I
W. Kenneth Musker University of California Davis. 95616
II
Textbook Errors, 83
Nitrogen Ylides from Tetramethylammonium Salts
I t is generally believed that the decomposition of tetramethylammonium hydroxide gives trimethylarnine and methanol (I), hut it has recently been shown that the products obtained from the "dry" decomposition' a t 135°C were trimethylamine and diiethyl ether (2). About 90% of the methyl group lost by the tetramethylammonium ion was found as diiethyl ether and only a trace of methanol was detected. Only when the reaction was carried out in the presence of excess water in a sealed tube a t 270DC,was methanol observed as the primary product (3).
functional grouping which they called a nitrogen ylide, or, more correctly, a trialkylammonium methylide. The simplest ylide, trimethylammonium methylide, is a member of the isoelectronic series: Mer&-%Ha
Mea&H2
+
Formation and Reactions of Nitrogen Ylides
In 1947, Wittig and Wetterling (4) discovered a new Suggestions of material suitable for this column and guest columns suitable for publication directly should be sent with as many details as possible, and particularly with references to modern textbooks, to W. H. Eberhardt, School of Chemistry, Georgia Institute of Technology, Atlanta, Ga., 30332. Since the purpose of this column is to prevent the spread and the continuation of errors and not the evaluation of individual texts. the sources of errors discussed will not be cited. In order to he presented an error must occilr in at least two independent recent standard books. 1 The compound cannot be made completely anhydrous witbout decomposition. 'The ylide is soluble in tetrahydrofuran and most of its reactions are carried out in this solvent. A comprehensive review of nitrogen ylide chemistry can be found in Joansoa, A. W., "Ylid Chemistry," Academic Press, Inc., New York, N. Y., 1966, p. 255.
200 / Journal o f Chemical Education
~ e A 6
The ylide was prepared (4) by shaking a suspension of tetramethylammonium chloride or bromide with phenyl lithium in diethyl ether for several weeks. The ylide was insoluble in ether and was held in suspension along with the starting material during the course of reacti~n.~ The presence of the ylide was shown by condensation with benzophenone, to give a 60% yield of the amino alcohol. I M ~ ~ G c H ~ ] B ~QLi -
In the introduction to the chemistry of ammonium salts, most textbooks and reviews discuss the thermal decomposition of tetramethylammonium hydroxide as a representative reaction of quaternary ammonium ions without &hydrogen atoms. Furthermore, all reactions are reported to occur by an S,2 process iuvolving a nucleophilic attack of the anion on the carbon atom of a methyl group. Nitrogen ylides are seldom mentioned. In this discussion, I will summarize the reactions of the tetramethylammonium ion and describe the evidence for the presence of nitrogen ylides in certain reactions. Because the reactions of the tetramethylammonium ion are so diverse, this unique species should be treated in detail as the initial member of the homologous series of quaternary ammonium compounds.
Mes61H
-
M~&-CH*
+ LiBr + QH 0-
Unfortunately, the formation of the ylide under these conditions was somewhat complicated by further attack of the phenyllithium on the ylide.
-
M ~ ~ & - C H+~@Li
CHzLi +/ Me2N
\-
f QII
CH,
In 1956, Wittig and Polster (6) noted that the structure of the ylide was not as previously reported but rather was a lithium halide complex, ~e$-&-~i~r
or
[MesN--CHaLi] +Br-
They indicated further that this simplest ylide could not be prepared salt free, for if the lithium ion was removed by complexation with dimethoxyethane or if phenylsodium was used in its preparation, the unstabilized ylide decomposed to give dimethylethylamine and polyethylene. Daniel and Paetsch (6),reported that the ylide should be formulated as [Me3NCHzLi]+ Br- on the basis of evidence that its reactivity was similar to organolithium reagents. However, since anion salvation in ethers is not extensive and complex aggregates of organolithium compounds (7) and lithium perchlorate (8) are present in ether solvents, this evidence does not necessarily rule out an intimate association between the bromide ion and lithium. Subsequent to this original work a number of sale free ylides were prepared (9-18) hut in all cases their enhanced stability was due to extensive delocalization of the positive or negative charge present in the ~lide.~
+-
Reoctions of the Tetramethylammonium Ion with Weak Bases
Reoctions of (CH&N-CHAiBr
To illustrate the behavior of the nitrogen ylide, some
+-
reactions of (CH&N-CHrLiBr are tabulated below. Decomposition (5,13): not stabilized by LiBr
-
(CH~)~&-CH*
CHa
CH-N
Oxidation (6) :
+ H,O,
(cH,),&
