yields 5-bromodibenzoborole (19). 

Copper-catalyzed systems have been developed that reduce ketones directly to silyl ethers. The reactions involve chiral biphenyl diphosphine type ligands and silane or siloxane hydride donors.187... 

Reduction potentials of silyl-substituted biphenyls have also been investigated [69], The half wave potentials of 4-trimethylsilylbiphenyl and 4,4 -bis(trimethylsilyl)biphenyl are slightly less negative than that of unsubstituted biphenyl. Reduction potentials of nitrophenylsilanes are less negative than that of nitrobenzene, but the difference is usually not so large [70],... 

An alternate approach, which utilizes lithium amides such as lithium hexamethyldisilazide or lithium amide, was also efficient in converting 2-chloropyridine into 2-aminopyridine (7.73.), In these reactions 2-(dicyclohexylphosphino)biphenyl was used as catalyst and the silyl protecting group was removed by TBAF.94... 

Veber et al.88 used Suzuki couplings on a solid support to generate simple biphenyls (Scheme 38), in the presence of a silyl linker, which was cleaved to release these structures without trace of the linkage position. 

In a simple strategy to biaryl formation, Han et al.89 showed that silicon-directed ipso-substitution and concomitant cleavage from supports could be used for formation of functionalized biphenyls. For this they used a tethered silyl aryl bromide in a Suzuki cross-coupling reaction, followed by the ipso-substitution/cleavage step (Scheme 39). A variety of boronic acids were coupled in this manner. The only difficulty occurred with electron-deficient nitrophenylboronic acid where the desired product was formed under anhydrous conditions in only 33% yield (the remainder being starting material). Reversion to the more usual conditions of aqueous base-DME (i.e., those used by Frenette and Friesen)70 improved the yield to 82%. 

Silyl carbamates have recently been shown to undergo photoinduced electron transfer with substituted alkenes, in the presence of catalytic amounts of dicyanoanthracene and biphenyl (BP), to yield more complex carbamates182. Two examples which illustrate the complex structures created in good yield are shown in Scheme 61. 

Tetrasubstituted silanes are also sources of silylene. Suginome and coworkers reported that palladium catalyzed the transfer of dimethylsilylene, formed from silylborane 44, to alkynes [equation (7.8)], 60 Exposure of silylborane 48 and alkyne 49 to substoichiometric amounts of palladium and P(7-Bu)2(2-biphenyl) afforded 2,4-disubstituted silole 50. This process tolerates a variety of functionality including silyl ether-, dimethylamino-, and trifluoromethyl groups. In addition to aliphatic terminal acetylenes, arylacetylenes were also competent substrates. For... 

Biphenyl 492 substituted by electron-donating groups reacts with sulfur dichloride to give dibenzothiophene 493 (Equation 20) <1995NJC65>. Similar reaction of 2,3-dimethoxy-l,3-butadiene affords 3,4-dimethoxythiophene 494 <2004TL6049>. Dienol silyl ethers 495 and elemental sulfur, when heated in the presence of molecular sieve (4 A), provide 3-siloxythiophenes 496 <2006T537>. 

Radical species were also generated from silyl carbamates by desilylation of their cation radicals. Steckhan and colleagues found that on irradiation in the presence of photosensitizers such as DCA and biphenyl (BP), a-silyl carbamates 57 add to strongly electron... 

Finally, the importance of GC/MS techniques for the analysis of hydroxyaromatic compounds generated during microsomal hydroxylation of benzene derivatives is mentioned here. Using various partially ring-deuteriated substituted benzenes, including biphenyl, evidence for direct aromatic hydroxylation was gained from the careful mass spectrometric tracing of the fate of the label in the various silyl-derivatized hydroxylation products . 

In the case of 0-silyl ethers, thermolysis of (triphenylsilyloxy)tetraphenylantimony (13) did not afford the ligand coupling product, phenyl(triphenylsilyl)ether. Instead, a disproportionation reaction took place to give the bis(silyloxy) ether (14) and pentaphenylantimony (3), which decomposed into triphenylantimony (4) and biphenyl (5) under the conditions of the reaction. 1... 

Polycyclic Aromatics. Extensive replacement of hydrogen by lithium in polycyclic aromatic hydrocarbons has been demonstrated by Halasa (15). Anthracene, biphenyl, fluorene, indene, and ferrocene (16) undergo polymetalation by n-butyllithium—TMEDA in hexane at 70°C for 24 hours. The products are insoluble mixtures of polylithio compounds containing up to 10 lithium atoms per molecule. Derivatization was accomplished using both D20 and trimethylchlorosilane and by analyzing the mixture of deuterated or silylated products by mass spectrometry. The results for anthracene, which are typical, appear in Table I. 

The product (183) is formed on irradiation of acridine (184) phenothiazine (185) crystals in which the ratio of the reactants is 3 4," This reactivity is different from the solution phase (in acetonitrile) process which affords both (183) and the dihydro dimer (186). The photoinduced electron transfer reactions of some a-silyl ethers has been investigated." The sensitizing system uses DCA/ biphenyl and irradiation at A, > 345 nm in acetonitrile/methanol. The irradiation brings about the formation of the radical cation (187) of the ether which undergoes cleavage to yield the radical (188), a hydroxymethyl equivalent. When these are generated in the presence of a,P-unsaturated esters such as (189) addition takes place affording the adducts (190). Additions to dimethyl maleate were also carried out successfully." ... 

Electrophilic trifluoromethylation reagents can be generated from the treatment of trifluoromethyl biphenyl sulfoxides with Tf20. These reagents will react with a wide range of nucleophiles including carbanions, aromatics, silyl enol ethers, thiolates, and phosphines (eq 90). ... 

Figure 6. SFC chromatogram of silylated ethoxylated allyl alcohol. Conditions 10 m X 50 fim ID open-tubular column,-30% biphenyl stationary phase CO2 mobile phase 100 °C FID. (Reprinted with permission from Ref 22).

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