Recent advances in radical-mediated intermolecular (4 + 2) cycloaddition.

(4 + 2) Cycloaddition plays an important role in the synthesis of versatile carbocyclic/heterocyclic compounds with its high atom- and step-economy. Additionally, with mild conditions and indispensable functional group compatibility, the radical reaction has been recognized as a useful tool in organic chemistry. Given the enormous impact of radical-mediated (4 + 2) cycloaddition processes and their promising applications, we summarize and highlight the recent works in this attractive area. On the basis of the types of radicals that initiate different (4 + 2) cycloaddition processes, we classify them into processes involving alkenyl cations or alkenyl radicals, aryl radicals, acyl radicals, alkyl radicals, and heteroatom radicals, and this review places special emphasis on the reaction design and mechanisms, which will stimulate future developments in radical-mediated intermolecular (4 + 2) cycloaddition.

Applications of sulfonyl hydrazides in radical cyclization of alkenes.

Radical cyclization is regarded as a powerful and promising strategy for the assembly of diverse important cyclic structures because of its high atom- and step-economy. As excellent radical acceptors, alkenes offer two potential directions, pushing the research domain of radical cyclization. In this context, as a radical precursor, sulfonyl hydrazide plays an important role in accomplishing radical cyclization of alkenes in a facile and efficient way. This review focuses on the applications of sulfonyl hydrazides in radical cyclization of alkenes, which generally has two radical conversion modes, sulfonyl radicals and sulfoxide radicals. In particular, the section of sulfonyl radicals consists of eight parts containing aromatic rings, alkenes, alkynes, cyanides, aldehydes, carboxylic acids, amides, and small ring compounds, according to the objects of cyclization after addition with alkenes. Within each category, representative instances are presented and discussed in terms of their general mechanistic perspectives when needed.

The polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs.

The novel polychloromethylation/acyloxylation of 1,6-enynes with chloroalkanes and diacyl peroxides through dual-role designs has been developed to prepare 2-pyrrolidinone derivatives with polychloromethyl units with the use of an inexpensive copper salt under mild conditions. This strategy includes two dual-role designs, not only improving atomic utilization but also allowing a cleaner process. The wide substrate scope and simple reaction conditions demonstrate the practicability of this protocol.

Cyclization/hydrolysis of 1,5-enenitriles initiated by sulfonyl radicals in the aqueous phase in the presence of the I2/TBHP system.

A novel cyclization/hydrolysis of 1,5-enenitriles for the synthesis of valuable pyrrolidine-2,4-diones in the aqueous phase using I2 as the catalyst and tert-butyl hydroperoxide (TBHP) as the oxidant is reported. In the presence of the I2/TBHP system, sulfonyl hydrazides produce sulfonyl radicals, which undergo radical addition, intramolecular cyclization, hydrogen abstraction, and hydrolysis to give the final products. The use of the inexpensive and environmentally friendly I2/TBHP catalytic oxidation system in the aqueous phase makes it a benign and sustainable strategy.

1-(2-Cyano-eth-yl)-1H-imidazole-4,5-dicarbonitrile.

In the title tricyano-nitrile compound, C(8)H(5)N(5), the N-substituted cyano-ethyl group is offset to the imidazole ring [dihedral angle = 75.41 (15)°].

2-[(E)-(4-Fluoro-benz-yl)imino-meth-yl]-6-meth-oxy-phenol.

In the title Schiff base, C(15)H(14)FNO(2), the dihedral angle between the benzene rings is 53.32 (8)°. In the crystal, mol-ecules related by a twofold rotation axis are linked by pairs of C-H⋯O hydrogen bonds into dimers with R(2) (2)(18) ring motifs. An intra-molecular O-H⋯N hydrogen bond is also observed.

N-[(E)-4-Fluoro-benzyl-idene]-3,4-di-methyl-aniline.

In the title Schiff base, C(15)H(14)FN, the N=C bond length of 1.263 (2) Šis shorter than the N-C bond [1.426 (2) Å], indicating a typical imine double bond. Moreover, the C-N-C angle is 118.5 (2)°. The benzene rings form a dihedral angle of 51.22 (5)°.

2-[(E)-(4-Fluoro-benzyl-imino)-meth-yl]-4-methyl-phenol.

In the title Schiff base compound, C(15)H(14)FNO, the benzene rings make a dihedral angle of 72.75 (13)°. The mol-ecular structure is stabilized by an intra-molecular O-H⋯N hydrogen bond. In the crystal, weak π-π stacking occurs between the phenol rings of inversion-related mol-ecules, the centroid-centroid distance being 3.7731 (14) Å.

1,2-Bis[4-(1H-imidazol-1-yl)benzyl-idene]hydrazine.

The title compound, C(20)H(16)N(6), is centrosymmetric with the mid-point of the N-N bond located on an inversion center. The imidazole ring is oriented at a dihedral angle of 28.03 (6)° with respect to the attached benzene ring. In the crystal, molecules are linked via C-H⋯N interactions.

Poly[[hexa-aqua-(µ(3)-2,2'-bipyridine-4,4',6,6'-tetra-carboxyl-ato-κ(6)O(4):N,O(6),O(6'),N':O(4'))dinickel(II)] dihydrate].

In the title complex, {[Ni(2)(C(14)H(4)N(2)O(8))(H(2)O)(6)]·2H(2)O}(n), the two Ni(II) atoms are located in different special positions (one on a twofold rotation axis and the second on a centre of symmetry) and have different distorted octa-hedral environments (one by two N atoms from a bipyridine unit, two O atoms from two water mol-ecules and two O atoms from two carboxyl-ate groups, and the second by four O atoms from four water mol-ecules and two O atoms from two carboxyl-ate groups). Thus, the environments of the Ni(II) atoms may be denoted as NiN(2)O(4) and NiO(6). In the crystal, there exists an extensive network of classical O-H⋯O hydrogen bonds.

