Diels-Alder reactions of masked o-benzoquinones with acrylonitrile

Regioselective Diels-Alder reactions of masked o-benzoquinones (MOBs) 2a-i derived from the corresponding 2-methoxyphenols 1a-i with acrylonitrile leading to highly functionalized bicyclo[2.2.2]octenone derivatives in high yields are described.     

Facile preparation of bicyclo[2.2.2]octenone derivatives via Diels-Alder cycloadditions of in situ-generated masked o-benzoquinones

Diels-Alder cycloadditions of in situ-generated, substituted 2,2-dimethoxycyclohexa-3,5-dienones with olefinic dienophiles resulted in the development of an efficient method for the preparation of highly functionalized bicyclo[2.2.2]oct-5-en-2-ones with good to excellent yields.     

Regio- and stereoselectivities in Diels-Alder cyclodimerizations of orthoquinonoid cyclohexa-2,4-dienones

The [4+2] cyclodimerization of cyclohexa-2,4-dienone derivatives of the orthoquinone monoketal and orthoquinol types has been the topic of numerous investigations over the last 50 years in the aim of rationalizing the extraordinary level of regio-, site-, and stereoselectivities observed in these processes. In particular, the double diastereo-π-facial differentiation expressed in cyclodimerizations of chiral orthoquinols (i.e., 6-alkyl-6-hydroxycyclohexa-2,4-dienones) is an important aspect of these transformations, for they relate to the construction of several natural products. The experimental and theoretical results that are described in this article offer a comprehensive understanding of the factors controlling these site-specific regio- and diastereoselectivities. Our interpretation of these results relies on a combination of Woodward-Hoffmann and Salem-Houk secondary orbital interactions and Cieplak-type hyperconjugative effects in bispericyclic C₂-symmetric transition states.     

Photolytic decarbonylation of o-benzoquinones

It was established that photodecarbonylation of o-benzoquinones occurs by irradiation not only by UV-light, but visible light (λ>520 nm) too. Study of the series of 4,5-di-substituted 3,6-di-tert-butyl-o-benzoquinones detected that the only product of photoreaction is the corresponding 3,4-di-substituted 2,5-di-tert-butyl-cyclopentadienone, which is formed in a yield close to quantitative. NMR monitoring of reaction of photodecarbonylation of o-benzoquinones detected that this is a two-stage process. In the first stage the photoexcited molecule of quinone rearranges into bicyclo compound (bicyclo[1.3.0]hexa-3-en-2,6-dione) containing five- and three- membered cycles, which spontaneously decomposes during the following dark stage into cyclopentadienone and a molecule of CO.     

Novel synthesis of o-naphthothiophenequinone derivatives via regioselective Diels-Alder reaction

A novel procedure to construct o-naphthothiophenequinones has been achieved from readily available o-benzothiophenquinones and N-dienes via Diels-Alder reaction-aromatization sequence as key steps. The absolute regioselectivity was established via Diels-Alder reaction of o-benzothiophenquinones with rich electron N-dienes.     

Formation of 2,6-dioxabicyclo[3.3.0]-octa-3,7-dienes or multiply substituted o-benzoquinones from reactions of 1,4-dilithio-1,3-dienes with dimethyl oxalate

Reactions of 1,4-dilithio-1,3-dienes with dimethyl oxalates afforded multiply substituted o-benzoquinones or stereodefined 2,6-dioxabicyclo[3.3.0]-octa-3,7-dienes in good yields.     

Formation of reactive o-quinone methides from the reaction of trimethylsilyl(methyl)-substituted 1,4-benzoquinones with nucleophiles

o-Quinone methides are formed from the reaction of nucleophiles with trimethylsilyl(methyl)-1,4-benzoquinones. These reactive intermediates are trapped by excess nucleophile to form substituted quinones following oxidation. In addition, varying amounts of a symmetrical dimer and a xanthen derivative were observed. The influence of different nucleophiles and ring substituents on the rate of reaction have been studied, and are consistent with rate-limiting formation of a vinylogous enolate initiated by attack of the nucleophile on the silyl group.     

An efficient route to 4-aryloxycoumarins via one-pot reactions of 4-hydroxycoumarins with hypervalent iodine reagents

Highly efficient reactions of 4-hydroxycoumarin with hypervalent iodine reagents under mild conditions are described, which give rise to 4-aryloxycoumarins in good to excellent yields.     

