Stereoelectronic effects intermediates

The substituent effects in aromatic electrophilic substitution are dominated by resonance effects. In other systems, stereoelectronic effects or steric effects might be more important. Whatever the nature of the substituent effects, the Hammond postulate insists diat structural discussion of transition states in terms of reactants, intermediates, or products is valid only when their structures and energies are similar. 

Sulphuranes - see also Dialkoxysulphuranes, Oxysulphuranes as intermediates 406 Sulphur bonding 484-493 Sulphur-containing groups, stereoelectronic effects of 584-594 Sulphur dioxide addition to 215-217 extrusion of 137, 140, 141, 158, 163, 397-402, 801, 805, 962, 1098 Sulphur monoxide, extrusion of 397-402 Sulphur trioxide, reactions of 217, 218 Sultenes, as intermediates 743 Sultines 679, 943 as photolytic products 881, 882... 

Chemoselectivity plays an important role in the benzannulation reaction as five-membered rings such as indene or furan derivatives are potential side products. The branching point is again the rf-vinylcarbene complex D intermediate which maybe formed either as a (Z)- or an ( )-metallatriene the (E)-configuration is required for the cyclisation with the terminal double bond. (Z)-Metallatriene D, however, leads to the formation of furan derivatives H (Scheme 8). Studies on the formation of (E)- and (Z)-isomers discussing stereoelectronic effects have been undertaken by Wulff [17]. 

C X bond, but not from B because only the has such an orbital. If the intermediate is in conformation B, the OR may leave (if X has a lone-pair orbital in the proper position) rather than X. This factor is called stereoelectwnic control Of course, there is free rotation in acyclic intermediates, and many conformations are possible, but some are preferred, and cleavage reactions may take place faster than rotation, so stereoelectronic control can be a factor in some situations. Much evidence has been presented for this concept. More generally, the term stereoelectronic effects refers to any case in which orbital position requirements affect the course of a reaction. The backside attack in the Sn2 mechanism is an example of a stereoelectronic effect. 

P. Deslongchamps, Stereoelectronic Control in the Cleavage of Tetrahedral Intermediates in the Hydrolysis of Esters and Amides , Tetrahedron 1977, 31, 2463 - 2490 P. Deslongchamps, Stereoelectronic Effects in Organic Chemistry , Pergamon Press, Oxford, 1983. 

This Chapter deals with the stereoelectronic effects which control the cleavage of tetrahedral intermediates during the formation or the hydrolysis of an ester. Since these effects are also operative in the ester function itself, a discussion of the functional group will first be presented. 

The stereoelectronically controlled reaction of hydroxide ion with an 0-labeled tertiary amide(J8 ) (Fig. 3) should give the intermediate 19 which can fragment in only two ways, yielding the starting labeled amide J8 or the hydrolysis products direct cleavage of W to give unlabeled amide 18 cannot take place with the help of the primary electronic effect. In order to form the unlabeled amide J8 with stereoelectronic control, intermediate Jj) must first be converted into another conformer such as 20. Oxygen... 

The formation or the hydrolysis of an acetal function proceeds by the mechanism described in Fig. 16 in which oxonium ions and hemiacetals occur as intermediates. It has also been established (76) that the rate determining step in acetal hydrolysis is generally the cleavage of the C—bond of the protonated acetal 100 to form the oxonium ion 111, This ion is then rapidly hydrated to yield the protonated hemiacetal 112 which can give the aldehyde product after appropriate proton transfers. It is pertinent therefore to find out if stereoelectronic effects influence the rate determining step (110 111) of this hydrolysis reaction. 

Intermediate 69 can either yield aZ (hydroxy-ester) or an E (lactone) ester. Intermediate 70 can only yield an E ester (lactone) whereas intermediate 7 can produce two esters, the hydroxy-ester having an E conformation and the lactone. Thus, primary stereoelectronic effects allow the cleavage of intermediates 69-71 to produce either the hydroxy-ester or the lactone prod-... 

The relationship between the conformation of the acetal function and its reactivity towards ozone has been described in detail (cf p. 41 ). It was shown that the insertion of ozone into the C —H bond of the acetal function to form the corresponding hydrotrioxide tetrahedral intermediate is subject to stereoelectronic control. This section deals with the next step of this reaction, i,e., the decomposition of the hydrotrioxide tetrahedral intermediate to yield the ester product. Experimental results will be presented to show that this step is also controlled by stereoelectronic effects. These results can therefore be used as evidence for the principle of stereoelectronic control in ester formation. 

