Exo orientation

Based on the above-mentioned stereochemistry of the allylation reactions, nucleophiles have been classified into Nu (overall retention group) and Nu (overall inversion group) by the following experiments with the cyclic exo- and ent/n-acetales 12 and 13[25], No Pd-catalyzed reaction takes place with the exo-allylic acetate 12, because attack of Pd(0) from the rear side to form Tr-allyl-palladium is sterically difficult. On the other hand, smooth 7r-allylpalladium complex formation should take place with the endo-sWyWc acetate 13. The Nu -type nucleophiles must attack the 7r-allylic ligand from the endo side 14, namely tram to the exo-oriented Pd, but this is difficult. On the other hand, the attack of the Nu -type nucleophiles is directed to the Pd. and subsequent reductive elimination affords the exo products 15. Thus the allylation reaction of 13 takes place with the Nu nucleophiles (PhZnCl, formate, indenide anion) and no reaction with Nu nucleophiles (malonate. secondary amines, LiP(S)Ph2, cyclopentadienide anion). 

Figure 1.24 Minimum-energy preinsertion catalytic intermediates leading to 1,2 and cis-1,4 polymerizations. In particular, (a) absolute minimum-energy intermediate, for which diene and of allyl group present an endo-endo orientation (that is, their concavities are oriented in opposite direction with respect to Cp ligand), can lead to 1,2-unlike or cis-1,4-like insertions, while (b) higher energy intermediate, for which diene and allyl group present an endo-exo orientation (that is, the concavity of allyl is toward Cp whereas diene is in opposite direction) can lead to 1,2-like or cis-1,4-unlike insertions.

It is to be noted that the endo addition rule is not as universal as the cis-addition rule. It can be finished by the more general rule according to which the reagents approach each other from the less hindered side. If that side appears to correspond to an exo-orientation, then exoaddition will occur both under and thermodynamic conditions. 

In all the above cases the attack from the less-hindered side (exo-orientation) was preferred and the ratio exo/endo varied from 19 (for a) to 4 (for c). The increase in weight of endo-orientation for c cannot be explained only on secondary overlap interactions. But they are accounted for by decrease of geometrical constraints. The steric requirements of saturated cyclohexane ring are higher than the steric requirements of sp2 carbon atoms in C. 

In the light of the appreciable puckering found in the seven-membered ring of [16], Childs (1984) recalculated the expected chemical shifts for the exo and endo H(8) protons of [12]. He calculated the difference in chemical shift (A6) to be 6.9 ppm which is in good agreement with the observed AS - 5.86 ppm. However, his calculations revealed that both the exo and endo protons are shielded. This surprising result is opposed to the accepted intuitive view that in an aromatic/homoaromatic system protons with the H(8)(exo) orientation should be deshielded and those with the H(8)(cndo) orientation shielded. This result closely parallels the analogous calculated data for the homocyclopropenyl cation [2] (Schindler, 1987). 

In the tetra-bridged phosphocavitands, the preorganized structure is imposed by the fixed boat-chair conformation of the four fused eight-mem-bered rings. Inwards (i) and outwards (o) configurations are defined relatively to the endo and exo orientations of the P=X bonds (X=0, S, electron pair), and six different stereoisomers arise from the equatorial or axial orientation of the substituents on the phosphorus atoms (Scheme 3). 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

If the system in question possesses a CH2 group located above the ring in a similar way to the case of the homotropenylium cation, the shift difference between endo- and exo-orient ed proton should also adopt a maximum value for the homoaromatic system. 

Whereas several X-ray structural analyses prove the endo orientation of the oxygen atom of the oxaziridine ring, the assumption of an exo oriented hydroperoxy group in 83 is based solely on the opposite sense of asymmetric induction observed for 82 and 83. 

These endo-exo preferences are energetically small and are of the order of a kcal mol-1. Consequently, factors such as dipole-dipole,27 electrostatic,28 steric29 and solvent effects27,30 can also influence the stereoselectivity. Secondary orbital interactions may not provide all of the answers, but no other theory can rationalize both the preferential endo orientation of 4 + 2 and 8 + 2 cycloadditions and the exo orientation of 6 + 4 cycloadditions so efficiently. See also Exercise 12. 

Recently, Boche et al. (5) have reported the synthesis of a highly interesting crystalline adduct of 1,3-diphenylallyllithium with diethyl ether. The crystal structure of this complex (I in Fig. 1) shows symmetrical it bonding between lithium atoms and allylic fragments. Each lithium atom interacts with two allylic 7t systems and further with the oxygen atom of a diethyl ether molecule. An exo, exo orientation of the phenyl ligands has been observed in this coordination polymer. 

The pentaammineosmium moiety occupies an exo orientation on all cycloadducts, indicating that cycloaddition takes place anti to the face of the pyrrole ring coordinated by the metal. The stereochemistiy of the cycloaddition appears to be governed by the steric environment about the... 

Scheme 5.1 Covalent prearrangement in the synthesis of [2]cate-nanes consisting of large, mobile rings. The structure of the first ring should not allow an exo orientation of the carbonate link.

Maleic anhydride reacts with cyclopenta-1,3-diene in a Diels-Alder reaction. Since there is a plane of symmetry, the reaction can lead to two achiral compounds, which are diastereomers of each other, containing an endo- or exo-oriented dicarboxylic anhydride group. These differ in absolute and relative configuration at the bond shared by both rings. Under normal conditions the Diels-Alder reaction proceeds stereospecifically to yield preferentially the endo product. Note that in the tricyclic product no trans fusion in the ring system is possible as a consequence of the reaction mechanism. Subsequent reduction of the products therefore affords two diols, which are also diastereomers of each other. These may be separated by chromatography on an achiral stationary phase. 

This compound is isobornyl acetate which has an exo-oriented acetate group and therefore the compound will exist as two enantiomers, i.e., the R,R,R isomer and its non-superimposable mirror image S,S9S isomer. The name given in the question designates the racemate. 

The stereochemistry of the cycloadducts in hetero Diels-Alder as well as of the all-carbon Diels-Alder reactions depends upon the different geometry of the possible transition structures [3,12,38]. According to an endo- or exo-orientation of the dienophile and an (E)- or (Z)-configuration of the diene, four different transition structures have to be discussed which are shown exemplary for 1-oxa-1,3-butadienes in the inter- and intramolecular mode (Schemes 1-2 and 1-3). 

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