Efficient synthesis of o-alkynyl-N-pivaloylanilines from o-acyl-N-pivaloylanilines and lithium trimethylsilyldiazomethane

Reaction of o-acyl-N-pivaloylanilines with lithium trimethylsilyldiazomethane efficiently gave the corresponding o-alkynyl-N-pivaloylanilines via alkylidenecarbene intermediates.     

Reversal of the regioselectivity in a cycloaddition of o-quinones by varying the position of alkoxy substituents

We have investigated the regioselective cycloaddition of o-quinones 1b-e with the protected sinapyl alcohol 2. It was found that the position of the alkoxy substituent on the o-quinone ring controlled the regioselectivity of the cycloaddition. In addition, our reported procedure for determining the location of the side chains on 1,4-benzodioxanes has been improved.     

The first application of an imidazole o-quinodimethane in Diels-Alder reactions leading to the synthesis of benzimidazoles

An efficient procedure for the generation of the imidazole-4,5-quinodimethane intermediate 4 from 2-bromo-4,5-bis(bromomethyl)imidazole derivative 3 in boiling toluene in the presence of 18-crown-6 is described. o-Quinodimethane 4 was captured for the first time by several symmetrically and unsymmetrically substituted dienophiles to afford the corresponding Diels-Alder benzimidazole adducts.     

Group 4 ansa-metallocenes derived from o-carborane and their luminescent properties

Luminescent group 4 complexes derived from o-carborane, M(η⁵:η¹-CpCMe₂CB₁₀H₁₀C)₂ (M = Ti; 1, Zr; 2, Hf; 3) and Me₂Si(η⁵-Ind)[η⁵:η¹-Cp-3-(CMe₂CB₁₀H₁₀C)]ZrCl (4) were explored. While the Ti complex 1 possessing C₂ molecular symmetry shows non-centrosymmetry in the unit cell as identically observed in 2 and 3, the double-ansa Zr complex 4 crystallizes in centrosymmetric P2₁/n space group despite its C₁ molecular symmetry. The non-centrosymmetric crystals of 2 and 3 exhibit blue mechanoluminescence similar to their solution and solid state photoluminescence. In contrast, 1 is lacking mechanoluminescence in crystal and shows much weaker emission than other congeners in solution. The electroluminescence devices based on 2 or 3 doped poly(N-vinylcarbazole) (PVK) as an emitting layer display green electroluminescence, which turned out to be originated from the exciplexes formed between the complex and PVK host. Although the double ansa-Zr complex 4 does not show mechanoluminescence owing to the centrosymmetric nature, it exhibits good electron-transporting properties when incorporated into the electron-transporting layer of Alq₃-based OLEDs.     

Dehalogenation of o-dihalogen substituted arenes and α,α′-dihalogen substituted o-xylenes with lanthanum metal

It was found that lanthanum metal caused the dehalogenation of o-dihalogen substituted arenes and α,α′-dihalogen substituted o-xylenes to generate the corresponding benzynes and o-quinodimethanes. When o-dihalogen substituted arenes were allowed to react with lanthanum metal in the presence of dienes, the Diels-Alder products between benzyne and dienes were formed in moderate to good yields. Similarly, the Diels-Alder adducts of o-quinodimethane with dienophiles were obtained, in the reaction of α,α′-dibromo-o-xylenes with lanthanum metal in the presence of dienophiles.     

Diels-Alder reactions of pyridinone o-quinodimethanes generated from substituted sulfolene[3,4-c]pyridin-4(1H)-ones

1-Benzyl-3-(bromomethyl)-2(1H)-pyrazinone was converted to [3,4-c] sulfolene pyridinone 8a and further (1- or 3-) substituted derivatives having a dienophilic side chain on the sulfolene ring. Thermolytic extrusion of sulfur dioxide from o-QDM precursor 8 led to generation of 3,4-dimethylene-2(1H)-pyrazinone 9, which was reacted in situ with various dienophiles. Thermolysis of the substituted precursors resulted in intramolecular cycloaddition of the corresponding o-QDM intermediates     

Enantioselective organocatalytic aryloxylation of aldehydes with o-quinones

An enantioselective organocatalytic inverse electron demand hetero Diels-Alder reaction of in situ generated enamines with o-quinone reagents is reported. The method, which is optimal in wet acetonitrile at ambient temperature, provides a new and direct asymmetric route to the aryl alkyl ether motif and an alternative metal-free strategy to S_N2 substitutions with phenolate nucleophiles.     

