Theoretical Study of Cp2Zr-, (MeO)2Zr-, and M(PH3)-Mediated Coupling Reactions of Acetylenes (M = Ni, Pt). Significant Differences between Early- and Late-Transition-Metal Complexes

Assembly of macrocycles by zirconocene-mediated, reversible carbon-carbon bond formation

Macrocyclic compounds have attracted considerable attention in numerous applications, including host-guest chemistry, chemical sensing, catalysis, and materials science. A major obstacle, however, is the limited number of convenient, versatile, and high-yielding synthetic routes to functionalized macrocycles. Macrocyclic compounds have been typically synthesized by ring-closing or condensation reactions, but many of these procedures produce mixtures of oligomers and cyclic compounds. As a result, macrocycle syntheses are often associated with difficult separations and low yields. Some successful approaches that circumvent these problems are based on "self-assembly" processes utilizing reversible bond-forming reactions, but for many applications, it is essential that the resulting macrocycle be built with a strong covalent bond network. In this Account, we describe how zirconocene-mediated reductive couplings of alkynes can provide reversible carbon-carbon bond-forming reactions well-suited for this purpose. Zirconocene coupling of alkenes and alkynes has been used extensively as a source of novel, versatile pathways to functionalized organic compounds. Here, we describe the development of zirconocene-mediated reductive couplings as a highly efficient method for the preparation of macrocycles and cages with diverse compositions, sizes, and shapes. This methodology is based on the reversible, regioselective coupling of alkynes with bulky substituents. In particular, silyl substituents provide regioselective, reversible couplings that place them into the α-positions of the resulting zirconacyclopentadiene rings. According to density functional theory (DFT) calculations and kinetic studies, the mechanism of this coupling involves a stepwise process, whereby an insertion of the second alkyne influences regiochemistry through both steric and electronic factors. Zirconocene coupling of diynes that incorporate silyl substituents generates predictable macrocyclic products in very high yields. In the absence of significant steric repulsion, the macrocyclization appears to be entropically driven, thereby providing the smallest strain-free macrocyclic structure. The scope of the reaction has been explored by variation of the spacer group between the alkynyl substituents and by incorporation of functional and chiral groups into the macrocycle. The size and shape of the resulting macrocycles are largely determined by the length and geometry of the dialkyne spacer, especially in the case of terminal trimethylsilyl-substituted diynes. For example, linear, rigid diynes with four or fewer phenylene rings lead to trimeric macrocycles, whereas bent or flexible diynes produce dimers. Depending on the reaction conditions, functional groups (such as N-heterocycles and imines) are tolerated in zirconocene coupling reactions, and in selected cases, they can be used to influence the shape of the final macrocyclic product. More recently, Cp(2)Zr(pyr)(Me(3)SiC≡CSiMe(3)) has been employed as a more general zirconocene synthon; it affords higher yields and increased functional group tolerance. Functional groups can also be incorporated through transformation of the zirconacyclopentadiene products, with acid hydrolysis to the corresponding butadiene being the most efficient derivatization. Furthermore, construction of chiral macrocycles has been accomplished by stereoselective macrocyclizations, and triynes have been coupled into three-dimensional cage compounds. We also discuss various design factors, providing a perspective on the utility of zirconocene-mediated couplings in the assembly of macrocyclic and cage compounds.

Enantioselective rhodium-catalyzed [2 + 2 + 2] cycloadditions of terminal alkynes and alkenyl isocyanates: mechanistic insights lead to a unified model that rationalizes product selectivity

This manuscript describes the development and scope of the asymmetric rhodium-catalyzed [2 + 2 + 2] cycloaddition of terminal alkynes and alkenyl isocyanates leading to the formation of indolizidine and quinolizidine scaffolds. The use of phosphoramidite ligands proved crucial for avoiding competitive terminal alkyne dimerization. Both aliphatic and aromatic terminal alkynes participate well, with product selectivity a function of both the steric and electronic character of the alkyne. Manipulation of the phosphoramidite ligand leads to tuning of enantio- and product selectivity, with a complete turnover in product selectivity seen with aliphatic alkynes when moving from Taddol-based to biphenol-based phosphoramidites. Terminal and 1,1-disubstituted olefins are tolerated with nearly equal efficacy. Examination of a series of competition experiments in combination with analysis of reaction outcome shed considerable light on the operative catalytic cycle. Through a detailed study of a series of X-ray structures of rhodium(cod)chloride/phosphoramidite complexes, we have formulated a mechanistic hypothesis that rationalizes the observed product selectivity.

Time-dependent density functional theory study on hydrogen-bonded intramolecular charge-transfer excited state of 4-dimethylamino-benzonitrile in methanol

The time-dependent density functional theory (TDDFT) method was carried out to investigate the hydrogen-bonded intramolecular charge-transfer (ICT) excited state of 4-dimethylaminobenzonitrile (DMABN) in methanol (MeOH) solvent. We demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O formed between DMABN and MeOH can induce the C[triple bond]N stretching mode shift to the blue in both the ground state and the twisted intramolecular charge-transfer (TICT) state of DMABN. Therefore, the two components at 2091 and 2109 cm(-1) observed in the time-resolved infrared (TRIR) absorption spectra of DMABN in MeOH solvent were reassigned in this work. The hydrogen-bonded TICT state should correspond to the blue-side component at 2109 cm(-1), whereas not the red-side component at 2091 cm(-1) designated in the previous study. It was also demonstrated that the intermolecular hydrogen bond C[triple bond]N...H-O is significantly strengthened in the TICT state. The intermolecular hydrogen bond strengthening in the TICT state can facilitate the deactivation of the excited state via internal conversion (IC), and thus account for the fluorescence quenching of DMABN in protic solvents. Furthermore, the dynamic equilibrium of these electronically excited states is explained by the hydrogen bond strengthening in the TICT state.