Selectivity in Reactions

The various important and cardinal t3q)es of selectivity in reactions, as observed in organic synthesis, may be summarized as under  

Generally, the C—C double bonds are found to be almost unreactive to the nucleophiles rmless 

It has been observed that the order of reactivity of carbonyl-containing functional moieties with nucleophiles invariably decreases in the following order  

It has been examined and observed that the cyclizations to produce five- and six-membered rings are significantly faster than their intermolecular counterparts. They are invariably favoured in equilibrium circumstances also. 

There are two commonly used reducing agents, namely first—sodium horohydride (NaBH ) which reduces exclusively aldehydes, ketones and acyl halides and secondly—lithium aluminium hydride (LiAlH ) which reduces the above compounds as well as compormds belonging to the carboxylic acid family. 

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High pressure and selectivity in reactions with formation of O- and N-heterocycles 97T2669. 

Alkoxy-substituted allylaluminum reagents diethyl[(Z)-3-methoxy-2-propenyl]- and -[(Z)-3-(l-methoxy-l-methylethyl)-2-propenyl]aluminum have been prepared by treatment of the corresponding alkoxyallyllithiums with diethylaluminum chloride in tetrahydrofuran at — 78 =C4. These reagents provide the syn-diastereomer with 9-11 1 selectivity in reactions with aldehydes at — 78 °C. The reaction of diethyir(Z)-3-methoxy-2-propenyl]a]uminum and acetophenone provided the iy -diastereomer with 4 1 selectivity. 

Recently, silicon-tethered diastereoselective ISOC reactions have been reported, in which effective control of remote acyclic asymmetry can be achieved (Eq. 8.91).144 Whereas ISOC occur stereoselectively, INOC proceeds with significantly lower levels of diastereoselection. The reaction pathways presented in Scheme 8.28 suggest a plausible hypo thesis for the observed difference of stereocontrol. The enhanced selectivity in reactions of silyl nitronates may he due to 1,3-allylie strain. The near-linear geometry of nitrile oxides precludes such differentiating elements (Scheme 8.28). 

These materials, generally prepared from the corresponding Grignard reagents by the addition of cadmium chloride, provide a milder reagent system than the Grignard that allows selectivity in reaction with phosphorus trichloride (Equation 4.21).34-43-63... 

Scheme 6.39. Variations in enantiofacial selectivity in reactions catalyzed by (ebthi)Zrbinol/MAO.

Table 4 Yields and selectivities in reactions with different catalyst loading ...

The use of organomagnesium reagents as bases leads to complexation of the nitrile imines (e.g., 141), which has been found to have a strong effect in promoting syn selectivity in reactions with methyl 2-(l-hydroxyalkyl)acrylates via coordination of the metal atom with the alcoholic oxygen (e.g., leading to the formation of 142). Lithium complexation had little effect (78). 

Katagiri et al. (48,322,323) applied L-menthone and an in situ generated nitroso ketene for the synthesis of nitrone 212, as shown in Scheme 12.61. The nitrone showed high selectivities in reactions with various allyl silanes. High pressure or the presence of BF3 Et20 was required for the reaction to proceed. In the reaction of 212 with allyltrimethylsilane in the presence of BF3 -Et20, isoxazolidine (213) was obtained as the sole product. After hydrolysis and hydrogenolysis. 

Substrate selectivities in reactions of aqueous sodium cyanide with alkyl halides in toluene and 17% RS onium ion catalysts are shown in Table 1 84). The data are particularly instructive about how intraparticle diffusion affects reactions that occur... 

Computational results were also able to account for the observed selectivity in reactions of crotylzinc reagents, which were found to react preferentially in a cisoid form (see previous examples). This could be accounted for by the energetic difference of the respective transition states 106 and 107 (equation 50). 

Reactivity of enols and enolates at carbon. Selectivity in reactions by ... 

Clifford, Rayner and co-workers at the University of Leeds [66,67] have used the density of SCCO2 to control the stereoselectivity of reactions in a novel way. Previous workers have observed an influence of density on selectivity in reactions very close to the critical point. The novelty of the Leeds work is that the effects are observed at densities considerably above pc and temperatures higher than Tc. This selectivity has no obvious counterpart in reaction chemistry in conventional solvents. Effects have been observed in a whole range of reactions, and this approach may well have widespread applicability. 

Aspects of selectivity in reaction pathways may be considered under the following topic areas chemoselectivity, regioselectivity, protection and, finally, stereoselectivi ty. 

Sterically and electronically tuneable and bifunctional organocatalysts based on diamides derived from proline are particularly selective in reactions of heterocyclic ketones with aldehydes.109... 

More important, they are less selective in reactions with tetralin. The presence of sulfur containing compounds in the macerals has a notable influence on their reactivity in hydrogen transfer reactions. 

Problem 1 highlights reagents for carbon-carbon 7t-system transformations. Problems 2-4 and 6 emphasize selectivity in reactions of carbon-carbon 71-bonds. The syntheses of TMs in Problems 5 and 7 require the selection of specific reagents to achieve chemo-, stereo-, or enantioselectivity. 

While carbenes are extraordinarily reactive it is important to remember that they exhibit an appreciable degree of selectivity in reactions with... 

J. M. Coxon, R. T. Luibrand, 71-Facial Selectivity in Reactions of Carbonyls A Computational Approach, in Modern Carbonyl Chemistry (J. Otera, Ed.), Wiley-VCH, Weinheim, 2000, 155-184. 

Since chemical reaction engineering considerations apply to nondcterministic as well as deterministic methods they will be briefly dealt with separately. The interaction of chemical kinetics and transport processes and their effect on catalyst activity and selectivity in reaction networks will be emphasized. Some attention will be also paid to catalyst deactivation. 

Although methods for stereocontrol of the aldol reaction are well documented, including diastereofacial selectivity in reactions of chiral enolates,25 stereocontrol in Mannich reactions appears to have received relatively little attention.26-30... 

This review describes some novel reaction types (Section F), but the more extensive area of activity has to do with variable adjustment of carbanion reactivity and selectivity in reactions with carbonyl compounds (Sects. C-E). Organotitanium... 

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