Transition state structures cyclic ether reactions

These phenomena can be explained as follows. First, in the absence of water or in the presence of a small amount of water, THF predominantly coordinates to Yb(OTf)3 and the activity of THF-coordinated Yb(OTf)3 as a Lewis acid is low. The reaction proceeds slowly via the cyclic six-membered transition state with anti preference [24]. On the other hand, when the moles of water are gradually increased, water is prone to coordinate to Yb(OTf)3 instead of tetrahydrofuran (THF), and Yb(OTf)3 dissociates to form the active Yb cation. The solid-state structure of Yb(0Tf)3 9H20 has been investigated [7dj. When Yb(0Tf)3 9H20 (prepared according to the literature) was used, the aldol reactions proceeded, but faster hydrolysis of the silyl enol ethers was observed. At this stage, intramolecular and intermolecular exchange reactions of water molecules occur frequently [25]. There is a chance for an aldehyde to coordinate to Yb instead of to water molecules and the aldehyde is activated ... 

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