Deprotonations mechanisms

Nevertheless, whereas the base-promoted isomerization of simple linear oxiranes and cyclohexene oxide occurs via a -deprotonation mechanism, recent denterinm-labeUng experiments demonstrate that the LDA-mediated rearrangement of cyclopentene oxide in nonpolar solvents furnishes the corresponding cyclopentenol through an a-deprotonation route (Scheme 7) . 

From a mechanistic point of view two possible hypotheses were discussed (Scheme 2). Since radical cations are intrinsically very acidic [29-32] one would expect enol radical cation 27 to deprotonate efficiently and rapidly thus providing an a-carbonyl radical (mechanism 1). In a further one-electron oxidation step the a-carbonyl cation is formed, that cyclizes to an intermediate cyclohexadienyl cation 28. After a 1,2-methyl shift and deprotonation the benzofuran 29 is obtained. The mechanistic proposal is in line with benzofuran formation from a-carbonyl cations as demonstrated by Okamoto [120]. Interestingly, the above mechanism was first proposed by Bailey to explain the formation of 3% of benzofuran 24 in the ozonization of enol 8 [121], Years later, however, the mechanistic hypothesis was proven to be untenable [122] under ozonization conditions. A priori, it cannot be excluded that intramolecular cyclization of the enol radical cation 27 " is faster than deprotonation (mechanism 2). The distonic radical cation formed is expected to lose a proton readily and after a second one-electron oxidation the same cyclohexadienyl cation intermediate as in mechanism 1 is formed. 

The complexes [Ru2(ju-CO)()u-CHR)(p,-dppm)Cp2] (R = H or Me, dppm = Ph2PCH2PPh2) also undergo reversible one-electron oxidation, but loss of a second electron leads to deprotonation of the bridging carbene and isolation of [Ru2(ju-CO)()u-CX)(ju-dppm)Cp2] (X = H or CHCH2) (211). A similar double-oxidation deprotonation mechanism accounts for the synthesis of [Ru3(u2-CO)3(ju3-CCH2)(Tj-C5Me5)3] from [RU3-(/Lt2-CO)3(ju3-CMe)(T,-CsMe5)3] (212) Section... 

A slow isomerization of the (E)-isomer of 54 was observed in CDCI3 at 25°C in the presence of DBU (81JOC2667). Because the TV-methyl derivative of ( )-54 did not isomerize during 48 hr, a deprotonation mechanism was suggested for the isomerization. In the first step the amide proton was abstracted to give a delocalized anion (55), which could rotate about the carbon-carbon single bond to yield the (Z)-isomer of 54. 

The best experimental estimate of the inversion barrier in aziridine itself is 19.5 kcal mol-1 after correction for zero-point energy differences and tunneling <75JA6990>. Caution must be used when considering the inversion barrier in A-unsubstituted aziridines, since a net inversion may occur by a protonation-deprotonation mechanism promoted by adventitious water, thus lowering the apparent... 

Gorelsky SI, Lapointe D, Fagnou K (2008) Analysis of the concerted metalation-deprotonation mechanism in palladium-catalyzed direct arylation across a broad range of aromatic substrates. J Am Chem Soc 130 10848-10849... 

A significantly more active catalytic system was recently reported, with a ruthenium complex generated from carboxylic acid 75. This allowed for direct arylations to occur also in apolar solvents likely via a concerted metalation-deprotonation mechanism (CMD) and set the stage for the use of aryl bromides, chlorides, and tosylates as electrophilic substrates (Scheme 9.26) [64],... 

The mechanism of type II IPP isomerase is currently not known. Two mechanisms have been proposed. Epoxy and fluorinated substrate analogues have been used to probe the mechanism of type II IPP isomerase. These compounds were irreversible inhibitors of the enzyme forming N5 covalent adducts with the reduced flavin. These studies were taken as evidence of a protonation/deprotonation mechanism in which protonation of the double bond of IPP is followed by deprotonation of the carbocation intermediate to form DMAPP, similar to the mechanism of type I IPP isomerase. Conversely, a radical mechanism has been... 

In addition to the deprotonation mechanism in Eq. (9.20), a mechanism involving exchange with the cis hydride is likely here. 

T. Iwamoto, M. Akita, T. Kozawa, Y. Yamamoto, D. Werst, D.A. Trifunac, and D. Alexander, Radi ation and photochemistry of onium salts acid generators in chemically amplified resists, Proc. SPIE 3999, 204 213 (2000) A. Nakano, K. Okamoto, Y. Yamamoto, T. Kozawa, S. Tagawa, T. Kai, H. Nemoto, and T. Shimokawa, Deprotonation mechanism of poly(4 hydroxystyrene) and its deriva tives, Proc. SPIE 5753, 1034 1039 (2005) T. Kozawa, A. Saeki, and S. Tagawa, Modeling and simulation of chemically amplified electron beam, x ray, and EUV resist processes, J. Vac. Sci. Technol. B 22(6), 3522 3524 (2004) T. Kozawa, A. Saeki, A. Nakano, Y. Yoshida, and S. Tagawa, Relation between spatial resolution and reaction mechanism of chemically amplified resists for electron beam hthography, J. Vac. Sci. Technol. B 21(6), 3149 3152 (2003). 

Our initial work in the oxidative aryiation of heteroarenes was an example of the discovery of new chemical territory. However, we sought to not just discover this new reactivity, but to understand it. Consequently, both experimental and computational studies were performed to elucidate the mechanism of the oxidative indole arylations. Both sets of data indicate a concerted metalation-deprotonation mechanism (CMD) for the palladation of both the... 

Gorelsky, S.I., Lapointe, D., Fagnou, K., 2012. Analysis of the palladium-catalyzed (aromatic) C—H bond metalation-deprotonation mechanism spanning the entire spectrum of arenes. I. Org. Chem. 77, 658-668. 

Cao Z, Mo Y, Thiel W (2007) Deprotonation mechanism of NH4-I- in the Escherichia coli ammonium transporter AmtB insight from QM and QM/MM calculations. Angew Chem Int Ed 46 6811... 

Before the advent of SIMS, formula recalculation of mica was performed using different normalization schemes [93]. Depending on the particular procedure utilized, the effect was to overestimate, for instance, vacancy-bearing substitutions (i.e., mechanisms 1 and 4 in the list above) or, on the contrary, underestimate them. This was because either perfect stoichiometry was assumed, neglecting deprotonation mechanisms, or the latter were taken into account but, in absence of a direct knowledge of the H content, they could not be properly constrained. 

C8Me8)(Tl-C5H5)2] and [M02(ii-C6Me6)( n5-indenyl)2] respectively the former undergoes C-H bond activation by a double-oxidation deprotonation mechanism. Finally, flyover ... 

When the supporting neutral ligand is switched from P[OCH(CF3)2l3 to 2,2 -bipyridyl (bipy), arylation is expected to occur at the most acidic a-position via ametallation-deprotonation mechanism. However, when arylboronic acids were... 

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