[Telamond]

The reaction between an alpha, beta-unsaturated carbonyl compound and a sulfur ylide can go with two different outcomes depending on if the sulfur ylide is stabilized or unstabilized.

With the stabilized sulfur ylide the product is the cyclopropane formation due to a slower but irreversible 1,4-addition vs. a fast but reversible 1,2-addition.

In the case of using an unstabilized sulfur ylide, the end product would be an epoxide through an 1,2-addition. My question is: Does the 1,4-addition happen at all? Is the 1,4-addition reversible or irreversible? Why is it slower than the 1,2-addition?

[g-bones]

1,2 addition is usually favored by harder nucleophiles.  softer nucleophiles are things that can delocalize their positive charge and tend to add 1,4 (a softer position of the electrophile).  unstabilized ylides are harder in nature and add 1,2 more often.  I tend to think that 1,4 addition is not usually thought of as reversible.

this is my take on the situation anyway.

hope this helps