Stereochemical behavior

Whereas the nucleophilic addition of vinylmagnesium bromide to a-alkoxy aldehydes (12, 16) proceeds with a low to moderate chelation-controlled diastereoselectivity, a remarkably high preference for the opposite stereochemical behavior is found with the jS-silyl phosphorus ylide 1477. Due to the electron-donating 4-methoxyphenyl substituents at the phosphorus atom, as well as the /i-methyldiphenylsilyl group, 14 is an excellent vinylation reagent which does not lead to any Wittig olefination products. 

I-Oialkoxy carbonyl compounds are a special class of chiral alkoxy carbonyl compounds because they combine the structural features, and, therefore, also the stereochemical behavior, of 7-alkoxy and /i-alkoxy carbonyl compounds. Prediction of the stereochemical outcome of nucleophilic additions to these substrates is very difficult and often impossible. As exemplified with isopropylidene glyceraldehyde (Table 15), one of the most widely investigated a,/J-di-alkoxy carbonyl compoundsI0S, the predominant formation of the syn-diastereomer 2 may be attributed to the formation of the a-chelate 1 A. The opposite stereochemistry can be rationalized by assuming the Felkin-Anh-type transition state IB. Formation of the /(-chelate 1C, which stabilizes the Felkin-Anh transition state, also leads to the predominant formation of the atm -diastereomeric reaction product. 

The results of the series of reactions shown in eq. 2 are listed in Table 1 together with our early reported data on the hydrogenation of 2-octanone (Z) [4]. The hydrogenation on all substrates proceeded smoothly and gave the corresponding chiral secondary alcohol. In the case of 3, 4, S, and 6, some amounts of lactone were produced as by-product. From this study, quite interesting stereochemical behavior... 

From the study of a series of substrates, 1 to 4 and Z/ siunmarized in Table 1, the (R,R)-TA-MNi found to show quite interesting stereochemical behavior in connection with the concept of stereocontrol mentioned above. From the stereochemistry of the... 

Aspects concerning the regio- and stereochemical behavior of these catalysts in the stereospecific polymerization of propene (or 1-olefins, in general) will be not discussed in details since these topics are at the center of several reviews recently published [11, 12, 14, 24, 25], Nevertheless, in the final sections we will briefly report about these points. 

Chlorination of olefins has also been achieved with SbCls in chlorinated solvents, which gives with mono-olefins vicinal dichloroalkanes by a syn addition. A concerted mechanism was initially proposed68 to rationalize this stereochemical behavior and the unexpectedly large amount of c -l,4-dichloro-2-butene found in the reaction of butadiene. In this case, however, because of orbital symmetry control it has been suggested that the addition occurs in an antarafacial direction69. 

The stereochemical behavior observed in the addition of BrCl has been compared with that related to the Br2 and CI2 additions to the same diene and discussed in terms of steric hindrance of the nucleophile approach (chloride ion with respect to bromide or tribromide ion) and different bridging in the bromonium or chloronium intermediates77. 

The difference in the stereochemical behavior of 62 and 63 as compared to that of 60 has been explained by assuming that the presence of the electron-withdrawing carbomethoxy substituents at C(9) and C(10) in the latter markedly decreases the availability of electrons from the participating C(7)—C(8) double bond, thus forcing the reaction to proceed mainly via the iodonium ion. 

In view of the stereochemical behavior in the additions to alkenes and dienes, the authors suggest that the reaction proceeds via a stepwise electrophilic addition126. However, in this case the two sulfur atoms of the dithioether dication are positively charged. In the reaction with multiple bonds, therefore, one of these sulfur atoms should be an electrophilic center whereas the other one should simultaneously be a nucleophilic center. In... 

The involvement of ion pairs in the addition process has also been related to the stereochemical behavior. The remarkable difference in configuration between the rearranged chlorides and acetates has been rationalized, as shown in equation 113, on the basis of a syn internal attack of Cl- on ion c and anti external attack of AcOH from the solvent pool. 

The same reaction scheme can also explain the stereochemical behavior of the addition of benzeneselenenyl chloride to 108 in methanol, which gives, in addition to the trans adduct 138, the analogous methoxy derivative 146, the cross-bonded chlorides 147 and 148, and the analogous epimeric methoxy adducts 149 and 150 (equation 130). 

The same stereochemical behavior has also been observed in the addition of ben-zeneselenenyl chloride to 1,5-cyclooctadiene (3) (equation 136). However, 3 reacts with... 

