Contrasting thermal reactions of allylic sulfilimines and phosphinimines

Allylic sulfilimines and phosphinimines undergo spontaneous but contrasting rearrangements at low temperatures. Whereas a sulfilimine is converted to an isomeric sulfenamide by (2,3)-sigmatropic rearrangement, a structurally analogous phosphinimine undergoes an unusually facile double bond shift to form an isomeric vinylic structure.     

A new synthesis of gamma-lactams based on the reaction of vinyl sulfilimines with dichloroketene

The reactions of several aryl-, furanyl-, and vinyl substituted sulfilimines with dichloroketene proceeded at 25 degrees C to yield thioalkyl substituted gamma-lactams which, in turn, were converted to a variety of nitrogen-containing substrates. [reaction: see text]     

A New Type of Claisen Rearrangement Involving 1,3-Dipolar Intermediates. Preliminary communication

During the reaction of allyl ethers, allyl sulfides and allyl selenides with in situ prepared dichloroketene ( 1 ), 2 competing pathways are observed. Besides [2+2]-cycloaddition, an unprecedented [3,3]-sigmatropic (Claisen) rearrangement via a 1,3-dipolar intermediate takes place. It leads to O-, S- or Se-esters of α,α-dichloro-γ, δ-unsaturated acids containing an inverted allylic group. Starting from cyclic n -membered α-vinyl-substituted ethers, lactones with n+4 -membered rings are formed. A very facile synthesis of the naturally occurring macrolides (±)-phoracantholide I ( 10 ) and (±)-phoracantholide J ( 11 ) illustrates the synthetic utility of this new ‘ketene’ Claisen rearrangement.     

Factors controlling the alkyne prins cyclization: the stability of dihydropyranyl cations

The relative stability of the intermediates involved in the alkyne Prins cyclization and the competitive 2-oxonia-[3,3]-sigmatropic rearrangement was studied. This rearrangement was shown to occur slowly under typical alkyne Prins cyclization conditions when the allenyl oxocarbenium ion that results from the rearrangement is similar to or higher in energy than the starting alkynyl oxocarbenium ion. The formal 2-oxonia-[3,3]-sigmatropic rearrangement may be disfavored by destabilizing the resultant allenyl oxocarbenium ion or by stabilizing an intermediate dihydropyranyl cation. The trimethylsilyl group as a substituent at the alkyne and electron-withdrawing groups (CH2Cl and CH2CN) located at the alpha-position to the carbinol center are shown to be effective. DFT calculations suggest that these substituents stabilize the dihydropyranyl cations, thus leading to a more uniform reorganization of the electronic density in the ring, and do not have a direct effect on the formally positively charged carbon atom.     

STRUCTURE,STEREOCHEMISTRY, AND MECHANISM OF FORMATION OF N-ACYLSULFILIMINES

Abstract

The methods for the preparation of sulfilimines are summarized, and the stereochemistry and electronic structure of N-acylsulfilmines are discussed. Sulfilimines have a non-planar structure, but the S^IV[sbnd]N bond character cannot be determined from their resolvability. N-Sulfonylsulfilimines (RR'SNSO₂Q) contain a delocalized S^IVNS^VI d bond system, the S^IVN and S^VIn bonds are of intermediate bond order. S^IV[sbnd]CH₃ hyperconjugation causes strong bond shortening. An asymmetric conformation of a sulfilimine containing two identical S^IV substituents was found in the crystalline state which could be accounted for theoretically. The rotation of the SN bond is not restricted in solution. The structure of N-carboacylsulfilimines (RR'SNCOQ) is strongly polar due to the strong NCO conjugation, and the instability of these compounds can be ascribed to the polar structure. The reaction between thioethers and chloramine-T is a nucleophilic substitution; the rate of the reaction is influenced by the nucleophilic character of the thioether sulfur atom. Thioether carboxylic acids are usually oxidized by chloramine-T and only give a sulfilimine if the carboxyl group is fixed sufficiently distant from the sulfur atom. The formation of sulfilimines from an optically active sulfoxide by the action of a sulfonamide (or N-sulfinylsulfonamide) proceeds via a trigonal bipyramidal intermediate with complete inversion. The intermediate of the sulfilimine synthesis starting from arylsulfonyl azides is a sulfonyl imene.     

Thermal sigmatropic rearrangements of isopyrazoles and pyrazolenines to pyrazoles

Thermal rearrangement of 4-allyl-, 4-dimethylallyl-, and 4-propargyl-isopyrazoles proceed by [3,3]-sigmatropic processes to pyrazoles. The migration terminus is the C(3) Me group, if present, and the rearrangement is preceded by imine-enamine tautomerism. When enamine formation is not possible the rearrangment is diverted to nitrogen. Thermal rearrangements of 4-alkyl- and 4-benzyl-isopyrazoles also occur although at higher temperatures and evidence is presented suggesting [1,5]-sigmatropic processes are involved. Some pyrazolenine to pyrazole rearrangements involving migration of ester and phenyl groups are also reported.     

