Stereochemical consideration, substitution

Stereochemical Considerations. Much attention has been directed toward the stereochemistry of substitution reactions of alkyl complexes. Substitution of MeMn(CO)s leads to cw-MeC(0)Mn(C0)4L initially, although the cis complex isomerizes to the trans eventually ... 

Furthermore, E) and (Z)-substituted alkenes will, depending on the actual mechanism of cycloaddition, yield stereochemically different triazolines or products in equilibrium. These stereochemical considerations are important for the elucidation of the mechanism of 1,3-dipolar cycloadditions involving azides as 1,3-dipoles (see Sect. 6.3 and the reviews by Huisgen, 1984, and Lwowski, 1984). 

Stable mesophases are produced by substituted p-terphenyls and trans-stilbenes, whereas mesophases of lower thermal stabihty are formed by Schilf bases, esters and biphenyls. Stereochemical considerations are important and affect the position in the order, e.g. stibenes are planar, azoxybenzenes are shghtly twisted and Schiff bases are considerably twisted. The relatively low position of the tolanes (Z= =C ) in the order is surprising since crystalline diphenyl-acetylene contains planar molecules and possibly in the mesophase the rings become non-coplanar. The consequences, however, could be less serious than those in Schilf bases, since the cylinder of electron density associated with the molecular orbital of the =C linkage may still allow conjugation to occur even when the rings are non-coplanar. 

The stereochemical features of the reactions of racemic 1-substituted (Z)-2-butenyl-boronates 2 are considerably different from those of the 1-substituted 2-propenyl- and 1-substituted ( )-2-butenylboronates discussed above. Transition state 5 (see p 1470) is destabilized by allylic interactions between X and the (Z)-methyl substituent26, and consequently diastcrcomcr 10 is the major product via transition state 6 (sec the following table)4,15. 

When enantiomerically pure allyl p-tolyl sulfoxide is deprotonated and then treated with electrophilic 2-cyclopentenone, a conjugate addition occurs forming a new carbon-carbon bond with very high control of absolute stereochemistry (equation 25)65. See also Reference 48. Similarly, using more substituted enantiomerically pure allylic sulfoxides leads to virtually complete diastereocontrol, as exemplified by equations 26 and 27 the double bond geometry in the initial allylic sulfoxide governs the stereochemistry at the newly allylic carbon atom (compare equations 26 vs. 27)66. Haynes and associates67 rationalize this stereochemical result in terms of frontier molecular orbital considerations... 

The mechanism for the union of hydrogen with benzene also has been illuminated by stereochemical studies. Linstead and his students demonstrated that the hydrogenation of substituted benzenes occurred with considerable stereospecificity (J6). Thus the hydrogenation of diphenic acid yielded mainly compounds with either cis (one ring reduced) or cis-syn-cis (both rings reduced) configurations. 

The foregoing considerations show that, in order to reproduce the stereochemical trends, it is not necessary to introduce either d orbitals for the silicon atom or pseudo-rotations for the transition state. Conversely, the stereochemistry of substitution reactions on silicon compounds cannot be taken as a proof of d orbitals intervention in silicon chemistry. 

The a-substituted tropolonates 2 which are tris chelate complexes with a M06 coordination core have received considerable study by DNMR.27, 4S 46) The complexes are of the M(A-B)3 and M(A-B )3 types and therefore the averaging sets of Eaton efa/.26, 27 in Table 1 are applicable. The metal ions and a-R-substituents used in these studies include M = Al(III), Ga(III), Co(III), V(III), Mn(III), Ru(III), Rh(III) and Ge(IV) R = isopropyl (C3H7) and isopropenyl (C3HS), however, only complexes of Al(III), Ga(III), and Co(III) have yielded definitive mechanistic information.27, 45 46> On the basis of line shape changes of the methyl resonances these complexes can be classed kinetically as follows stereochemically nonrigid complexes which attain the fast-exchange limit of inversion and/or isomerization... 

Optically pure (+)-(i )-3-phenylsulfanyl-l,3-diarylpropan-l-ones are readily available by the enantioselective Michael addition of thiophenols to chalcones. After reduction, acidic dehydration of the racemic alcohol affords a mixture of the racemic cis- and trans- 2,4-disubstituted thiochromans (Scheme 164). A detailed consideration of the stereochemical outcome of the reaction with unsymmetrically substituted diaryl derivatives suggests the involvement of a [1,3] PhS shift via a four-membered sulfonium intermediate and this is backed up by theoretical calculations <2003T3621>. 

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