Selectivity regioselectivity, and

One of the most profound influences a solvent system can have on a reaction is a change of selectivity. Enzymes in organic solvents have been discovered in many cases to feature altered selectivity, including substrate specificity, enantioselectivity, prochiral selectivity, regioselectivity, and chemoselectivity (Wescott, 1994 Hirose, 1995). 

The reactive species generated by the photoexcitation of organic molecules in the electron-donor-acceptor systems are well established in last three decades as shown in Scheme 1. The reactivity of an exciplex and radical ion species is discussed in the following sections. The structure-reactivity relationship for the exciplexes, which possess infinite lifetimes and often emit their own fluorescence, has been shown in some selected regioselective and stereoselective photocycloadditions. However, the exciplex emission is often absent or too weak to be identified although the exciplexes are postulated in many photocycloadditions [11,12], The different reactivities among the contact radical ion pairs (polar exciplexes), solvent-separated radical ion pairs, and free-radical ions as ionic species... 

The use of a class of pentafluorophenyl Pt(ll) complexes as catalysts allows the efficient epoxidation of simple terminal alkenes with environmentally benign hydrogen peroxide as the oxidant. Key features of this system are very high substrate selectivity, regioselectivity, and enantioselectivity, at least for this class of substrates. These properties are related to the soft Lewis acid character of the metal center that makes it relatively insensitive to water but, at the same time, capable of increasing the electrophilicity of the substrate by coordination. The reversal of the traditional electrophile/nucleophile roles in epoxidation helps explain the unprecedented reactivity observed. 

For example, MPc complexes are synthesized in the zeolite framework by subjecting the zeolite to metal ion exchange and then treating it with molten dicyanobenzene. These "ship-in-a-bottle complexes cannot leave the zeolite without destroying the framework. Such zeolite catalysts, whose super-cages serve as a sort of reaction flask with molecular dimensions, continue to possess shape selectivity, reactant selectivity, regioselectivity and stereoselectivity. 

Enzymes generally show four kinds of selectivity substrate selectivity, reaction selectivity, regioselectivity, and stereoselectivity. An enzyme selects one substrate out of a mixture in solution and performs a desired chemical reaction on it. It also directs that reaction to a particular region of the substrate even though there may be others of equal or greater intrinsic chemical reactivity, and achieves the transformation with good stereoselectivity. This can include the formation of a single enantiomer, as in the synthesis of natural amino acids. 

When using aliphatic alkynes the lactam products are also preferentially obtained in good yields. Through modification of the phosphoramidite ligand, high levels of enantio-selectivity, regioselectivity and product selectivities are obtained The uncatalyzed reaction of heterocyclic formamidines 201 with two equivalents of phenyl isocyanate affords the [2+2+2] cycloadduct 202 in 48 % yield... 

Aryl and heteroaryl amidines underwent oxidative C-H bond functionalization with alkynes with the aid of ruthenium catalysis [48]. In the presence of [RuCl2(p-cymene)]2/KPF5 as catalyst, the cyclization reactions led to the formation of a variety of 1-iminoisoindolines with high site selectivity, regioselectivity, and... 

The TiX2-TADD0Late-catalyzed 1,3-dipolar q cloaddition reactions were extended to include an acrylate derivative [66]. In the absence of a catalyst, the reaction between nitrones 1 and acryloyl oxazolidinone 19b proceeded to give a mixture all eight regio-and stereoisomers (Scheme 6.23). However, application of in this case only 10 mol% of Ti(OTs)2-TADDOLate 23d as catalyst for the reaction of various nitrones 1 with alkene 19b, led to complete regioselectivity and high endo selectivity in the reaction and the endo products 21 were obtained with 48-70% ee (Scheme 6.23) [66]. 

The action of aqueous potassium hydroxide on chloronitrile 14 in DMSO at 25-30°C accomplishes the hydrolysis of the chloronitrile moiety and furnishes ketone 13 in a yield of 80%. Treatment of a solution of ketone 13 in CH2CI2 with mCPBA and sodium bicarbonate results in a selective Baeyer-Villiger oxidation to give bicyclic lactone 12 in >95% yield. It is noteworthy that the Baeyer-Villiger oxidation is completely regioselective and that the... 

Bartoli recently discovered that by switching from azide to p-anisidine as nucleophile, the ARO of racemic trans- 3-substituted styrene oxides could be catalyzed by the (salen)Cr-Cl complex 2 with complete regioselectivity and moderate selectivity factors (Scheme 7.36) [14]. The ability to access anti-P-amino alcohols nicely complements the existing methods for the preparation of syn-aryl isoserines and related compounds [67] by asymmetric oxidation of trans-cinnamate derivatives [68]. 

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

However, the particular usefulness of enzymes stems from their selectivity. There are three types of selectivity exerted by enzymes chemoselectivity, regioselectivity, and stereoselectivity. 

This study demonstrates that the addition of the 2-diazopropane with the triple bond of propargyl alcohols is regioselective, and affords new antibacterial 3H-pyrazoles. The photochemical reaction of these 3H-pyrazoles selectively leads to a- and 6-hydroxy cyclopropenes. The overall transformation constitutes a simple straightforward route to substituted cyclopropenyl alcohols without initial protection of the propargyl alcohol hydroxyl group. 

Homogeneous transition-metal catalysts are usually chosen because of their high selectivity. A distinction is made between chemoselectivity, regioselectivity, and stereoselectivity, of which a special form is enantioselectivity. The latter is e.specially important in the production of pharmaceuticals. 

The differences in the steric effect between catecholborane and pinacolborane, and the valence effect between a cationic or neutral rhodium complex reverse the re-gioselechvity for fluoroalkenes (Scheme 1-4) [26]. The reaction affords one of two possible isomers with excellent regioselectivity by selecting borane and the catalyst appropriately, whereas the uncatalyzed reaction of 9-BBN or SiaiBH failed to yield the hydroboration products because of the low nucleophilicity of fluoroalkenes. The regiochemical preference is consistent with the selectivity that is observed in the hydroboration of styrene. Thus, the internal products are selectively obtained when using a cationic rhodium and small catecholborane while bulky pinacolborane yields terminal products in the presence of a neutral rhodium catalyst. 

As is true for most reagents, there is a preference for approach of the borane from the less hindered face of the alkene. Because diborane itself is a relatively small molecule, the stereoselectivity is not high for unhindered alkenes. Table 4.4 gives some data comparing the direction of approach for three cyclic alkenes. The products in all cases result from syn addition, but the mixtures result from both the low regioselectivity and from addition to both faces of the double bond. Even 7,7-dimethylnorbornene shows only modest preference for endo addition with diborane. The selectivity is enhanced with the bulkier reagent 9-BBN. 

The regioselectivity and stereoselectivity of electrophilic additions to 2-benzyl-3-azabicyclo[2.2.1]hept-5-en-3-one are quite dependent on the specific electrophile. Discuss the factors that could influence the differing selectivity patterns that are observed. 

The Heck reaction has been applied to synthesis of intermediates and in multistage syntheses. Some examples are given in Scheme 8.9. Entries 1 and 2 illustrate both the (3-regioselectivity and selectivity for aryl iodides over bromides. Entries 3 and 4 show conditions that proved favorable for cyclohexene. These examples also indicate preferential syn Pd-H elimination, since this accounts for formation of the 3-substituted cyclohexene as the major product. 

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