Additions steric course

The addition of lithium dimethyl-, dibutyl-, diphenyl- or 1-butenylcuprate to 2d produced (JiS) 94% ee, ifiS) 95% ee, (jiR) 96% ec, and ifiR) 90% ee, respectively. In this case the difference between S and R results from the CIP selection rules and not to the steric course of the reactions. The conformation of the chiral auxiliary in 1 d is such that one IV-methyl group is axially, the other equatorially, arranged on the bicyclic structure. The reagents attack the double bond from the side opposite to the equatorial iV-methyl group. Other chiral auxiliaries such as a-c14 were less effective15. 

Change of base (pyridine, aniline, water) had no obvious effect on the steric course of the reaction (59). Rather surprisingly, it appears that the reactions of the cis and trans isomers of 1-bromopropene with the hydride [Co(CN)5H] give different products this eliminates an initial addition of Co—H to the double bond, since this would lead to the same product for the two isomers 105). Vinyl chloride apparently reacts only with [Co salen] , and not with the hydride, to form the CH2=CHCo complex 43) the same may be the case with the BAE complexes 40). 

The significance of B—C ir-bonding in determining the course and the ease of chemical reaction with organoboranes is obviously a more difficult question for both the experimentalist and the theoretician. Again, both inductive and resonance effects can, in principle, play a role but in addition, steric effects now become important, either for intramolecular reactions, as in Eq. (5), or for intermolecular reactions, as in Eqs. (7, 8). 

The frontier-electron density was used for discussing the reactivity within a molecule, while the superdelocalizability was employed in comparing the reactivity of different molecules 44>. Afterwards, the applicability of the frontier-electron theory was extended to saturated compounds 50>. The new theoretical quantity "delocalizability was introduced for discussing the reactivity of saturated molecules 60>. These indices satisfactorily reflected experimental results of various chemical reactions. In addition to this, the conspicuous behavior of HO and LU in determining the steric course of organic reactions was disclosed 44.51). 

The steric course of electrophilic additions of BrN3, IN3, and iV-bromosuccinimide (NBS) to the tetrahydropyridazine ring of benzo[g]pyridazino[l,2- ]phthalazine-6,13-dione system, for example, 130, has been studied and the results are shown in Scheme 12 <1997CJC348>. 

Toromanoff, E., Steric Course of the Kinetic 1,2 Addition of Anions to Conjugated Cyclohexenones, 2, 157. 

Additional information about the effect of amine structure and solvent on the steric course of the reaction of amines with acetylenecarboxylic esters has recently been reported by Huisgen [93]. 

In a study of the addition of nitrosyl chloride or nitrosyl bromide to norbor-nene and norbomadiene, it was observed that (a) there was no structural rearrangement during the reaction, (b) a cis addition had taken place, (c) nucleophilic solvents such as ethanol or acetic acid were not incorporated in the products. These facts seem to speak against an ionic addition mechanism, while a free radical initiated by NO radicals was considered unlikely since nitric oxide is inactive toward norbomadiene. Therefore a four-center mechanism has been suggested [70], However, when a relatively simple, unstrained olefin such as A9-octalin was subjected to the reaction, only blue, crystalline, monomeric 9-nitroso-10-chlorodecalin was produced. This product had a trans configuration. Thus it is evident that the structure of the olefin has a significant bearing on the steric course of the addition [71]. 

The steric course of the addition of the two hydrogen atoms is generally syn. Accordingly, the reduction of cis- and frans-stilbenes with dideuterodiimide affords meso- and racem-1,2-dideutero-l,2-diphenylethanes, respectively, with 97% selectivity 188... 

G. Fronza, C. Fuganti, P. Grasselli, G. Pedrocchi-Fantoni, and C. Zirotti, On the steric course of the addition of diallylzinc onto a,P-dialkoxy chiral carbonyl compounds Stereospecific synthesis of 2,6-dideoxysugars of the L-series, Tetrahedron Lett., 23 (1982) 4143 1146. 

A similar stereochemical question as in the /8-replacement reactions can be asked in the a, /8-eliminations where the group X is replaced by a hydrogen, i.e., is the proton added at C-/8 of the PLP-aminoacrylate on the same face from which X departed or on the opposite face This question has been answered for a number of enzymes which generate either a-ketobutyrate or pyruvate as the keto acid product. Crout and coworkers [119,120] determined the steric course of proton addition in the a,/8-elimination of L-threonine by biosynthetic L-threonine dehydratase and of D-threonine by an inducible D-threonine dehydratase, both in Serratia marcescens. Either substrate, deuterated at C-3, was converted in vivo into isoleucine, which was compared by proton NMR to a sample prepared from (3S)-2-amino[3-2H]butyric acid. With both enzymes the hydroxyl group at C-3 was replaced by a proton in a retention mode. Although this has not been established with certainty, it is likely that both enzymes, like other bacterial threonine dehydratases [121], contain PLP as cofactor. Sheep liver L-threonine dehydratase, on the other hand, is not a PLP enzyme but contains an a-ketobutyrate moiety at the active site [122], It replaces the hydroxyl group of L-threonine with H in a retention mode, but that of L-allothreonine in an inversion mode [123]. Snell and coworkers [124] established that the replacement of OH by H in the a, /8-elimination of D-threonine catalyzed by the PLP-containing D-serine dehydratase from E. coli also proceeds in a retention mode. They... 

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