Stereoelectronic special

J.E. Baldwin (1976 B) also introduced a set of rules for closure reactions of 3- to 7-membered rings which are derived from similar arguments about stereoelectronic control . The specific rules are clearly explained in the original paper and need not be repeated here since their utility seems to be restricted to some specialized cases. The assumed... 

Stereoelectronic effects and nonbonded interactions are non-cooperative in the reactions of (E)-allylboronates and x-heteroatom-substituted aldehydes. Thus, while transition state 8 experiences the fewest nonbonded interactions (gauche pentane type, to the extent that X has a lower steric requirement than R3), transition state 9 is expected to benefit from favorable stereoelectronic activation (Felkin-type)58f. This perhaps explains why the reaction of 2,3-[iso-propylidenebis(oxy)]propanal and ( >2-butenylboronate proceeds with a modest preference (55%) by way ol transition state 9. This result is probably a special case, how ever, since C-3 of 2.3-[isopropylidenebis(oxy)]propanal is not very stcrically demanding in 9 owing to the acetonide unit that ties back the oxygen substituent, thereby minimizing interactions with the... 

This chapter describes the chemical behavior of a-sulfinyl and a-sulfonyl carbanions. The stereoelectronic effects of these sulfur-containing groups have been the subject of much controversy for more than a decade which has now gradually settled down. Meanwhile, the special features of the chemical behavior of these groups have been utilized for syntheses of thousands of useful organic substances. This chapter deals with the... 

C NMR spectroscopy of 1,7-dioxa- (314), l-oxa-7-thia- (315), and 1,7-dithiaspiro[5.5]undecane (316) and many of their derivatives, with special emphasis on stereoelectronic and steric effects governing the configurational and conformational properties of these compounds (484-486). This work showed that chemical-shift parameters developed for six-membered carbocyclic compounds in chair conformations (149) could be applied successfully only to sulfur and not to oxygen heterocycles (486). 

The relative energy barrier for stereochemical change from one tetrahedral intermediate to another and that for a stereoelectronically controlled cleavage must also be taken into consideration when analyzing experimental results. Evidence will be presented that in certain cases (specially when the tetrahedral intermediate exists in the T ionic form) conformational change of the intermediate does not compete with its cleavage. 

D-(+)-galactose (15) is an example of the consecutive numbering of the carbon ring atoms in a monosaccharide (disaccharide see p. 253). Carbohydrates can exist in a cyclic and an acyclic structure. For this reason there is a special position in the structure of a monosaccharide, the carbon atom C-l and so called anomeric center. You can see that there is an equilibrium between er-anomer a-15 and / -anomer /3-15 of D-(+)-glucopyranose over the acyclic aldehyde structure 16. Both are cyclic hemi-acetals. The /Tanomer is the preferred conformation, but there are a few effects, like sterical or stereoelectronical effects (anomeric effect, inverse anomeric effect), which have influence on the a /i rate. 

The formation of bi- and tricyclic compounds by transannular photochemical cyclization of monocyclic reactants mostly follows special rules that are caused by the molecular rigidity, equilibria of ring conformers and distinct stereoelectronic effects and, therefore, will be discussed in a separate section. Naturally, it is no longer possible to strictly structure the reactions according to the size and the type of the ring and, furthermore, vastly different examples are known. Therefore, only some representative examples will be discussed in this section and the selection does not claim to be complete. 

The electronic structure of 11 sparteines has been studied by Galasso et a/. by the use of experimental and DFT calculated NMR parameters, special attention being paid to spin-spin couplings. In particular, the influence of stereoelectronic hyperconjugative effects on A JMCeq/Vncax has been correctly accounted for by the DFT results. 

The chelation of small molecules described above may be extended to metal ions. The convergence of the carboxyls within the molecular clefts provides a microenvironment ideal for divalent metals. A special structural feature of the new ligands involves stereoelectronic effects at carboxyl oxygen. Classical chelates such as EDTA present the less anti lone pairs to the metal ion, but the new structures offer the more basic syn lone pairs. Metals such as Ca and Mg are tightly bound and readily transported across liquid membranes (Scheme 6). In addition, the mode of binding within the new ligands is exclusively trans, a feature which is likely to lead to altered reactivity of the bound metal ions as catalysts. 

Retrosynthetic analysis primarily considers the electronic properties of synthons, i.e., the stabilizing effect of neighboring groups on the charged species in the ground state. This approach is not amenable to analysis of asymmetric syntheses since their outcome is determined by the stereoelectronic properties of the transition state. Stereoselective reactions are kinetically controlled, and the rational approach to analysis of the stereoelectronic properties of the energetically preferred transition state is difficult and usually supported by specialized computer programs. 

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