Synthesis of 3-substituted-4-hydroxyquinoline N-oxides from the Baylis-Hillman adducts of o-nitrobenzaldehydes

The reaction of the Baylis-Hillman adducts 1b-f derived from o-nitrobenzaldehydes in trifluoroacetic acid in the presence of triflic acid (0.2 equiv.) afforded 3-substituted-4-hydroxyquinoline N-oxides 2b-e and 2a in good to moderate yields. The reaction mechanism was evidenced by the experiment with 1f, the Baylis-Hillman adduct of 2-nitrobenzaldehyde N-tosylimine, as the one involving N-hydroxyisoxazoline as the key intermediate.     

Oxidative addition reaction of o-quinones to triphenylantimony: novel triphenylantimony catecholate complexes

New catecholate Sb(V) complexes triphenyl(3,6-di-tert-butylcatecholato)antimony(V) Ph₃Sb(3,6-DBCat) (1) and triphenyl(perchloroxanthrenecatecholato)antimony(V) Ph₃Sb(^OXCat_Cl) (2) were synthesized by the oxidative addition reaction of corresponding o-quinones (3,6-di-tert-butyl-o-benzoquinone and perchloroxanthrenequinone-2,3) with triphenylantimony. Catecholates 1 and 2 can alternatively be synthesized by reacting the appropriate thallium catecholate with triphenylantimony dichloride. The oxidative addition reaction of an equimolar ratio of 4,4′-di-(3-methyl-6-tert-butyl-o-benzoquinone) and triphenylantimony yielded 4-(2-methyl-5-tert-butyl-cyclohexadien-1,5-dion-3,4-yl)-(3-methyl-6-tert-butyl-catecholato)triphenylantimony(V) Ph₃Sb(Cat-Q) (3); in the case of a 1:2 molar ratio, complex 4,4′-di-[(3-methyl-6-tert-butyl-catecholato)triphenylantimony(V)] Ph₃Sb(Cat-Cat)SbPh₃ (4) resulted. Complexes 1-4 were characterized by IR- and ¹H NMR spectroscopy. Molecular structures of 1, 2 and 4 were determined by X-ray crystallography to be a distorted tetragonal-pyramidal.     

o-Thioquinones on [2.2]paracyclophanes: an example of totally stereocontrolled hetero Diels-Alder reactions

The reaction of 4-hydroxy[2.2]paracyclophane with phthalimidesulfenyl chloride allowed the preparation of a suitable precursor for a paracyclophane-o-thioquinone. This species participates in an inverse electron demand hetero Diels-Alder reaction with different electron-rich alkenes to give the expected benzoxathiin cycloadducts with complete control of regio- and stereochemistry.     

Preparation and oxidative coupling of bis[o-(hydrosilyl)phenyl]cuprates and bis[o-(fluorosilyl)phenyl]cuprates

Bis[o-(hydrosilyl)phenyl]cuprates and bis[o-(fluorosilyl)phenyl]cuprates were prepared by reacting [o-(hydrosilyl)phenyl]lithiums and [o-(fluorosilyl)phenyl]lithiums, respectively, with copper salts, such as CuCN and Cu(OPiv)₂. The phenylcuprates underwent oxidative coupling to afford 2,2′-bis(hydrosilyl)biphenyls and 2,2′-bis(fluorosilyl)biphenyls.     

PtCl2-catalyzed reactions of o-alkynylanilines with ethyl propiolate and dimethyl acetylenedicarboxylate

The PtCl₂-catalyzed reactions between indoles and ethyl propiolate gave rise to mono and double addition products. The composition of the products was largely influenced by the substituents on the indoles as well as the amount of ethyl propiolate used. o-Alkynylanilines reacted with ethyl propiolate and dimethyl acetylenedicarboxylate under the catalysis of PtCl₂ to generate the corresponding 2,3-disubstituted indoles. The reaction proceeded by following a sequential cyclization/intermolecular addition pathway.