Competition experiment between

As Table 20 shows, the yields of the Rh(II)-promoted reaction are temperature-dependent. Furthermore, competitive experiments between pairs of alkanes revealed a marked dependence on the alkoxy group of the diazoester and on the perfluoroalkyl carboxylate part of the catalyst. The observed relative selectivities have been taken as evidence for the importance of lipophilic interactions between carbenoid and alkane. 

B-Allenyl-9-BBN has also been shown to react cleanly and efficiently with other electrophiles [27]. Not surprisingly, aldehydes show the highest reactivity. In a competition experiment between benzaldehyde and acetophenone at -78 °C, the aldehyde adduct predominated by more than 30 l(Eq. 9.22). Competition experiments with other carbonyl compounds showed a similar bias for aldehyde adducts. 

Cyclohexyl xanthate has been used as a model compound for mechanistic studies [43]. From laser flash photolysis experiments the absolute rate constant of the reaction with (TMS)3Si has been measured (see Table 4.3). From a competition experiment between cyclohexyl xanthate and -octyl bromide, xanthate was ca 2 times more reactive than the primary alkyl bromide instead of ca 50 as expected from the rate constants reported in Tables 4.1 and 4.3. This result suggests that the addition of silyl radical to thiocarbonyl moiety is reversible. The mechanism of xanthate reduction is depicted in Scheme 4.3 (TMS)3Si radicals, initially generated by small amounts of AIBN, attack the thiocarbonyl moiety to form in a reversible manner a radical intermediate that undergoes (3-scission to form alkyl radicals. Hydrogen abstraction from the silane gives the alkane and (TMS)3Si radical, thus completing the cycle of this chain reaction. 

Primary alcohols were oxidised to aldehydes and (less readily) secondary alcohols to ketones by Ru(N0)Cl(salen = )/03//UV (incandescent or halogen lamp), hi competitive experiments between 1- and 2-decanol or benzyl alcohols only the primary alcohol was oxidised [827]. With Ru(NO)Cl(salen )/(Cl2pyNO) or TMPNO or Oj/C H /UV (TMPNO=tetra-methylpyridine-iV,iV -oxide) racemic secondary alcohols were asymmetrically oxidised to ketones [828]. A Ru(NO)(salen " ) complex was used as Ru(N0)Cl(salen " )/02/UV/CgH3Cl to oxidise racemic secondary alcohols to the ketones in the presence of l,3-bis(p-bromophenyl)propane-l,3-dione e.e. of 55-99% were achieved [829], Chiral Ru(NO)Cl(salen ) complexes were made... 

A second puzzling observation was the inverse dependence of hydrosilation rates on the concentration of substrate. In a rate study in which the concentration of acetophenone was varied, rate suppression was observed as [acetophenone] increased. Since raising [acetophenone] is expected to shift the equilibrium for adduct formation towards the adduct, this phenomenon argues against direct involvement of the adduct in the catalytic cycle for hydrosilation. Finally, it was observed that, in a competition experiment between benzaldehyde and ethylbenzoate, benzaldehyde was hydrosilated almost exclusively at a comparable rate to the non-competitive hydrosilation of the aldehyde. This observation shows that the relative basicity of the substrate does play an important role in the reaction, determining the chemoselectivity of the reaction. 

The electronic nature of the amine incorporated into these imine complexes should play an important role in determining the stability of Cu1 complexes, and therefore the composition of equilibrium mixtures when several amines compete as subcomponents. To investigate the influence of electronic effects, we ran a series of competition experiments between unsubstituted and substituted anilines [46] (Scheme 1.19). 

The substrate-specific condensation of aromatic carboxylates 52 and 53 having strong affinity for the CD cavity of 54 was realized with in-situ activation by formation of an acyloxytriazine, which undergoes aminolysis to give an amide [43]. Competitive experiments between 52 and 53 showed high selectivity, resulting in the major formation of compound 55 over compound 56 (Scheme 13.13). The observed selectivity can be attributed to differences in the affinity of the substrates for the CD cavity (Scheme 13.13). Most importantly, the CD-based catalyst 54... 

The electronic nature of silylsilver intermediate was interrogated through inter-molecular competition experiments between substituted styrenes and the silylsilver intermediate (77).83 The product ratios from these experiments correlated well with the Hammett equation to provide a p value of —0.62 using op constants (Scheme 7.19). Woerpel and coworkers interpreted this p value to suggest that this silylsilver species is electrophilic. Smaller p values were obtained when the temperature of the intermolecular competition reactions was reduced [p = — 0.71 (8°C) and —0.79 (—8°C)]. From these experiments, the isokinetic temperature was estimated to be 129°C, which meant that the product-determining step of silver-catalyzed silylene transfer was under enthalpic control. In contrast, related intermolecular competition reactions under metal-free thermal conditions indicated the product-determining step of free silylene transfer to be under entropic control. The combination of the observed catalytically active silylsilver intermediate and the Hammett correlation data led Woerpel and colleagues to conclude that the silver functions to both decompose the sacrificial cyclohexene silacyclopropane as well as transfer the di-terf-butylsilylene to the olefin substrate. 

