Inversion mechanism

Entry 4 shows that reaction of a secondary 2-octyl system with the moderately good nucleophile acetate ion occurs wifii complete inversion. The results cited in entry 5 serve to illustrate the importance of solvation of ion-pair intermediates in reactions of secondary substrates. The data show fiiat partial racemization occurs in aqueous dioxane but that an added nucleophile (azide ion) results in complete inversion, both in the product resulting from reaction with azide ion and in the alcohol resulting from reaction with water. The alcohol of retained configuration is attributed to an intermediate oxonium ion resulting from reaction of the ion pair with the dioxane solvent. This would react until water to give product of retained configuratioiL When azide ion is present, dioxane does not efiTectively conqiete for tiie ion-p intermediate, and all of the alcohol arises from tiie inversion mechanism. ... 

Asano and co-workers have reported die kinetic effects of pressure, solvent, and substituent on geometric isomerization about die carbon-nitrogen double bond for pyrazol-3-one azomethines 406 (R = H), 406 (R = NO2) and 407, (Scheme 93). The results demonstrate the versatility of die inversion mechanism. The rotation mechanism has been invalidated. First-wder rate constants and activating volumes for diermal E-Z isomerization for 406 (R = H) and 406 (R = NO2) are given at 25°C in benzene and methanol (89JOC379). 

A site-inversion mechanism (the key feature of which is that isomerization between diastereomeric and A configurations is rapid on the propylene-insertion time scale) based on theoretical calculations was proposed by Cavallo and coworkers in order to explain the ligand-directed chain-end controlled polymerizations (Fig. 35) [42]. The site-inversion mechanism allows chain-end control to work in concert with the site control effects. Our experimental results and the expected catalytic behavior resulting from the site-inversion mechanism concur with each other very well. 

Fig. 35 Site-inversion mechanism for propylene polymerization promoted by FT catalysts. Reproduced with permission from Furuyama et al. [68]. Copyright 2005, Elsevier...

Whenever a mechanism dissipates mechanical energy as heat there exists an inverse mechanism by which heat can generate a mechanical fluctuation. There is always a connection between fluctuation and dissipation. 

Walden first published his observations on inversion in Berichte 1893, 26, 210 1896, 29, 133 and 1899, 32, 1855, long before the inversion mechanism was proposed by Ingold in J. Chem. Soc., 1937, 1252. The idea that the addition of one group could occur simultaneously with the removal of another was first suggested by Lewis in 1923, in Valence and Structure of Atoms and Molecules, Chemical Catalog Company, New York, 1923, p. 113. Olsen was the first to propose that a one-step substitution reaction leads to inversion, in J. Chem. Phys., 1933, 1, 418. 

Mechanism of Racemisation Walden Inversion Mechanism of Walden Inversion... 

All diaryl sulfoxides, some alkyl aryl sulfoxides (e.g., methyl p-tolyl sulfoxide, 41), and some dialkyl sulfoxides (e.g., methyl 1-adamantyl sulfoxide), were found to undergo racemization according to the pyramidal inversion mechanism. Mislow and co-workers (248) have found that for most of the compounds investigated, irrespective of the nature of the substituents attached to sulfur, the first-order rate constant for racemization in p-xylene at 210 C is about 3 X 10" sec", corresponding to a half-life of about 6 hr. Moreover, the activation parameters do not show significant differences and their values were contained in a narrow range 35 to 42 kcal/mol and... 

AS -8 to +4 e.u. Even though the racemization rate constants differ slightly, their distinct dependence on the steric and to a lesser extent on the electronic effects of the substituents bonded to the sulfinyl sulfur atom was noted. It deserves adding that the activation volume for racemization of methyl p-tolyl sulfoxide 41, A F 0 ml/mol, is also consistent with the pyramidal inversion mechanism (249). 

A pyramidal inversion mechanism was proposed for the race-mization of 1-adamantyIethylmethylsulfonium 105, benzylethyl-methylsulfonium (112), p-nitrobenzylethylmethylsulfonium (214) and phenacylethylmethylsulfonium 2 perchlorates (152,153). 