A novel, easy and mild preparation of sulfilimines from sulfoxides using the Burgess reagent

A novel preparation of sulfilimines from the corresponding sulfoxides using the Burgess reagent is described. The reaction is general to dialkyl- and aryl alkyl sulfoxides and proceeds under mild conditions in benzene. A variety of protecting groups can be introduced on the nitrogen of the sulfilimine by choosing the appropriate Burgess reagent.     

PUMMERER-TYPE REACTION OF SULFILIMINES WITH ACETIC ANHYDRIDE

Abstract

Methyl phenyl N-p-tosylsulfilimine was found to undergo Pummerer type rearrangement affording α-acetoxymethyl phenyl sulfide similar to the corresponding sulfoxide. In order to clarify the mechanism of the reaction of methyl p-substituted phenyl sulfilimines with acetic anhydride, kinetic experiments were carried out. A large negative value of activation entropy, a relatively small value of activation enthalpy and the influence of acetic acid seem to reveal that the initial intermediate, i.e., acetylated sulfilimine, receives the nucleophilic attack of acetate ion to replace imino group, and this substitution reaction is the rate determining step.     

A facile access to N-sulfonylimidates and their synthetic utility for the transformation to amidines and amides

[reaction: see text] It is shown that N-sulfonylimidates can be efficiently prepared by a three-component coupling of terminal alkynes, sulfonyl azides, and alcohols with use of a copper catalyst and an amine base. The reaction is characterized by mild conditions, high selectivity, and tolerance with various functional groups. Facile transformation of imidates to amidines was also achieved by sodium cyanide. Additionally, a protocol for the extremely efficient Pd-catalyzed [3,3]-sigmatropic rearrangement of allylic sulfonimidates to N-allylic sulfonamides has been developed.     

A direct and rapid route to α,α-difluoroacylsilanes from trifluoroethanol

α,α-Difluoroacylsilanes were synthesised directly from a range of allyl ethers of trifluoroethanol via dehydrofluorination/metallation procedures, followed by thermal [3,3]-sigmatropic rearrangement of the intermediate vinylmetals. The scope and limitations of ether synthesis and dehydrofluorination/metallation are described.     

Creation of quarternary stereocentres via [3,3]-sigmatropic rearrangement of allylic thiocyanates. A synthetic approach to (+)-myriocin

A stereoselective approach to the advanced precursor of (+)-myriocin, 3-deoxy-1,2:5,6-di-O-isopropylidene-3-methoxycarbonylamino-α-d-glucofuranose 3-C-carboxylic acid, via the [3,3]-sigmatropic rearrangement of allylic thiocyanates prepared from d-glucose is presented. From the observed results, supported by DFT calculations, we can conclude that the [3,3]-sigmatropic rearrangement of the thiocyanato group in allylic hexofuranosides is strongly influenced by the steric interaction of the 1,2-O-isopropylidene group in the transition states.     

EPR support for the ketyl radical-anion "triggered" [3,3]-sigmatropic rearrangement

New physical evidence to support a ketyl radical-anion mechanism for the [3,3]-sigmatropic rearrangement is presented. With use of EPR spectroscopy, spectra are observed that can be attributed to 8, an acyl radical-anion species resulting from a [3,3]-rearrangement; this also functions as a key intermediate in the process. The spectrum of an additional paramagnetic species resulting from further addition of tin-centered radicals to the reaction product was also observed.     

Development of the 1,2-oxaza-Cope rearrangement

A method for a concise assembly of oxazine rings based on a Lewis acid-promoted hetero-Cope rearrangement is described. The hetero-Cope rearrangement features the first examples of the [3,3]-sigmatropic transpositions involving the nitroso group. A variety of functionalized substrates have been subjected to the reaction. The described method has a strong potential for the synthesis of marine natural products containing oxazine rings.     

Trans influence on the rate of reductive elimination. Reductive elimination of amines from isomeric arylpalladium amides with unsymmetrical coordination spheres

