Stereochemical Data

Brown has questioned whether the spatial structure of the solvolysis products of 2-exo-norbornyl brosylate is a proof of the intermediate nonclassical ion He believes the classical 2-norbornyl cation to yield, when attacked by the nucleophile, only exo compounds for two resons  

In the unsubstituted classical 2-norbornyl cation the steric hindrances to the endo attack must yield the same exo products as the attack from the backside of a non- 

These data support the assumption about an intermediate nonclassical ion. 

An uncommonly high content of endo isomer has been recorded by Brown on solvolysis of 2-tert-butyl-2-exo-norbornyl-p-nitrobenzoate (exoiendo product ratio is 95 5) The Goering-Schewene diagram (Fig. 6) allows to recognize a higher stability of the ground state of the endo isomer (by 1.9 kcal/mole) and a small difference in transition state energies (1.7 kcal/mole). 

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The kinetics and stereochemical data definitely point to a transient aziridinone of inverted configuration 2 among the further potential intermediates (Fig. 5). Reactions of aziridinone 2 with a nucleophile would allow the crucial bond of the product Ifl to form through a second inversion (overall retention). This pathway... 

This chapter is concerned initially with kinetic results and mechanistic interpretations of the CO insertion (Section III) and extrusion (Section IV) reactions. A discussion of the stereochemical data follows (Section V), and a comprehensive survey of these reactions by the triads (Section VI) rounds out the review. Carbon monoxide insertion reactions were discussed in 1967 by Basolo and Pearson (21). Since then they have been mentioned in several reviews (49, 118, 203, inter alios) but have not been treated comprehensively. 

Since the inception of our work Jere, Miller and Jackson have published kinetic and stereochemical data on the hydrogenation of alanine (19). Important in their analysis is the observation that amino acids must be in their protonated form to undergo facile hydrogenation since reduction of carboxylate anions is significantly more endothermic than protonated acids (19). Control of pH is important for two reasons at neutral pH amino acids exist as zwitterions and the resultant hydrogenation products are basic. For these reasons a full equivalent of phosphoric acid (or similar acid) is required to obtain high yields. 

Further experimental studies involved the determination of the rate constant of the reaction of several alkyl halides with a series of electrochemically generated anion radicals so as to construct activation driving force plots.39,40,179 Such plots were later used to test the theory of dissociative electron transfer (Section 2),22,49 assuming, in view of the stereochemical data,178 that the Sn2 pathway may be neglected before the ET pathway in their competition for controlling the kinetics of the reaction. 

The preponderance of stereochemical data in the literature has been obtained from studies using 2-pentene, which now appears to have been a rather poor substrate for emphasizing steric aspects of the reaction. Recent experiments utilizing 4-methyl-2-pentene (76) have given much clearer indications of steric control in metathesis reactions (vide infra). 

Later on, product distribution studies15 of the ionic addition of chlorine to conjugated dienes, and in particular to cyclopentadiene, 1,3-cyclohexadiene, cis,cis-, trans,trans-and c ,fraws-2,4-hexadienes, and cis- and trans-1,3-pentadienes have supplied the first stereochemical data, showing that the stereochemistry of 1,4-addition is predominantly syn, although to an extent smaller than that of bromine addition. Moreover, the 1,2-addition is generally non stereoselective, except for the addition to the 3,4-bond of cis-and trans-1,3-pentadienes where the attack is 89-95% anti. Finally, appreciable amounts of cis- 1,2-dichlorides were obtained from the two cyclic dienes, whereas 2,4-hexadienes showed a preference for anti 1,2-addition, at least in the less polar solvents (carbon tetrachloride and pentane). On the basis of all these results the mechanism shown in equation 29 was proposed. 

The experimental regio and stereochemical data as a function of substitution on cyclopentadiene may be summarized as follows ... 

These experimental facts show mainly parallel chemical and stereochemical data for mono- and bifunctional organosilanes and allow us to conclude that the relative ease of displacement is... 

Stereochemical data show that the above empirical order can generally be extended to various acyclic organosilanes (2, 5). Moreover, cyclic... 

Our purpose is now to show that the Anh and Minot approach gives a qualitatively rational explanation for most of the stereochemical data at silicon. 

