Selectivity toward various

Some transition metal systems M(CO)R react with a wide range of L, including phosphites, phosphines, arsines, stibines, organic amines, iodide, and CO, to mention a few, yielding the corresponding acyls. Other systems, e.g., CpFe(CO)2R (2S), display a marked selectivity toward various L. Certain unsaturated molecules L [SOj (239), CF2=Cp2 (238), inter alia] insert themselves into the M—R bond instead of effecting the reaction shown in Eq. (8). 

Few quantitative data are available on the relative nucleophilicities of L toward various alkyl carbonyls. The rates of the reaction of CpMo(CO)3Me with L in toluene (Table II) decrease as a function of the latter reactant P( -Bu)3 > P( -OBu)j > PPhj > P(OPh)j, but the spread is relatively small (<8). The above order is that customarily observed for 8 2 reactions of low-valent transition metal complexes (J, 214). Interestingly, neither CpMo(CO)3Me nor CpFe(CO)2Me reacts with 1 or N, S, and As donor ligands 28, 79). This is in direct contrast to the insertion reactions of MeMn(CO)5 which manifest much less selectivity toward various L (see Section VI,B,C,D for details). 

Additional flexibility in the control over the selectivity of heterolytic reactions is provided in the diversity of electrophilic reagents that formally correspond to the same electrophile. For example, reagents such as RCO BF, RCOO-SO2CF3, RCOCl, and (RC0)20 are employed in synthesis as equivalents of the acyl cation RCO. However, a tremendous difference in the reactivity of these acylating species enables one to choose a reagent specifically adjusted to the peculiarity of the nucleophilic counterpart. In a similar way, such unlike compounds as trialkyloxonium salts, R30 BF7, alkyl halides, tosylates, or acetates can serve as transfer agents of the same alkyl cation, R, but they differ drastically in their activity and pattern of selectivity toward various nucleophiles. 

Activated carbon cloth (ACC) is used for the removal of volatile organic compounds fix)m effluent gas streams [1], Specific micropore structures in ACCs make them real candidates for use in adsorption processes where a high rate of adsorption is accompanied with short contact time between adsorbent and adsorbate. It is very important for manufacturers to produce ACC with desirable pore size distributions, and be able to control the development of various pores from primary micropores to mesopores during activation processes. Adsorption selectivity towards various gases is another very important property, which can be achieved by introducing oxygen complexes onto the surface during activation [2-4]. 

Other matters that are important include the ability of the electrophile to select among the alternative positions on a substituted aromatic ring. The relative reactivity of different substituted benzenes toward various electrophiles has also been important in developing a firm understanding of electrophilic aromatic substitution. The next section considers some of the structure-reactivity relationships that have proven to be informative. 

The enantioselectivity a is defined as the distribution ratio of one single enantiomer over the two chiral phases and has been determined experimentally for a variety of compounds (Table 5-1). It has been known from work by Prelog [66, 67] that tartaric acid derivatives show selectivities towards a-hydroxyamines and amino acids. However, from Table 5-1 it is obvious that tartaric acid derivatives show selectivity for many other compounds, including various amino bases (e.g. mirtazapine (10)) and acids (e.g. ibuprofen (11)). The use of other chiral selectors (e.g. PLA)... 

Deligoz and Yilmaz [52] reported that the selective liquid-liquid extraction of various alkali and transition metal cations from the aqueous phase to the organic phase as carried out by using p-tert-h iy calix[4]arene (1), p-tert-b x. y calix[6]arene (2), tetra-ethyl-p-tm-butylcalix[4]arene-tetra-acetate (3), tetra-methyl-p-/< /-/-butyl calix[4]arene-tetraketone (4), calix[n]arenes ( = 4 and 6) bearing oxime groups on the lower rim (5 and 6) and a polymeric calix(4]arene (8). It was found that compounds 5 and 6 showed selectivity towards Ag, Hg, Hg, Cu, and Cr and the order of the ex-tractability was Hg > Hg > Ag > Cu > Cr. The polymeric calix[4]arene (8) was selective for Ag, Hg, and Hg , unlike its monomeric analog. 

