Q: In this task, you will be given a passage to read. A fill in the blank question will be given to you. Your answer should fit the blank appropriately.
The reaction mechanism for the Johnson Corey Chaykovsky reaction consists of nucleophilic addition of the ylide to the carbonyl or imine group . A negative charge is transferred to the heteroatom and because the sulfonium cation is a good leaving group it gets expelled forming the ring . In the related Wittig reaction , the formation of the much stronger phosphorus @-@ oxygen double bond prevents oxirane formation and instead , olefination takes place through a 4 @-@ membered cyclic intermediate . The trans diastereoselectivity observed results from the reversibility of the initial addition , allowing equilibration to the favored anti betaine over the syn betaine . Initial addition of the ylide results in a betaine with adjacent charges ; density functional theory calculations have shown that the rate @-@ limiting step is rotation of the central bond into the conformer necessary for backside attack on the sulfonium . The degree of reversibility in the initial step ( and therefore the diastereoselectivity ) depends on four factors , with greater reversibility corresponding to higher selectivity : Stability of the substrate with higher stability leading to greater reversibility by favoring the starting material over the betaine . Stability of the ylide with higher stability similarly leading to greater reversibility . Steric hindrance in the betaine with greater hindrance leading to greater reversibility by disfavoring formation of the intermediate and slowing the rate @-@ limiting rotation of the central bond . Solvation of charges in the betaine by counterions such as lithium with greater solvation allowing more facile rotation in the betaine intermediate , lowering the amount of reversibility . fill in the blank : The reaction mechanism for the Johnson Corey Chaykovsky reaction consists of nucleophilic addition of the ylide to the carbonyl or __________________ group .
A:
imine