An unusual reaction of dimethyl azodicarboxylate with H-dimethylphosphonate

Article abstract of DOI:10.1080/10426500390220998

When the commonly used phosphorus components of the cocktail of the classical Mitsunobo reaction are changed from triphenylphosphine and trialkylphosphite to H-dimethylphosphonate, the course of the reaction changes and leads to products arising from the

Full text of DOI:10.1080/10426500390220998

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Phosphorus, Sulfur, and Silicon and the Related
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An Unusual Reaction of Dimethyl Azodicarboxylate with
H-Dimethylphosphonate
S. Munavalli a , D. K. Rohrbaugh b & H. D. Durst b
a Geo-Centers, Inc. , Aberdeen Proving Ground, Maryland
b U. S. Army, Edgewood Chemical Biological Center , Aberdeen Proving Ground, Maryland
Published online: 23 Aug 2006.
To cite this article: S. Munavalli , D. K. Rohrbaugh & H. D. Durst (2003) An Unusual Reaction of Dimethyl Azodicarboxylate
with H-Dimethylphosphonate, Phosphorus, Sulfur, and Silicon and the Related Elements, 178:9, 1871-1879, DOI:
10.1080/10426500390220998
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Phosphorus, Sulfur, and Silicon, 178:1871–1879, 2003
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Copyright Taylor & Francis Inc.
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426500390220998
AN UNUSUAL REACTION OF DIMETHYL
AZODICARBOXYLATE
WITH H-DIMETHYLPHOSPHONATE
S. Munavalli,a D. K. Rohrbaugh,b and H. D. Durstb
Geo-Centers, Inc., Aberdeen Proving Ground, Maryland;a
and U. S. Army, Edgewood Chemical Biological Center,
Aberdeen Proving Ground, Marylandb
(Received February 4, 2003; accepted February 27, 2003)
When the commonly used phosphorus components of the cocktail of the
classical Mitsunobo reaction are changed from triphenylphosphine and
trialkylphosphite to H-dimethylphosphonate, the course of the reaction
changes and leads to products arising from the involvement of the free
radical reaction. This article presents the mass spectral characteriza-
tion of the novel compounds along with the probable mechanism of their
formation.
Keywords: CI characterization; diethyl azodicarboxylate; free radical
participation; GC-MS-EI; H-dimethylphosphonate; Mitsunobu reac-
tion; novel compounds
Hydrogen mono- and diphosphonates (H-phosphonates) have found
particular application in phosphorylation reactions and in the syn-
thesis of oligonucleotides.1 The hydrogen directly attached to the
phosphorus atom is readily abstractable.1c Also, H-phosphonates are
known to add to carbon-carbon double bonds.2a,b Recently we re-
ported that H-phosphonate adds to the carbon-carbon double bond of
perfluoromonoalkenes to give various products.3 The H-phosphonates
also attack the carbonyl group.4 Free radical catalyzed reaction of
H-phosphonate with styrene oxide has recently been published.5 Air ox-
idation of trialkylphosphines and trialkylphosphites has been described
to yield products including phosphates via the free radical process.6
During the free radical reaction and the exposure to microwaves,
Address correspondence to S. Munavalli, Geo-Centers, Inc., PO Box 68, Gunpowder
Branch, Aberdeen Proving Ground, MD 21010.
1871

