Wilhelm P. Neumann

New ways of genterating organotion reactive intermediates for organic synthesis

Abstract Progress in generating and applying two important organotin reactive species for organic synthesis is reported. - Stannyl radicals R 3 Sn . , at present the most used tools for free radical synthesis of complicated target molecules, are generally obtained from the hydrides R 3 SnH. In a number of cases, however, this is a drawback because of the high radical scavenging power of the latter. New ways of generating R 3 Sn . , decoupling them from the presence of R 3 SnH, are developed and discussed: the spontaneous thermal dissociation of distannanes with bulky substituents, the thermal fragmentation of bisstannyl benzpinacols, the ketone sensitized mild photolysis of simple distannanes, and the mild photolysis of benzyltin compounds such as 9-stannyl-9,10-dihydro anthracene. Examples of advantageous applications, also in combination with adjusted H donors, are given. - Stannyl cations R 3 Sn + are used as superior leaving groups in electrophilic aromatic ipso substitution. Examples for their application under very mild conditions include the considerable broadening of the scope of the Friedel-Crafts acylation, the Vilsmeier formylation, sulfinations, and sulfonations. The directing influence of certain other substituents can be overcome by this ipso substitution.

Zur papierelektrophoretischen Fraktionierung tierischer Gifte

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Synthesen mit verbindungen R3MHgMR3 und (R2MHg)n (MC, Si, Ge, Sn) : IX. Reduktive silylierung von carbonylverbindungen Mittels bis(trimethylsilyl)-quecksilber

Abstract Aromatic aldehydes are reductively dimerised by Me 3 SiHgSiMe 3 (I), yielding bis( O -silyl)hydrobenzoin derivatives. 1,2-Diketones undergo 1,4-addition with formation of bis( O -silyl)enediols, while esters of conjugated dicarboxylic acids give O -silyl ketene acetals: maleic yields mainly the C , O -bissilyl derivative (1,4-addition), whereas fumaric and ethylene tetracarboxylic esters yield mainly the corresponding O , O -bissilyl products (1,6-addition). 1,2- and 1,4-quinones are readily transformed by (I) to the corresponding bis O -silyl)benzenoid system. Because of length and the high polarisability of teh SiHg bonds, it is not surprising, that a variety of reaction mechanisms is involved here. All these reactions proceed much faster in HMPT as solvent than in benzene. The rate determining step has been shown in several cases to be a nucleophilic attack of (I): presumably this is generally valid for most of the reactions studied here. The subsequent reaction steps may involve free radicals. In other cases, however, the primary step is a radical one, e.g. with quinones. The reactivity of (I) is greatly affected by traces of impurities e.g. still present from the synthesis.

Synthesen mit verbindungen R3M-Hg-MR3 und (R2M-Hg)n (M=C, Si, Ge, Sn) XII. Darstellung von und umsetzungen mit verbindungen R3Sn-Hg-SnR3 und (R2Sn-Hg)n⋆

Summary The yellow or red compounds R 3 Sn-Hg-SnR 3 (I) (R =Me, Et, Pr, t-Bu, Ph) are prepared from t-Bu 2 Hg and 2 R 3 SnH. They are oxidized immediately by air, forming stannoxanes. At -10° they decompose giving distannanes and mercury, the stability of the Ph compound is somewhat higher, and surprisingly stable is the crystalline t-Bu compound (m.p. 196°). Stannyl residues undergo exchange reactions easily, especially with R′ 3 SnH. (I) adds to electron deficient or polar azo compounds and alkynes, presumably by way of a polar four-centre mechanism. With mercury containing intermediates, the corresponding N,N′ -bis-stannyl hydrazines or 1,2-bis-stannyl alkenes are formed. Alkynic hydrogen is substituted by R 3 SnHg residues. Several subsequent reactions are observed. The isolation of (I) is not necessary in these syntheses. It is preferable to mix the reactant with t-Bu2Hg (or R 3 SnH) and to then add slowly R 3 SnH (or t-Bu 2 Hg, respectively) at 0–20°. t-Bu 2 Hg forms an intermediate (R 2 Sn-Hg) n with R 2 SnH 2 (R=Et, Bu, i-Bu, t-Bu, cyclohexyl, Ph). (R 2 Sn-Hg) n is stable only when R=t-Bu, and decomposes immediately in all other cases to form Hg and a mixture of the cyclostannanes (R 2 Sn) n .

