Triazoles silylated

On silylation with TCS 14/triethylamine then heating in chlorobenzene in the presence of triflic acid the amidrazone 1540 cyclizes to the fused 1,2,4-triazole 1541 in nearly quantitative yield [68] (Scheme 9.38). 

Substituted (5R,6A,)-6-(dimethyl(phenyl)silyl)-2-phenyldihydropyrazolo[l,2- ][l,2,4]triazole-l,3(2//,5//)-dione 716, synthesized via the [3+2] annulation of a-substituted allylic silanes 715 with PTAD, were oxidized to the corresponding hydroxy substituted urazoles 717. This work shows that allylsilanes with a single substituent at the allylic carbon undergo exclusive stereoselective [3+2] annulation (Scheme 114) <2007TL6671>. 

In a quite different approach, shown in Scheme 204, cycloaddition of nitrile 1232 to trimethylsilyldiazomethane provides silylated triazole 1233, isolated in 75% yield. Treatment with tetrabutylammonium fluoride removes the trimethylsilyl group and simultaneously the silyl protection of the carboxylic group to afford 4-substituted triazole derivative 1234 in 81% yield <2003PEN699>. 

In many cases, the yields of these products are high. However, the use of /V-silylated triazoles as nucleophiles or the use of cyclic nitroso acetals (475) substituted at the C-3 atom leads to a noticeable decrease in the yield of the oximes. Therefore, steric hindrance in nitroso acetals and a decrease in nucleophilicity of A-centered nucleophiles result in an increase in the contribution of side reactions. It should be emphasized that C -nucleophiles, such as anions of nitro compounds, are not involved in coupling reactions with cyclic nitroso acetals (475). However, the products, which formally correspond to the C,C-coupling mechanism, can be prepared by the nucleophilic substitution of chlorine in compound (476 d) by a Sa/2 mechanism (Scheme 3.254, product (483c), the yield was 79%). 

Attempted formation of the 4-silyl-substituted nucleophilic carbene (111) by deprotonation of the corresponding triazolium salt with KH led to the triazole (112), the product of apparent [1,2]-Si migration.A crossover experiment indicated that silyl transfer is intermolecular. 

Methyl- and 2-phenyl-triazole 1-oxide (207) are silylated selectively at the 5-position with trimethylsilyl triflate to afford stable 5-trimethylsilyl derivatives (208) (Scheme 35). If the 5-position is blocked with a substituent, the 4-position is silylated. Thus, 5-chlorotriazole 1-oxide is converted to 5-chloro-4-trimethylsilyl-l,2,3-triazole 1-oxide in the presence of lithium tetramethylpiperidine. 

Phenyl-1,2,3-triazole 1-oxide (209) is selectively silylated at C(5) to afford (210) by treatment with trimethylsilyl triflate and diisopropylethylamine. If the C(5) position is blocked, electrophilic attack occurs at the C(4) position <93JCS(PI)625>. 

Silyl-substituted diazoketones 29 cycloadd with aryl isocyanates to form 1,2,3-triazoles 194 (252) (Scheme 8.44). This reaction, which resembles the formation of 5-hydroxy-l,2,3-triazoles 190 in Scheme 8.43, has no analogy with other diazocarbonyl compounds. The beneficial effect of the silyl group in 29 can be seen from the fact that related diazomethyl-ketones do not react with phenyl isocyanate at 70 °C (252). Although the exact mechanistic details are unknown, one can speculate that the 2-siloxy-1-diazo-1-alkene isomer 30 [rather than 29 (see Section 8.1)] is involved in the cycloaddition step. With acyl isocyanates, diazoketones 29 cycloadd to give 5-acylamino-l,2,3-thiadiazoles 195 by addition across the C=S bond (252), in analogy with the behavior of diazomethyl-ketones and diazoacetates (5). 

Ketenes rarely produce [3+ 2]-cycloaddition products with diazo compounds. The reaction possibilities are complex, and nitrogen-free products are often obtained (5). Formation of a cyclopropanone represents one possibihty. Along these lines, the synthesis of (Z)-2,3-bis(trialkylsilyl)cyclopropanones and (Z)-2-trialkylsilyl-3-(triethylgermyl)cyclopropanones from diazo(trialkylsilyl)methanes and appropriate silyl- or germylketenes has been reported (256,257). It was found that subsequent reaction of the cyclopropanone with the diazoalkane was not a problem, in contrast to the reaction of diazomethane with the same ketenes. The high cycloaddition reactivity of diazomethylenephosphoranes also extends to heterocumulenes. The compound R2P(C1)=C=N2 (R = N(/-Pr)2) reacts with CS2, PhNCO and PhNCS to give the corresponding 1,2,3-triazole derivative (60). 

The monoadduct from norbornadiene and the bulky trimethylsilyl azide is less stable than the bisadduct and spontaneously yields a 1-silylated triazole that isomerizes to a 2-silyltriazole (Scheme 139).104... 

Another class of heterocyclic compounds are silyl-substituted triazoles which can be obtained by the addition of TMS-azide 14) and manifold substituted acetylenes 1, 35, 350 (Scheme 50)33,209 2ll Those in the first step resulting l-TMS-4-phenyl (3Ji)-, l-TMS-4,5-bis(methoxycarbonyl)- 352) and l-TMS-4-alkyl-l, 2,3-triazole 353) are then hydrolyzed to form 4-phenyl- 354)-, 4,5-bis(methoxycarbonyl)- 355), 4-alkyl-l,2,3-triazole 356). 

Substituted 1,2,3-triazole 1-oxides 326 are silylated at the oxygen atom to give moisture sensitive silyloxytriazolium salts, which have been characterized by their NMR spectra (1993JCS(P1)625, 1992JCS(P1)2555). 

Both ring positions and lateral positions - both at C4 and C5 - are activated by the O-silylation. Substituents can be introduced at the lateral 5-position by O-silylation followed by abstraction of the activated lateral proton with a weak non-nucleophilic base. The neutral species 428 formed is subject to nucleophilic allylic displacement of the silyloxy anion rendering laterally substituted triazole 429 in one pot (Scheme 121). 

In reality, 4-silyl-[l,2,3]-triazoles are the true addition products, silylaziridines being formed after decomposition with loss of nitrogen. 

Sila- and germatriazolines. Not included are triazolines and triazoles, which have only silyl, germyl, or stannyl ring substituents (cf. ref. /9a). 

Trimethylsilyl-l,2,3-triazole (140), easily obtained by silylation of 1,2,3-triazole (67CB3485), can be smoothly acylated in the 1-position to give (141) by means of acetic... 

Similarly, silylated alkjmes react with HN3 to afford TMS-substituted 1,2,3-triazoles (79CB2829). 1,2,3-Triazoles with alkenic side chains can also be synthesized from l,3-en)me hydrocarbons and their silane derivatives (78ZOB705). 

Silylated 3-nitro-1,2,4-triazole derivative 242 is readily aminated by ethylenediamine to give dimeric cytidine 243, as well by other oi-diamines. 

---