Kinetic of protonolysis

The kinetic of protonolysis of (9-BBN)2 with representative alcohols and phenols are followed [1] in THF and CCl at 25 °C. The protonolysis of (9-BBN)2 hi CCI4 with tert-butyl alcohol exhibits first-order kinetics, first order in (9-BBN)2, and is in good agreement with those obtained in alkenes [1], alkynes [1], and reduction of aldehydes and ketones [2]. 

Dienes form very stable complexes with a variety of metal caibonyls, particularly Fe(CO)s, and the neutral V-diene metal carbonyl complexes are quite resistant to normal reactions of dienes (e.g. hydrogenation, Diels-Alder). However, they are subject to nucleophilic attack by a variety of nonstabilized carbanions. Treatment of -cyclohexadiene iron tricarbonyl with nonstabilized carbanions, followed by protonolysis of the resulting complex, produced isomeric mixtures of alkylated cyclohexenes (Scheme 15).24 With acyclic dienes, this alkylation was shown to be reversible, with kinetic alkylation occurring at an internal position of the complexed dienes but rearranging to the terminal position under thermodynamic conditions (Scheme 16).2S By trapping the kinetic product with an electrophile, overall carbo-... 

R,R-diphenyl ethylene carbonate CR,R-DPEC)) with a racemic zirconaaziridine. (C2-symmetric, cyclic carbonates are attractive as optically active synthons for C02 because optically active diols are readily available through Sharpless asymmetric dihydroxylations [67].) Reaction through diastereomeric transition states affords the two diastereomers of the spirocyclic insertion product protonolysis and Zr-mediated transesterification in methanol yield a-amino acid esters. As above, the stereochemistry of the new chiral center is determined by the competition between the rate of interconversion of the zirconaaziridine enantiomers and the rate of insertion of the carbonate. As the ratio of zirconaaziridine enantiomers (S)-2/(R)-2 is initially 1 1, a kinetic quench of their equilibrium will result in no selectivity (see Eq. 32). Maximum diastereoselec-tivity (and, therefore, maximum enantioselectivity for the preparation of the... 

However, intramolecular nucleophilic participation by the conjugate base during protonolysis of a C—Hg bond is questionable. A study of the acidolysis of the carbon-mercury bond in unsymmetrical di-alkylmercurials rather suggests that the reaction proceeds via a three-center transition state.In any case, substantial kinetic and stereochemical evidence has led to the idea that reaction occurs by a concerted, front side attack with a transition state that involves a pentacoordinate carbon center. In some cases unimolecular mechanisms, SeI, also have been observed. 

Organo-/-element-catalyzed hydroaminations have been extensively investigated for more than 10 years.1034-1038 Lanthanide metallocenes catalyze the regiospecific intermolecular addition of primary amines to acetylenic, olefinic, and diene substrates at rates which are —1/1000 those of the most rapid intramolecular analogs. Kinetic and mechanistic data argue for turnover-limiting C=C/C=C insertion into an Ln-N bond, followed by protonolysis of... 

The kinetic analysis demonstrated an unusual dependence of the cyclization rate on the rare-earth metal ion size, with maximum turnover rates observed for the medium-sized yttrium and slower rates for the larger lanthanum and smaller lutetium [144]. Similar to aminoalkynes, catalysts with more open ligand frameworks are less active. DFT calculations indicate that protonolysis is the ratedetermining step of the process [113], although this notion is contrary to some experimental observations [143,144]. 

Reactions of 2-trialkylsilyl- and trialkylstannyl-substituted furans with benzhydryl cations provided 2,5-disubstituted furans and ipso-substituted furans. Kinetic investigations of the reactions revealed that the monosubstituted product was produced from the protonolysis of the 2,5-disubstituted furylstannane, while the 2,5-disubstituted furan was derived from an electrophilic substitution of the mono-substituted furan. Introduction of a trialkylsilyl and a trialkylstannyl group to the 2-position of furan hardly affected the reactivity of this position towards carbenium ions ipso attack), while the 5-position is somewhat activated <01OL1629,01OL1633>. 

One might presume that Cu(II) acts as a one-electron oxidant, which would require a two-step oxidation sequence (the reaction of Cuta with [Pt Clg] appears from kinetics to be stepwise, although the results were not completely conclusive [25]), making this finding even more remarkable. Conceivably, the actual oxidant involves a cluster of Cu(II) centers, as has been suggested for other oxidations of Pt(II) by Cu(ff), where the intermediacy of Pt(III) may appear unattractive [26] similar consideradmis may apply to the reoxidation of Pd(0) in the Wacker system (a step that is not very well characterized mechanistically). Mixed Pt(II)-Cu(II) clusters are also known [27] and might play a part here. However, other one-electron oxidants such as [Ir Cl6] and Ce(IV), where such clustering seems most unlikely, also oxidize [(CH3)Pt°Cl3] at rates competitive with protonolysis [16]. 

Hydrocarboxylation of alkenes or alkynes involves the formal addition of a carboxylic acid O—H bond across a C=C or C=C bond (Scheme 2.25) [63]. In particular, intramolecular hydrocarboxylation provides an atom economical strategy for lactone synthesis. The aforementioned reaction is thermodynamically favorable but there is a large intrinsic kinetic barrier for this type of cydization, thus requiring the addition of a catalyst. Catalysts for hydrocarboxylation typically facilitate addition by alkene or alkyne binding. This process increases the inherent electrophilicity of the C=C and C=C bonds, respectively. Subsequent protonolysis (or 3-H elimination under Pd catalysis) regenerates the catalytic spedes. 

Table 4.24 Kinetic data for the protonolysis of (9-BBN)2 by unhindered alcohols in CCf at 25 °C [1]...

The protonolysis of (9-BBN)2 in THF with hindered and unhindered alcohols and phenols follow first-order kinetics (Table 4.25) [1], indicating that the dominant pathway involves prior dissociation of (9-BBN)2- Table 4.26 Relative rates for the protonolysis of (9-BBN)2 by representative alcohols and phenols in THF at 25 °C [1] ... 

A study has been reported regarding the ruthenium-catalysed reaction of benza-mides with alkynes, which yields ort/io-alkenylated derivatives. Here, the mechanism is likely to involve rate-limiting metalation, followed by alkyne insertion to form intermediates such as (63) which on protonolysis yield the alkenylated products. An allylic carbon-carbon double bond has also been used as a coordination site in palladium-catalysed alkenylation reactions, as shown in Scheme 3. Here measurement of kinetic isotope effects suggests that coordination of the palladium with the allylic double bond occurs before palladation to give (64). Insertion of the alkene into the carbon-palladium bond gives (65) and -hydride elimination " leads to the product... 

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