Oxirane opening conjugate

Molecular orbital calculations predict that oxirane forms the cyclic conjugate acid (39), which is 30 kJ moF stabler than the open carbocation (40) and must surmount a barrier of 105kJmoF to isomerize to (40) (78MI50500). The proton affinity of oxirane was calculated (78JA1398) to be 807 kJ mol (cf. the experimental values of 773 kJ moF for oxirane and 777-823 kJ moF for dimethyl ether (80MI50503)). The basicity of cyclic ethers is discussed in (B-67MI50504). 

Epoxide opening with nucleophiles occurs at the less substituted carbon atom of the oxirane ring. Catalytic hydrogenolysis yields the more substituted alcohol. The scheme below contains also an example for frans-dibromination of a C—C double bond followed by dehy-drobromination with strong base for overall conversion into a conjugated diene. The bicyclic tetraene then isomerizes spontaneously to the aromatic l,6-oxido[l0]annulene (E. Vogel, 1964). 

Trans hydroxylation of an alkene involves sequential conversion to an oxirane (epoxide), and its conjugate acid, followed by nucleophilic opening of the oxirane by water... 

Ryabov, B.V., lonin, B.L, and Petrov, A.A., Conjugated opening-closing of the oxirane ring in the reactions of (4-chloro-2,3-epoxyalkyl)phosphonates with nucleophiles, Zh. Obshch. Khim., 56, 2647, 1986 J. Gen. Chem. USSR (Engl. Transl.), 56, 2344, 1986. 

A sequence of several steps, first discussed in connection with Scheme 6.13, installs chlorine at Cg. Reaction of 23-8 with basic hydrogen peroxide starts by selective conjugate addition of peroxide anion to the distal end of the diene. That transient intermediate goes on to form the a-epoxide. Diaxial opening of the oxirane with chlorine gives chlorohydrin... 

The reaction of oxiran with HF to yield CH2FCH2OH was chosen as the model for the ring-opening of epoxides that is caused by halogen acids, and theoretical ab initio calculations for the reaction were performed. For the gas-phase reaction, the preferred mechanism leads to retention of configuration at the carbon atoms of the ring. An alternative pathway via preliminary formation of the conjugate acid of the oxiran was found to fit a borderline A2 mechanism. 

Cationic ring-opening polymerization of oxiranes can also be carried out photochemically (photochemical reactions are discussed in Chap. 10). Yagci and coworkers reported polymerizations of cyclohexene oxide with the aid of highly conjugated thiophene derivatives [12]. The reaction is illustrated as follows ... 

It will be recalled (Chapter 8) that epoxides (oxiranes) can be induced to undergo acid-catalyzed ring opening to diols. Thus, formation of an epoxide (oxirane) in place of the conjugated double bond of the a,p-unsaturated ketone puts three oxygen-substituted carbon atoms in adjacent positions. 

---