Coupling reactions of arylsilanes

The coupling reaction of arylsilanes with aryl iodides is also mediated by a palladium catalyst and a fluoride ion [23,24]. Optimized reaction conditions are as... 

Allyl carbonates (Scheme 5) and diene monoxides (Eq. 32) were also employed in the palladium-catalyzed coupling reaction of arylsilanes 136, 37]. The reaction does not require activation by a fluoride ion or an additional base like a hydroxide ion. 

The coupling reaction of arylsilanes with aryl iodides is also mediated by a palladium catalyst and a fluoride ion.f " Optimized reaction conditions are as follows (i) two fluorine atoms on silicon are required (ii) an ethyl or propyl group as a dummy alkyl ligand is preferred, because a methyl group competitively participates in the crosscoupling reaction and (iii) TBAF, a highly effective fluoride ion source, can be replaced by inexpensive KF. Various unsymmetrical biaryls are synthesized under the conditions (Scheme 10). 

Cross-coupling reactions of arylsilanes with aryl halides... 

Protiodesilylation of the starting material is occasionally encountered in the coupling reactions of arylsilanes with a nucleophihc promoter. This side-reaction can be suppressed by the use of a stoichiometric amount of a copper(I) salt to promote the cross-coupling of arylsilyl ethers with iodoarenes (Scheme 7.36) [63]. The preferred salt is CuOCgFj, generated in situ by mixing Cul and NaOC Fj, to effectively promote this reaction. Unhke other arylsilyl ether cross-couphng reactions wherein a trialkoxysilane is used, the mono-methoxysilane reacted much more efficiently than either the di- or trimethoxysilane. Despite the harsh conditions used, this is an impressive fluoride- and paUadium-ffee reaction. 

Table 1 Pd-catalyzed cross-coupling reactions of alkenyl- and arylsilanes...

Synthetic reactions involving activation of alkenyl- and arylsilanes by copper salts have also been reported. A copper-mediated system is effective in the cross-coupling reaction of aryl- and heteroarylsilanes with aryl halides under Pd-free conditions (Equation (13)).66 Alkenyl- and arylsilanes as well as alkynylsilanes undergo homocoupling in the presence of Cul and air.65,65a Functionalized alkenyl- and arylsilanes bearing a coordination site in the vicinity of silicon (e.g., 7) are easily activated by copper salts even at room temperature (Scheme 5).67,67l 67e 68 q-jle organocoppers such as 8 are useful for carbon-carbon bond-forming reaction with carbon electrophiles. 

We have found that alkynylsilanes are smoothly converted into alkynylcopper compounds by treatment with CuCl in l,3-dimethyl-2-imidazohdinone (DMI) the copper reagents can be isolated in good yields [563]. This study was the first example of preparation and isolation of organocopper compounds by use of organosilicon reagents. The Si-Cu transmetalation is applicable to the synthesis of alkynyl ketones by Cu-catalyzed alkynylation of acid chlorides (Scheme 10.217). We have also shown that a Cu-mediated system is effective in the cross-coupling reaction between arylsilanes or heteroarylsilanes and aryl hahdes [564]. 

The cross-coupling reaction of aryl halides with arylsilane reagents is an important alternative aryl-aryl bond forming strategy. Thus the reaction of various aryltrialk(ox)ylsilanes readily proceeds with aryl iodides [130] and bromides [131] in the presence of palladium(0)-catalyst to give the biaryls in good to excellent yields. Scheme 32. 

The only side-reaction of considerable interest is the homo-coupling reaction of the parent arylsilane. For that reason, Masuda s method is usually conducted with a slight excess, e.g. 20 mol%, of arylsilane. Once again, methods involving significantly higher excesses of arylsilanes cause several difficulties during the purification procedure of the final biaryl product. 

Silylation of Aryl and Alkenyl Halides. Aryl halides can be successfully silylated in the presence of palladium and rhodium catalysts. The palladium(0)-catalyzed coupling reaction of triethoxysilane with aryl iodides and bromides was developed by Masuda and Murata (eq 21), and later improved on by DeShong. This is an efficient process in most cases, resulting in facile formation of arylsilanes. One limitation of this palladium(0)-catalyzed process is the decreased effectiveness in the presence of ort/zo-substituted and electron-deficient aryl halides. The successively developed rhodium(I)-catalyzed process by Murata and Masuda is broadly applicable to a wide range... 

Hosomi and coworkers described the cross-coupling reaction between arylsilanes or heteroarylsilanes and aryl halides mediated by a copper(I) salt. In particular, the cross-coupling reaction of 2-TST (1) with iodobenzene under fluoride- and palladium-free conditions (eq 31) was examined employing different solvents and copper(I) salts (Table 1). The yield of the product was increased in polar solvents such as 1,3-dimethyl-2-imidazolidinone (DMI). In addition, it was found that the reaction was best promoted by CuOCgFs because of the strong affinity of pentafluorophenoxide ion to the silicon atom, resulting in an accelerated transfer of the thiazole group to copper. 

