Nucleophilic substitution three-member ring compounds

Unlike most ethers, epoxides (compounds in which the C—O—C unit forms a three-membered ring) are very reactive substances. The principles of nucleophilic substitution are important in understanding the preparation and properties of epoxides. 

A final example of this class of allyl-X compounds is a vinylcyclopropane that is geminally substituted on the three-membered ring with electron withdrawing groups. Upon addition of a Pd° complex, the allylic C—C bond ruptures, with the stabilized carbanion serving as a leaving group (equation 11).96 The complex can be independently reacted with electrophiles at the carbanion as well as with nucleophiles on the allyl moiety. 

Three-membered cyclic compounds can generally be prepared from difunctional compounds by 1,3-elimination. In the formation of oxiranes from 2-substituted alkanols, the alkoxide produced in the basic medium participates in an internal nucleophilic attack, which promotes the departure of the substituent on the adjacent carbon atom leading to ring-closure. A stereochemical condition for the reaction is that the reacting groups should be in an antiperiplanar conformation relative to each other (Eq. 51). 

Nucleophiles will open acceptor-substituted vinylcyclopropanes either by attacking the three-membered ring resulting in a 1,5-addition or by a 1,7-addition to the olefin moiety . In most cases two acceptor groups are a prerequisite for sufficient activation (see Chapter 9). An interesting nucleophile-dependent dichotomy is demonstrated by equation 143. Organoaluminium compounds add in a 1,5-fashion whereas the corresponding ate-complexes prefer the 1,7-mode . 

What accounts for the much faster reaction of the trans-substituted compound In this reaction, the thio substituent is an intramolecular nucleophilic catalyst. It displaces the chloro substituent by attacking the back side of the carbon to which the chloro substituent is attached. Back-side attack requires both substituents to be in axial positions, and only the trans isomer can have both of its substituents in axial positions (Section 2.14). Subsequent attack by water or ethanol on the sulfonium ion is rapid because the positively charged sulfur is an excellent leaving group and breaking the three-membered ring releases strain. 

Three-membered rings can be constmcted by several methods. Of rather general occurrence is the intramolecular nucleophilic substitution process depicted for oxiranes and aziridines in Scheme 10.6. Many compounds have been prepared by these methods. Phosphiranes are... 

Kinetic and product structure studies of the reaction of cyclopropanes and bicyclic compounds containing three-membered rings have shown that the protonation of the cyclopropane ring is followed by addition of the nucleophile at the most substituted carbon. The product composition is determined by the ability of the more highly substituted carbons to sustain more of the positive charge. The substitution at the incipient carbocation is the most important factor in determining the degree of reactivity. The relative rates of cyclopropane and its methyl, 1,1-... 

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