Hydrogen chloride, adventitious

It was speculated that 7 obtained in the reaction with (S)-3 is actually the enantiomeric structure (ent-1), and that it derives from (7 )-2-methylbutanal that may have been produced by racemization of the 5-isomer in situ11. It is also possible, however, that 7 derives from the reaction of (5)-2-methylbutanal and (R)-3, since the reagent that was used is not enantiomer-ically pure (ca. 95% ee). In addition, in situ racemization of 3 via reaction with adventitious chloride ions (or trace amounts of hydrogen chloride) is also possible36. 

Two pieces of direct evidence support the manifestly plausible view that these polymerizations are propagated through the action of car-bonium ion centers. Eley and Richards have shown that triphenyl-methyl chloride is a catalyst for the polymerization of vinyl ethers in m-cresol, in which the catalyst ionizes to yield the triphenylcarbonium ion (C6H5)3C+. Secondly, A. G. Evans and Hamann showed that l,l -diphenylethylene develops an absorption band at 4340 A in the presence of boron trifluoride (and adventitious moisture) or of stannic chloride and hydrogen chloride. This band is characteristic of both the triphenylcarbonium ion and the diphenylmethylcarbonium ion. While similar observations on polymerizable monomers are precluded by intervention of polymerization before a sufficient concentration may be reached, similar ions should certainly be expected to form under the same conditions in styrene, and in certain other monomers also. In analogy with free radical polymerizations, the essential chain-propagating step may therefore be assumed to consist in the addition of monomer to a carbonium ion... 

The decomposition of acid-sensitive substrates during Swern oxidations can also be explained by the presence of adventitious hydrogen chloride. This can be avoided by the use of freshly distilled oxalyl chloride and carefully dried DMSO.174... 

Upon addition of TMSCl, the yield dramatically increased from 21 % to 91 % for the palladium-catalyzed intramolecular conversion of methyl 3-oxo-6-heptenonate to 2-carbomethoxycyclo-hexanone (eq 96). In this particular case, however, hydrogen chloride, generated from hydrolysis of TMSCl with adventitious moisture, was identified as the active promoter. Thus, in a similar cyclization reaction, addition of hydrogen chloride, instead of TMSCl, provided the desired product with comparable yield (eq 97). 

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