Arylsilanes coupling

Phenyltrimethoxysilane 75 is activated by fluoride anion to 76, which undergoes palladium-catalyzed couphng with 4-methyliodobenzene 77 to give a nearly quantitative yield of 4-methyldiphenyl 78 [90]. Such Heck-, Stille-, or Suzuki-type C-C coupling of arylsilanes such as 75 were recently reviewed [91] (Scheme 2.12). 

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. 

In subsequent reactions, the arylsilyl anion (VI) can react with other arylsilanes to give the 1,4-coupling product or with DME to give a methyl-substituted derivative that, under the reaction conditions, may be further reduced to new radical anions (135) ... 

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. 

The monosilanes that produce polymeric material tend to be limited. Thus far, primary arylsilanes are the only well documented silanes that react to provide polymer. Primary alkylsilanes couple more slowly and tend to give relatively short chains (chain lengths up to about 12 silicon atoms). 

The future challenges in this area will be the development of catalyst sytems that promote the formation of higher molecular weight material and that also have the ability to control the microstructure of the polymer. Other features for new catalyst systems would involve the ability to distinguish between primary, secondary, and tertiary centers and couple both alkylsilanes and arylsilanes efficiently. To achieve these goals will require a better understanding of the mechanistic pathways for dehydrocoupling than is currently available. 

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. 

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. 

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. 

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. 

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]. 

Scheme 36. Pd-catalyzed cross-coupling of arylsilanes with aryl chlorides [129]...

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