Pyrazoles, silylated

Withasomnine Pyrazole, 1-phosphoryl-reactions, 5, 271 Pyrazole, 1-silyl-synthesis, 5, 236 Pyrazole, 1-stannyl-synthesis, 5, 236 Pyrazole, 1-styryl-synthesis, 5, 233 Pyrazole, 1-thienyl-reactions, 5, 268 Pyrazole, 4-(2 -thienyl)-nitration, 5, 238 Pyrazole, 4-(3 -thienyl)-nitration, 5, 238 Pyrazole, trifluoromethyl-synthesis, 5, 284... 

From silylated iV-sulfamoyl-protected imidazoles or pyrazoles, imidazolyl or pyr-azolyl anions can be generated with the strong base CsF (carbodesilylation) and subsequently treated with electrophiles. In this way 5-substituted imidazoles or pyrazoles can be prepared after the deprotection of N(l) [181... 

An efficient route to 4-aryloxy pyrazoles 74 bearing a trifluoromethyl group has been developed from 4-hydroxypyrazole 72 under basic conditions with 3,5-dicyanofluorobenzene 73 with concomitant removal of the silyl group to give pyrazoles 75 <06SL1404>. Fries-type... 

Figure 12 shows the reaction profile for the hydrosilylation process involving the most stable fi3-sily 1-ally 1 complex, 10a-anti, calculated with model B. Examination of the reaction profile suggests that the rate determining step of the catalytic cycle is the reductive elimination. More specifically, the transfer of the silyl moiety to the (J-carbon of the styrene. Since recoordination of the pyrazole ligand occurs in this step, it is possible that enhancement of this ligands ability to recombined with the Pd center may lead to improved activities. 

Metalation of 2 with lithium diisopropylamide (LDA) generates diazo(trimethyl-silyl)methyl hthium (3), which reacts with a,p-unsaturated nitrile 36 and phenyl-sulfones (37) to form 3(or 5)-trimethylsilyl-l//-substituted pyrazole 4 that can be desilylated to furnish pyrazoles 5 (Scheme 8.3). 

D Synthesis of Silylated Heterocycles 1 Ring Formations a) Pyrazoles... 

The desilylation of C-silylpyrazoles can be achieved selectively the replacement of silyl groups in 4-positions by hydrogen is smoothly effected by concentrated sulfuric acid. If concentrated nitric acid is used instead, 4-nitro-3(5)-TMS-pyrazole (344) can be isolated (Scheme 47)205 ... 

Alkyl- or aryl-substituted pyrazole 1-oxides 94 can be obtained in acceptable yields by oxidative cyclization of O-silylated 3-oximimines like 1 -tert-hutyldimethyIsilyloxy-4-methylamino-1 -azab nta-1,3-diene 93 using copper(II) sulfate as the oxidant and pyridine and acetonitrile as the solvent. The oximimines are prepared from 1,3-dicarbonyl compounds 92 in a one-pot process. The method also gives access to 2-alkyl and aryl-pyrazole 1-oxides R=H devoid of substituents at the ring carbon atoms (94 Ri = R2=H) (1995JCS(P1)2773) (Scheme 27). 

O-Silylation of 2-substituted pyrazole 1-oxides also activates lateral protons at the 3- and 5-positions. Thus the 5-methyl derivative 189 upon silylation furnishes the silyloxypyrazolium ion 190, which is deprotonated at the methyl group by PMP giving rise to a neutral species 191. Next, the iodide ion replaces the trimethylsilyloxy group of the intermediate 191 in an allylic type substitution to give iodomethyl-substituted pyrazole 192. The whole sequence 189 -> 192 takes place in one pot (1992JCS(P1)2555) (Scheme 54). 

A-Methyl-/V-nitroso-a-(trimethylsilyl)benzylamine reacts thermally with DMAD to give 3,4-dicarbomethoxy-l-methyl-lH-pyrazole. The reaction is highly sensitive to the temperature upon increasing the temperature from 25 to 110 °C, the reaction time drops from 168 h to 3 min and yields increase from 30 to 100%. No reaction occurs from non-silylated /V-methyl-/V-nitrosobenzylamine. Other acetylenic esters have been successfully tested. In contrast, phenylacetylene gives a poor yield of the adduct and diphenylace-tylene does not react.473... 

Silylation of benzyl C-H bonds using hydrosilanes can also be performed with the aid of Ru3(CO)12-catalyst (Table 1) [9]. This silylation occurs only at benzylic CH3 groups. Pyridine, pyrazole, and hydrazones function as good directing groups. Benzylamines, oxime ethers, dimethylanilides, and aryl pyridyl ethers have no activity in this silylation. 

Silylation can alternatively be used to protect readily lithiated positions. For example, deprotonation and silylation of the pyrazole 105 with LDA/Me3SiCl gives an intermediate 106 which is readily transmetallated, functionalised and desilylated.95... 

Straightforward 5-lithiation results using -BuLi with ethyl 3-mcthoxy-l -methyl-17/-pyrazol-4-earboxy late and halides, zincates, or boronic acids can thus be produced <2002SL769>. A neat device allows 3-lithiation of indazole silylation at N(2) allows the desired 3-metallation (Scheme 60) <2006EJ02417>. 

Diazoacetic acid silyl esters can be prepared by fra t-esterification of tert-butyl diazoacetate with trialkylsilyl triflate <1985JOM33>. Analogously prepared (alkenyloxy)silyl 203 and (alkynyloxy)silyl diazoacetates 206 underwent silicon-tethered 1,3-dipolar cycloaddition reactions as shown in Scheme 37 and Equation (38). Compound 205 resulted from a lateral criss-cross cycloaddition of the intermediate azine 204, which was formed from two molecules of 203 by diazo + diazo or diazo + carbene reaction <2000T4139>. On the other hand, when silyl diazoacetates 206 were kept in xylene at 142 °C for 1 h, bicyclic pyrazoles 207 were obtained (Equation 38). 

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