1872
S. Munavalli et al.
H-dimethylphosphonate (1) was transformed into trimethyl-phosphate
and tetramethylpyrophosphate.3
Diethyl azodicarboxylate (DEAD, 2) and triphenylphosphine or
trialkyl-phosphites form the necessary constituents of the cocktail
used in the classical Mitsunobu reaction. The versatility and wide
applicability of the Mitsunobu reaction in constructing carbon-carbon
bond have been described in detail.7 The reaction of PPh3 and DEAD
has been stated to form N-alkyl-Nꢁ-bis-(alkoxy)phosphoryl-hydrazine-
carboxylate via the phosphonium salt intermediate.8a Although this
suggestion has been questioned,8b the formation of the phospho-
nium salt intermediate during the Mitsunobu reaction has been well
documented.8c In fact, a stable oxy phosphonium salt has been isolated
and identified.8d Several studies of the mechanism of the Mitsunobu
reaction have appeared.9 The formation of the adduct appears to occur
within seconds.9a It has been suggested that the reaction follows a dual
mechanism in the presence of CF3CO2H.9c
The DEAD-PPh3 adduct has been stated to cause dehydrative decar-
boxylation of β-hydroxy carboxylic acids to yield cis and trans olefinic
products.10a The combination of DEAD-PPh3 can be conveniently used,
instead of dicyclohexylcarbodiimide, in the preparation of esters from
acids and alcohols.10b Secondary hydroxyl group can be transformed
into amino group by reacting it with phthalimide under the condi-
tions of the Mitsunobu reaction followed by reduction.10c Also, the
secondary alcoholic group can be replaced with N3 by conducting the
Mitsunobu reaction in the presence of HN3.10d,e Interestingly, DEAD
also reacts with the carbon-carbon double bond.11 Alkylation of ac-
tive methylene compounds under the Mitsunobu conditions yields both
mono- and dialkylated products accompanied by diethyl N-alkylated
hydrazinedicarboxylate.12a The reaction also has found application in
the preparation of β-lactams.12b,c A novel application of DEAD is its
use in the steroespecific synthesis of aminated products, in particular
biologically active and useful amino acids.13 Propargyl alcohols have
been transformed into allenes.11d
In continuation of our interest in the chemistry of H-dimethyl-
phosphonate (1),3,5,14 its reaction with DEAD (2) has now been inves-
tigated. Among the novel compounds identified are products derived
from the free radical reaction of H-dimethylphsphonate (1) with DEAD
and toluene, the latter being the solvent. The GC-MS characterization
of various compounds [3∼14, cf. Figure 1] and the probable mecha-
nism of formation of the unusual compounds are presented in this
article.

An Unusual Reaction of Dimethyl Azodicarboxylate with H-Dimethylphosphonate 1873
FIGURE 1 Structures of compounds cited in the text.
RESULTS AND DISCUSSION
In the light of the reported addition of H-dimethylphosphonate (1)
to carbon-carbon double bond of α-nitrostyrene,15 the first reaction
we explored was its reaction with cinnamonitrile. Both microwave
cooking of a mixture of cinnamyl nitrile and H-dimethylphosphonate
(1) in the presence of activated molecular sieve powder for 10 min and
refluxing a solution of the two in toluene overnight gave only the start-
ing material. The GC of the reaction product failed to detect the pres-
ence of any other compounds. Since we recently had observed the
addition of difluorocarbene across the nitrogen-nitrogen double bond
of DEAD,16 it was interesting to see whether H-dimethylphosphonate
(1) would react with DEAD (2). Accordingly, a solution of DEAD
(2) and H-dimethylphosphonate (1) in toluene was heated for 8 h at
80∼90◦C (oil bath temperature) under nitrogen and with stirring. The
GC analysis of the reaction product showed it to be a complex mixture of
compounds. The GC-MS analysis of the product after the removal of the
solvent under reduced pressure showed it to consists of 12 components.
A careful study of the mass spectral fragmentation behavior enabled

