Relative reactivities, epoxides

Sulfonate Esters. Sucrose sulfonates are valuable intermediates for the synthesis of epoxides and derivatives containing halogens, nitrogen, and sulfur. In addition, the sulfonation reaction has been used to determine the relative reactivity of the hydroxyl groups in sucrose. The general order of reactivity in sucrose toward the esterification reaction is OH-6 OH-6 > OH-1 > HO-2. 

The radical versus electrophilic character of triplet and singlet carbenes also shows up in relative reactivity patterns given in Table 10.1. The relative reactivity of singlet dibromocarbene toward alkenes is more similar to electrophiles (bromination, epoxidation) than to radicals ( CCl,). 

Moreover, using the cobalt catalyst S,S-(50c) it was possible to perform substrate screening45 so that the chiral epoxides (48a-c) could be studied in parallel.55 It was found that //-(48a) is the most reactive substrate followed by //-(48c) and //-(48b). The relative reactivity of //-(48b) and //-(48c) corresponds to that reported in the literature for laboratory scale reactions,83 whereas the reaction... 

Both diastereomers are relatively reactive, complicating their isolation and purification. The half-lives of anti-and syn-BPDE in water at pH 7 are 2 hr and 30 min., respectively. Although they tend to decompose on chromatographic absorbants, these diol epoxides can be purified by rapid chromatography on low activity alumina columns or by HPLC in the presence of triethylamine as a stabilizer. 

Epoxidation of the 1,2- and 7,8-dihydrodiols of 5-MC with m-chloroperbenzoic acid furnished the corresponding anti diol epoxides 26 and J27. Compound 26 was the first diol epoxide bearing a methyl group in the same bay region as the epoxide function to be synthesized. While the diol epoxide 26 is relatively reactive (104), it is more stable than the structurally analogous DMBA 1,2-diol-3,4-epoxide (21) it was obtained as a white crystalline solid. 

The reaction of the epoxide with a thiol group yields a thioether linkage, whereas reaction with a hydroxyl gives an ether and reaction with an amine results in a secondary amine bond. The relative reactivity of an epoxy group is thiol > amine > hydroxyl, and this is reflected by... 

Isodesmic reactions 2 and 3 were calculated as a measure of the relative ease of formation of the aza-epoxides, in order to obtain some insight as to the effect of nitrogen atom on metabolic activation of aza-PAHs. It was presumed that formation of the epoxides is not as relevant as carbocation formation in determining the relative reactivities of Phe, BhQ and BfQ, because almost no energy differences were observed for the isodesmic reactions involving these compounds. 

Epoxide ring opening of 7-Me-B[c]ACR was more exothermic than for B[c]ACR, in accord with the relative reactivities reported in the literature (20). 

The radical versus electrophilic character of triplet and singlet carbenes also shows up in relative reactivity patterns shown in Table 10.1. The relative reactivity of singlet dibromocarbene toward alkenes is more similar to that of electrophiles (bromination, epoxidation) than to that of radicals ( CC13). Carbene reactivity is strongly affected by substituents.61 Various singlet carbenes have been characterized as nucleophilic, ambi-philic, and electrophilic as shown in Table 10.2. This classification is based on relative reactivity toward a series of different alkenes containing both nucleophilic alkenes, such as tetramethylethylene, and electrophilic ones, such as acrylonitrile. The principal structural feature that determines the reactivity of the carbene is the ability of the substituents to act as electron donors. For example, dimethoxycarbene is devoid of electrophilicity toward... 

The acylperoxy radical was found to epoxidize olefins much faster than peracids also formed under reaction conditions. The result ruled out the role of the latter.267 The addition of RCO3 was observed to occur 105 faster than that of ROO. The relative reactivity of alkenes suggests a strongly electrophilic radical forming the polar transition state 30 ... 

The relative reactivity of secondary and primary amines with epoxide, k2/kt, can affect the overall kinetics. If k2/kj is not close to 0.5 the parameter f in Eqs. (4-7) and (4-13) becomes significant. In this respect there are differences between aliphatic and aromatic or alicyclic amines. For the aliphatic amines the ratio k2/kj is reported to be in the range 0.6-0.7 83 90), whereas for aromatic amines values in the range 0.2 to 0.5 have been observed 90 9S). For the reaction of BADGE with DDS, Dobas et al. 94) reported a value of 0.21 for k2/k, at 80 °C. This factor is likely to account for part of the observed non-linearity in reduced rate plots at higher levels of conversion. 

Further support for Heck s suggestion that HCo(CO)4 reacts as an acid with epoxides was found by Takegami et al. (145), who pointed out that the relative reactivities of propylene oxide/epichlorohydrin is 30/1 for cobalt hydrocarbonyl. The same order of reactivity is shown to acids (propylene oxide/epichlorohydrin = 70 for perchloric acid) while the reactivity propylene oxide/epichlorohydrin for bases is in the reverse order (0.25 for ammonia). 

Shea, K. J. Kim, J. S. Influence of strain on chemical reactivity. Relative reactivity of tor-sionally distorted double bonds in MCPBA epoxidations. J. Am. Chem. Soc. 1992, 114, 3044-3051. 

We find that j3-cyclodextrin reacts smoothly with 1-butyldimethyl-silylchloride to afford the heptasilyl derivative in which all of the primary hydroxyls are blocked. This then can be sulfonated on the secondary face for ease of separation, our preferred reagent is triisopropylbenzene sulfonyl chloride. The resulting monotrypsilated cyclodextrin is, from NMR evidence, sulfonated on hydroxyl-2. This is not easily displaced, but on treatment with base it is converted to the 2,3-epoxide, and this is relatively reactive towards nucleophiles. 

The relative reactivity of the alcohol and amine in the example just given could be overturned by conducting a reaction under thermodynamic control. In kinetically controlled reactions, the idea that you can conduct chemoselective reactions on the more reactive of a pair of functional groups— carbonyl-based ones, for example—is straightforward. But what if you want to react the less reactive of the pair There are two commonly used solutions. The first is illustrated by a compound needed by chemists at Cambridge to study an epoxidation reaction. They were able to make the following diol, but wanted to acetylate only the more hindered secondary hydroxyl group. 

The transformation of aziridines that do not involve ring opening are rare due to the reactivity of the aziridine ring. Considering the somewhat more difficult synthesis of aziridines (relative to epoxides), the ability to convert one aziridine into another represents a significant expansion of the scope of any aziridine synthesis. 

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