The BS2 catalyst was more selective toward the formation of the dialkylated product than the Pd catalysts tested. The activity of BS2 for DAE-MIBK reaction was slower than that with acetone due to steric effects posed by the larger ketone. Here again, the imine tends to rapidly cychze to form imidazolidines or pyrimidines. Figure 17.2 shows the stepwise formation of various side products observed during the reductive alkylation of DAE with acetone. 

It is interesting to note that the photosubstitution intermediate 11 appears to be significantly more selective toward reaction with various two electron donor substrates than is the photofragmentation intermediate 1. One speculative rationalization of this is that the Ru3(C0) intermediate has the opportunity to "delocalize" its unsaturation by having one CO bridge an edge of the metal triangle with concomitant formation of a multiple metal-metal bond. 

The overall conclusion from the reaction of BP and 6-substituted BP radical cations with nucleophiles of various strengths is that weak nucleophiles display higher selectivity toward the position of highest charge localization. Thus another important factor in the chemical reactivity of radical cations is represented by the strength of the nucleophile. 

Transition metals can display selectivities for either carbonyls or olefins (Table 20.3). RuCl2(PPh3)3 (24) catalyzes reduction of the C-C double bond function in the presence of a ketone function (Table 20.3, entries 1-3). With this catalyst, reaction rates of the reduction of alkenes are usually higher than for ketones. This is also the case with various iridium catalysts (entries 6-14) and a ruthenium catalyst (entry 15). One of the few transition-metal catalysts that shows good selectivity towards the ketone or aldehyde function is the nickel catalyst (entries 4 and 5). Many other catalysts have never been tested for their selectivity for one particular functional group. 

Until now, only a few versatile, selective and effective transition-metal complexes have been applied in nicotinamide cofactor reduction. The TOFs are well within the same order of magnitude for all systems studied, and are within the same range as reported for the hydrogenase enzyme thus, the catalytic efficiency is comparable. The most versatile complex Cp Rh(bpy) (9) stands out due to its acceptance of NAD+ and NADP+, acceptance of various redox equivalents (formate, hydrogen and electrons), and its high selectivity towards enzymatically active 1,4-NAD(P)H. 

Rh complexes with ChiraPhos, PyrPhos, or ferrocenyl phosphines lacking amino alkyl side chains (such as BPPFA) are much less active toward tetrasubstituted olefins. Table 6-1 shows that in asymmetric hydrogenations catalyzed by 5a-d, the coordinated Rh complex exerts high selectivity on various substrates. It is postulated that the terminal amino group in the ligand forms an ammonium carboxylate with the olefinic substrates and attracts the substrate to the coordination site of the catalyst to facilitate the hydrogenation. 

Vinylboronates are generally less reactive than vinylzirconocenes towards various electrophiles and hence selective reactions of the latter should be possible. It was found that selective cleavage of the carbon—zirconium bond in 45 by N-halosuccinimides provides (a-haloalkenyl)boronic esters 53 in excellent chemical yields and with complete re-gioselectivity (Scheme 7.17) [54], An X-ray crystal structure determination of 45 confirmed the configuration of the four-coordinate Zr complex, with two cyclopentadienyl rings, Cl, and C(sp2) as the four ligands (Fig. 7.5) [54,126]. 

As an example of the selective reactivity of borazirconocene alkenes, their hydrolysis was examined [1]. The carbon—zirconium bond is more reactive than the carbon—boron bond towards various electrophiles, and so hydrolysis can be expected to occur with preferential cleavage of the former bond. Since hydrolysis of alkenylzirconocenes is known to proceed with retention of configuration [4,127—129], a direct utility of 45 is the preparation of (Z)-1-alkenylboronates 57 (Scheme 7.17) [12]. Though the gem-dimetalloalkenes can be isolated, in the present case it is not necessary. The desired (Z)-l-alkenylboronates can be obtained in a one-pot procedure by hydrozirconation followed by hydrolysis with excess H20. The reaction sequence is operationally simple and is compatible with various functional groups such as halides, acetals, silanes, and silyloxy protecting groups [12]. 

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