Valentin D. Gvozdev

A highly delocalized triplet carbene, 5-Methylhexa-1,2,4-triene-1,3-diyl: matrix IR identification, structure, and reactions.

The first representative of highly delocalized triplet carbenes bearing both vinyl and ethynyl groups at the formal carbene center, 5-methylhexa-1,2,4-triene-1,3-diyl, has been generated in a low-temperature Ar matrix upon UV photolysis of 5-ethynyl-3,3-dimethyl-3H-pyrazole and detected by FTIR spectroscopy. The transformation of 3H-pyrazole into the carbene proceeds in two stages via intermediate 3-diazo-5-methylhex-4-en-1-yne. According to DFT PBE/TZ2P calculations, 5-methylhexa-1,2,4-triene-1,3-diyl possesses an effective conjugation along the five-carbon chain and shows the same type of the bond length alternation as the HC(4m+1)H-type polyacetylenic carbenes. The carbene readily reacts with molecular oxygen, producing carbonyl oxides, which undergo further transformations typical of this type of compound upon irradiation in the UV-visible region. Two major photolytic rearrangements of 5-methylhexa-1,2,4-triene-1,3-diyl represent reactions characteristic of vinyl carbenes and resulting in the formation of 1-ethynyl-3,3-dimethylcyclopropene and 3E-2-methylhexa-1,3-dien-5-yne. A minor reaction is that typical of ethynylcarbenes; this leads to the formation of singlet 2-(2-methylpropenyl)cyclopropenylidene. Fragments of singlet and triplet potential energy surfaces of the C(7)H(8) system have been explored in DFT PBE/TZ2P calculations.

Synthesis of bicyclic N,N-enaminals by cyclization of alk-4-ynals with aliphatic diamines in DMSO upon treatment with KOH

A cyclization of alk-4-ynals with aliphatic diamines in DMSO upon treatment with KOH was found to lead to bicyclic N,N-enaminals. The studies of this reaction showed that 1,3-diaminopropane and N-methyl-1,3-diaminopropane gave (E)-6-(arylmethylidene)octahydropyrrolo[1,2-a]pyrimidines in 45—78% yields, whereas 1,2-diaminoethane gave 5-(arylmethylidene)hexahydropyrrolo[1,2-a]imidazoles as mixtures of E- and Z-isomers in up to 75% total yield. The mechanism of these new cascade cyclization reactions includes formation of the equilibrium mixtures of imines and cyclic aminals with subsequent intramolecular hydroamination of the triple bond having considerable ionic character.

Alkynylhalocarbenes. 4. Generation of (alk-1-ynyl)halocarbenes from 3-substituted 1,1,1,3-tetrahalopropanes under the action of bases

When treated with KOH under phase-transfer catalysis or with ButOK, 3-substituted (alkyl or phenyl) 1,1,3-tribromo-1-fluoropropanes 1a—c exclusively generate previously unknown (alk-1-ynyl)fluorocarbenes 5a—c, which react with olefins to give 1-(alk-1-ynyl)-1-fluorocyclopropanes 6a—h in 12—69% yields. Under analogous conditions, 3-alkyl- and 3-aryl-3-bromo-1,1,1-trichloropropanes 2a—c selectively afford (alk-1-ynyl)chlorocarbenes 7a—c, which are trapped by olefins to form the corresponding 1-(alk-1-ynyl)-1-chlorocyclopropanes 8a—k in 35—70% yields. (Phenylethynyl)chlorocarbene 7a is also selectively generated from 1,1,1,3-tetrachloro-3-phenylpropane (3a) upon its treatment with ButOK. With an excess of 2,3-dimethylbut-2-ene or 2-methylpropene, carbene 7a yields 1-chloro-1-(phenylethynyl)cyclopropanes 8a or 8c, respectively. In contrast, 1,1,1,3-tetrachloroheptane 3b and 3-alkyl- and 3-phenyl-1,1,1,3-tetrabromopropanes 4a,c,f react with bases in the presence of olefins to give, along with the corresponding 1-(alk-1-ynyl)-1-halocyclopropanes 8a,c,d and 11a—f, vinylidenecyclopropanes 12a,c—g, which suggests the generation, under these conditions, both (alk-1-ynyl)halocarbenes 7b and 9a—c and vinylidenecarbenes 10 and 11a—c. The composition and structures of intermediate products in the reactions of tetrahalides 1b, 2a, 2b, 3a, and 3b with ButOK were determined and the mechanisms for carbene generation in these reactions were proposed.

