Transition boat-shaped

TMS enol ether has been shown to rearrange with good enantioselectivity, presumably because the boat-shaped transition state is sterically disfavoured. Rearrangement of the enantiomeric starting material has been used as a key step in a total synthesis of (+)-alline.13... 

The value of q3 = (6) V2R (R is the CC bond length) is 0.63 A. Under pseudorotation the equatorial boat-shaped structures B (0 = 90°, = 0, 60°, 120°,.. . ) turn into a twist-boat structure TB (0 = 90°, = 30°, 90°,.. . ). The transitions between the chair and twist boat structures involve the intermediate formation of half boat (HB) and half chair (HC) structures. Quantum chemical calculations carried out by Dixon and Komornicki [1990] show that the axial structure C with symmetry D3d is stable. The energies of structures B and TB are 7.9 and 6.8kcal/mol higher than C. The barrier for transition from C to TB is 12.2-12.4 kcal/ mol. Because of the high barriers for pseudorotation, only thermally activated conformational transitions occur in cyclohexane. 

Fig. 5.45 One might have guessed that the chair cyclohexane conformations 1 and I are connected by a boat-shaped intermediate 2. However, this C2v structure shows an imaginary frequency it is a transition state which wants to twist toward 3 (arrows) or 3 (arrows in opposite directions, not shown), which are the actual intermediates (no imaginary frequencies) between 1 and 1. The chair conformation reaches the twist via a half-chair 4...

The hexaaza [ISJaneNe forms complexes with transition metal ions and with certain alkali and alkaline earth and lanthanide ions. For the higher aza macrocycles with seven or more donor atoms, dinuclear complexes become possible. A systematic investigation of both the structural and thermodynamic aspects of copper complexes formed with the larger polyaza macrocycles from heptaaza to dodecaaza has been published. All of the macrocycles were found to form hydroxo species as well as polynuclear complexes. A number of structures have been determined for the higher polyaza macrocycles, both in complexed and noncomplexed forms, and structures range from highly boat shaped to nearly planar. ... 

These examples have been discussed in the framework of the Zimmermann-Traxler chair models, without considering the distortions of the chairs that can noticeably modify the relative free-energy differences. Moreover, in some aldol reactions, boat-shaped transition models better account for the observed stereoselectivities [122, 123], Ab initio calculations have supported such boat transition models however, the calculated energy differences are rather small [41, 124], Molecular mechanics calculations have also been applied to these problems, by, among others, Bemardi, Gennari and Paterson [124, 125, 126], and some general trends were highlighted. 

The Bp ligand is the simplest of all the heteroscorpionate ligands. It is basically bidentate, forming the typical boat-shaped H2B( x-pz)2M ring, in which the pseudoaxial B-H may form an agostic bond with the metal.4,5 [M(Bp)2] complexes of first row transition metals are usually square planar or tetrahedral.6,7 Some octahedral anionic [M(Bp)3]-species of low stability can be also isolated.8,9 Five-coordinate anionic species [MX(Bp)2]- (M = Cr, Mn X = halides or pseudohalides) have been also known.10,11... 

With a cyclic substrate, for example in which three or more atoms of the allyl vinyl ether are constrained in aring, then the boat-shaped transition state maybe favoured. Formation of the silyl ketene acetal from the lactone 295 and rearrangement on warming gave the carboxylic acid 296 (3.190). The reaction occurs via a boatshaped transition state and was used in a synthesis of the sesquiterpene widdrol. 

The cyclopropane intermediate results from the rhodium-carbenoid addition to the less-hindered terminal double bond of the diene. This generates the cyclopropane in which both alkenes are cis related, which allows the subsequent Cope rearrangement of the divinyl cyclopropane (see Section 3.6.1). The conformation of the boat-shaped transition state for the rearrangement must have the methyl and acetate groups trans to one another. See W. R. Cantrell and H. M. L. Davies, J. Org. Chem., 56 (1991), 723. 

As can be seen from the diagram, the reaction probably proceeds by P-hydride transfer via a boat-shaped transition state in which the less bulky substituent buttresses the methyl group of the pinene, accounting for the high selectivities seen with ketones containing the slender ethynyl group. Note that the pinene is regenerated and can be reconverted to Alpine borane as shown. In this way (+)-a-pinene can be seen merely as an asymmetric carrier for the hydride of 9-BBN. 

The two dimensional potential energy surface for the chair to chair rearrangement of [Cp2Ti(/x-C2S4)4] has been described to be via a boat shaped transition state. Extended Huckel calculations have been carried out on a range of complexes e.g. Cp2TiSiHPh, [(CpiTijC/i-HSi(HPh)( t-H)] which are important polymerisation intermediates. ... 

Similarly, thiochromones are converted into their 1,1-dioxides even using a deficiency of DMD very little 1-oxide is produced and it appears that the sulfoxide is very much more reactive towards DMD than the parent sulfide. This feature has been attributed to the different shapes, with the transannular stabilization of the transition state involving the lone pair of electrons on sulfur and the carbonyl function in the sulfoxide boat conformation not being possible for the planar sulfide. The amount of sulfoxide is increased in hydrogen bonding solvents <2001JOC2275>. 

The twist-boat can also interconvert with the boat form itself. The effect is to make all the protons isochronous if the twist-boat is also in rapid equihbrium with the boat-boat form, as is extremely likely from the previous discussion. The boat is probably the transition state in this pathway (TB B - TB) and thus the activation energy calculated is 9.4 kcal/mole. This pathway has a sufficient barrier for study by nmr hne shape methods if cyclooctane actually exists in these forms. At high temperatures e.g. room temperature) all the protons should be isochronous and should give... 

Another explanation takes into account that boat- and twist-shaped six-membered, closed transition states can successfully compete with the chair model. " Evans et al. pointed out that in a-unsubstituted enolate reactions, missing allyl strain interactions lead to lower selectivity in diastereoselective aldol reactions.Calculations indicate that a twist-boat can easily be formed from the U-configuration of a-unsubstituted enolates. The possible transition state in this case has a geometry like 34 and is favored by the chelating character of the complexation mode for the zinc cation and the outward-pointing substituents of the oxazolidinone moiety. This twist-boat transition state correctly predicts the stereochemical outcome of the reaction. 

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