Self-fulfilling cavitands: packing alkyl chains into small spaces

Abstract

The present research was undertaken to determine the conformations of alkyl groups that are forced into small spaces. Unbranched alkyl groups assume fully extended conformations in the free space of solutions, because any bends in the structure create steric clashes between hydrogens along the chain. We synthesized a series of alkyl esters of a carboxylic acid attached to the inner surface of a vase-shaped container structure. The shorter esters (ethyl and propyl) can be accommodated in extended conformations, and even small solvent molecules can share the container's space. Longer (butyl-octyl) esters adopt increasingly coiled conformations that writhe rapidly in the limited space of the cavity. Even longer esters (nonyl and decyl) can be synthesized, but their containers become distorted, and their spectra indicate slowed internal motions of the alkyl groups within the space. In general, alkyl groups are readily contorted when their internal strains are compensated by attraction with the inner surfaces and the proper filling of space.

Fig. 1.

Alkanes adopt a conformation suitable for the available space. (Left) Two views of encapsulated decane in an extended conformation. (Right) Encapsulated tetradecane coils into a helical conformation.

Fig. 2.

Structure of the introverted acid cavitand H1, the introverted esters R1, and control molecules 2.

Fig. 3.

Molecular models (maestro; AMBER force field) of the introverted ester cavitands R1.

Fig. 4.

The upfield window of the ¹H NMR spectra of linear alkyl introverted ester cavitands. The solvent is mesitylene-d₁₂. The spectra of Me1, Et1, Pr1, and Bu1 were acquired at 300 K; the others were acquired at 273 K.

Fig. 5.

NOESY spectrum of heptyl ester C₇H₁₅1 at 273 K in mesitylene-d₁₂. A NOE between the terminal methyl group at −3.1 ppm and (CH₂)_i+4 at −0.7 ppm is indicative of coiling.

Fig. 6.

Side views of the molecular models (maestro; AMBER force field) of selected introverted ester cavitands. The decyl ester C₁₀H₂₁1 is modeled in its folded conformation for illustrative purposes; NMR structural studies show that the ester chain is too large, and the cavitand is unfolded.

Fig. 7.

¹H NMR spectra (Lower) and model structures (Upper) of introverted ester cavitands in mesitylene-d₁₂ and CDCl₃. The upfield region of the NMR spectra, shown in Left Upper, indicate that the ester chain is displaced from the cavity by solvents of appropriate size and shape, in this case chloroform, as shown in Right Upper.