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.

Mesityl alkyne substituents for control of regiochemistry and reversibility in zirconocene couplings: new synthetic strategies for unsymmetrical zirconacyclopentadienes and conjugated polymers.

Reaction of 2 equivs of MesC[triple bond]CPh with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) afforded the zirconacyclopentadiene Cp(2)Zr[2,5-Ph(2)-3,4-Mes(2)C(4)]. The regiochemistry of this isomer (betabeta with respect to the mesityl substituents) was determined through single-crystal X-ray analysis and 2D (NOESY, HSQC, HMBC) NMR experiments. This selectivity is attributed largely to a steric-based directing effect of the o-methyl ring substituents since coupling of 1,3-dimethyl-2-(phenylethynyl)benzene with zirconocene gave a single regioisomer (o-xylyl groups in both beta-positions) while coupling of 1,3-dimethyl-5-(phenylethynl)benzene gave a statistical distribution of zirconacyclopentadiene regioisomers. The coupling reaction of 2 equivs of MeC[triple bond]CMes or PrC[triple bond]CMes with Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) at ambient temperature gave the betabeta regioisomers, Cp(2)Zr[2,5-Me(2)-3,4-Mes(2)C(4)] and Cp(2)Zr[2,5-Pr(2)-3,4-Mes(2)C(4)], respectively, as the major products. Heating solutions of these zirconacycles at 80 degrees C for several hours resulted in an increase in the amount of the unsymmetrical product. For reaction mixtures of PrC[triple bond]CMes and Cp(2)Zr(eta(2)-Me(3)SiC[triple bond]CSiMe(3))(pyr) the major (and apparently thermodynamic) product under these reaction conditions was Cp(2)Zr[2,4-Mes(2)-3,5-Pr(2)C(4)]. The steric strain in the mesityl-substituted zirconacycles allowed for facile substitution reactions of MesC[triple bond]CPh or PrC[triple bond]CMes by less bulky alkynes (i.e., tolan and 3-hexyne) to give the unsymmetrical ziconacyclopentadienes Cp(2)Zr[2,4,5-Ph(3)-3-MesC(4)], Cp(2)Zr[2-Ph-3-Mes-4,5-Et(2)C(4)], and Cp(2)Zr[2-Pr-3-Mes-4,5-Ph(2)C(4)]. Reaction of a mesityl-terminated diyne containing a rigid dihexylfluorenylene spacer with zirconocene afforded poly(p-fluorenylenedienylene) after demetalation with benzoic acid.

Stereoselective macrocyclization through zirconocene-mediated coupling of achiral dialkynes.

1,4-Bis[trimethylsilyl(ethynyl)]naphthalene () and 1,4-bis[trimethylsilyl(ethynyl)]anthracene () undergo diastereoselective coupling with Cp2Zr(py)(Me3SiC[triple bond, length as m-dash]CSiMe3) to give trimeric macrocycles in good yield.

9,10-Disubstituted octafluoroanthracene derivatives via palladium-catalyzed cross-coupling.

9,10-Dichlorooctafluoroanthracene (1) reacts with aryl boronic acids and terminal alkynes under palladium-catalyzed cross-coupling conditions to afford 9,10-diaryloctafluoroanthracenes (2a-e) and 9,10-dialkynyloctafluoroanthracenes (6a,b), respectively. Optical spectroscopy and cyclic voltammetry indicate that octafluoro-9,10-di(thiophen-2-yl)anthracene (2d) exhibits donor-acceptor character and a LUMO energy level of -3.27 eV relative to vacuum. A functionalized 5-bromothiophen-2-yl derivative (2e) was obtained in high yield by bromination of 2d with NBS. X-ray crystallographic analysis of octafluoro-9,10-bis[(trimethylsilyl)ethynyl]anthracene (6a) reveals a solid-state structure that mimics the packing of columnar liquid crystals, with a pi stacking distance of 3.39 A between the octafluoroanthracene cores. In addition, octafluoro-9,10-bis(mesitylethynyl)anthracene (6b) displays a LUMO energy level of -3.50 eV, which approaches the value of -3.65 eV measured for perfluoropentacene, making 9,10-dialkynyloctafluoroanthracenes a promising new class of n-type organic materials.

9,10-Dichlorooctafluoroanthracene as a building block for n-type organic semiconductors.

9,10-Dichlorooctafluoroanthracene (1) was synthesized from commercially available tetrafluorophthalic acid by an optimized solution-phase route. To establish 1 as a synthon for n-type organic semiconductors, the compound was reacted with phenylboronic acid under modified Suzuki-Miyaura coupling conditions to generate octafluoro-9,10-diphenylanthracene (7) in high yield. Cyclic voltammetry and X-ray crystallography indicate that 7 has a stabilized LUMO energy level and exhibits extended pi stacking, which should lead to efficient electron transport in solid-state devices. 1,2,3,4,5,6,7,8-Octafluoroanthracene (2) was also synthesized as a potential n-type building block, but suitable C-C coupling conditions for this compound were not found, and 2 could not be converted into 9,10-dibromooctafluoroanthracene or octafluoro-9,10-diiodoanthracene.