Dimethyl hydrazine-1,2-dicarboxylate–triphenylphosphine oxide (1/1)

In the crystal structure of the title compound, C4H8N2O4·C18H15OP, two triphenylphosphine oxide molecules and two dimethyl hydrazine-1,2-dicarboxylate molecules are connected via N—H⋯O hydrogen bonds of moderate strength and are related via a twofold rotational axis. Weak Car—H⋯ O contacts strengthen the crystal structure.

In the crystal structure of the title compound, C 4 H 8 N 2 O 4 Á-C 18 H 15 OP, two triphenylphosphine oxide molecules and two dimethyl hydrazine-1,2-dicarboxylate molecules are connected via N-HÁ Á ÁO hydrogen bonds of moderate strength and are related via a twofold rotational axis. Weak C ar -HÁ Á Á O contacts strengthen the crystal structure.

Comment
The two main by-products of the Mitsunobu reaction are bis-substituted hydrazines RO 2 CNHNHCO 2 R and phosphine oxides O=PR 3 . Both types of compounds can interfere with the isolation of the desired products as in our work using N3-benzoylthymine. In such cases it was necessary to skip isolation of the initially formed product and to perform debenzoylation in basic medium and eventually isolate the much more polar deprotected final compound. We recently alkylated N3-benzoylthymine and uracil with allyl and propargyl alcohol, Ph 3 P and iPrO 2 CNNCO 2 iPr in dioxane or tetrahydrofuran for further transformations. After treatment of the crude reaction mixture with sodium methoxide in methanol, we isolated a crystalline compound identified by the present study as a 2:2 complex of the substituted hydrazine with triphenylphosphine oxide.
Evidently, a transesterification occurred during the sodium methoxide treatment.
Even though the Mitsunobu reaction has been known for over 40 years, only in 1996 has an X-ray structure of a similar complex (Anderson et al., 1996, Héroux & Brisse, 1997 been published and its stability was attributed to strong association via hydrogen bonding. The present communication shows a second compound of this kind. In the adduct the bond lengths of the hydrazine molecule are the same within 0.01 Å as in the crystal structure of pure dimethyl hydrazine-1,2-dicarboxylate (Wang et al., 2007) while the H-N-N-H torsion angle increases from 95 to 105°w ith adduct formation. This value is almost identical to that in the di(isopropyl) hydrazine-1,2-dicarboxylate adduct (Héroux & Brisse, 1997).
That there is little or no change in component geometry with adduct formation suggests that the hydrogen bonds formed are of only moderate strength (Table 1). This is also supported by the modest shift of the P=O stretching frequency from 1190 cm -1 in pure triphenylphosphine oxide to 1166 cm -1 in the adduct which is one property expected to be sensitive to the strength of the hydrogen bonding.

Experimental
The compound is a by-product of the Mitsunobu reaction: allyl alcohol was employed for alkylation of thymine using Ph 3 P and iPrO 2 CNNCO 2 iPr in dioxane. After treatment of the crude reaction mixture with sodium methoxide in methanol, a crystalline compound was isolated (m.p. 127-129° C). Parent peaks at 148 and 278 were clearly visible in a mass spectrum of the crystal dissolved in dichloromethane. Evidently, a transesterification occurred during a sodium methoxide treatment. supplementary materials sup-2 A comparatively large block was cut for the data collection due to the relatively low sensitivity of the CCD detector. Data collection was limited to θ=25° because of the geometry of the instrument.