Die chemie der schweren carben-analogen R2M, M = Si, Ge, Sn: IX. Eigenschaften und thermolyse von neuen 7-silabicyclo[2.2.1]heptadienen☆

Abstract Factors governing the ease and mechanism of 7-silabicyclo[2.2.1]heptadienes thermolysis in order to generate free silylenes and the corresponding benzene derivatives are investigated. For this purpose, 29 new compounds of the types VII–X have been prepared. No indications for a polar mechanism or an intermediate biradical could be found. The degradation is exactly of first order in all cases investigated sofar, and is enhanced by phenyl groups at the bridgehead C atoms, if a conformation coplanar with the basic ring is allowed by the neighbouring substituents, but is not enhanced by phenyl groups at the Si. The X-ray structure of two typical derivatives is discussed with this respect. A special mechanism is operating in the easy thermolysis of carbomethoxy-substituted compounds leading to cyclic sila enolether intermediates.

Dimethyl germylene insertion into a strained C-Ge bond and matrix isolation of tetramethyl digermene Me2Ge=GeMe2

Abstract Free tetramethyl digermene Me 2 Ge=GeMe 2 is generated thermally from molten 7,8-digerma bicyclooctadienes[2.2.2] which are obtained by germylene (Me 2 Ge) insertion into the C-Ge bond of corresponding 7-germa norbornadienes. The structure of the digermene follows from MS, matrix IR and Raman spectroscopy.

Additions of free dimethylgermylene to vinyl ketones and α-diketones

Abstract Derivatives of 1-oxa-2-germacyclopent-4-ene are obtained from germylenes with a number of cyclic or acyclic vinyl ketones, and 1,3-dioxa-2-germacyclopent-4-enes with different non-enolizable α-diketones. Structure-mechanism relationships are discussed.

Synthesen mit verbindungen R3M-Hg-MR3 und (R2M-Hg)n(M = C, Si, Ge, Sn) : XI. Mercurierung und demercurierung mittels di-t-butylquecksilber, (CH3)3C-Hg-C(CH3)32

Abstract Me 3 CHgCMe 3 (I) adds to strongly polar CC groups as well as to some CC and NN groups resp., forming t-BuHg containing cis -products. Its reactivity is higher than that of Et 2 Hg and equals that of Me 3 SiHgSiMe 3 to some extent. In most examples, polar four-center transition states are assumed, but radical reactions occur, too. In the Hg containing adducts investigated, the t-BuHg group can be replaced by the Me 3 Sn group easily, by using Me 3 SnH. Hg is split off, and i-C 4 H 10 is evolved quantitatively.

[2+4] Cheletropic cycloadditions of stannylenes R2Sn (R = alkyl, amino, halogen)

Free stannylenes Me2Si(tBuN)2Sn 1a and [(Me3Si)2CH]2Sn 1b as well as SnCl2, SnBr2, and SnI2 give 1,4-cycloadditions to different 1,3-dienes yielding 1-stannacyclopent-3-enes. A singlet heavy carbene analogue behaviour is shown with the two isomers of the diallene 2 : only those stereoisomers are found to be postulated for a [2+4] cheletropic addition. In the other cases, a similar mechanism is likely. 1b acts as a donor towards E,E-1,4-disubstituted 1,3-dienes, reacting faster with electron deficient ones.

Synthesen mit verbindungen R3MHgMR3 : XV. Photolytische umsetzungen von R3MHgMR3 mit organohalogenverbindungen. SH2-reaktionen am quecksilber (M = Si, Ge)☆

Abstract Prim. alkyl halides RCl and RBr react quickly with (Me 3 M) 2 Hg (M = Si, Ge) under irradiation (daylight), forming Me 3 MHal and Me 3 MHgR. This is a radical chain reaction with S H 2 mechanism and attack of R at the Hg. Aryl bromides behave quite similarly, but continue the reaction to Ar 2 Hg. Benzyl, s- and t-alkyl radicals (arising from the corresponding halides), are, however, not able to displace Me 3 M from the Hg.