Sterically Congested in-Methylcyclophanes

Abstract

The crowded in-methylcyclophane 9 was prepared by condensation of 1,8,13-tris(bromomethyl)-9-methyltriptycene and 1,3,5-tris(mercaptomethyl)benzene under high dilution conditions. Oxidation of 9 gave the highly crystalline trisulfone 10, and its X-ray structure was determined. The in-methyl carbon atoms of the two independent molecules in the structure are only 2.90 and 2.87 A from the centroid of the basal aromatic ring, the closest such contacts ever observed. In addition, the C-CH3 bonds in these cyclophanes are compressed; the two independent bond distances are 1.475 and 1.495 A, significantly shorter than the ca. 1.54 A distances found in similar but uncongested molecules.

New [4.4]Cyclophane Diketals, Monoketones, and Diketones: Design, Synthesis, and Structural Analysis

“…The 1 H NMR spectra of these compounds (Figure 10) reveal a shielding of the ortho , ortho′ protons in 7 ( δ = 6.28 ppm), a greater shielding of more than 0.5 ppm in the monoketone 13 ( δ = 5.62 ppm), and a quite incredible position of this signal ( δ = 4.96 ppm) in the spectrum of diketone 15 . The shielding of the signal for this proton is important and comparable with spectacular cases presented in the literature 3a,22…”

Squeezing CC Bonds

“…The steric congestion at the center of this molecule leads to a shortened bond length of 1.43 Å 16. Song, Ho, and Pascal recently synthesized related in‐cyclophanes with short CMe bonds of 1.475 and 1.495 Å 17. Tightening the cage around such an in‐methyl group, as in 10 , leads to a corresponding CC bond length of just 1.41 Å.…”

A New Approach to the Design of Neutral 10‐C‐5 Trigonal‐Bipyramidal Carbon Compounds: A “π‐Electron Cap” Effect

“…Probably because of a sterical compression effect (SCE; “π‐electron cap” effect),17f,h in the in ‐cyclophanes the MX bonds are shorter than in its absence in model acyclic compounds. For example, in in ‐methylcyclophane (MX=CC Me )17h such shortening of the CC Me bond is equal to 0.07 Å .…”