High yield selective acylation of polyamines: proton as protecting group.

An advance in the selective acylation of polyamines having identical or similar amine functions is reported. While nucleophilicity differences between the various amine functions are slight, the corresponding conjugate acids exhibit pKa values over a significant range. We have used proton as polyamine protecting group: the monoamine resulting from single deprotonation of a polyammonium compound has allowed for high yields of selective acylation.

Improved physical properties and rotational dynamics in a molecular gyroscope with an asymmetric stator structure.

[structure: see text] A molecular gyroscope consisting of a 1,4-diethynylphenylene rotator linked to trityl and triptycyl groups (3) showed significantly improved physical properties and faster rotational dynamics than analogous symmetric bis(trityl) (1) or bis(triptycyl) (2) structures. An activation energy of 7.9 kcal/mol for 3 was determined by 2H NMR. This is ca. 4-6 kcal/mol lower than that of compound 1. The different dynamics of the three compounds can be qualitatively understood in terms of their different packing coefficients.

Crystalline molecular machines: a quest toward solid-state dynamics and function.

Complex molecular machinery may be envisioned as densely packed, multicomponent, self-assembling systems built with high structural precision to control the dynamics of one or more internal degrees of freedom. With molecular gyroscopes as a test, we describe a general strategy to design crystals capable of supporting structurally programmed molecular motions, a practical approach to their synthesis, convenient strategies to characterize their solid-state dynamics, and potential applications based on polar structures responding collectively to external fields.

Hinged molecular capsules: synthesis and conformational control via temperature, pH, or solvent composition.

Three new covalently linked molecular capsules were synthesized from their resorcinarene cavitand precursors in good yields. The capsules undergo reversible conformational switching between the closed "vase" form and the open "kite" form upon temperature or pH variation. The kite conformation obtained via either method in CDCl(3) switches to vase conformation upon addition of polar solvents such as acetone-d(6) or THF-d(8).

Molecular crystals with moving parts: synthesis, characterization, and crystal packing of molecular gyroscopes with methyl-substituted triptycyl frames.

We report a highly convergent synthesis for the preparation of molecular gyroscopes consisting of para-phenylene rotors linked by triple bonds to methyl-substituted triptycenes acting as pivots and encapsulating frames. The desired 1,4-bis[2-(2,3,6,7,12,13-hexamethyl-10-alkyl-9-triptycyl)ethynyl]benzenes were prepared from 2,3-dimethyl-1,3-butadiene using Diels-Alder cycloadditions and Pd(0)-catalyzed coupling as the key reactions. The main challenge in the synthesis came about in the preparation of 9-alkynyl-triptycenes by Diels-Alder reaction of benzynes and 9-alkynyl-2,3,6,7-tetramethylanthracenes. These reactions occurred with chemical yields and regioselectivities that were strongly influenced by steric and electronic effects of substituents at C10 of the anthracene core. Anthracenes with methyl, propyl, and phenyl substituents were utilized to complete the synthesis of their corresponding molecular gyroscopes, and their solid-state structures were determined by single-crystal X-ray diffraction analysis. Examination of these results indicated that, as expected, the bulky triptycyl groups encourage crystallization motifs that create more free volume around the phenylene rotor, as needed to facilitate fast gyroscopic motion in the solid state.

Molecular compasses and gyroscopes. II. Synthesis and characterization of molecular rotors with axially substituted bis[2-(9-triptycyl)ethynyl]arenes.

We have developed a simple convergent procedure for the synthesis of molecular rotors consisting of a central aromatic group coupled with two axially positioned ethynyltriptycenes. Molecular rotors with 1,4-phenylene (1), 1,4'-1,1'-biphenylene (2), 9,10-anthracenylene (3), and 2,7-pyrenylene (4) groups were prepared by Pd(0)-catalyzed coupling of ethynyl triptycenes with the corresponding dibromoarenes. Although compounds 1-4 were not expected to have free rotation in the solid state, the rotational potentials of 1 and 3 were analyzed by semiempirical methods and the crystal packing of 1 was analyzed to design the structures most likely to yield a functional rotor in the solid state. Semiempirical PM3 calculations predict compounds 1, 2, and 4 to have frictionless internal rotation even at temperatures as low as 25 K, while compound 3 is expected to have a barrier of ca. 4 kcal/mol.