Recent X-ray and BC NMR results on macrocyclic compounds containing oxyethylene units (1,4 dioxa groupings) are reviewed. When such "saturated" crown ethers and related compounds form complexes with cations and uncharged protonacidic molecules, they adopt conformations which contain chiefly ag "a units or g ±g±a units (genuine comers) and are thus of the types encountered, or conceived for corresponding cycloalkanes. Uncomplexed or incompletely complexed crown ethers, on the other hand, adopt as a rule unusual conformations never encountered in cycloalkanes. These latter crown ethers contain g±g'a units (pseudo corners) characterized by close 1,5 CH· .. 0 interactions. Such conformational units are also found in uncomplexed cryptands. In open chains, there is no evidence of this type of chain folding, except in the gas phase for 1,2-dimethoxyethane. It is concluded that pseudo-corner formation is resorted to only when solvation of ether oxygen is excluded, such as in the gas phase or in the inaccessible interior of macrocyclic compounds.
Cyclic boric esters of 1,2-diols show a much greater heat of reaction with amines than do cyclic esters of 1,3-diols and non-cyclic borates.Isolation of the acetylation products of the mixed ester resulting from a competitive reaction between ethylene glycol and propane-1 ,S-diol for boric acid shows that the cyclic ester of the 1,3-diol is formed preferentially.It is concluded that the five-membered ring must be strained with trigonal boron but not with tetrahedral boron, whereas the six-membered ring is unstrained in both cases. The strain in the five-membered ring approximately offsets the energy gain in ring formation, so that 1,2-diol borates, although distillable as cyclic esters, rearrange more or less to open-chain polymers on condensation. The unstrained 1,3-diol borates remain in the cyclic form.FROM the work of Boeseken and his collaborators it is known that many 1,2-but not 1,3-diols raise the acidity and conductivity of boric acid by complex formation. On the other hand, Hermans,2 and later other workers,3 have shown that boric esters are formed more readily from 1,3-than from 1,2-diols. In an early stereochemical treatment Hermans4 suggested that the difference in stability is caused by the different 0-0 distances for trigonal (2.32 A) and tetrahedral boron (2.42 A), the first of which fits better with 1,3-diols, the latter better with 1,2-diols.show that the two 0-0 distances are really much closer and the difference probably too small to account for such a strong effect.
Newer results
Bu"0, R * CH -0, Me.CH -9 Bu"0' R-CH -0 ' Me-CH -0 CHMe+O-B, I B -0 B u "
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.