Samuel Braverman

Cycloaromatization and cyclodimerization of bridged diallenes

Aus dem Ather (I) erhalt man mit Dibromcarben (II) das Bis-Addukt (III), das mit Methyllithium zum Diallenyl-Derivat (V) und weiter zum Furan (VI) reagiert.

The [2,3]-sigmatropic rearrangement of propargyl benzenesulphinates to allenyl phenyl sulphones☆

Abstract α-Methyl-, α-phenyl-, α,α -dimethyl-, and α-ethyl-α-methylpropargyl benzenesulphinates were found to undergo thermal rearrangement in high yields to sulphones, accompanied by a simultaneous acetylene-allene isomerization. The allenic sulphones produced by the rearrangement of the α-monosubstituted propargyl esters underwent further rearrangement under the reaction conditions, to γ-substituted propargyl phenyl sulphones, by way of a base-catalyzed [1,3] - prototropic shift. A kinetic study of the rearrangement reaction was carried out using two different esters. This study revealed that the rearrangement exhibited a relatively low sensitivity to the effect of solvent ionizing power and substituents. These data and other pertinent evidence suggest a concerted [2,3] - sigmatropic rearrangement.

A novel synthesis of isocyanates and ureas via β-elimination of haloform

Abstract A novel synthesis of isocyanates via base-induced β-elimination of haloform N-monosubstituted trihaloacetamides is described. The rate of reaction exhibits a strong dependence on the nature of the trihalomethyl group. Thus, while the reaction of tribromoacetamides proceeds at room temperature and the reaction of trichloroacetamides requires heating in polar solvents, no reaction could be observed for any of the corresponding trifluoro derivatives. This novel β-elimination of haloform from stable and readily available trihaloacetamides was applied to a “one-pot” synthesis of ureas which avoids the use of phosgene and isolation of isocyanates.

A novel synthesis of monosubstituted sulfines via an unusual β-elimination of chloroform from allylic and benzylic trichloromethyl sulfoxides

Abstract A new method for the synthesis of thioaldehyde S-oxides by base-induced elimination of chloroform from allylic and benzylic trichloromethyl sulfoxides is described. The reaction proceeds smoothly under mild conditions. A mechanism for this remarkable sulfine synthesis and apparently unprecedented β-elimination of chloroform is presented.

Base catalyzed rearrangement of π-conjugated sulfur and selenium bridged propargylic systems

Abstract A series of novel π-conjugated bis-propargylic sulfides, selenides, sulfoxides and sulfones have been prepared. In the presence of amine bases these compounds underwent facile isomerization to the corresponding allenes, followed by a tandem cyclization and aromatization of the latter via a diradical intermediate. While the reactions of sulfones and sulfoxides were almost spontaneous and proceeded in practically quantitative yield, the sulfides and selenides exhibited modest reactivity and required more polar solvents to afford the corresponding thiophenes and selenophenes. Some mechanistic studies and DNA cleaving properties are also presented.

[2,3]Sigmatropic Rearrangements of Propargylic and Allenic Systems

[2,3]Sigmatropic rearrangements of propargyl sulfonium ylides along with recent advances in the correspondingsulfenate-sulfoxide and sulfinate-sulfone type rearrangements are reviewed. Some new additionalexamples of these types of transformations, including rearrangements of propargylic dialkoxy disulfidesare also surveyed.

Facile synthesis and rearrangement of propargylic trifluoromethanesulfinates

Abstract The synthesis and reactivity of propargylic trifluoromethanesulfinates under various conditions have been investigated. These esters readily undergo [2,3]-sigmatropic rearrangement to the corresponding allenyl trifluoromethyl sulfones, even under solvolytic conditions. An unusually facile nucleophilic addition of the solvent to the allenyl sulfone under the latter conditions has also been observed.

Tandem rearrangement, cyclization and aromatization of sulfur bridged propargylic systems

Abstract The reactivity of some novel π-conjugated bis-propargylic sulfides, sulfoxides and sulfones under basic conditions has been investigated. These compounds undergo isomerization to the corresponding diallenes, followed by a tandem cyclization and aromatization of the latter via a diradical intermediate. Surprisingly, we have found that the rate of the cyclization step was independent of the nature of the bridging functionality.

The rearrangement of benzylic trichloromethane-sulfenates to sulfoxides and chlorides☆

Abstract While benzyl trichloromethanesulfenate undergoes no rearrangement to sulfoxide even at high temperatures, the corresponding anisyl ester rearranges to p -anisyl trichloromethyl sulfoxide in hexane under mild conditions. Substitution of hexane by chloroform under similar conditions, lead to the formation of p -anisyl chloride and dichlorosulfine as main reaction products. This process is enhanced by the use of more polar solvents and higher temperatures. The conversion of trichloromethanesulfenate to chloride has also been observed with the benzyl and benzhydryl esters on heating in various solvents, though at very differing rates. Both rearrangements are suggested to take place by an ionization mechanism. Depending on the reaction conditions and nature of the substrate, the sulfenate anion can either recombine with the cation to give sulfoxide, or further dissociate to dichlorosulfine and chloride ion, which gives the benzyl chloride. The observation of an S N 1 type mechanism for rearrangement of sulfenates appears to be unique.

Ramberg-Bäcklund rearrangement vs. β-Elimination of haloform from trichloro and trifluoromethyl sulfones

Abstract A new and convenient method for the preparation of trichloro and trifluoromethanesulfinates is described. These esters readily undergo rearrangement to the corresponding sulfones at room temperature, in high yields. In contrast to trichloromethyl sulfoxides which undergo base-induced β-elimination of chloroform to sulfines, the corresponding sulfones undergo an unusually facile Ramberg-Backlund rearrangement with formation of dichloromethylene products. Replacement of CCl3 by CF3 results in complete loss of reactivity, even under drastic basic conditions.