6185-76-8Relevant articles and documents
The effect of a para substituent on the conformational preference of 2,2-diphenyl-1,3-dioxanes: Evidence for the anomeric effect from X-ray crystal structure analysis
Uehara, Fumiaki,Sato, Masayuki,Kaneko, Chikara,Kurihara, Hiroyuki
, p. 1436 - 1441 (1999)
The molecular structures of 2,2-di(para-substituted phenyl)-1,3-dioxanes were elucidated for the first time by X-ray crystallographic analysis, which revealed two important structural features: (1) These compounds have the chair conformation in which electron-withdrawing aryl groups [viz. p-nitro-or p-(trifluormethyl)phenyl] are always axial and electron-donating aryl groups (viz. p-methoxyphenyl) are always equatorial. (2) In these compounds as well as in symmetrically substituted 2,2-diphenyl-1,3-dioxane the axial C2-aryl bond is longer than the equatorial C2-aryl bond. The axial preference of the electron-withdrawing aryl group was also demonstrated in solution by 1H and 13C NMR spectroscopy. The anomeric carbon substituted with an electron-withdrawing aryl group resonates at an unusually high field, as does the aromatic carbon bearing the electron-withdrawing substituent. The observed 13C NMR data clearly indicate enhanced electron density at these carbons due to the anomeric effect. Semiempirical molecular orbital calculations by the MOPAK PM3 method reproduced the bond lengthening for axial C2-aryl, while the heat of formation derived from this calculation failed to support the axial preference of electron-withdrawing aryl groups. The X-ray crystallographic data on the conformational preference and bond lengths at the anomeric carbon, as well as the solution NMR spectroscopic data, clearly indicate the anomeric effect that is best rationalized in terms of stabilizing interaction between the lone-pair electrons on the ring oxygens and the antibonding orbital of the axial C2-aryl bond.
Systematic Evaluation of 1,2-Migratory Aptitude in Alkylidene Carbenes
Dale, Harvey J. A.,Nottingham, Chris,Poree, Carl,Lloyd-Jones, Guy C.
supporting information, p. 2097 - 2107 (2021/02/01)
Alkylidene carbenes undergo rapid inter- and intramolecular reactions and rearrangements, including 1,2-migrations of β-substituents to generate alkynes. Their propensity for substituent migration exerts profound influence over the broader utility of alkylidene carbene intermediates, yet prior efforts to categorize 1,2-migratory aptitude in these elusive species have been hampered by disparate modes of carbene generation, ultrashort carbene lifetimes, mechanistic ambiguities, and the need to individually prepare a series of 13C-labeled precursors. Herein we report on the rearrangement of 13C-alkylidene carbenes generated in situ by the homologation of carbonyl compounds with [13C]-Li-TMS-diazomethane, an approach that obviates the need for isotopically labeled substrates and has expedited a systematic investigation (13C{1H} NMR, DLPNO-CCSD(T)) of migratory aptitudes in an unprecedented range of more than 30 alkylidene carbenes. Hammett analyses of the reactions of 26 differentially substituted benzophenones reveal several counterintuitive features of 1,2-migration in alkylidene carbenes that may prove of utility in the study and synthetic application of unsaturated carbenes more generally.
Photocontrolled Radical Polymerization from Hydridic C-H Bonds
Stache, Erin E.,Kottisch, Veronika,Fors, Brett P.
supporting information, p. 4581 - 4585 (2020/03/05)
Given the ubiquity of carbon-hydrogen bonds in biomolecules and polymer backbones, the development of a photocontrolled polymerization selectively grafting from a C-H bond represents a powerful strategy for polymer conjugation. This approach would circumvent the need for complex synthetic pathways currently used to introduce functionality at a polymer chain end. On this basis, we developed a hydrogen-atom abstraction strategy that allows for a controlled polymerization selectively from a hydridic C-H bond using a benzophenone photocatalyst, a trithiocarbonate-derived disulfide, and visible light. We performed the polymerization from a variety of ethers, alkanes, unactivated C-H bonds, and alcohols. Our method lends itself to photocontrol which has important implications for building advanced macromolecular architectures. Finally, we demonstrate that we can graft polymer chains controllably from poly(ethylene glycol) showcasing the potential application of this method for controlled grafting from C-H bonds of commodity polymers.