It is noteworthy that a complete stereoselectivity toward the cA-isomer, which is opposite to that found in aminomercuration of the same dienes173 characterizes these reactions. The following mechanism has therefore been proposed to rationalize the stereochemical behavior. After the addition to one of the double bonds, the electron pair of the nitrogen should interact with the mercury atom. In a second step, another mercury(II) ion from an additional molecule of mercury(II) nitrate is similarly complexed by the electrons of the nitrogen atom, requiring an approach from that same side and resulting in a cis... 

A stereochemical behavior similar to that of the 1-bromo-l-lithio aUcene 164 with regard to chiral aldehydes is shown by the hthiated methoxyallene 183. When added to iV,iV-dibenzylated a-aminoaldehydes 188, it reacts with non-chelate control so that awh -carbinols 189 are obtained predominantly. Diastereomeric ratios of 189 190 range from 80 20 to 95 5. As outlined above, the hydroxyalkylated allenes 189/190 can be converted into furanones 191/192 upon treatment with potassium f-butoxide and subsequent acid hydrolysis" . When, on the other hand, the adducts of 183 to the aldehydes 193 are submitted to an ozonolysis, A-protected a-hydroxy-/3-amino esters 194/195 result (Scheme 25)"" . 

Therefore, it is clear that protonation with retention will occur cis to oxygen but alkylation with inversion will place the new C-C bond trans to oxygen. Interestingly, when a second lithium cation is allowed to complex to the sulfinyl oxygen, the intermolecular Li—O bond has the dominant structural effect. In the presence of excess lithium ions, the stereochemical behavior of an a-lithio sulfoxide should, therefore, resemble that of an a-sulfinyl carbanion. 

However, examination of Tables I and II shows clearly that this explanation is not sufficiently general. For instance, the Si—SR and Si—F bonds show very similar stereochemical behavior, leading either to retention or inversion. Following the Sommer s rule, the SR group (for instance MeSH, pKa 10) would be a poor leaving group and be displaced mainly with retention. At the opposite extreme, the fluoro substituent (HF, pKa ... 

We can also introduce the OTs and OCOR leaving groups in the above classification near chloro and bromo groups (2). They show high reactivity and a stereochemical behavior similar to that of chlorosilanes, leading only to inversion (34). 

It is noteworthy to find chlorine more apicophilic than fluorine, although the latter is more electronegative. For instance, it explains the stereochemical behavior of 1-NpFcSiClF (29) only chlorine is displaced by carbon nucleophiles and inversion is the predominant stereochemistry. 

Scheme 7. Stereochemical behavior of organometallics (R - alkyl or aryl).

The stereochemical behavior of Ph2CHLi, i.e., cleavage of the Si—H bond with predominant inversion, can be explained as follows. We are faced with a very soft nucleophile of low level (Scheme 10). The substrate superjacent MO-nucleophile HOMO interaction prevails. The big lobe of the Si hybrid orbital points to the rear in the superjacent MO (Scheme 11), and the inversion is therefore favored. However, this reaction is quite slow (10). 

The concept of an intermediate phenonium ion was, at first, controversial, and its chief detractor was H. C. Brown.25 Although 3-phenyl-2-butyl tosylate showed the stereochemical behavior expected if an intermediate phenonium ion were formed, it did not, in his opinion, show the rate acceleration that should attend anchimeric assistance to ionization of the tosylate.26 Brown said that the stereochemical results could be accounted for by invoking rapidly equilibrating open carbocations (15). According to his explanation, ionization of the tosylate... 

The effect of different metals on the stereochemistry was studied149,150,151 in the hydrogenation of 4-ferf-butyhnethylenecyclohexane (9). Whereas Pt, Rh, Ir and Ru yield predominantly the cis isomer, fraws-l-terf-butyl-4-methylcyclohexane is the main product on Pd. The stereochemical behavior of 9 was very similar to that found for the hydrogenation of the isomeric 4-ferf-butyl-l-methylcyclohexene (10) (increasing cis selectivity... 

Despite the low reactivity and poor stereoselectivity of compound 1 as a di-enophile, the main interest of the Maignan and Raphael s paper [5] derives from the fact that it was the first one involving the use of enantiomerically pure sulfoxides in Diels-Alder reaction, which would be used profusely later in asymmetric synthesis. For this reason it deserves some additional comments. From Scheme 2 can be deduced a moderate endo orientating character of the sulfinyl group [endo-adducts are the major ones (66%) in the mixture]. Although no explanation about the stereochemical behavior of compound 1 was offered in... 

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