Intermolecular competition

Methylene, produced by photolysis of diazomethane in mixtures of liquid alkanes, does not discriminate between the components to any extent which exceeds intramolecular discrimination . Deviations from random insertion were within experimental error. 

Relative rates of reaction of a number of olefins with singlet and triplet methylene have been determined in the gas phase Singlet methylene was generated by direct photolysis of ketene at 2600 A and the triplet by mercury-photosensitized decomposition of ketene. The evaluation of relative 

At very low conversions of the olefins, the following expressions can be used alternatively for the determination of the relative rate coefficients (k-Jk- ). 

Where values of kjki from (41 a) and (41b) were compared, the differences between the two sets were quite small. The kjki values from eqn. (41b), based on the determination of only one product, are listed in Table 15. The values in column 2 of Table 15 are affected only slightly if allowance is made for 10% of triplet methylene which may be produced in the direct photolysis of ketene at 

This correction has been achieved with the aid of data given in Table 13. For some olefins not listed in Table 13 allowance was made for C-H insertion by analogy with the other olefins. 

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Smooth and efficient cyclopropanation also occurs with copper(II) triflate and diazomethane. Intra- and intermolecular competition experiments show that, in this case, the less substituted double bond reacts preferentially251. The same is true for CuOTf and Cu(BF4)2, whereas with CuX P(OMe)3 (X = Cl, I), CuS04 and cop-per(II) acetylacetonate, cyclopropanation of the more substituted double bond predominates. An example is given for cyclopropanation of 1. 

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

In the hydroxycyclopropanation of alkenes, esters may be more reactive than N,N-dialkylcarboxamides, as is illustrated by the exclusive formation of the disubstituted cyclopropanol 75 from the succinic acid monoester monoamide 73 (Scheme 11.21) [91]. However, the reactivities of both ester- as well as amide-carbonyl groups can be significantly influenced by the steric bulk around them [81,91]. Thus, in intermolecular competitions for reaction with the titanacydopropane intermediate derived from an alkylmagnesium halide and titanium tetraisopropoxide or methyltitanium triisoprop-oxide, between N,N-dibenzylformamide (48) and tert-butyl acetate (76) as well as between N,N-dibenzylacetamide (78) and tert-butyl acetate (76), the amide won in both cases and only the corresponding cyclopropylamines 77 and 79, respectively, were obtained (Scheme 11.21) [62,119]. 

The intramolecular kinetic isotope effect determined in reaction of BTNO with p-MeO-C6H4CH(D)0H gave a h/ d ratio of 5.6 in MeCN , consistent with a rate-determining H-abstraction step. Additional determinations gave a h/ d of 7 with PhCH(D)OH, and 12 for the intermolecular competition of fluorene vs. 9,9-dideuteriofluorene. The latter value supports the contribution of tunnelling already commented on for reaction of PINO with various C—H donors ( h/ d values in the 11-27 range) . ... 

Intermolecular Competition Acylation Experiment of Ethanol, Isopropanol and tert-Butanol... 

Recent comparative studies (intermolecular competition experiments using two different bismuth(V) oxidants and intramolecular competition experiments with unsymmetrically substituted bismuth(V) oxidants [80, 81]) revealed that the substituents affect the triarylbismuth(V) oxidants as follows ... 

To estimate the relative reactivity of allylic, benzylic, and nonconjugative aliphatic alcohols toward the Ar3BiCl2/DBU system, intermolecular competitive oxidations were examined. As summarized in Scheme 20, cinnamyl and benzylic alcohols were preferentially oxidized in the presence of ethyl alcohol. The chemos-electivities observed for the Ar3BiCl2/DBU oxidant (Ar = o-tolyl) are considerably higher than those achieved by Dess-Martin periodinane [83, 84]. 

Interestingly, the newly developed Ar3BiCl2/DBU oxidants [Ar = p-nitrophenyl, p-(trifluoromethyl)phenyl] rapidly oxidize 2,2,2-trifluoro-l-phenylethanol [81], which is generally known to resist oxidation [85-87], to the corresponding trifluor-omethyl ketone within 5-50 min at room temperature (Scheme 21). The difference in the reaction rates among the bismuth(V) oxidants is in good agreement with the results obtained for the intermolecular competition experiments. 

