Salts carbenium

Direct alkylation or acylation of the oxygen of THF by exchange or addition occurs with the use of trialkyl oxonium salts, carboxonium salts, super-acid esters or anhydrides, acyhum salts, and sometimes carbenium salts. 

Velichkova, R. S., Toncheva, V. D., and Panayotov, I. M., Macromonomers fi om vinyl and cyclic monomers prepared by initiation with stable carbenium salts, J. Polym. Sci., Part A Polym. Chem.. 25. 3283-3292, 1987. 

The foregoing analysis shows how a new potential carbenium salt initiator can be rapidly assessed in terms of its relative solubility, the relative position of its iommolecule... 

Explanation The ester ends react with the salt and AgC104 is precipitated, with the formation of a propagating carbenium salt of SbF6, which cannot form an ester. 

PYRROLIDINO INSTEAD OF DIMETHYLAMINO SUBSTITUENTS FOR THE ENVIRONMENTAL ACCEPTABILITY OF PHOSPHONIUM AND CARBENIUM SALT-BASED REAGENTS... 

FIGURE 2.19 Pyrrolidino instead of dimethylamino substitutents for the environmental acceptability of phosphonium and carbenium salt-based reagents.56 Tetramethylurea from O-benzotriazol- l -yl/V./V./V./V -tetramethyluronium hexafluorophosphate and tetrafluoroborate is more volatile and is cytotoxic. The product released from PyBOP is not environmentally objectionable. PyBOP = benzotriazol-l-yl-oxytripyrrolidinophosphonium hexafluorophosphate. 

BENZOTRIAZOL-1 -YL-OXYPHOSPHONIUM AND CARBENIUM SALT-MEDIATED REACTIONS... 

The phosphonium and carbenium salts are efficient reagents for activating and coupling A-alkoxycarbonylamino acids as well as peptide acids. However, the requirement for tertiary amine to effect the reaction has several implications. The base renders hydroxyl groups subject to acylation. Hence, the side chains of serine and threonine and any hydroxymethyl groups of a resin that have not been derivatized... 

Method 1 addition of a coupling reagent (carbodiimide, EEDQ, phosphonium and carbenium salts, trisubstituted phosphates, etc.) and tertiary amine, if necessary, to a mixture of the acid and the amine nucleophile that are to... 

During the past decades, the scope of Lewis acid catalysts was expanded with several organic salts. The adjustment of optimal counter anion is of significant importance, while it predetermines the nature and intensity of catalytic Lewis acid activation of the reactive species. Discovered over 100 years ago and diversely spectroscopically and computationally investigated [131-133], carbocations stiU remain seldom represented in organocatalysis, contrary to analogous of silyl salts for example. The first reported application of a carbenium salt introduced the trityl perchlorate 51 (Scheme 49) as a catalyst in the Mukaiyama aldol-type reactions and Michael transformations (Scheme 50) [134-142]. 

In order to enhance the catalytic activity of a carbocationic center, the novel Lewis acid 54 was designed by Mukaiyama [149-152]. The 1-oxoisoindolium-based carbenium salt 54 [149], possessing a weak coordinating borate counter anion, proved to be a very active catalyst in the aldoUzation (Scheme 58) [150]. The Mukaiyama aldol reaction was catalyzed by 1 mol% of salt 54 and proceeded in up to 97% yield in 30 min. 

In contrast to these results, the group of Sammakia [156] reported that the reaction can be actually catalyzed by the protic acid TfOH, released either by the decomposition of the carbenium salt or by the nucleophilic attack of the diene on the cation center with evolution of the proton. 

Very useful transformations of the 4 -methylcarbenium salts (49) are possible in the presence of electrophiles and trace amounts of water. The 4-CH2+ group first gives the unstable 4-CH2OH derivatives (62), which with electrophiles, e.g. the carbenium salts (49) themselves or aryldiazonium salts, give the 4-E substituted A -phosphorin derivatives (63) or (64), respectively. The CH2OH+ group leaves position 4, and stable 4-E substituted A5-phosphorins are formed (equation (36)) (72TL843). 

The reaction of alkyl isothiocyanates, RNCS, with diphenylphosphinic hydrazide (338) in benzene has been reported.308 The bis(diethylamino)[(methylthio)thiocarbon-yl]carbenium salts (339 X = I or BF4) display ambident reactivity and can react either at carbenium carbon (hard nucleophiles) or at the thiocarbonyl sulfur atom (soft nucleophiles).309 Electrochemically generated superoxide reacts with dithioic S,S -diesters (dicarbothiolates) (340 Ar = C5H3N or C6H4) to give the monocarboxylate anions in 100% yield before giving the dicarboxylate anions.310... 

The application of further suitable carbenium salts offers a wide field of synthetic routes to derivatives of dicoordinated phosphorus. 

Rate Constants for Hydride Transfer of Et3SiH with Various Carbenium Salts... 

Table 2 Rate Constants for the Reactions of Various Bis(p-methoxyphenyl) Carbenium Salts with 2-Methyl-l-pentene in CHzClj at -70° C...

Because Reaction (24) is reversible, eventually all the carbenium salt is consumed in irreversible Reaction (25). Similar behavior was observed for polymerization of tetrahydrofuran initiated with trityl salts. In this case, however, hydride transfer from tetrahydrofuran molecule is followed by proton expulsion to form 2,3-dihydrofuran, which complicates the initiation mechanism [27,28] ... 

The studies of initiation of tetrahydrofuran polymerization with differently substituted carbenium salts shed some light on the reasons of low... 

Unravelling the mechanism(s) of initiation, whereby chain carriers are formed in a system. This topic includes such studies as the mode of proton transfer frran a Br nsted acid to a monomer, the steps involrred in the interaction of a Lewis acid with an olefin (in the absence and/or in the presence of a cocatalyst), the reaction of stable carbenium salts with variais vinylic moieties, etc. 

The second category of carbenium salts includes much less stable entities which cannot be easily isolated. t-Butyl-, benzyl- and 1-phenylethyl carbenium salts with such anions as BF , C10 , etc., are typical examples of such highly reactive initi-... 

The above observations are obviously not intended as a rejection of this very useful technique. We shall have numerous opportunities to underline the important contributions made by its judicious exploitation. Perhaps the most interesting application of conductivity in cationic polymerisation relates to measurements in model systems such as the study of the self-ionisation of initiators, the interactions of Lewis acids with cocatalysts, and of course the extent of dissociation of stable carbenium salts. 

The application of ultraviolet and visible spectroscopy to the identification and measurement of carbenium ions derived from aromatic and dienic monomer has already been discussed (see Sect. II-G-2). The use of this technique to monitor stable carbenium salts is also well known. We have finally stressed in a preceding section that the fate of certain anions could be followed spectrophotometrically during a cationic polymerisation. The limits of detection allowed by the values of the extinction coefficients of all these species and by the sensitivity of present-day instruments is 10 to 10 M. 

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