A new synthesis of bicyclic N,O- and N,S-enaminals by the anionic cyclization of alk-4-ynals with amino alcohols and amino thiols

A reaction of alk-4-ynals with aliphatic amino alcohols or 2-aminoethanethiol in the system DMSO—KOH gives bicyclic N,O- and N,S-enaminals: 6-methylidenehexahydro-2H-pyrrolo[2,1-b][1,3]oxazines, 5-methylidenehexahydropyrrolo[2,1-b]oxazoles, or 5-methyl-idenehexahydropyrrolo[2,1-b]thiazoles. The reaction proceeds through the formation of equilibrium mixtures of the corresponding imines and monocyclic aminals with subsequent 5-exodig-cyclization catalyzed by the superbasic system DMSO-KOH.

Cyclization of alk-4-ynals with o-diaminoarenes as a selective one-pot synthesis of arylmethylidene-substituted 2,3-dihydro-1H-pyrrolo[1,2-a]benzimidazoles and 7,8-dihydro-6H-pyrrolo[1’,2’:1,2]imidazo[4,5-b]pyridines

A one-pot synthesis of arylmethylidene-substituted 2,3-dihydro-1H-pyrrolo[1,2-a]benzimidazoles based on the reaction of alk-4-ynals with 1,2-diaminobenzenes in DMSO at sequential catalysis with NH4Br and bases was suggested. The use in these processes of KOH as a base led to the selective formation of E-isomers of the final products in 48–66% yields, whereas less basic K2CO3 gave the corresponding Z-isomers in 32–82% yields. Similar cyclization reactions involving 2,3-diaminopyridine gave 7,8-dihydro-6H-pyrrolo[1’,2’:1,2]imidazo[4,5-b]pyridines in up to 56% yields. The distinguishing feature of the processes under study is proceeding of their key step that is intramolecular hydroamination of the triple bond, as a 5-exo-dig-cyclization. The necessity for the application of drastic conditions required for these processes is in agreement with the results of quantum chemical calculations of PES of their most probable rate-determining step, namely, the cyclization of the corresponding benzimidazolide anions to cyclic vinyl anions.

(Alk-1-ynyl)organylthiocarbenes: generation and reactions with olefins

Abstract1-Substituted 1-chloro-3-organylthiopropa-1,2-dienes 1a–f belonging to previously unknown type of allenic compounds were synthesized by chlorination of 1-organylthioalk-1-ynes 3a–f with N-chlorosuccinimide. The reactions of compounds 1a–f with ButOK in hexane at −20 °C are accompanied by γ-elimination of HCl to give new alk-1-ynyl(organylthio)carbenes 2a–f, which add to olefins to form the corresponding 1-(alk-1-ynyl)-1-organylthiocyclopropanes 5 in yields of up to 60%. The electrophilic properties of carbenes 2 were confirmed experimentally.

Stereoselective functionalization of 1-alkoxy-2-(phenylethynyl)cyclopropanes via lithiation and subsequent reactions with electrophilic reagents

Abstracttrans-1-Alkoxy-2-(phenylethynyl)cyclopropanes undergo lithiation at the hydrogen atom in the α-position to the triple bond on treatment with BuLi in THF at–(65—70) °C. The resulting organolithium derivatives react with acetaldehyde, acetone, dimethyl disulfide, and methyl chloroformate giving the corresponding alcohols, sulfides, and esters with the yields up to 69% with complete retention of cyclopropane ring stereoconfiguration. The obtained methyl 3-alkoxy-2,2-dimethyl-1-(phenylethynyl)cyclopropanecarboxylates and the corre-sponding acid readily undergo ring opening with addition of water molecule or HCl.