To determine the trans effect on the rates of reductive eliminations from arylpalladium(II) amido complexes, the reactions of arylpalladium amido complexes bearing symmetrical and unsymmetrical DPPF (DPPF = bis(diphenylphosphino)ferrocene) derivatives were studied. THF solutions of LPd(Ar)(NMeAr') (L = DPPF, DPPF-OMe, DPPF-CF3, DPPF-OMe,Ph, DPPF-Ph,CF3, and DPPF-OMe,CF3; Ar = C6H4-4-CF3; Ar' = C6H4-4-CH3, Ph, and C6H4-4-OMe) underwent C-N bond forming reductive elimination at -15 C to form the corresponding N-methyldiarylamine in high yield. Complexes ligated by symmetrical DPPF derivatives with electron-withdrawing substituents on the DPPF aryl groups underwent reductive elimination faster than complexes ligated by symmetrical DPPF derivatives with electron-donating substituents on the ligand aryl groups. Studies of arylpalladium amido complexes containing unsymmetrical DPPF ligands revealed several trends. First, the complex with the weaker donor trans to nitrogen and the stronger donor trans to the palladium-bound aryl group underwent reductive elimination faster than the regioisomeric complex with the stronger donor trans to nitrogen and the weaker donor trans to the palladium-bound aryl group. Second, the effect of varying the substituents on the phosphorus donor trans to the nitrogen was larger than the effect of varying the substituents on the phosphorus donor trans to the palladium-bound aryl group. Third, the difference in rate between the isomeric arylpalladium amido complexes was similar in magnitude to the differences in rates resulting from conventional variation of substituents on the symmetric phosphine ligands. This result suggests that the geometry of the complex is equal in importance to the donating ability of the dative ligands. The ratio of the differences in rates of reaction of the isomeric complexes was similar to the relative populations of the two geometric isomers. This result and consideration of transition state geometries suggest that the reaction rates are controlled more by substituent effects on ground state stability than on transition state energies. In addition, variation of the aryl group at the amido nitrogen showed systematically that complexes with more electron-donating groups at nitrogen undergo faster reductive elimination than those with less electron-donating groups at nitrogen.     

[3,3]-Sigmatropic rearrangement of allyl and 2-butenyl 1-naphthyl sulfides

The [3,3]-sigmatropic rearrangement of alkenyl 1-naphthyl sulfides in solutions with various polarities was investigated at 138–190 °C. The reaction proceeds through the formation of 2-alkenyl-1-naphthalene thiols, which subsequently undergo cyclization to compounds of the 2,3-dihydronaphtho[1,2-b]thiophene and naphtho[1,2-b]dihydrothiopyran series. 2-Butenyl 1-naphthyl sulfide, in addition to its passing directly through a [3,3]-sigmatropic rearrangement, to a considerable extent undergoes a prior [1,3]-sigmatropic rearrangement, which ultimately leads to the formation of four cyclic products. The kinetic parameters of the rearrangement of the sulfides were determined. The more negative entropies of activation constitute evidence for the high symmetry of the transition state.Translated from Khimiya Geterotskilicheskikh Soedinenii, No. 5, pp. 611–614, May, 1981.     

Stereocontrolled formation of amino acids and N-heterocycles bearing a quaternary chiral carbon

[STRUCTURE: SEE TEXT] Stereocontrolled formation of tertiary or quaternary chiral carbons bearing nitrogen was achieved using the [3,3]-sigmatropic rearrangement of cyanate to isocyanate as a key element. A short and highly selective sequence of reactions, starting from p-menthane-3-carboxaldehyde, was developed leading to alpha,alpha-dialkylated alpha-amino acids or N-heterocycles, depending on the method of cleavage of the auxiliary.     

Competing Diels-Alder reactions of activated nitroethylene derivatives and [3,3]-sigmatropic rearrangements of the cycloadducts

Diels-Alder reaction of nitroethylene derivatives with cyclohexa-1,3-diene afforded three pericyclic products some of which could be converted to others via a new [3,3]-sigmatropic rearrangement or via a Claisen rearrangement.     

Sterically biased 3,3-sigmatropic rearrangement of azides: efficient preparation of nonracemic alpha-amino acids and heterocycles

[reaction: see text] Homochiral alpha-amino acids, heterocycles, and carbocycles are efficiently constructed via a short sequence of reactions starting from the chiral auxiliary p-menthane-3-carboxaldehyde. The key feature of the sequence is a highly selective tandem Mitsunobu/3,3-sigmatropic rearrangement of hydrazoic acid that procures enantiomerically enriched allylic azides. The sequence is either terminated by oxidative cleavage to provide amino acids or by ring-closing metathesis to provide heterocycles or carbocycles bearing nitrogen.     

N-Hydroxy indoles as flexible substrates in rearrangements—a novel reaction with activated triple bonds

A novel rearrangement of N-hydroxy indole derivatives obtained from addition of N-hydroxy indoles to the activated triple bonds of alkynes was found to coexist with 3,3-sigmatropic rearrangements to the indolic ring. A mechanism involving an intermediate oxaziridinium ring rationalizes the transformation.     

Mechanism of the fischer reaction. Effect of electronic factors on the kinetics of the rearrangement of m-substituted cyclohexanone arylhydrazones to tetrahydrocarbazoles

The kinetics of the thermal and acid-catalyzed Fischer indolization of m-substituted cyclohexanone arylhydrazones were studied. It was shown that substituents with different natures (-CH₃, -Cl) and the polarity of the solvent have little effect on the rate of the rearrangement. The results obtained were interpreted within the framework of a concerted mechanism for the formation of the carbon-carbon bond ([3,3]-sigmatropic rearrangement).Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1207–1212, September, 1985.