Silicon arenes

The substituent effect of vinylsilanes is similar to that of allylsilanes. The reactivity of vinylsilanes increased as the number of chlorine atoms on the silicon increased, but decreased as the number of methyl groups increased. However, vinyltrimethylsilane does not react with benzene to give alkylated products. " In the aluminum chloride-catalyzed alkylation of arenes with allylsilanes or vinylsilanes, one or more chlorine substituents on the silicon atom of silanes are required. 

This section will describe the Friedel-Crafts alkylation reactions of aromatic hydrocarbons with alkenylchlorosilanes containing short chain alkenyl groups such as allyl and vinyl. The reaction will be discussed in terms of the substituent effect on silicon and the arene rings. 

The highly dispersible calix[4]arene neutral carriers can also improve the durability for neutral-carrier-type ion sensors. Time-course changes in both sensitivity (slope for Na+ calibration graph) and selectivity (selectivity coefficient for Na" " with respect to K+) were followed in the Na -ISFETs based on ion-sensing membranes of silicone rubber-(l), plasticized PVC-(l), and plasticized PVC-(2) (Fig. 2). Deterioration proceeded quite quickly in the Na -ISFETs of plasticized PVC-(2) both the Na+ sensitivity and selectivity... 

As neutral carriers for the chemical modification, 16-crown-5 and calix[4]arene derivatives possessing a triethoxysilyl group (7) and (8) were designed for Na sensors. Triethoxysilylethyl-16-crown-5(7) was then mixed with a silicone-rubber precursor for the membrane fabrication accompanying covalent bonding of the neutral carrier. Comparison of IR spectra before and after extraction of the nonbonded neutral carrier... 

FIG. 11 Potential response of Na+-selective electrodes based on silicone-rubber membranes modified chemically by triethoxysilylated calix[4]arene (8) (O) without anion excluder ( ) with TFPB ( ) modified chemically by triethoxysilylated tetraphenylborate (9) as well. (From Ref. 44.)... 

The Na -selective electrodes based on silicone-rubber membranes modified chemically by (8) and (9), were also investigated for Na assay in control serum and urine [22]. The found values for the Na concentrations in both of the serum and urine samples are in good agreement with their corresponding actual values with a relative standard deviation of about 1%. These results suggest that the Na -selective electrodes based on silicone-rubber membranes modified chemically by calix[4]arene neutral carrier (8) are reliable on assay in human body fluid. 

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. 

Among the latter group, iridium complexes (though less common than rhodium) and perhaps also ruthenium play crucial roles in many of the above-mentioned transformations of silicon compounds, leading to the creahon of sihcon-carbon bonds. Examples include the hydrosilylation or dehydrogenahve silylation of alkenes and alkynes, the hydroformylahon of vinylsilanes, and the silyhbrmylation of alkynes as well as activation of the sp C—H of arenes (by disilanes) and alkenes (by vinylsilanes). 

Allylchlorosilanes undergo Friedel-Crafts alkylation with aromatic compounds such as benzene derivatives and ferrocene to give [p-(chlorosilyl)alkyl]arene compounds in the presence of Lewis acid catalyst. Allylsilanes containing two or more chlorine atoms on silicon react smoothly with benzene under mild conditions to give alkylation products in good yields [Eq. (15)]. In alkylations of benzene, the reactivity of the allylsilanes increases as the number of chlorine atoms on the silicon increases, but decreases as the number of methyl groups increases. Because the reactivity of allylsilanes is sensitive to the electronic nature of the substituents on the silicon atom, allylsilane selection is an important factor for alkylation reactions. 

The catalyst, although applied in 1.5 equiv., also worked well with heteroarenes in the alkylation reactions. A simple and most plausible mode for the enantioselectivity of the Friedel-Crafts reaction has been shown in Scheme 33. It is evident from the model that the arene would approach from the front (Si) face, as the back (Re) face is blocked by the phenyl group present on the silicon. 

Much of the interest in the polysilanes, polygermanes, and polystannanes involves their sigma delocalization and their sigma-pi delocalization when coupled with arenes or acetylenes. This is not unexpected since silicon exists as a covalent network similar to diamond. In exhibiting electrical conductivity, germanium and tin show more typical metallic bonding. Some polystannanes have been referred to as molecular metals. ... 

