Cyclic ethers transition state structures

With the non-C2-symmetrical dialdehyde 11, reductive cyclisation on treatment with Sml2 led to the preferential formation of the cyclic cz s-diol 12. The intervention of the transition state structure 13 was invoked to rationalise the uratz-selectivity observed with the bulky silyl ether-protected hydroxyl group. 

A high degree of stereoselectivity can be realized under chelation control, where an oxygen atom of an ether function (or more generally a Lewis base) in the a-, P- or possibly y-position of carbonyl compounds can serve as an anchor for the metal center of a Lewis acid. Since Cram s pioneering work on chelation control in Grignard-type addition to chiral alkoxy carbonyl substrates [30], a number of studies on related subjects have appeared [31], and related transition state structures have been calculated [32], Chelation control involves Cram s cyclic model and requires a Lewis acid bearing two coordination sites (usually transition metal-centered Lewis acids). 

The loss of rotational freedom when the cyclic transition state is formed from the acyclic starting material accounts for the observed negative entropies and volumes of activation. It has been established that the transition state of the closely related Cope rearrangement (see below) resembles the chair conformation of cyclohexane. An analogous transition state structure for the Claisen rearrangement would account for the fact that isomerization of vinyl a-methylallyl ether yields 97% trans 4-hexQm and 3% cw-4-hexenal, since there should be a preference for an equatorial orientation of the methyl group in a cyclohexane chair-like transition state . 

Djerassi, C. Fenselau, C. Mass Spectrometry in Structural and Stereochemical Problems. LXXXIV. The Nature of the Cyclic Transition State in Hydrogen Rearrangements of Aliphatic Ethers. J. Am. Chem. Soc. 1965, 87, 5747-5762. 

L forming intermediate 8. Trapping 8 with a silyl enol ether 9 leads to the aldol addition product 5. The open-chain structure 10a and the cyclic structure 10b are discussed as transition states in the literature. Release of the Lewis acid L is the final step of the catalytic cycle. 

The formation of ds-propenyl ether is probably a consequence of a cyclic intermediary structure. The formation of trans-propenyl ether is probably a consequence of free rotation of bonds in the transition state. The rate of the transformation of allyl ether groups in propenyl ether groups is higher at higher temperatures (150-160 °C). This rearrangement of ally ethers to propenyl ether groups is catalysed not only by alkaline alcoholates [60, 77, 78] but also by some complex compounds of ruthenium (such as ruthenium dichloride-triphenylphosphine complex [89]) ... 

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 ... 

The silicon centers in silyl enol ethers and allylsilanes gain Lewis acidity when Si is incorporated into small cyclic structures. A representative example is enol silacyclobutyl ethers, which react with aldehydes at ambient temperatures possibly through a six-membered transition state (Scheme 3-122). Crotylbis(catechol)silicates react with aldehydes smoothly at the reflux temperature of THF also through six-membered transition states involving hexacoordinate silicates (Scheme 3-123). ... 

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