S—H bond, cleavage

The smallest molecule in the class is methanethiol, which has been studied at several surfaces as an indicator for the behaviour of larger thiols. On all metals, except gold, adsorption results in the dissociation of the S H bond. Yates and co-workers suggested47 that the S-H bond of methanethiol is also stable on silver, dissociation being catalysed by adsorbed sulfur present at defect sites. However, this contradicts Jaffey and Madix s results,56 which showed S-H bond cleavage below 350 K in their study of ethanethiol on Ag(lll). 

The products are believed to be formed by S-H bond cleavage resulting in phenylthio radicals that subsequently couple to form... 

Hypervalent sulfonium radicals, H3S [188] and (CH3)2SH [189], have been shown to be metastable on the microsecond time scale when generated as partially deuterated species and studied by variable-time NR mass spectrometry. The metastability was attributed to the formation of excited electronic states, as the ground doublet states were calculated to be unbound and predicted to dissociate exothermically and without a barrier by S-H bond cleavage [188,189]. Hypervalent sulfonium radicals of the R3S type have been the long sought-after intermediates of radical substitution in sulfides in solution, but have never been detected [190]. The NRMS studies, combined with ab initio calculations, show that such intermediates most likely do not exist on the ground state potential energy surface, but may be of importance in photochemical reactions. 

In each reaction, oxidative addition of the thiol to the cluster via S-H bond cleavage is observed. The mechanism by which this process occurs is unknown but one proposal features initial formation of the cyclic product, [M3(CO)i2(RSH)] (8), in which S and H are bound to two different metals (8a, 8b). The acyclic species [M3(CO)i2(SR)(H)] might then be produced before loss of two carbon monoxide molecules to give the product [M3(CO)io(yU-SR)(//-H)]. ... 

We might conceive of a reaction in which S—H bond cleavage occurs long before the thiol reaches the transition state, such as in the base-catalysed addition of RSH to an olefin, equations (10) and (11). 

Dmuchovsky, Vineyard and Zienty observed a quite unusual inverse isotope effect for k jkj of 0-65 for the base catalysed addition of n-butane-thiol-S-dj to maleic anhydride. While inconsistent with any model of a transition state involving S—H bond cleavage, the inverse isotope effect could be accounted for by postulating a pre-reaction equilibrium between butanethiol and triethylamine, much h ke the one in equations (10) and (11). In fact, substitution into equations (16) and (17) of 2566 and 1850 cm for the S—H and S—D stretching frequency, respectively, and 3253 and 2380 cm for the N—H and N—D stretches of the amine-thiol complex, yields an equilibrium isotope effect of 0-68 . 

B. Tracers of Atoms and Free Radicals during S—H Bond Cleavage rv. Tracing S-labelled Thiols in Biological Systems... 

The observation of a large kinetic isotope effect indicates that isotopically substituted thiol hydrogen participates directly in a vibrational mode, whose frequency changes on going to the transition state, i.e. that S—H bond cleavage is an integral part of the transition state. The fact that a value for of 2-80 was obtained for the addition of benzene-... 

Cleavage of the S-H bond is central to this monolayer formation ... 

Both schemes accommodate the kinetics, the primary isotope effect and the induction factor, which indicates that Cr(IV) is the initial stage of reduction of the oxidant. RoCek s mechanism does not accord with the solvent isotope effect of 2.5 per proton, which has just the value to be expected for acid-catalysis, for the O-H bond cleavage should be subject to a primary isotope effect of about 7. The ester mechanism is not open to this criticim. 

Time-resolved PAC has also been applied to investigate the thermochemistry of reactions involving the cleavage of C-H, O-H, and S H bonds by a method similar to the one described by equations 13.23 and 13.24 [304-310]. In these cases, the hydrogen abstraction reactions were too slow to be examined by non-time-resolved PAC. Other advantages of using TR-PAC were described by Correia et al. [308]. Finally, it is noted that TR-PAC can also be used to derive rate constants of hydrogen abstraction reactions [311],... 

Exploiting the Lewis basic phosphoryl oxygen of Im, Terada reported the direct alkylation of a-diazoesters with N-acyl imines to afford P-amino-a-diazoesters in high yields and ee s (Scheme 5.12) [23]. Earlier, Johnston had observed that catalytic TfOH promoted aziridine formation (Aza-Darzens reaction) between diazoacetates and N-benzyl imines [24]. The authors propose that aziridine formation is circumvented through C—H bond cleavage by the phosphoryl oxygen of 1 (Intermediate A). However, as noted by the authors, the low nucleophilicity of N-acyl imines might also be considered as the cause of this selective transformation. 

The hydrolysis of a series of spiro-/4-sulfanes such as (330)-(332) leads to sulfoxides in dioxane-water solutions. A mechanism involving slow nucleophilic attack of water on the positively polarized sulfur atom and simultaneous 0—H and S—N bond cleavage is proposed.298 The photo-oxidation of the sulfenamides (333 R = Me, Et, Ph, Bn, etc.) has been reported.299... 

The presence of a styryl group (R1 = Ph) at C-6 confers to penicillins a /3-lactamase inhibition activity (see Section 2.03.12.4) by acidifying H-6, which provokes the C(5)-S(l) bond cleavage (see Section 2.03.5.3.2) toward the formation of l,4-thiazepin-7-ones 36 (Scheme 20) <1996BML2289>. 

The bond dissociation energy (BDE) of the S-H bond of aryl thiols is approximately 75 kcal/mole (14), which would result in the formation of a resonance-stabilized phenylthio radical. The BDE of the aromatic C-S bond is approximately 85 kcal/mole (14), and cleavage of thiophenol at the C-S bond would result in the formation of an unstable and reactive phenyl radical. Very little decomposition of neat thiophenol occurred at a reaction temperature of 375°C for 30 minutes, confirming a report in the... 

As described in Section III.1.4.1.1, the catalytic direct arylation reactions of aromatic compounds occurs effectively via C-H bond cleavage when the substrates are appropriately functionalized. On the other hand, various five-membered heteroaromatic compounds involving one or two heteroatoms, even without a functional group, are known to undergo arylation, usually at their 2- and/or 5-posi-tion(s), on treatment with aryl halides under the action of palladium catalysis. The coupling has recently been developed significantly [1, 2]. Representative examples with some mechanistic discussion are summarized in this section. 

For the hydride transfer the effect of the selenium substitution goes in the opposite direction compared to the H-H bond cleavage, with a 1.5 kcal/mol lower barrier for the selenium case. This effect can be explained by a larger mobility and acidity of selenium. The mobility is indicated by the bond distance to nickel which is 2.30 A for the hydride in the sulfur case and 2.40 A for the selenium case. At the transition states the Ni-S and Ni-Se distances increase to 2.44 A and 2.61 A, respectively. It is also interesting to note that the Se-H distance of 1.71 A at the transition state is shorter than the S-H distance of 1.83 A. Since selenium is more acidic this indicates a dominating hydride character of the hydrogen at the transition state. 

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