In the biogenesis of steroids, the enzyme-catalyzed polycyclization of squalene (225) produces the tetracyclic substance lanosterol (225) which is eventually converted into cholesterol (227) Eschenmoser, Stork, and their co-workers (80-82) have proposed that the squalene-1anosterol conversion can be rationalized on the basis of stereoelectronic effects. The stereochemical course of this biological cyclization (83, 84) can be illustrated by considering the transformation of squalene oxide (228) (an intermediate in the biosynthesis of cholesterol (83, 84)) into dammaradienol 229. This transfor-... 

Treatment of polyolefinic ketal 230 with stannic chloride in pentane gave a mixture (30% yield) of about equal amounts of the two racemic D-homoster-oidal tetracyclic isomers 231 (88). In this cyclization, the first cationic intermediate is not chiral and the two faces of the 5,6-double-bond can react with equal facility with the carbonium ion as a consequence, the product obtained (231) is necessarily racemic. The conversion of the open-chain tetraenic acetal 230 having no chiral centers into a tetracyclic system having seven such centers and producing only two (231) out of a possible 64 racemates is a striking tribute to the power of stereoelectronic effects. 

The reaction of nucleophiles with the conformationally rigid piperidinium ion lj>, like that with cyclic oxonium ions, can also be controlled by stereoelectronic effects. On that basis, the addition of a nucleophile on the upper face of Jjj must lead to the boat-like intermediate whereas that from the lower face must lead to the chair-like intermediate V7. The transition state leading to must be less favorable than that leading to Yl and product V7 should therefore be favored. 

The 1,6-addition to a,s, y, a-dienones is also subject to stereoelectronic effects. Addition on the bottom face of dienone 137 leads to a chair-like intermediate while that on the top face leads to a boat-like intermediate 140 in order to maintain maximum orbital overlap. Also, in 140 the R group encounters an eclipsed 1,2-R/H interaction and more importantly, a 1.4-CH3/R steric interaction which resembles the bowsprit flagpole arrangement of a twist-boat form of cyclohexane. This analysis of Marshall and Roebke (48) predicts that the trans product 139 should prevail over the cis product 141. 

These reactions likely proceed vi a the formation of an intermediate carban-ion. Indeed, the carbanion 496 generated by treatment of 491 and 492 with lithium di isopropyl amide gave 81% of 491 (H=D) and 19% of 492 (H=0). The preferential formation of 491 can be explained on the basis of stereoelectronic effects which influence the reactivity of the intermediate carbanion... 

The knowledge of photoreaction mechanisms and of the structures of intermediate radicals is also of particular interest in the understanding of many structural modifications of sugars. For instance, radicals which are not solvated and less susceptible to steric factors adopt specific conformations determined by stereoelectronic effects which can explain some regio and stereoselectivities of many photochemical reactions in carbohydrates. 

Stereochemical data on reactions involving radicals substituted with one, two or three oxygen atoms at the radical centre is more extensive than data on the corresponding nitrogen species, and apparent stereoelectronic effects of around an order of magnitude in relative rates are known in a number of systems. A problem has been, however, that until recently the geometry of the radical intermediates has not been well defined. 

Stereoelectronic effects also play a role in the photoreactivity of the chlorides 56230, in which the halide is y to the aryl chromophore230. Their irradiation in methanol yields primarily products derived from carbocation intermediates, but here the endo-isomer is more photoreactive than the exo-isomer, in contrast with the situation observed for the / -aryl compounds. The inverted reactivity pattern is attributed to the favourable aryl/chlo-rine relationship in the endo-isomer. 

The optimized geometry of the biphenol MOM ether-SnCU was obtained from a partial PM3 calculation of the MOM units on the basis of a B3LYP/LANL2DZ-opti-mized geometry for a biphenol-SnCU complex (Fig. 5). It is noteworthy that the C5-04 bond is almost perpendicular to the C1-C3 axis, presumably because of the steric repulsive interactions with apical and equatorial chlorines. Also of interest is the observation that the C5-06 bond is shorter than the 04-C5 bond. This indicates that the C5-06 bond has a partial double-bond character because of the stereoelectronic effect and its most stable conformer would be the reactive oxonium intermediate. 

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