Hydrogen bonds facial selectivity

In contrast with exo (top) facial selectivity in the additions to norbomene 80 [41], Diels-Alder reaction between isodicyclopentadiene 79 takes place from the bottom [40] (see Scheme 32). To solve this problem, Honk and Brown calculated the transition state of the parent Diels-Alder reaction of butadiene with ethylene [47], They pointed ont that of particular note for isodicyclopentadiene selectivity issue is the 14.9° out-of-plane bending of the hydrogens at C2 and C3 of butadiene. The bending is derived from Cl and C4 pyramidalization and rotation inwardly to achieve overlap of p-orbitals on these carbons with the ethylene termini. To keep the tr-bonding between C1-C2 and C3-C4, the p-orbitals at C2 and C3 rotate inwardly on the side of the diene nearest to ethylene. This is necessarily accompanied by C2 and C3 hydrogen movanent toward the attacking dienophile. They proposed that when norbomene is fused at C2 and C3, the tendency of endo bending of the norbomene framework will be manifested in the preference for bottom attack in Diels-Alder reactions (Schane 38). 

These catalysts are believed to function through an acyclic TS. In addition to the normal steric effects of the open TS, the facial selectivity is probably influenced by tt stacking with the aryl ring and possibly hydrogen bonding by the formyl hydrogen.152... 

Mandelate and lactate esters have been found to generate diastereoselectivity in reactions of hydroxy-substituted quinodimethanes generated by thermolysis of benzo-cyclobutenols.88 The reactions are thought to proceed by an exo TS with a crucial hydrogen bond between the hydroxy group and a dienophile carbonyl. The phenyl (or methyl in the case of lactate) group promotes facial selectivity. 

In the epoxidation of acyclic allylic alcohols (Scheme 6), the diastereoselectivity depends significantly on the substitution pattern of the substrate. The control of the threo selectivity is subject to the hydroxyl-group directivity, in which conformational preference on account of the steric interactions and the hydrogen bonding between the dioxirane oxygen atoms and the hydroxy functionality of the allylic substrate steer the favored 7r-facial... 

Step b Jt-Facial selectivity may be determined by the stabilizing effect of the H-bond between the benzoate oxygen with the aldehyde hydrogen (see Tetrahedron Lett. 1997, 38, 33). 

The six-membered transition-state is stabilized by hydrogen-bonding between the nitrogen of the imine and the carboxyl group of proline. Switching of the facial selectivity is disfavored, because of to steric repulsion between the PMP group of the imine and the pyrrolidine moiety of the enamine. This is opposite to similar direct asymmetric aldol reaction in which re-facial attack occurs [27, 30, 36]. 

Bach, T., Bergmann, H., and Harms, K. (1999) High facial selectivity in the photocycloaddition of chiral aromatic aldehyde and enamide induced by intermolecular hydrogen bonding. Journal of the American Chemical Society, 121, 10650-10651. 

During the photocycloaddition of 88 with cyclopentene (Reaction 1), de of the major isomer 89 increased from 30% in nonpolar solvents up to 68 in a mixture of methanol and acetic acid. When prochiral enone 91 was irradia in the presence of a cyclopentene linked to the 8-phenylmenthol (Reaction the best selectivity was now obtained in nonpolar solvents. To explain this eff< it was proposed that the facial selectivity is high in every case and that diastereoselectivity depends on an s-cis s-trans ratio of the conjugated es influenced by hydrogen bonding [65]. Similar results were obtained with c... 

The epoxy alctrfiol (97), a key intermediate in the synthesis of maytansine, has been prepared through Ti-catalyzed epoxidadon of (95 equation S6). The alcohol (95) exists predominantly in confoimation (162), with the allylic hydrogen at C-4 and the ir-bond very nearly eclipsed. The oxygens of the alcohol and silyl ether which are located below the plane the ir bond complex with Ti this complex blocks the approach of the epoxidizing reagent fitom the a-fu and hence the P-epoxide is fmmed. It is of interest to note that the ir-facial selectivity resulting fiom this route is the opposite of the ir-facial selectivity observed in MCPBA epoxidadon (see equadon 33). 

The facial selectivity can be explained on the basis of mechanistic considerations and the models are supported by molecular mechanics calculations. It has been proposed that, when a free hydroxy group is present, the acylnitroso group is hydrogen bonded to the hydroxy group via the oxygen atom of the nitroso group. The presence of a methoxy group accounts for the lesser selectivity observed (Scheme 1). 

MD simulations of halide anions [X]- and their inclusion complexes [X] c [L4+] with a macrotricyclic tetrahedral host built from four quaternary ammonium sites dissolved in [C4mim][PF6] were carried out [118]. In the dry IL the uncomplexed halides were surrounded by four to five [C4mim]+ cations which bond via hydrogen bonding to facial coordination. The first solvation shell of [Cl]-, [Br], and [I] comprised of three to four cations next to four H20 molecules for the humid system. The solvation of the [L4+] host and of its [X]-1 [L4+] complex mainly involved anions in the dry IL, and additional H20 molecules in the humid IL. Free energy perturbation calculations predicted that in the dry liquid [F] is preferred over [Cl] , [Br] and [I]- in contrast to the aqueous solution where [L4+] was selective for [Cl]-. In the humid liquid no [F] /[C1] discrimination was observed, showing the importance of small amounts of water on the complexation selectivity [118]. 

Excellent facial selectivity observed using a cyclic alkene has been explained on the basis of a hydrogen bond-directed intermolecular cycloaddition. 

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