Selectivity of reactions

Tannor D J, Kosloff R and Rice S A 1986 Coherent pulse sequence induced control of selectivity of reactions exact quantum mechanical calculations J. Chem. Rhys. 85 5805-20, equations (1)-(6)... 

Reaction with phenol gives hydroxymethylphenols as the principal products (44). Through proper selection of reaction conditions and catalyst a 1 1 mixture of 0- and -isomers is obtained (45) ... 

A study of the Diels-Alder reaction was carried out by Earle et al. [42]. The rates and selectivities of reactions between ethyl acrylate (EA) and cyclopentadiene (CP) in water, 5 m lithium perchlorate in diethyl ether (5 m EPDE), and [BMIM][PE(3] were compared. The reactions in the ionic liquid [BMIM][PE(3] were marginally faster than in water, but both were slower than in 5 m EPDE [42, 43] (see Table 5.1-1 and Scheme 5.1-18). It should be noted that these three reactions give up to 98 % yields if left for 24 hours. The endo. exo selectivity in [BMIM][PE(3] was similar to that in 5 M EPDE, and considerably greater than that in water (Table 5.1-1). 

Scheme 12.10 Selectivity of reactions of cyclohexadiene and benzene diepoxides with amines [2].

The selection of reaction conditions for ATRP is dependent on many factors including the particular monomer, initiator and catalyst. 

In this section we briefly discuss a selection of reactions of aromatic triazenes that are directly related to diazo chemistry. A broader coverage of triazene reactions can be found in Saunders and Allen s monograph (1985, Ch. 7). 

Table 6 gives a selection of reactions of sulfonyl halides with different unsaturated systems. 

Scheme 17 HOMO amplitude controls the selectivities of reaction modes...

The orbital mixing theory was developed by Inagaki and Fukui [1] to predict the direction of nonequivalent orbital extension of plane-asymmetric olefins and to understand the n facial selectivity. The orbital mixing rules were successfully apphed to understand diverse chemical phenomena [2] and to design n facial selective Diels-Alder reactions [28-34], The applications to the n facial selectivities of Diels-Alder reactions are reviewed by Ishida and Inagaki elesewhere in this volume. Ohwada [26, 27, 35, 36] proposed that the orbital phase relation between the reaction sites and the groups in their environment could control the n facial selectivities and review the orbital phase environments and the selectivities elsewhere in this volume. Here, we review applications of the orbital mixing rules to the n facial selectivities of reactions other than the Diels-Alder reactions. 

A concept named molecular manufacturing, which was originally proposed by K. Eric Drexler [99] in 1992, has attracted the attention of some investigators [100, 118-121]. Molecular manufacturing is defined as the production of complex structures via non-biological mechanosynthesis (and subsequent assembly operations) [99]. A chemical synthesis controlled by mechanical systems operating on the atomic scale and performing direct positional selection of reaction sites by atomic-precision manipulation systems is known as mechanosynthesis. 

Mechanosynthesis. Chemical synthesis controlled by mechanical systems operating on the atomic scale and performing direct positional selection of reaction sites by atomic-precision manipulation systems. 

Catalysis zeolites possess acid sites that are catalytically active in many hydrocarbon reactions, as we shall discuss in Chapter 9. The pore system only allo vs molecules that are small enough to enter, hence it affects the selectivity of reactions by excluding both the participation and formation of molecules that are too large for the pores. 

The spatio-temporal variations of the concentration field in turbulent mixing processes are associated wdth very different conditions for chemical reactions in different parts of a reactor. This scenario usually has a detrimental effect on the selectivity of reactions when the reaction time-scale is small compared with the mixing time-scale. Under the same conditions (slow mixing), the process times are increased considerably. Due to mass transfer inhibitions, the true kinetics of a reaction does not show up instead, the mixing determines the time-scale of a process. This effect is known as mixing masking of reactions [126]. 

The catalyst does not affect the chemical nature of the products. This must be qualified if more than one reaction (set of products) is possible, because the catalyst usually affects the selectivity of reaction. 

CHEC-II(1996) <1996CHEC-II(8)345> covered a range of reactions of substituents, including hydrolyses of esters to carboxylic acids and geminal dihalides to aldehydes, reduction of aldehydes and halogens directly attached to the ring, O-methylation with diazomethane, and reduction of nitroso compounds. A selection of reactions which have... 

Generation of Alkylideneallyl Cations from Alkylidenecyclopropanone Acetals Selectivity of Reaction with Nucleophiles... 

We have recently developed a novel method for the generation of alkylideneallyl cations from alkylidenecyclopropanone acetals (8, 9). This method provides a nice opportunity to examine the selectivity of reactions of the ambident cation with various nucleophiles including siloxyalkenes (10) and furans (11). The reaction of the cation with the carbon nucleophiles gives [4 + 3] and [3 + 2] cycloaddition products as well as simple nucleophilic addition products. These results are summarized in this chapter. 

This selection of reactions, by way of example, is not meant to imply that, for instance, a second-order termination reaction of ion-pairs, say... 

Supramolecular concepts involved in the size- and shape-selective aspects of the channels and cavities of zeolites are used to control the selectivity of reactions of species produced by photoexcitation of molecules encapsulated within zeolites. The photochemistry of ketones in zeolites has been extensively studied. Photoexcitation of ketones adsorbed on zeolites at room temperature produces radical species by the Norrish type 1 reaction. A geminate (born together) radical pair is initially produced by photolysis of the ketone, and the control of the reaction products of such radicals is determined by the initial supramolecular structure... 

The selectivity of reactions can be controlled thermodynamically or kinetically. In the first case, the pathways leading from a starting material or a common intermediate to the products are reversible. Here, the relative portion of the products is determined by their different heats of formation. This constitutes the minor... 

Pig. 4 Dependences of toluene conversion via alkylation by ethylene (1) and para-selectivity of reaction (2) upon radius of cation introduced. 

Entry H.T.-X Weight of H.T.-X- / g Reaction time / h trimethoxysilane conversion / % Selectivity of reaction / %... 

Although this early synthesis of the fluvastatin template (Scheme 12.1) was important for strucmre-activity smdies, a second synthesis (Scheme 12.2) was later developed to facilitate large-scale production of fluvastatin (Repic et al., 2001). As is typical in process development work, this second approach sought to shorten the reaction sequence, eliminate toxic reagents (such as tri-n-butylstannylvinylethoxide), and improve the yield and/or selectivity of reactions in order to reduce the amount of chromatography required to isolate the products. 

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