Bis{2,4-dibromo-6-[(E)-(4-fluoro-benz-yl)imino-meth-yl]phenolato-κ(2)N,O}cobalt(II).

The complete mol-ecule of the title complex, [Co(C(14)H(9)Br(2)FNO)(2)], is generated by crystallographic twofold symmetry, with the Co(II) atom lying on the rotation axis. The coordination of the metal atom by the two N,O-bidentate ligands results in a squashed CoN(2)O(2) tetra-hedron. The six-membered chelate ring is an envelope, with the metal atom as the flap. The dihedral angle between the planes of the aromatic rings within each ligand is 84.1 (6)°.

Bis{2,4-dibromo-6-[(E)-(4-fluoro-benz-yl)imino-meth-yl]phenolato-κ(2)N,O}zinc.

In the title Schiff base complex, [Zn(C(14)H(9)Br(2)FNO)(2)], the Zn(II) atom is located on a twofold rotation axis and is coordinated by two O and two N atoms from two symmetry-related bidentate Schiff base ligands in a compressed tetrahedral geometry. The bond lengths and bond angles are within normal ranges. The dihedral angle between the least-squares planes of the aromatic rings within each ligand is 82.76 (17)°.

1,3-Bis(3,5-dimethyl-phen-yl)-5-methyl-benzene.

In the title compound, C(23)H(24), the dihedral angles formed by the central benzene ring with the peripheral benzene rings are 29.90 (5) and 34.95 (5)°. The crystal packing is stabilized by π-π stacking inter-actions with centroid-centroid distances of 3.815 (4) Å.

1-Methyl-3,5-bis-(3-methyl-phen-yl)benzene.

In the title compound, C(21)H(20), the dihedral angles formed by the central benzene ring with the outer benzene rings are 21.43 (6) and 31.70 (4)°. The crystal packing is stabilized by a weak π-π stacking inter-action, with a centroid-centroid distance of 3.843 (3) Å.

Bis[µ-3-(1H-benzimidazol-2-yl)benzoato]dicopper(I).

The dimeric title complex, [Cu(2)(C(14)H(9)N(2)O(2))(2)], resides on a center of symmetry. In the crystal, the mol-ecules are packed via π-π stacking inter-actions alternating between imidazole and benzene rings [mean inter-planar distances = 3.754 (3) and 3.624 (3) Å]. An inter-molecular N-H⋯O hydrogen bond links the dimers together. The two-coordinate Cu(I) atom displays an O-Cu-N bond angle of 176.3 (2)°. The Cu⋯Cu distance within the dimer is 5.100 (2) Å.

2,4-Dibromo-6-{(E)-[(R)-1-phenyl-ethyl]imino-meth-yl}phenol.

In the title Schiff base, C(15)H(13)Br(2)NO, the benzene and phenyl rings form a dihedral angle of 75.18 (13)°. The N=C bond length of 1.263 (6) Šis shorter than of the N-C bond [1.476 (5) Å], indicating a double bond. In the crystal, there is some pseudosymmetry. This occurs because most of the two mol-ecules are centrosymmetrically related. The mol-ecular structure is stabilized by intra-molecular O-H⋯N hydrogen bonds.

2-Hydr-oxy-N'-(4-isopropyl-cyclo-hexyl-carbon-yl)-3-methyl-benzohydrazide.

The crystal structure of the title compound, C(18)H(26)N(2)O(3), is stabilized by inter-molecular N-H⋯O and O-H⋯O hydrogen bonds. One of the methyl groups is disordered with occupancies of 0.51 (3):0.49 (3).

Erratum: 2-Hydr-oxy-N'-(4-isopropyl-cyclo-hexyl-carbon-yl)-3-methyl-benzohydrazide. Corrigendum.

The author list in the paper by Shu, Wen, Chen & Lei [Acta Cryst. (2009), E65, o575] is corrected.[This corrects the article DOI: 10.1107/S160053680900556X.].

N-(4-Fluorobenzoyl)-2-hydroxy-4-methyl-benzohydrazide.

In the title compound, C(15)H(13)FN(2)O(3), the aromatic rings are aligned at an angle of 10.15 (3)°. The mol-ecules are packed with π-π stacking inter-actions [mean inter-planar distances of 3.339 (2) and 3.357 (3) Å] and the crystal structure is stabilized by inter-molecular N-H⋯O and O-H⋯O hydrogen bonds. An intramolecular N-H⋯O interaction also occurs.

The cocrystal 2-hydr-oxy-4-methyl-N-propanoylbenzohydrazide-2-hydr-oxy-N-(2-hydr-oxy-4-methyl-benzo-yl)-6-methyl-benzohydrazide (2/1).

The asymmetric unit of the title compound, 2C(11)H(14)N(2)O(3)·C(16)H(16)N(2)O(4), contains one mol-ecule of 2-hydr-oxy-4-methyl-N-propanoylbenzohydrazide and one-half of a mol-ecule of 2-hydr-oxy-N-(2-hydr-oxy-4-methyl-benzo-yl)-6-methyl-benzohydrazide. The latter is located on a centre of inversion. Intra-molecular N-H⋯O inter-actions stabilize the conformations of both mol-ecules. The crystal structure is stabilized by inter-molecular N-H⋯O and O-H⋯O hydrogen bonds.