Bonding in ethers and epoxides

Nomenclature of Ethers, Epoxides, and Sulfides 650 Structure and Bonding in Ethers and Epoxides 652 Physical Properties of Ethers 652... 

In Chapter 9, we take the principles learned in Chapters 7 and 8 about leaving groups, nucleophiles, and bases, and apply them to alcohols, ethers, and epoxides, three new functional groups that contain polar C—O bonds. In the process, you will discover that all of the reactions in Chapter 9 follow one of the four general mechanisms introduced in Chapters 7 and 8—SnI, Sn2, El, or E2—so there are no new mechanisms to learn. 

The ease of interaction of Ni(0) complexes with organic substrates has been shown to depend upon both the ligands on nickel and the solvent. The presence of a,a-bipyridyl with the Ni(0) complex and the alkyne led to the isolation of a nickelacyclopropene, an observation in accord with the recently proposed metallocyclic pathway for the Ni(0)-cata-lyzed trimerization of alkynes. Allylic and benzylic ethers and epoxides have been observed to undergo oxidative insertion of Ni(0) into their C-0 bonds with solvent (TMEDA > THE > Et O > CeHe) and ligand (EtsP > PhsP a,a-bipy > COD) effects consistent with an electron-transfer attack by Ni(0). With such sulfur heterocycles as dibenzothiophene, phenoxathiin, phenothiazine, and thian-threne, a 1 1 admixture of (COD)2Ni with a,a-bipyridyl gave as the principal product the desulfurized, ring-contracted cyclic product. 

We have seen that the compounds in group II—alkyl halides, alcohols, ethers and epoxides—contain a carbon that is bonded to a more electronegative atom. The carbon, therefore, is electrophilic and reacts with a nucleophile. 

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

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