1874
S. Munavalli et al.
the characterization of all but one component. The unknown compound
(14) failed to give its molecular ion peak. It underwent extensive
fragmentation in the mass spectrometer before its molecular weight
was recorded. In view of the unusual nature of the structures of some
of the compounds, the sample was reexamined using both EI and CI
modes to confirm the molecular weights of all compounds. Even under
the GC-MS CI mode, the unknown compound (14) was observed to
undergo extensive fragmentation. The GC-MS-CI enabled us to confirm
the molecular ion peaks of all of the compounds except 14 (cf. Table I).
Toluene and trimethylphosphate (4) are the first to come off
(Figure 1). The mass spectrum of 4 already has been recorded.3,5
Diethyl hydrazinedicarboxylate (5), carbethoxydimethylphosphonate
(6), and 1,1-diethylhydrazinedi-carboxylate (7) are the next to come
off the GC-MS column. There is nothing unusual about compound 5.9d
TABLE I Mass Spectral Fragmentation of the Compounds Cited in the Texta
5. Compound 5: M+ = 176 (r.t. = 5.55 min; 2.0%); 130 (M CO2-2H); 103
(M CO2C2H5); 91 (M CH3 CO2C2H5); 87 (NCO2C2H5); 84 (C4H8N2); 73
(CO2C2H5, 100%); 59 (C2H7N2); 57 (C2H5N2) and 45 (C2H7N or OC2H5).
6. Compound 6: M+ = 182 (r.t. = 6.03 min; 0.6%); 153 (M C2H5); 139 (153-CH2); 109
(M CO2C2H5, 100%); 95 [(HO)P(O)(OCH3)]; 79 [PH(O)(OCH3)]; 65 (PH2O2); and
47 (PO).
7. Compound 7: M+ = 176 (r.t. = 7.52 min; 6.0%); 148 (M C2H4); 130 (M-2H CO2);
117 (M CO2 NH); 104 (148-CO2, 100%); 89 (104-CH3); 76 (C4H9N2); 59 (C2H7N2)
and 45 (C2H7N or OC2H5).
8. Compound 9: M+ = 248 (r.t. = 8.18 min; 0.2%); 221 (M C2H3, 100%); 203
(M OC2H5); 190 (221 OCH3); 171 (203-OCH2); 159 (191-CH3OH); 145 (159-CH2);
127 [(CH3O)2P(OH)2]; 109 [(CH3O)2P(O) ]; 95 [(HO)P(O)(OCH3)]; 81 (H2PO3); 79
(PO3); 65 (PH2O2); 63 (PO3) and 47 (PO).
10. Compound 10: M+ = 248 (r.t. = 10.05 min; 7.5%); 203 (M OC2H5); 176
(M CO2C2H4, 100%); 158 (M CO2H or OC2H5); 130 (H5C2NNCO2C2H5); 104
(176-C3H4O2); 89 (C3H7NO2); 75 (C2H7N2O); 59 (C2H7N2) and 45 (OC2H5).
11. Compound 11: M+ = 284 (r.t. = 11.14 min; 66.9%); 239 (M CO2H); 212
(M CO2C2H4); 194 (239-OC2H5); 166 [H5C2N2P(O)(OCH3)2]; 158 (C6H10N2O3);
155 [CH3N2H3P(O)(OCH3)2]; 140 [N2H3P(O)(OCH3)2 or 155-CH3]; 127
[P(HO)2(OCH3)2, 100%]; 110 [PH(O)(OCH3)2]; 109 [P(O)(OCH3)2]; 93
[CH3P(O)(OCH3)2]; 65 [P(OH)2] and 47 (PO).
12. Compound 12: M+ = 298 (r.t. = 11.42 min; 2.1%); 253 (M OC2H5 or CO2H); 226
(M CO2C2H4); 180 (180-C2H4); 169 (198-C2H4); 158 C6H10N2O3), 153
(C3H11N2O3P); 126 (C3H11NO3P, 100%); 95 (CH4O3P); 81 (H2PO3); 79 (PO3); 65
(H2PO2); 57 (C2H7N2) and 47 (PO).
13. Compound 13: M+ = 266 (r.t. = 11.98 min; 8.5%); 193 (M CO2C2H5); 178
(C6H5CH2NCO2C2H5); 150 (178-C2H4); 133 (178-OC2H5); 121 (193 CO2C2H4);
106 (C6H5CH2NH); 91 (C6H5CH2, 100%); 77 (C6H5) and 51 (C4H3).
14. Compound 14: M+ = Not Seen (r.t. = 12.9 min; 0.2%).
aMass spectra were run both in EI and CI modes.

An Unusual Reaction of Dimethyl Azodicarboxylate with H-Dimethylphosphonate 1875
Compound 5 and its derivatives are frequently encountered in work-
ing with DEAD.9c However, the formation of compounds 6 and 7 needs
some explanation (cf. Figure 2).
There are not many examples of the free radical reactions of DEAD
(2). It has been stated that cyclic monoalkenes react with DEAD (2) via
FIGURE 2 Probable mechanism of formation of compounds cited in the text.