Cross-Coupling Reaction of 2-(2-Hydroxyprop-2-yl)cyclohexyl-Substituted Arylsilanes with Aryl Chlorides A General Procedure ... 

Arylsilane radical anions undergo cleavage and coupling reactions, usually under conditions where excess reducing agent is available. Reduction of phenylsilane, diphenylsilane, or triphenylsilane with sodium-potassium alloy under preparative conditions gives high yields of tetraphenylsilane (7). In the reduction of phenylsilanes, the appearance of 1,4-bis(silyl)benzene radical anions is frequently observed (135, 35, 86, 97, 75, 120, 100). Typical results are shown in Table II. 

Han, Y. Walker, S. D. Young, R. N. Silicon Directed ipso-Substitution of Polymer Bound Arylsilanes Preparation of Biaryls via the Suzuki Cross-Coupling Reaction, Tetrahedron Lett. 1996, 37, 2703—2706. 

Han Y, Walker SD, Young RN, Silicon directed ipso-substitution of polymer bound arylsilanes preparation of biaryls via the Suzuki cross-coupling reaction, Tetrahedron Lett., 37 2703-2706, 1996. 

Under an atmospheric pressure of carbon monoxide, aryl- and alkenylsilanes undergo a carbonylative coupling reaction with aryl and alkenyl halides [44,45]. The optimized conditions for arylsilanes involve the use of N,M-dimethyl-2-imidazolidinone (DMI) as a solvent and KF as a fluoride ion source (Eq. 38), whereas alkenylsilanes prefer THF and TBAF (Eq. 39). 

Allyl silanes react with epoxides, in the presence of Bp3 OEt2 to give 2-allyl alcohols.The reaction of a-bromo lactones and CH2=CHCH2Si(SiMe3)3 and AIBN leads to the a-allyl lactone.On the other hand, silyl epoxides have been prepared from epoxides via reaction with iec-butyllithium and chlorotri-methylsilane. ° a-Silyl-A-Boc-amines were prepared in a similar manner from the A-Boc-amine. " Arylsilanes were prepared by reaction of an aryl-lithium intermediate with TfOSi(OEt)3. In the presence of BEs etherate, allyl silane and a-methoxy A-Cbz amines were coupled. Benzyl silanes coupled with allyl silanes to give ArCHa—R derivatives in the presence of VO(OEt)Cl2 " and allyltin compounds couple with allyl silanes in the presence of SnCl4. Allyl silanes couple to the a-carbon of amines under photolysis conditions. 

Similar palladium-catalyzed arylation reactions using arylsilane and arylzinc reagents are also possible by the judicious choice of bases and additives. As shown in Equation 5.9, the addition of a fluoride donor BU4NF promoted efficient cross-couplings of primary aUcyl bromides and iodides with aryltrimethoxysilanes [12]. While the reaction showed good functional group compatibility, electron-deficient arylsilanes, such as 4-fluorophenyltrimethoxysilane, showed a rather poor reactivity. 

Many arylations are assisted by functional groups that promote ortho-metallation. Thus, for example, acetanihdes react with arylsilanes at the ortho position via pal-ladacycles 78 and 79 to form derivatives 80 (Scheme 11.27) [84]. Mechanistically, this transformation is similar to a cross-coupling reaction, in which the oxidative addition step is replaced by the ortho-metallation, although in this case the Pd(0) intermediate must be oxidized in situ to generate the reactive Pd(ll) species. Unsubstituted benzylamines and N-methylbenzylamine are ortho-arylated with Pd(OAc)2 in the presence of trifluoroacetic acid (TFA) and silver acetate [85], and a mechanism which differs from the usual Pd(0)/Pd(II) catalytic cycle was suggested for this. Ortho-alkylation was also observed in the palladium-catalyzed... 

Aroyl chlorides undergo silylative decarbonylation to give arylsilanes by the reaction of disilanes. Oxidative addition of aroyl chlorides and decarbonylation are followed by transmetallation with disilane to generate (aryl)silylpalladium, and its reductive elimination gives arylsilanes. For example, neat trimellitic anhydride add chloride reacted with dichlorotetramethyldisilane at 145 °C to afford 4-(chlorodimethylsilyl)phthalic anhydride by decarbonylation and reductive elimination. Also, trimellitic anhydride add chloride was converted to biphenyltetracarboxylic anhydride at 165 °C in refluxing mesitylene by reaction of the disilane. Thus, decarbonylation-coupling of aroyl chlorides offers a good synthetic method of biaryls (Scheme 7). ... 

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