Recent advances in transition-metal-catalyzed Büchner reaction of alkynes.

Medium-sized ring-containing organic molecules, especially seven-membered rings, are significant structural motifs. However, such frameworks are considered difficult structures to access owing to entropic effects and transannular interactions. Compared to the construction of five and six-membered rings, the synthesis of seven-membered rings can be more challenging through traditional cyclization pathways. Büchner reactions are particularly attractive and efficient synthetic strategies to construct functionalized seven-membered ring products from the benzenoid double bond with carbene. In recent years, the field of transition-metal-catalyzed Büchner ring expansion reactions of alkynes has experienced a speedy development and a diverse array of efficient synthetic procedures have been disclosed under mild experimental conditions, as the synthesis of synthetically challenging seven-membered rings is easily achieved. In this review, we will focus on the recent progress in the transition-metal-catalyzed Büchner reaction of alkynes and the mechanistic rationale is depicted where possible, with the reactions being sorted according to the type of catalyst.

Bis[N'-(3-cyano-benzyl-idene)isonicotino-hydrazide]silver(I) trifluoro-acetate.

In the title compound, [Ag(C(14)H(10)N(4)O)(2)]CF(3)CO(2), the Ag(I) ion is coordinated by two N atoms of the pyridine rings of two N'-(3-cyano-benzyl-idene)isonicotinohydrazide ligands in a nearly linear geometry. In the crystal structure, a combination of close contacts formed via Ag⋯N inter-actions [Ag⋯N = 3.098 (2) and 3.261 (2) Å] from symmetry-related mol-ecules and inter-molecular N-H⋯O hydrogen bonds between CF(3)CO(2) (-) anions and the hydrazone groups of two ligands give rise to chains. Furthermore, there are Ag⋯O inter-actions with a separation of 2.765 (2) Šbetween chains. The F atoms of the CF(3)CO(2) (-) anion are disordered over two sites with refined occupancies of 0.593 (5) and 0.407 (5).

Dichlorido(2,9-diprop-oxy-1,10-phenanthroline-κN,N')cadmium(II).

In the title complex, [CdCl(2)(C(18)H(20)N(2)O(2))], the Cd(II) ion is coordinated by two N atoms from a bis-chelating 2,9-diprop-oxy-1,10-phenanthroline ligand and two Cl atoms in a distorted tetra-hedral environment. The two Cd-Cl bond distances are significantly different from each other and the N-Cd-N bond angle is acute. In the crystal structure, there are π-π stacking inter-actions between symmetry-related phenanthroline ring systems, with a centroid-centroid distance of 3.585 (3) Å.

Dichlorido(2,9-dieth-oxy-1,10-phenanthroline-κN,N')zinc(II).

All non-H atoms except for the Cl atoms lie on a mirror plane in the title complex, [ZnCl(2)(C(16)H(16)N(2)O(2))]. The Zn(II) ion is coordinated by two N atoms from a bis-chelating 2,9-dieth-oxy-1,10-phenanthroline ligand and two symmetry-related Cl atoms in a distorted tetra-hedral environment. The two Zn-N bond lengths are significantly different from each other and the N-Zn-N angle is acute. In the crystal structure, there are weak but significant π-π stacking inter-actions between phenanthroline rings, with a centroid-centroid distance of 3.764 (1) Å.

catena-Poly[[[diaqua-bis(2-methyl-6-oxo-1,6-dihydro-3,4'-bipyridine-5-carbo-nitrile)copper(II)]-µ-sulfato] tetra-hydrate].

In the title polymer, {[Cu(SO(4))(C(12)H(9)N(3)O)(2)(H(2)O)(2)]·4H(2)O}(n), both the metal center and the sulfate anion are located on a twofold axis. The Cu(II) ion is coordinated by two pyridyl N atoms from two symmetry-related organic ligands, two O atoms from two symmetry-related water mol-ecules, and two O atoms from two symmetry-related sulfate anions, resulting in a distorted octa-hedral geometry. The sulfate anions act as µ(2)-bridges and connect metal ions, forming a one-dimensional chain along the b axis. The three-dimensional crystal structure is established through inter-molecular N-H⋯O and O-H⋯O hydrogen bonds involving the organic ligands, sulfate anions, coordinated and uncoordinated water mol-ecules, and through π-π inter-acting 2-pyridone rings, with centroid-centroid separations of ca 3.96 Šand tilt angles of ca 2.62°.

catena-Poly[[[N'-(4-cyano-benzyl-idene)nicotinohydrazide)silver(I)]-µ-N'-(4-cyano-benzyl-idene)nicotinohydrazide] hexa-fluorido-phosphate].

In the title polymer, {[Ag(C(14)H(10)N(4)O)(2)]PF(6)}(n), each Ag(I) ion is coordinated by two N atoms from two pyridyl rings of independent N'-(4-cyano-benzyl-idene)nicotinohydrazide ligands, and one N atom from one carbonitrile group of a symmetry-related ligand in a distorted T-shaped geometry. The ligands exhibit two modes of coordination. One acts as a bridge connecting Ag atoms to form one-dimensional chains along [01]. The other acts as a terminal monodentate ligand, coordinating to Ag through its pyridyl N atom. Two neighbouring anti-parallel chains in the crystal are connected through N-H⋯O hydrogen bonds. Other adjacent chains are packed via Ag⋯O inter-actions, with Ag⋯O separations of 2.876 (2) Å. In addition, PF(6) (-) counter-anions inter-act with the hydrazone groups through N-H⋯F hydrogen bonds. The PF(6) (-) anion is disordered over two sites, with occupancies of 0.773 (8) and 0.227 (8).