Stereochemical Considerations

In a cycloaddition reaction, the two active n systems may approach each other in either of two orientations, for example, head to head or head to tail. If one combination dominates, the reaction is said to be regioselective. In the course of the reaction, 4 new saturated centers are formed. With maximum labeling, a total of 16 (=2 ) stereoisomeric forms, consisting of 8 enantiomeric pairs of diastereomers if neither polyene is chiral, may be formed. In pericyclic reactions, the stereochemistry is determined by specifying the stereochemical mode in which each component reacts. Each of the two 

The reverse process is also useful in synthesis. The active components of the cycloreversion reaction are the two g bonds which will be broken and any n systems to which both G bonds are allylic (i.e., at least one of m, n is an even number greater than 2). The stereochemistry of the reaction may also be specified in terms of the stereochemical mode of reaction of the active components of the reaction. 

Isomerization of butene via a 7r-allyl species introduces an added dimension to the stereochemistry. The 7r-allyl species from propylene is presumed to be planar with its plane approximately parallel to the surface. Since it is attached to the electropositive zinc, it may have considerable carbanion character. A corresponding structure for adsorbed butene would lead to two isomeric forms, viz  

Cfs-butene should lead initially to the anti form trrms-butene should lead initially to the syn form and 1-butene should give rise initially to both. The equilibrium distribution of syn and anti forms usually differs greatly from the equilibrium distribution of cis- and frans-butene for cobalt complexes 59, 60) the syn form, precursor of irans-butene, is by far the most stable. By way of contrast for the corresponding carbanion, the cis anion seems by far the more stable. This preference for the cis carbanion is presumed to be the source of the high initial cis-to-trans ratio in the initial products of base catalyzed isomerization. In the base catalyzed isomerization of more complex cf-s-olefins (cfs-S-methyl-stilbene), the ions corresponding to syn and anti are not interconvertible and cis-trans isomeriza- 

A particular feature of several biphenol-based diphosphites is their ability to form stereoisomers due to temperature-dependent atropisomerism. Formation of diastereomers may complicate mechanistic studies and influence the 

When the 2,2 -bis(3- erf-butyl-4-methoxyphenol) moiety is part of a cyclic phosphite, the rotation barrier is lowered [107]. In the relevant diphosphate, even at a low temperature (-74°C) two diastereomers could be detected by NMR spectroscopy. 

A similar situation arises when three different oxy groups are linked to the phosphorus. As a consequence, it becomes stereogenic. In combination with atropi-somerism, diastereomers are formed as found with acylphosphite phosphites 1 [44]. Attempts to separate them in order to elucidate their individual contribution to rhodium-catalyzed hydroformylation were unsuccessful. However, the different effects of diastereomers on catalyst formation and hydroformylation results could be evidenced by individual synthesis of all three configurationally stable tris(binaphthol) phosphites 2 [108]. 

A comparison in the literature of arylphosphites with a MeO or tert-Bu group shows rather disparate results. In the hydroformylation of octenes with BIPHEPHOS-type ligands, the tert-Bu ligand induced in the corresponding Rh catalyst a TOF (turnover frequency) of approximately twice as high as the MeO 

Chiral acylphosphites have been prepared from aliphatic dicarboxylic acids and used in asymmetric hydroformylation [113]. 

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Moving on to some wider stereochemical considerations, just as enantiomers are indistinguishable as far as their physical and chemical properties are concerned (except, of course, as regards their reactions with other optically active reagents) so their spectra, acquired under normal conditions, are identical. The NMR spectrometer does not differentiate between optically pure samples and racemic ones. Note there is a way of differentiating between enantiomers by NMR but it involves using certain chiral reagents which we ll discuss in detail later. 

Kokes and Dent by combining IR spectroscopy with hydrogen isotope techniques, and by applying kinetic and stereochemical considerations were able to determine the intermediate surface species in the hydrogenation and isomerization of simple olefins over zinc oxide. 

Norbomene-type dipolarophiles offered yet another stereochemical consideration owing to the facial selectivity posed by the dipolarophile. While no examples of endo-face attack on... 

The use of several different experimental techniques to investigate a given system is likely to be particularly revealing. Two noteworthy examples in which stereochemical considerations had a part are provided by Taylor and Dibeler (8) on the reactions of deuterium with the butenes on nickel wires and by Meyer and Burwell (9, 10) on the deuteration of multiply unsaturated hydrocarbons. 

Beckett, A.H., Casey, A.F. Synthetic analgesics stereochemical considerations./. Pharm. Pharmacol, 1954, 6, 986-999. 

Two models for the mechanism of the isomerization have been proposed. The first model was originally proposed because of similarities between enzymes involved in the activation of C-H bonds next to a carbonyl function (Fig. la). Stereochemical considerations were used to suggest that the ring structure of the sugar is opened to... 

If tlie 1,2-addition is reversible (the nucleophile is a good leaving group), then we get thermodynamic control and the conjugate addition product predominates. When the 1,2-addition is not reversible (the nucleophile is a poor leaving group), we get kinetic control and simple addition. Stereochemical considerations are also partly responsible, since it will be easier for larger nucleophiles, especially enolate... 

Stereochemical Considerations Concerning the Catalytic Protonation Process 214... 

Stereochemical considerations in the E2 reactions The E2 follows a concerted mechanism, where removal of the proton and formation of the double bond occur at the same time. The partial 7t bond in the transition state requires the parallel alignment or coplanar arrangement of the p orbitals. When the hydrogen and leaving group eclipse each other (0°), this is known as the 5 yn-coplanar conformation. 

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