The influence of the alcohol on the reaction was evaluated (Scheme 26). The results of a competition experiment between the alcohols are shown in Table 7. Both alcohols were treated with mono-alkoxysilane le using 10 % Pd/C as the catalyst. The silyl ketals of both alcohols were isolated as a mixture and the area under the methine protons, from the (+)-ethyl lactate moiety of both silyl ketals, was compared by NMR analysis. The difference in reactivity of primary, versus secondary, versus tertiary alcohol was small. The differences in reactivity range from 1.5 1 for 1° vs 2°, to 3 1 for 1° vs 3°. The reactivity of a benzyl alcohol is slower than the aliphatic alcohol as shown in entries 4 to 6. Entries 4 and 5 show an increase in the ratio of 1° 2° alcohol and a decrease in ratio for the 2° 3° for the secondary benzyl alcohol. Entries 6 and 7 confirm that benzyl alcohols are less reactive than aliphatic alcohols. The inductive electron withdrawing effect of the aryl group in the benzyl alcohol renders it less nucleophillic and this may affect the rate of reaction with the silane. Although the difference in reactivity is small, this trend may be informative. The influence of the alcohol s nucleophilicity on the reaction mechanism will be addressed in a later section. 

By competition experiments between nucleophile 155, (EtO)2PO and PhS toward radical 158, it was concluded that 155 is more reactive than (EtO)2PO and as reactive as PhS ions197. 

We compared combinations of encapsulated carboxylic acids, boronic acids, and carboxamides to determine the strongest interactions - at least in the context of the capsule [55], Direct competition experiments between guests of the same size (the p-ethyl derivatives) were used to eliminate the effects of fit . 

In a competition experiment between chiral N-sulfinyl oxazolidinone and Andersen s menthyl sulfinate ester, it has been shown that the former is at least two orders of magnitude more reactive than the latter. This finding is being used to avoid some of the problems involved in sulfinate esters, related to the nature of the alkoxide leaving group in the nucleophilic substitution. 

Imines and their derivatives could be used in an analogous way to aldehydes, ketones, or their derivatives this subject has been reviewed [79]. A competition experiment between an aldimine and the corresponding aldehyde in the addition to an enol silyl ether under titanium catalysis revealed that the former is less reactive than the latter (Eq. 14) [80]. In other words, TiCU works as a selective aldehyde activator, enabling chemoselective aldol reaction in the presence of the corresponding imine. (A,0)-Acetals could be considered as the equivalent of imines, because they react with enol silyl ethers in the presence of a titanium salt to give /5-amino carbonyl compounds, as shown in Eqs (15) [81] and (16) [79,82]. 

Competition experiments between two ir-bonded nucleophiles within the same molecule were studied in an attempt to identify reaction parameters and factors responsible for regioselectivity. These experiments, summarized in heme 12, demonstrated that the dithioacetal (initiating carbocation) is in competition with two nucleophilic functional groups within the same molecule, a silyl enol ether and a vinylsilane. In this instance, when the thioacetal (29) was treated with DMTSF, complete chemoselectiv-... 

Recently, Sen has reported two catalytic systems, one heterogeneous and the other homogeneous, which simultaneously activate dioxygen and alkane C-H bonds, resulting in direct oxidations of alkanes. In the first system, metallic palladium was found to catalyze the oxidation of methane and ethane by dioxygen in aqueous medium at 70-110 °C in the presence of carbon monoxide [40]. In aqueous medium, formic acid was the observed oxidation product from methane while acetic acid, together with some formic acid, was formed from ethane [40 a]. No alkane oxidation was observed in the absence of added carbon monoxide. The essential role of carbon monoxide in achieving difficult alkane oxidation was shown by a competition experiment between ethane and ethanol, both in the presence and absence of carbon monoxide. In the absence of added carbon monoxide, only ethanol was oxidized. When carbon monoxide was added, almost half of the products were derived from ethane. Thus, the more inert ethane was oxidized only in the presence of added carbon monoxide. 

Exceptions are aromatic aldehydes attached to an electron-withdrawing group (EWG). They have low reactivity comparable to that of acetals and ketals. The competition experiments between benzaldehyde derivatives having an electron-donating... 

Further improvements in palladium catalysis were achieved with a larger excess of benzene as co-solvent, and also with DavePhos (95) as ligand and pivalic acid as additive (Scheme 9.31) [70]. This catalytic system tolerated various valuable functional groups, such as a nitro substituent. These reaction conditions allowed not only for the achievement of better yields of biaryls with aryl bromides as electrophiles, but also improved chemoselectivies of these transformations. Thus, in competition experiments between benzene (87) and fluorobenzene (96), the latter reacted preferentially in a ratio of >11 < 1 (Scheme 9.31) [70],... 

Competition experiments between C2H4 and alkyl-substituted ethylenes for the capture of CTF show that the relative efficiencies increase by a factor of six in going from ethylene to tetramethylethylene. The temperature dependence of these relative rates indicates that the variation is almost entirely the result of differences in activation energy for monofluorocarbene addition to alkenes. 

The stability of the resulting complexes depends on the polymer,as shown by desorption by polybrene. From competition experiments between heparin and polymers, it is proposed that the T-AT desorption parallels the anticoagulant activity of the polymers. 

Competition between heparin and polymers. In order to better document differences in the abilities of the various polymers to adsorb T-AT, competition experiments between natural and synthetic anticoagulants were performed in two different ways. 

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