This agrees well with the stereochemical data, and in particular, for the general trend observed experimentally. 

In the case of the p -methoxyphenoxide anion, Taft et al. (77) have shown that the oxygen atom has a high degree of sp3 character. This nucleophile is quite similar to hard alkyl anions from an electronic point of view, i.e., it is a hard nucleophile with contracted valence orbitals around oxygen, unfavorable out-of-phase overlap with the leaving group is minimized (Scheme 9), and a front-side attack leading to retention is therefore possible. The stereochemical data are summarized in Table XI. 

The stereochemical data reported with 1-NpPhMeSiX are given in parenthesis. 

The stereochemical data reported in the case of germanium compounds by Eaborn et al. 90), Brook and Peddle 91), or Carre and Corriu (92) are quite parallel to those discussed here. The nature of the leaving group and the electronic character of the nucleophile, rationalized in terms of hard and soft reagents, are also the dominant factors that govern the stereochemistry. 

The mechanism depicted in Scheme 10 satisfactorily accounts for most of the kinetic and stereochemical data concerning this reaction,367,400,401 although some important details are still subject to debate.367... 

The first example of Iiving polyolefin with a uniform chain length was found in the low-temperature polymerization of propylene with the soluble catalyst composed of V(acac)3 and Al(C1Hi)2Cl. The mechanism of the living coordination polymerization is discussed on the basis of the kinetic and stereochemical data. Subsequently, some applications of living polypropylene are introduced to prepare tailor-made polymers such as terminally functionalized polymers and block copolymers which exhibit new characteristic properties. Finally, new types of soluble Ziegler-Natta catalysts are briefly surveyed in connection with the synthesis of living polyolefins. 

The use of the 2R,3R isomer led to formation of only 2R,3R-2-ethoxy-3-phenylbutane. Thus the configuration at each chiral center was retained in the product. These stereochemical data rule out simple ionization and solvent capture as a reaction mechanism since this would lead to a mixture of 2R and 2S configurations. From these observations it has been postulated that the phenyl group assists ionization of the leaving group by electron donation to produce a bridged ion. 

As in substitution at carbon, stereochemical and kinetic data provide the means of differentiating between these two possibilities. The stereochemical data are examined first. Substitution at silicon leads to both retention and inversion of configuration and the stereochemical outcome depends upon the nature of the leaving group, the nucleophile, solvent, complexing agents and whether or not the silicon is part of a ring. 

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. 

As already mentioned, the stereochemistry of simple olefin hydrogenation can usually be understood by utilizing the classic Horiuti-Polanyi mechanism (1,2). A number of different mechanistic rationales have been put forth, however, to account for the stereochemical data obtained on hydrogenation of a, /3-unsaturated ketones in different media. Actually, no single explanation can be used to account for all of the stereochemical observations, but it is possible to blend the various proposals to give a mechanistic framework from which it is possible by extrapolation to obtain the desired stereochemical information. 

None of these mechanistic proposals is sufficiently general to use to rationalize all of the stereochemical data observed on the hydrogenation of a,[3-unsaturated ketones. By a judicious combination of segments of each of these proposals along with the Horiuti-Polanyi mechanism (2), it is possible, however, to develop a uniform mechanistic rationale that can be useful in determining the effect of solvent on product stereochemistry. In addition, the influence of hydrogen availability, the type and quantity of catalyst, and the nature of other substituents on the reacting molecule on the product isomer distribution can also be more readily understood. 

Tables IX-XIII show the stereochemical results obtained with each catalyst system and give some additional experimental detail. Table XIV is a composite table that pulls together the stereochemical data for all of the substrates and catalysts used.

The stereochemical data discussed for the protonation/deprotonation at C-fi catalyzed by the above enzymes are consistent with the results of trapping experiments with JV-ethylmaleimide carried out by Flavin and Slaughter [160]. This reagent will react with enamines, like the PLP-aminocrotonate intermediate, with formation of an adduct, a-keto-3-[3 -(A"-ethyl-2, 5 -diketopyrrolidyl)]butyric add (KEDB) This compound has two chiral centers which are generated during the reaction. 

Table 7.3. Stereochemical Data on C-Methyl Derivatives of Reversed Esters of Pethidine...

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