Ameline, G. Vaultier, M. Mortier, J. Directed metalation reactions. Intermolecular competition of the carboxylic acid group and various substituents. Tetrahedron Lett. 1996, 37, 8175— 8176. 

A key feature of the competitive isotope fractionation measurements is the use of natural abundance O2. Isotope effects are, therefore, determined for the reactions of the most abundant isotopologues 160-160 and 180-160. It is the intermolecular competition of these species that is reflected in the isotope effect. Aside from the obvious advantage of not requiring costly enriched materials, the competitive measurements also avoid the error that could arise from small leaks in the vacuum manifold and dilution due to ambient air. 

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. 

Scheme 7.19. Hammett correlation of intermolecular competition experiments.

Scheme 7.45. Intermolecular competition reactions that examine the relative rates of cycloaddition and cyclization.

Fig. 9. The structures of L-methionine, GSMe, GSH, 5 -GMP and dGpG used as model compounds in intermolecular competition reactions...

Reactions ofN-acetyl-L-Methionine Complexes ofPtu(en) with 5 -GMP and dGpG. Recently, intermolecular competition was studied in more de-... 

In this respect it should be mentioned that Pt-methionine species are generally not antitumor active However, methionine is known to play an important role in the metabolism of cisplatin. The bischelate [Pt(Met-S,A02 has been isolated form the urine of patients treated with cisplatin [85]. On the other hand, intermolecular competition studies have shown that formation of a bifunctional G-N(7),G-N(7) adduct is possible for the cisplatin analog PtCl2(en)[19][86],... 

The effect of different substituents on the chlorine replacement by EC-produced recoil 211 At has been studied by carrying out intermolecular competition experiments with equimolar mixtures of PhCl and m- or / -ClC6H4X (where X = F, Cl, Br, I, Me, N02) diluted with 0.5 mole fraction of Et3N558. The results have been evaluated with the use of Hammett equation 251 ... 

To evaluate the effect of the substituent more accurately, a competition study with equimolar mixtures of C6H6-C6H5X(X = OH, F, Cl) was carried out by Norseyev and colleagues50. Quantitative evaluation of intermolecular competition for hydrogen replacement was performed on the basis of the Hammett relationship applying the yields of replacement products instead of reaction rate constants, according to equation 10 ... 

In intermolecular competition, a mixture of labeled and unlabeled reactants compete for a limited amount of reagent reactions (1) and (2) thus go on in the same mixture, and we measure the relative amounts of H—Z and D—Z t. oduced. (Sometimes, larger amounts of the reagent Z are used, and the relative amounts of the two reactants—ordinary and labeled—left unconsumed are measured the less reactive will have been used up more slowly and will predominate. The relative rates of reaction can be calculated without much difficulty.)... 

By intermolecular competition of an allylsilane and a homoallylstannane for reaction with an electrophile, the P-effect of silicon has been shown to be more effective than the y-effect of tin in activating the double bond towards addition.40 41... 

This simple model is valid in fact for P isotope effects and does not take into account the existence of intermolecular competitive reactions which occur frequently in transformations involving C-H bond breaking (refs. 7, 8). Indeed the abstraction of a hydrogen atom from a group in the substrate having n equivalent germinal and m equivalent vicinal positions AHn BHm, involves a set of different isotope effects... 

A remarkable feature of the Fe(TPP)Cl/PhIO system in olefin epoxidations was the stereospecific epoxidation of cA-stilbene and the high reactivity of cA-stilbene compared to trara-stilbene. With respect to the latter, cA-stilbene showed 15 times greater reactivity than trara-stilbene in intermolecular competitive epoxidation of cA- and tra .s-stilbenes by Fe(TPP)Cl and Phio (Equation (S)). " The preference of cA-stilbene over tra .s-stilbene was ascribed to nonbonded interactions between the phenyl groups of trara-stilbene and the phenyl groups of the Fe(TPP)Cl catalyst. In the same year, Chang and Kuo reported that a green intermediate. 

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