Diazinium carbalkoxy methylides

In this paper we present study concerning the structure, the stability and the reactivity of some new diazinium ylides. Pyrimidinium-and pyridazinium carbalkoxy methylides prove to be stable compounds with the exception of 3-( p -chlorophenyl)pyridazinium carbethoxymethylide 12 which traps athmospheric CO 2 leading to a ylide-betaine. A selective way to increase the yield of ylide or betaine has been found. An interesting correlation between structure, stability and reactivity has been found. The structure of ylides and betaine has been proven through elemental and spectral (IR, NMR, MS) analysis as well as by chemical methods. Thus, the ability of diazinium carbalkoxy methylides to react with symmetrical substituted activated alkynes (DMAD) has been studied. Nine new compounds derived from diazines have been obtained.


Introduction
5][6][7] In previous research work we presented a detailed study concerning the syntheses, structure, stability and reactivity of some new 3-p-halophenylpyridazine salts and ylides 8,9 and 4methylpyrimidine salts and ylides. 10In these ylides the substituent of the ylide carbanion was a p-R-benzoyl radical with a medium electron-withdrawing effect.Continuing our research in this field we decide to change the substituents of the ylide carbanion with carbalkoxy (carbmethoxy or carbethoxy) groups and, consequently, to see the influence concerning the synthesis, structure, stability, reactivity and properties in the diazinium ylides series.

Scheme 1
Diazinium carbalkoxy ylides 10-15, proved to be stable compounds, more stable than the diazinium-p-R-benzoylmethylides. 10The higher stability of these ylides could be explained through the convergent effect of two factors: the delocalization of ylide carbanion charge to the carbalkoxy groups as well as through the enol-betaine structures of these ylides, Scheme 2. Unexpectedly, in the case of salts 7, in addition to the ylide, 13, the betaine compound 16, was also formed (in a ratio ylide/betaine 30:70).The mechanism that we propose for this reaction can account for these experimental results.Thus, in this case, a part of ylide 13 are trapping atmospheric CO 2 , leading to betaine 16, Scheme 3. Analogous trapping reactions were found in related cases. 3,13,14

Scheme 3
In order to check our hypothesis, we generate the ylide 13 (using an aqueous solution of alkaline carbonates), either in an inert atmosphere (under nitrogen) either with continuously bubbling CO 2 in to the reaction medium.As expected, in first case the percentage of betaine 16 decreased from 70% (as resulted in normal conditions) to 5% while in the second case the percentage of betaine 16 increased from 70% to 85%.
The structure of the new compounds, ylides and betaine, has been proven through elemental (N%) and spectral (IR, 1 H-NMR, MS) analysis as well as by chemical methods.If we consider compounds 13 and 16 as representatives for the series, the spectral analysis is the following: In the IR spectrum of ylide 13 the most important signal is that of the ketone group at ν ∼ = 1730 cm -1 ; in the IR spectrum of betaine 16 at ν ∼ =1555 cm -1 and at ν ∼ = 1420 and 1370 cm -1 appears the bands corresponding to unsymmetrical respectively symmetrical vibrations of the CO 2 -group (medium intensity).At ν ∼ =1550 cm -1 appear the band of ketone group from betaine.
In the 1 H NMR spectrum of ylide 13 the most important signals are those one of the H e (αendocyclic) and H f (ylidic) protons.The H e proton appears around 11.00 ppm (doublet, J= 5.5 Hz) and H f appears in the multiplet from 8.10-7.20 ppm.These protons appear at such high chemical shifts because of the deshielding effect of the positive nitrogen, and, in the case of the ylidic proton H f has to be added the deshielding effect of the carbalkoxy group.In the 1 H NMR spectrum of betaine 16 the most important signals are also, those of the H e (α-endocyclic) and H f (betaine) protons.The H e proton appears around 10.10 ppm (d, J= 5.5 Hz) and H f at 6.00 ppm (s); H f appears at lower chemical shift by more that one ppm as compared with the ylide because this time H f is an aliphatic proton (not ylidic).
Further we have carried out the reactions of ylides with the dipolarophile DMAD both to obtain further chemical evidence for their structure and to gain access to new azabicyclic compounds.Thus, we treated ylides 13 and 15 (generated in situ from the corresponding cycloimmonium salts), with DMAD, when reactions occur as a [3+2] dipolar cycloaddition leading to the azabicyclic compounds 17 and 18, Scheme 4.
In the case of pyrimidinium carbalchoxy methylides the cycloaddition reaction could involve either the 2-or 6-position of the pyrimidine ring.However, position 6 is less electron-deficient than position 2, therefore it is more suitable for reaction with an electron-poor dipolarophile.
The structure of the new compounds was established by elemental (N) and spectral (IR, 1 H-NMR) analysis.For instance, if we consider compound 18, in the 1 H-NMR spectrum the most important signals are those of the H 1 , H 4 and methyl H-atoms of COOCH 3 groups.The H 1 -atom resonated at δ=10.41 ppm (singlet, 1H) while H 4 -atom resonated at δ=8.02 ppm (singlet, 1H), which excludes the cyclisation to the carbon between the two nitrogen atoms.The signal of the CH 3 ester groups appear as non-equivalent at δ=3.85 ppm (singlet, 3H, CH 3 from 6 position) and respectively at δ=3.30 ppm (singlet, 3H, CH 3 from 5 position).
In conclusion, pyrimidinium-and pyridazinium carbalkoxy methylides prove to be stable compounds with the exception of 3-(p-chlorophenyl)pyridazinium carbethoxymethylide 12 which traps athmospheric CO 2 leading to a ylide-betaine.A selective way to increase the yield of ylide or betaine has been found.The structure of ylides and betaine has been proven through elemental and spectral (IR, NMR, MS) analysis as well as by chemical methods.Pyrimidiniumand pyridazinium carbalkoxy methylides react with DMAD, when reactions occur as a [3+2] dipolar cycloaddition leading to the azabicyclic compounds.

Experimental Section
General Procedures. 1 H NMR spectra were run on a Bruker 80 MHz spectrometer and were recorded in ppm downfield from an internal standard, SiMe 4 .The coupling constants are given in Hz.The mass spectra were recorded by electron impact.The IR spectra were recorded with a SPECORD-71 spectrometer in KBr.The melting points are uncorrected.Technical nitrogen has been employed (98%).
General procedure to obtain diazinium ylides 1. Salts (10 mmol) was dissolved in 50 ml water and treated with an aqueous solution of K 2 CO 3 40% when the ylide was obtained.The product was filtered off under vacuum, washed with a large amount of water and dried under vacuum.2. Salts (10 mmol) was dissolved in 50 ml water, than an aqueous solution of K 2 CO 3 40% was added, dropwise in 10 min.(stirring, under nitrogen (for ylides) or CO 2 (for betaines)).The product was filtered off under vacuum, washed with a large amount of water and dried under vacuum.

General procedure to obtain 3+2 cycloadducts
The cycloimmonium salt(2 mmol) and DMAD(2 mmol) were suspended in 20 ml of benzene.The mixture was heated for 2 h on a steam bath and triethylamine (2 mmol, dissolved in 3 ml benzene) was added dropwise (in 30 min.).The resulting mixture was filtered hot, to eliminate triethylamine bromhydrate.The clear solution was evaporated on a steam bath.The crude products recrystallized from acetone for compound 17, and for compound 18 we done flash chromatography on silica using dichloromethane-methanol 99:1.