1876
S. Munavalli et al.
a free radical process, while the acyclic monoalkenes do not.11 Allylic
radicals are said to add across the nitrogen-nitrogen double bond of
DEAD to initially give the “bicarbamyl radical,” which then is described
to abstract hydrogen from the olefin.11b
The formation of 6 and 7 can be rationalized as shown in Figure 2.
The hydrazinyl radical (15) formed by the addition of hydrogen spilt off
from H-dimethylphsophonate (1) (cf., Figure 2) can then abstract hy-
drogen from 1 to form the product, namely 5 and the phosphoryl radi-
cal. Functionalized diazenes have been reported to form radical species
photolytically and thermally.17 Else, the hydrazinyl radical (15) can
split off the carbethoxy radical to give ethyl diazocarboxylate (16). The
carbethoxy radical thus split off from 15 can then go on to add to the hy-
drazinyl radical (15) to furnish triethyl hydrazenecarboxylate radical
(10). The same carbethoxy radical also can add to the dimethylphos-
phoryl radical to yield carbethoxy dimethylphosphonate (6). The bicar-
bamyl radical (15) can lose carbethoxy radical to give ethyl diazacar-
boxylate (16), which then can pick up the carbethoxy radical (Figure 2)
to give the hydrazyl radical (17). The latter can then abstract hydrogen
to form 1,1-diethyl hydrazinecarboxylate (7).
Compounds 5 and 7 are isomers and have the same molecular weight,
namely 176. However, the retention time of compound 7 is greater
than that of compound 5. This implies that compound 7 is more polar
than compound 5. The mass spectrum of compound 7 shows a peak at
m/e = 59, which corresponds to [H2NN+C2H5] moiety. There is another
peak which corresponds to the loss of the NH2 fragment. These consid-
erations lend support to the suggested structure of compound 7. The
formation and the origin of both trimethylphosphate (5) and tetram-
ethylpyrophosphate (8) already have been rationalized.3,5 Figure 2 ex-
plains the formation of ethyl trimethylpyrophosphate (9). The ethyl
methylphosphoryl radical formed from 6, can add to the bicarbamyl
radical (15) to give diethyl-N-[ethylmethylphosphonyl]-hydrazine di-
carboxylate (12). The methyl radical formed in Figure 2, can abstract
hydrogen from the methyl group of toluene to form the benzyl radical
(22), which then adds to the bicarbamyl radical (15) to yield diethyl
N-benzylhydrazinecarboxylate (13).
There are precedents for the addition of the phosphoryl radical to
the carbon-carbon double bond.18 The phosphoryl radical itself has
been implicated in the formation of trimethylphosphtate.19 And again,
there are precedents for the splitting off the methyl radical from H-
dimethylphosphonate (2) and trimethylphosphate (5).20 Table I gives
the mass spectral fragmentation of the compounds described in the
narrative.

An Unusual Reaction of Dimethyl Azodicarboxylate with H-Dimethylphosphonate 1877
EXPERIMENTAL
All solvents were dry and freshly distilled prior to use. Mass spec-
tra were obtained using a Finnigan TSQ-7000 GC/MS/MS equipped
with a 30 m × 0.25 mm. i.d. DB-5 capillary column (J and W Scientific,
Folsom, CA) or a Finnigan 5100 GC/MS equipped with a 15 m × 0.25
mm. i.d.Rtx-5 capillary column (Restek, Bellefonte, PA). The conditions
on 5100 were: oven temperature 60–270◦C at 10◦C/min, injection tem-
perature was 210◦, interface temperature 230◦C, electron energy 70 eV,
emission current 500 µA and scan time 1 sec. The conditions on the TSQ-
7000 were: oven temperature 60–270◦C at 15◦C/min, injection tempera-
ture 220◦, interface temperature 250◦C, source temperature 150◦, elec-
tron energy 70 eV (EI) or 200 eV (CI) and emission current 400 µA
(EI) or 300 µA (CI) and scan time 0.7 sec. Data was obtained in both
the electron ionization mode (range 45–450 da) and chemical ionization
mode (mass range 60–450 da). Ultrahigh purity methane was used as
the CI agent gas with a source pressure of 0.5 Torr (5100) or 4 Torr
(TSQ-7100). Routine GC analyses were accomplished with a Hewlett-
Packard 5890A gas chromatograph equipped with a J and W Scientific
30 m × 0.53 mm i.d. DB-5 column (J and W Scientific, Folsom, CA).
Reaction of H-dimethylphosphonate (1) with Diethyl
Azodicarboxylate (2)
A solution of a stoichiometric amount diethyl azodicarboxylate (1) in
toluene (this was used as supplied by the Fluka Chemical Company)
was added to stoichimetric amounts of H-dimethylphsophonate (2)
and the whole was heated with atirring for 8 hr in an oil bath kept at
80∼90◦C. After that, the reaction mixture was allowed to come to
ambient temperature and was analyzed by GC. Since the reaction
product was shown to be a highly complex mixture of compounds
(3∼14), the solvent was removed under reduced pressure and the
residue was subsequently analyzed and reanalyzed by GC-MS-EI.
To confirm the molecular ion peaks and to determine the molecular
weight of the unknown compound (14), the analysis was done in the
GC-MS-CI mode also. The mass spectral fragmentation of the various
compounds is given in Table I.
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An Unusual Reaction of Dimethyl Azodicarboxylate with H-Dimethylphosphonate 1879
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