Mislow and co-workers (258) and Hammond (259) have shown that optically active diaryl sulfoxides, which are configurationally stable in the dark at 200°C, lose their optical activity after 1 hr at room temperature on irradiation with ultraviolet light. Similarly, an easy conversion of the trans isomer of thianthrene-5,10-oxide 206a into the thermodynamically more stable cis isomer takes place upon irradiation in dioxane for 2 hr. However, the behavior of a-naphthylethyl p-tolyl sulfoxide under comparable irradiation conditions is different, namely, it is completely decomposed after 4 min. These differences are not surprising because the photochemical racemization of diaryl sulfoxides occurs by way of the pyramidal inversion mechanism whereas decomposition of the latter sulfoxide occurs via a radical mechanism with the cleavage of the sulfur-carbon bond. It is interesting to note that photoracemization may be a zero-order process in which the rate depends only on the intensity of the radiation and on the quantum yield. 

A few empirical and theoretical studies to postulate a general set of rules for the fabrication of asymmetric membranes by phase inversion mechanism (in which the polymer solution is coagulated within a nonsolvent bath) have been attempted. Thus, for example, from the literature which described the formation of asymmetric membranes, Klein and Smith (5) compiled working rules in the early 1970s regarding the requirements of a casting solution ... 

Depending on the configurations, the attempted cyclizations of ureas 123 and 125 with thionyl chloride yielded different products. Thionyl chloride treatment of the cis thiourea derivatives 123 (n = 1,2 R = H, Me) resulted in products 124 by water elimination (85T5981). Similar treatment of the tram ureas 125 (n = 1, 2 R = H, Me) furnished the cis oxazines 127. The reaction proceeded by an inversion mechanism, via the intermediate 126 (77MI1). 

The electron energy term of the reduced dye may then be so energetic that direct electron injection is possible as indicated in Fig. 20a. The energy conditions for the inverse mechanism with hole injection by the oxidized dye (which reacts in the excited state primarily with an oxidizing supersensitizer) are shown in Fig. 20b. 

Brown PO.. and N.R. Cozzarelli "A Sign Inversion Mechanism for Enzymatic Supercoiling of DNA." Science. 206. IIX - 1083 < t979]... 

The reason for the lack of SN2 reactivity in ethenyl or ethynyl halides may be that the attacking nucleophile is unable to react by the concerted inversion mechanism that invariably is observed with alkyl halides ... 

Fig. 7.120. Spring-diaphragm actuator (a) air-to-open (direct) mechanism (6) air-to-close (inverse) mechanism...

A net C—O to C—C bond chirality transfer has been demonstrated in the preparation of a cyclopropane (equation 344).428 Another C—O to C—C chirality transfer was accomplished starting with an optically active alkylidenetetrahydrofuran 429 Not only was complete stereospecificity via a double inversion mechanism observed, but, in addition, regioselectivity had to be exercised to avoid the alternate cycliza-tion to give a seven-membered ring product (equation 345). 

These stereochemistry observations for S02 insertion, taken together with first-order kinetics with respect to metal substrate, retardation by steric bulk in R, and rate enhancement with increasing electron density at R, have been interpreted in terms of an 5 2 (inversion) mechanism involving a configurationally stable ion pair (24) as depicted in Scheme 6 (86-88). 

An inversion mechanism had been previously observed in the hydroxylation of caranine (2) to lycorine by Wildman and Heimer. They observed 7% incorporation of [2/ -3H] caranine into lycorine in Zephyranthes Candida Herb., the 3H being retained at C-2 of 1 as shown by the conversion into the inactive 385. The stereospecifically labeled precursor was obtained through LiAl3H4 reduction of lycorine-1,2-a-epoxide prepared from lycorine via its cis-chlorohydrin and chromatography on Florisil. The structure of the a-epoxide rests on physical and chemical grounds, whereas the stereochemistry of the... 

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