Lead isomers factors

Protonation of the a-carbanion (50), which is formed both in the reduction of enones and ketol acetates, probably first affords the neutral enol and is followed by its ketonization. Zimmerman has discussed the stereochemistry of the ketonization of enols and has shown that in eertain cases steric factors may lead to kinetically controlled formation of the thermodynamically less stable ketone isomer. Steroidal unsaturated ketones and ketol acetates that could form epimeric products at the a-carbon atom appear to yield the thermodynamically stable isomers. In most of the cases reported, however, equilibration might have occurred during isolation of the products so that definitive conclusions are not possible. 

If a monoarylacetylene (ArC = CH) is taken as a model for a transition state of an arenediazonium ion with a nucleophile Nu, two types of transition state can be visualized the first, 7.13, leads to the (Z)-azo compound 7.14, whereas the second, 7.15, results in the (E )-isomer 7.16 (Scheme 7-3). If the transition state is reactantlike (i.e., early on the reaction coordinate), repulsive interaction between the nucleophile and the aryl nucleus is small because the distance Nu-Np is still large. Therefore, the repulsion between the lone pair on Np and the aryl nucleus becomes the decisive factor. It favors an (E )-configuration of the Np lone pair with respect to the aryl nucleus (obviously it is energetically dominant compared with the repulsion between the lone pairs on Na and Np) therefore, transition state 7.13 is at a lower energy level, and Nu attacks NB in the (Z)-configuration. 

The solubility of LAS is dependent on various factors. For homologs with different molecular weights, it is normally the case that the higher the molecular mass, the lower the solubility. If the homologs have the same molecular weights, those with symmetrical isomer distribution will be easily dissolved branched homologs lead to a deterioration of solubility. The Na salts of the 2-and 3-phenylalkanes are less soluble then those of the internal phenyl isomers (for calcium salts the opposite is true) [187,188]. 

E) alkenes. One explanation for this is that the reaction of the ylid with the carbonyl compound is a 2-1-2 cycloaddition, which in order to be concerted must adopt the [rt2s+n2al pathway. As we have seen earlier (p. 1079), this pathway leads to the formation of the more sterically crowded product, in this case the (Z) alkene. If this explanation is correct, it is not easy to explain the predominant formation of ( ) products from stable ylids, but (E) compounds are of course generally thermodynamically more stable than the (Z) isomers, and the stereochemistry seems to depend on many factors. 

In Entry 4 the silyl group appears to introduce a controlling steric factor, leading to the observed stereoisomer. The unsubstituted terminal alkyne, which reacts through the dianion, gives the alternate isomer. 

Consider the trans isomer of butadiene. Both the symmetry operations that define the group < 2h and the characters of the representation r are given in Table 3. The reduction of this representation leads to Tn =2Bg 2Aa. Thus, two linear combinations of the atomic orbitals can be constructed of symmetry Bg and two others of symmetry A. Their use will factor the secular determinant into two 2x2 blocks, as described in the following paragraph. 

Semiempirical and ab initio calculations were performed on the cyclodimerization of 1-methylphosphole oxide (86). The relative order of the values of the heat of formation for the transient states leading to the possible isomers (87-94) confirmed that the formation of the isomer prepared (87) is indeed favored to a high extent (Scheme 24) [67], The selectivity can be explained by steric reasons and kinetic factors. 

Low-temperature 31P NMR studies showed that the reaction of (PCP)Ir with nitrobenzene leads to three products of the type (PCP)Ir(aryl)H and meta- and /> ra-additions are kinetically favored. However, refluxing the trans-isomer afforded the f-product, designated the thermodynamic product. Nevertheless, chelation to the metal still remains an important regiochemical factor in these reactions, which will be termed chelation controlled or directed hereafter.92... 

The diastereoselectivity is striking. Even when steric factors are not overwhelming (eq. Table 2, entries 5, 9, 22, 29, 30, and 33) only a single oxaspiropentane was detected. A particularly useful aspect of this reaction deals with carbonyl partners that are easily epimerized at the a-carbon. It appears that epimerization is faster than carbonyl addition. However, since one of the two epimers reacts faster than the other, only a single diastereomeric oxaspiropentane still results. For example, 2-isopropyl-5-methylcyclopentanone exists as an Zs.Z-mixture (see Eq. 29)47). For steric reasons, the Z isomer reacts faster than the E isomer which leads to 12 as the... 

The regiochemistry of the acid-catalyzed water addition to cis- (8c) and trans- (8t) 1-ethoxy-l,3-butadienes leading to 9c and 9t, respectively33, has been investigated in deuterium incorporation experiments (equations 5 and 6). The c/s-isomer incorporated deuterium at the 2-position as well as the 4-position whereas deuterium was added to the fraws-isomer exclusively at the 4-position. This result has been interpreted in terms of equations 7 and 8 y-protonation in the frans-isomer was assumed to be controlled mainly by thermodynamic factors whereas a-protonation was assumed to arise from charge control... 

The low selectivity affects the synthetic interest of homolytic arylation from two points of view. The first concerns the position of substitution generally all the free positions are substituted, giving very complex mixtures of isomers. Thus, for example, quinoline gives all the seven possible isomers in appreciable amounts.This is in contrast to all the homolytic substitutions described in the previous sections, which lead to exclusive attack at the 2- and 4-positions. The other aspect concerns the conversions of the heterocyclic compounds, which are always very low, usually lower than 1%. If the conversions are high, the mixture of the reaction products becomes much more complex. Thus with quinoline it can be easily foreseen from the partial rate factors (Table IX) that not only all the possible 21 diphenylquinoline isomers, but also... 

Factors which affect the oxepin-benzene oxide equilibrium positions are similarly expected to influence the thiepin-benzene episulfide distribution at equilibrium. Since however the thianorcaradiene tautomer has not to date been detected, the main evidence for this form is based upon the thermal instability and reactions of the thiepin system. Thus it is assumed that where the thianorcaradiene isomer is present, a spontaneous thermal decomposition involving extrusion of a sulfur atom will occur. Substitution at the 2,7-positions in the oxepin-arene oxide system leads to a preference for the seven-membered ring form and this effect was further enhanced by bulky substituents (e.g. Bu ). A similar effect was observed in thiepins and thus the remarkable thermal stability of (49) (2,7-r-butyl groups) and (51) (2,7-hydroxyisopropyl groups) contrasts with the behavior of thiepin (55)(2,7-isopropyl groups), which was thermally unstable even at -70 °C (78CL723). The stability of thiepin (49) results from the 2,7-steric (eclipsed) interactions which obtain in the thianorcaradiene form but which are diminished in the thiepin tautomeric form (relative to the episulfide tautomer). 

The direct reduction itself may be suppressed by adding the tributyltin hydride slowly over several hours, thus keeping its concentration as low as possible. In this case, however, consecutive rearrangements sometimes lead to different products.1,2 Stereochemistry may be a decisive factor for the outcome of the rearrangement.1-1 4 For example, in the bicyclo[3.2.0]hept-2-en-6-one series, the exo-derivative exo-1 underwent ring enlargement to a cycloheptanone 2, while the e c/o-diastereomer enclo-1 reacted at the C —C double bond to form a tricyclic isomer 3.1-2... 

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