Protolytic cleavage

The protolytic cleavage of THF-2,2-dimethyl-l-boraadamantane 23 with pivalic acid also proceeds regiospecifi-cally giving rise, after treatment with 8-hydroxyquinoline, to boron chelate 55 (Scheme 17) <2004MI1>. 

Reaction of alkenes with boryl and silyl groups c/r-located at the C=C bond with alcohols is known to proceed in two different ways <1973S309, 1986JOM(310)151> by protolytic cleavage of the B-C bond and by complexation... 

Treatment of 89 with methanol gives rise to two products 112 and 113 (Scheme 46). The minor compound 113 is formed by protolytic cleavage of 1-boraadamantane, while the major product 112 is obtained as a result of coordination of alcohol to 89 and subsequent 1,2-anionotropic rearrangement <2001JOM(620)51>. 

Table 10.2 Protolytic cleavage reactions of homoleptic Group 15 metal(lll) amides... 

Aliphatic and cycloaliphatic monocarboxylic acids react with sulfur tetrafluoride to give, in general, 1,1,1-trifluoroalkanes as the main products together with considerable amounts of bis(l,1-difluoroalkyl) ethers. Yields of the ethers are related to the nature of the acid and to the reaction conditions. The optimum conditions for the formation of the ethers are dependent on the stability of the ethers towards protolytic cleavage in highly acidic reaction media and on the reactivity of the acids towards sulfur tetrafluoride. 

Fig. 25. Two possible pathways proposed by Sommer and co-workers explaining the observed H/D exchange of the alkanes. Pathway 1 the carbenium ion Rh (nondeuterated) is formed by protolytic cleavage of a C-H bond (via a carbonium ion intermediate). Pathway 2 the olefin R-is formed by acid-base bifunctional dehydrogenation. (Reprinted with permission from Sommer et al. (130a). Copyright 1995 American Chemical Society.)...

H202 in superacids at —78°C converts simple straight-chain alkanes into primary alcohol (ethane), or secondary alcohols and ketones (propane, butane).1,62 89 9° Electrophilic hydroxylation of the secondary C—H bond by the incipient hydroxyl cation formed through the protolytic cleavage of hydroperoxo-nium ion accommodates these observations ... 

However, with vinyl- or phenyltrichlorosilanes, the protolytic cleavage of the Si—C bond becomes the main process, leading to the formation of 1-chlorosilatrane or its C-derivatives62. 

This is why cracking is not significant in butane isomerization. In contrast, prolonged reaction of pentane and hexane in this acid system resulted in predominant protolytic cleavage. 

Considering all possibilities, the protolytic cleavage of propane can be summarized according to Scheme 5.42. Since the butyryl cation was not detected, path a (involvement of n-propyl cation 34) can be excluded. Pathway b (protonation of the secondary C—H bond) is kinetically disfavored compared with protonation of the more electron-rich C—C bond (pathways c andrf). The large amount of methane leaves only path c as the major activation route of propane leading to the formation of ethylcarboxonium ion 112 (formation of the ethyl cation followed by carbonylation with CO). 

The synthesis of hydroxycarbamates from secondary aliphatic amines, C02 and epoxides has been found to be catalyzed by (5,10,15,20-tetraphenylporphinato) aluminum(III) acetate, AI(TPP)(02CCH3) [80], Scheme 6.14 illustrates the mechanism proposed for the catalytic process, which can be carried out under not severe conditions (293-343 K 0.1-5 MPa C02 pressure). The key step here is the insertion of epoxide into the Al-O bond of the A1-carbamate A (Scheme 6.14), which preliminarily forms by the reaction of A1(TPP)(02CCH3) with the amine and C02. Protolytic cleavage of the Al-alkoxide bond in the insertion product, C, by dialkylcarbamic acid regenerates the catalytkally active carbamato-species A and... 

Protonolysis of alkenylboranes by carboxylic acids occurs readily. The stereochemistry of the alkenyl group is retained during the reaction and so hydroboration/protolytic cleavage of alkynes leads to cis alkenes. Deuterated... 

On the basis of these findings a new level had been reached, from which a very efficient pursuit of the stereochemical and other questions pertinent to those classes of compounds became possible. In this respect, the key reaction consisted in the protolytic cleavage of phosphate ion 57 to give 2-biphenylyl-bis-2,2 -biphenylylene-phosphorane 25 d. On the assumption that 57 very probably had an octahedral structure, it was then hoped that the fate of the optical activity on protolytic cleavage of optically active forms of this ion would also enable some conclusions on the stereochemistry of the resulting phosphorane 25 d to be made. The resolution of 57 could readily be achieved with methylbrucinium iodide and finally yielded the optically active potassium salts K-57 with = 1930 and [M] = 10164° 66). 

The following reactions proceed with the participation of the allylic boron system (i) allylboration and protolytic cleavage of organic compounds with multiple bonds, (ii) allylboron-alkyne condensation,598 599 (iii) reductive mono-and trans-a,a -diallylation of nitrogen aromatic compounds, (iv) disproportionation processes between tribut-2-enylborane and BX3 (X = C1, Br, OR, SR). Allylboration of carbonyl compounds, thioketones, imines, or nitriles leads to the homoallylic alcohols, thiols, or amines (Equations (136) and (137). It is most important that 1,2-addition to aldehydes and imines proceeds with high diastereoselectivity so that ( )-allylic boranes and boronates give the anti-products, while -products are formed preferentially from (Z)-isomers. 

Protolytic cleavage of the Sn—Cp bond of stannocene is a reaction of great synthetic utility in the synthesis of a variety of divalent tin compounds. In some cases the partially substituted product CpSnX can be isolated, but in most reports SnX2 is synthesized according to Eq. (24) (43-49). 

The methycyclopentadienyl analog of stannocene undergoes similar reactions. Plumbocene (18) undergoes the protolytic cleavage reactions shown in Eqs. (25) and (26). 

Bispentamethylcyclopentadienyltin(II) undergoes protolytic cleavage of one pentamethylcyclopentadienyl group to yield the salt Me5C5-Sn+BF4 as shown in Eq. (31) (23). 

Regioselective protolytic cleavage of the acetal moiety in compound 1406 which contains a hydrolytically unstable vinyloxy group was carried out in the presence of an acid (aq dioxane, HCl, 5°C, 1 min), and it led to formation of compound 1407 in 91% yield. Treatment of pyrrole 1406 (R = CH2=CHOCH2CH2) with hydrochloric acid in aqueous dioxane at 30-35 °C for 0.5 h resulted in removal of the acetal protection and hydrolytic cleavage of the vinyloxy group to afford pyrrole 1408 in 92% yield. 

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