Ana M. González-Nogal

The silyl-cupration and stannyl-cupration of allenes

Abstract The stoichiometric silyl-cupration of allene 7, followed directly by treating the intermediate cuprate with a proton, with a range of carbon electrophiles, and with chlorine gives the vinylsilanes 8–13. Alternatively, when iodine is the electrophile, the product is the vinyl iodide 16. This can then be metallated and treated with a proton or a range of carbon electrophiles to give the allylsilanes 18–21. More-substituted allenes also undergo silyl-cupration followed by protonation, phenylallenes giving vinylsilanes, and alkylallenes giving, on the whole, allylsilanes. Stoichiometric stannyl-cupration of allenes takes place, with similar but less reliable regiocontrol to that of the corresponding silyl-cupration.

Regiospecific synthesis of 5-silyl azoles

5-Silylisoxazoles bearing other silyl groups different to the more usual trimethylsilyl have been prepared by condensation of silylalkynones with hydroxylamine hydrochloride. The reaction with hydrazines is more complex and leads to 5-silylpyrazoles or the corresponding hydrazones, which can be cyclized by reaction with electrophiles. This has allowed us to synthesize 5-silylpyrazoles functionalized at C-4 by groups impossible to introduce by electrophilic substitution of the pyrazole nucleus.

Synthesis of silylated β-enaminones and applications to the synthesis of silyl heterocycles

Abstract Silyl β-enaminones have been synthesized by reductive cleavage of silylisoxazoles. These versatile synthons bearing the silyl group in different positions of the enamino ketonic system are of great interest in the construction of a variety of penta- and hexaheterocycles, which, in general, retain the silyl group attached at the ring or in a side chain.

Regio- and Stereospecific Cleavage of α,β-Epoxysilanes with Lithium Diphenylphosphide

Abstract Trimethyl- or dimethylphenylsilylepoxides react with lithium phenylsulfide to give regio- and stereodefined vinyl sulfides resulting from α-ring opening and Peterson elimination. When the epoxide bears the bulky tert -butyldiphenylsilyl group the reaction is more puzzling. Depending on the β-substitution and the presence of aluminium chloride, we obtained silyl enol ethers, α-silylaldehydes or α-hydroxy-β-phenylthiosilanes, all resulting from β-opening.

Reactions of pyrazoles and pyrazolium salts with complex metal hydrides and organometallic reagents. Synthesis of pyrazolines and pyrazolidines

Abstract 2-Pyrazolin-4-oximes have been synthesized by reaction of 4-nitroso- and 4-nitropyrazoles with complex metal hydrides and organometallic reagents. Furthermore, 4-nitropyrazolium tetrafluoroborates are reactive substrates towards organolithium and Grignard compounds leading to new and highly substituted 4-nitro-3-pyrazolines and 4-nitropyrazolidines. The formation of 3-pyrazolines is regioselective and, when diastereoisomer pyrazolidines are possible, only the most stable 3,4-trans and/or 4,5-trans isomers were obtained.

Reactions of pyrazolium salts with silyllithium reagents. Regioselective synthesis of 5-silylated 3-pyrazolines

1,3,5-trisubstituted-2-methylpyrazolium iodides react with dimethylphenylsilyl- and tert-butyldiphenylsilyllithium leading, in general, to one of the corresponding 5-silyl-3-pyrazolines. The silyl group is easily substituted by electrophiles to give 5-functionalized 3-pyrazolines.Moreover, 5-silyl-3-pyrazolines undergo thermic ring opening with silicon-rearrangement affording α-silylated β-diimines.

THE REDUCTION OF FUNCTIONALIZED PYRAZOLIUM SALTS AS A STEREOSELECTIVE ROUTE TO FUNCTIONALIZED PYRAZOLIDINES

Abstract Functionalized pyrazolium salts 1 react with metal complex hydrides (LiAlH4 and NaBH4) to give pyrazolidines 2 with good-to-excellent levels of diastereoselectivity. The stereochemical course of the reduction is essentially anti, leading to trans,trans-pyrazolidines. The relative configuration of the crystalline pyrazolidine 2c has been determined by X-ray crystallographic analysis.

New evidence on the regiochemistry of the tert-butyldiphenylsilylcupration of allene using the silylcuprate or silylcopper reagent

Abstract The regiochemistry in the tert -butyldiphenylsilylcupration of the allene depends on the temperature and also on the nature of the electrophile. Thus, the intermediate generated by addition of the silylcuprate at −78°C reacted with electrophiles to give allylsilanes, except with oxo compounds which afforded vinylsilanes. On the other hand, the silylcopper reagent was added at −40°C leading, in all cases, to the corresponding allylsilanes. When enones were used as electrophile the vinylsilanes were the 1,2-addition products and the allylsilanes those from 1,4-addition. These functionalized vinyl or allyl tert -butyldiphenylsilanes are interesting synthons for the preparation of conjugated tert -butyldiphenylsilyltrienes and functionalized exocyclic alkylidenecyclopentenes.

Regio- and stereospecific cleavage of stannylepoxides with lithium phenylsulfide

Abstract Unsubstituted and α or β C-substituted epoxystannanes react with lithium phenylsulfide to give regio- and stereodefined α-phenylthio-β-hydroxystannanes resulting from α-opening with inversion of configuration. On the other hand, α- or β- trans -silyl epoxystannanes afford stereospecific α- or β-silylated vinylsulfides formed by nucleophilic attack at the carbon which bears the tin group and subsequent syn -elimination of HOSnBu 3 . Both types of products are interesting synthons in organic chemistry.