Boat-like structure

In the transition state a boat like structure appears and there will be a suprafacial cis addition to the termini of the n bond. The ene reaction does not have a symmetrical transition state and it is a thermally allowed concerted reaction. Its transition state involves a suprafacial interaction of six electrons (4 from the k bonds and two from the o bond) So it is a Huckel system and transition state is aromatic. In the terminatlogy of Woodward and Hoffmann it can be regarded as o2s + n2s + 7t2s reaction. 

Barriers for ring inversion, shown in Table 2, were obtained in this study <1997PCA3371> and other work, as well as nucleus-independent chemical shifts (NICSs) of thiepine 1 <2000MI177>. From NICS values of thiepine, the planar structure appeared to be antiaromatic and the more stable boat-like structure as nonaromatic. The structure and energies were compared with those of 1,2- and 1,4-dithins <2000MI177>. 

However this may be, it does not help to explain the course of the alkali-catalysed reaction of the lya-hydroxy isomer (2). The alkali-catalysed reactions are formulated [204] as rearrangements of the i7 alkoxy anions, and the stereochemistry found in the products corresponds to reactions in the side chain conformation with the C(i7) and C(20) oxygen functions ri s-orientated by the mutual interactions of the anionic charge and the electric dipole of the carbonyl function. Both isomers react by migration of C i3), leading to the lya-epimeric-17-keto derivatives. In the ly/if-hydroxy isomer (i), this mode of reaction is the one favoured by a conformational preference for the chair-like transition state (E), but as Wendler has emphasised, the lycc-hydroxy-ao-ketone (2) appears to react through a transition state (G) of boat-like structure. Indeed,... 

One, which results from clockwise rotation of the sidechain around the breaking C-0 bond, adopts a chair-like configuration the other results from anticlockwise rotation, which leads to a boat-like structure. Both are asymmetric structures in which the breaking C-0 bond (ca. 1.45 A) is significantly shorter than the making C-C bond (ca. 1.95 A). 

In [3,3] sigmatropic shifts, a chair-like transition state is energetically preferred to a boat-like structure. Rearrangement of the meso diene through such a transition state then would give the cis—trans isomer (Scheme 3.11), while in the case of the rearrangement of the racemic mixture the trans—trans isomer is the major product (Scheme 3.12). 

Figure 4 The chair-to-chair rearrangment in Cp2Ti[C2S4lTiCpi. The chair-like structures are local minima, while the boat-like structure is the transition state. Copyright 1994, American Chemical Society. Reprinted with permission of the American Chemical Society...

Instead, transannular strain between the silylether and one of the Cp ligands leads to a product 77 from a boat-like insertion transition structure (76 bottom). 

We also performed a single-crystal X-ray structure analysis of this lead compound. The solid state structure of this compound depicted in Fig. 3-15 shows a half-boat-like ( sofa ) conformation with the 9-phenanthryl group in a quasi-axial or r/Mf/.v/-flagpole position, and the a, 3-unsaturated exocyclic ester in a s-cis conformation. This cleft-like conformation is advantageous for the creation of centers with a high recognition ability, since one enantiomer fits in better than the other thus leading to selectivity. 

The transition state leading to the observed major product is in fact preferred (by 2.1 kcal/mol) according to the 3-2IG calculations. Inspection of its structure reveals a chair-like geometry, compared to a boat-like geometry for the transition state leading to the minor product. 

Protonated forms of the large-ring macrocycle [24]Ng02 (5) and related compounds have been shown to be active as synthetic phosphorylation catalysts in ATP synthesis. It is likely that in this case the substrate enters the macrocyclic cavity to some extent, or is enveloped by it. Evidence for this possibility comes from the crystal structure of the chloride salt of 5-6H (Figure 1) in which a chloride ion is enveloped within a cleft formed by the boat-shaped conformation of the macrocy-cle. The crystal structure of the nitrate salt of 5-4H has also recently been determined and the host again adopts a boat-like conformation as it interacts with the anion. The hydrochloride salt of the smaller [22]Ng binds two chloride anions above and below the host plane in a similar way to 1. Molecular dynamics simulations indicate that the pocket-like conformation for 5-6H is maintained in solution, although Cl NMR experiments demonstrate that halide ions are in rapid exchange between the complexed and solvated state. 

Various bimolecular assemblies that have been proposed for the transition state are shown in Scheme 13 (14, 19a, 20g). Bicyclic transition state A involves transfer of bridging alkyl group (R) to the terminally located aldehyde, while transition structure B involves reaction between terminal R and bridging aldehyde. The reaction may proceed via mono-cyclic, boat-like six-membered transition state C. Transition structures of types B and C were originally proposed for the reactions of orga-noaluminum compounds and carbonyl substrates (26, 27). Ab initio calculations suggest that methyllithium dimer reacts with formaldehyde through a bicyclic transition state related to A (28). The dinuclear Zn... 

The reaction is symmetry-allowed when it is suprafacial on all three components, but there are two reasonably accessible transition structures for an all-suprafacial reaction, chair-like 5.52 and boat like 5.53, both of which are [ g+ g+ J. In the reaction 5.48 - 5.49, it must be boat-like to give two cis double bonds in the product, but this is probably constrained by the... 

In the absence of constraints, chairlike transition structures are preferred to boat-like transition structures. 

Wentrup and co-workers reported the first X-ray study of a monocyclic N-unsubstituted l//-l,3-diazepine 5 (Figure 5) <20040BC1227>. The structures of these compounds are boat-like. 

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