Nitro-arene allgemeine Arbcitsvorschrift5 Zu 5-10 ml Aren werden aquivalente Mengen (0,1-0,5 g) Metall(IV)-nitrat [beim Titan(l V)-nitrat werden 25 ml einer 0,1 M Losung eingesetzt] bei 20° unter Ruhren addiert. Obwohl die Reaklion nach 1 min beendet ist, fiihrt man die quantitative GLC erst nach 30 min durch. Fiir Nitro-pyridine bzw. Nitro-chinoline wild cine Polyethylen-glykol-Saule bei 150° hzw. eine Silicon SE30-Saule bei 140° verwendet. 

Silicon vapor reacts with alkanes, alkenes, alkynes, arenes, ketones, alkyl halides, and ethers to give mainly polymers (104). 

Some novel silicon-containing arene-chromium tricarbonyl complexes (IV-VII), derived from phenyl-substituted silacyclobutanes (118), 1,1-dimethylsilaindane (120) and alkynylarenes (45), have been prepared by essentially identical methods. It is worth mentioning here that the organosilicon complex (VI) is somewhat more thermally stable than the silicon-free 1,1-dimethylindane complex (120). 

The stabilization of a silicon-containing arene has not as yet been achieved. Nevertheless, silabenzene (7), 1-silatoluene (8) and hexamethyl-l,4-disilabenzene (9) have all been generated as transients.42"47 Both silabenzene and 1-silatoluene are stable when condensed in argon matrices. The UV spectrum exhibits three bands at 212, 272 and 320 nm expected for a jr-perturbed benzene. In its photoelectron spectrum, the lowest ionization energies are at 8.0, 9.3 and 11.3 eV (cfcalculated values derived by SCF methods of 8.2, 9.2 and 11.5 eV).48... 

Alkynes, and Arenes, 1985 Paul N. Rylander, Hydrogenation Methods, 1985 Ernest W. Colvin, Silicon Reagents in Organic Synthesis, 1988 Andrew Pelter, Keith Smith and Herbert C. Brown, Borane Reagents, 1988 Basil Wakefield, Organolithium Methods, 1988... 

In 1982, Curtis and co-workers reported that Vaska s complex promotes the formation of phenylsiloxanes from the reaction of hydridosiloxanes and benzene in a catalytic, albeit low-yield, process.93 Catalytic arylsilane formation has also been reported by Tanaka and co-workers. Under photo-lytic conditions, RhCl(CO)(PMe3)2 catalyzes the C-H bond activation of arenes in reactions with hydrosilanes or disilanes, leading to the formation of new silicon-carbon bonds.94 More recently, C-H bond activation of arenes resulting in arylsilane formation has been observed in the... 

In direct analogy to their work in the nickel-catalyzed double silylation of unsaturated substrates with 3,4-benzo-l,l,2,2-tetraethyl-l,2-disilacyclo-butene, Ishikawa and co-workers have studied the catalytic formation of arene-silicon bonds. The Ni(PEt3)4 catalyst activates the C-H bond of the arene to result in net Si-H and Si-Ar bond formation [Eq. (34)].96... 

Perhaps the most industrially feasible approach has been developed by Rich and co-workers at General Electric, a palladium-catalyzed silylative decarbonylation reaction of aromatic acid chlorides with disilanes [Eq. (35)].97 One of the silicon centers from the disilane is transferred to the arene whereas the other acts as a chloride acceptor to produce the chlorosi-... 

Addition of silyl enol ethers to nitroarenes2 In the presence of 1 equiv. of TASF, silyl enol ethers add to nitroalkenes to form unstable ortho and/or para nitronates, which are oxidized in situ by Br2 or DDQ to nitroaryl carbonyl compounds. The position of substitution depends on the substitution pattern of the arene and the size of the silicon reagent. With less hindered silyl derivatives ortho addition is strongly favored. 

The alkylation reaction is limited to nitro-substituted arenes and heteroarenes and is highly chemoselec-tive nucleophilic displacement of activated halogens, including fluorine, was not observed. The regio-selectivity is determined by the bulkiness of the silicon reagent. With unhindered silyl derivatives a strong preference for ortho addition was observed, as in the example of equation (6). With bulkier reagents attack took place exclusively at the para position (Scheme 1). The success of this reaction, which could not be reproduced with alkali enolates, was attributed at least in part to the essentially nonbasic reaction conditions under which side processes due to base-induced reactions of nitroarenes can be effectively eliminated.12... 

Silicon-substituted arenes have particular usefulness in organic synthesis. Ipso substitution always gives a single isomer, and desilylation can be carried out on deactivated systems, or systems that would undergo side-reactions under normal conditions (equations 34-36). Ipso substitution also provides a useful route to labelled compounds (equation 37)95-97. 

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