Synthesis and Molecular Structure of 2-(diphenylphosphano)phenyl Benzoate Borane Adduct

The crystal and molecular structure of 2-(diphenylphosphano)phenyl benzoate borane adduct are reported. The title compound crystallizes from a petroleum ether/ethyl acetate mixture in the triclinic space group P1 ̅ with two molecules in the unit cell. The unit cell parameters are: Bond lengths and angles are typical for this phosphane borane adduct.


Introduction
Staudinger Ligation reactions comprise a conjugation method for the introduction of various labels such as fluorescence dyes or radionuclides or for the connection of biologically active (macro-) molecules like peptides or carbohydrates under the formation of a carboxylic amide (peptide) bond between both ligation partners [1][2][3].Functionalized phosphanes are widely applied as building blocks for the traceless variant of this ligation type.For this purpose, modified 2-(diphenylphosphano)phenyl derivatives are the basis which consist of a phosphanophenol unit where the OH group is located in ortho position to the central phosphorus atom of oxidation state +III.Prior to an application via the traceless

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Staudinger Ligation the OH group is esterified [4,5].To modify the appropriate acid residue it is sometimes necessary to block the phosphorus atom, especially, when working with alkylating agents which can lead to phosphonium salts or under oxidative conditions.Borane (BH3) is commonly used as protecting group and forms a stable air-resistant Lewis acid-base pair with the phosphane to prevent the aforementioned side reactions [6].
To introduce the borane protection group, two synthesis ways were pointed out recently.The first consists of the direct reaction of the borane protected phosphanophenol with acid chlorides under basic conditions and the second way involves the introduction of the BH3 group into the functionalized phosphane using BH3• THF [7].Two crystal structures of closely related compounds resulting from such chemical reactions have been published before, see [4,5].

Results and Discussion
Recently, we have shown the formation of the title compound using the first synthesis way starting from acid chloride 1 and phosphanophenol 2 [4].Here, we demonstrate the introduction of the BH3 group in the final step.For this purpose, starting compound 3 was dissolved in anhydrous THF and the BH3• THF complex solution was added at −78 °C.The resulting mixture was warmed to ambient temperature over 2 h.After purification via column chromatography, compound 4 was isolated in 79% yield.Crystals of 4 were grown during the purification step from a saturated petroleum ether/ethyl acetate solution.The reaction procedure is shown in Scheme 1.

Scheme 1. Two synthesis ways for the preparation of phosphane borane adduct 4.
The crystal and instrumental parameters used in the unit cell determination, the data collection, and structure refinement parameters are summarized in Table 1.The molecular structure of 4 is shown in Figure 1 with the used atom-labeling scheme.The displacement thermal ellipsoids are drawn at the 50% probability level.Selected bond lengths comprising key features of 2-(diphenylphosphano)phenyl benzoate borane adduct 4 are given in Table 2.The bond angles around the central P atom (P1) deviate from ideal tetrahedral and range from 105.23(5)° for C1-P1-C13 to 116.97(5)° for B1-P1-C7.The benzoate group is not arranged parallel to the P-bonded phenyl group.The torsion angle C13-C18-O1-C19 is 124.4(1)°.The packing of the molecules in the unit cell in a view along the crystallographic b direction is shown in Figure 2. No mentionable interactions between the molecules exist besides those of the van-der-Waals type.

Data Collection and Refinement
Crystallographic data were collected with a Bruker-Nonius Apex-II CCD-diffractometer (Bruker-Nonius GmbH, Karlsruhe, Germany), with monochromatic Mo-Kα radiation (λ = 0.71073 Å) and a CCD detector.Preliminary data of the unit cell dimensions was obtained from the reflection positions of 36 frames, measured in three different directions of the reciprocal space.After completion of the data measurements the reflex intensities were corrected for Lorentz, polarization and absorption effects.The structures were solved by direct methods using SHELXS-97 and refined against F 2 on all data by full-matrix least-squares methods using SHELXL-2014/1 [8,9].All non-hydrogen atoms were refined anisotropically; all hydrogen atoms bonded to carbon atoms were placed on geometrically calculated positions and refined using riding models.The H atoms bonded to the B1 atom were refined isotropically.Crystallographic data has been deposited with the Cambridge Crystallographic Data Centre, CCDC-1029083.These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html.

Conclusions
The crystal and molecular structure of the title compound 2-(diphenylphosphano)phenyl benzoate borane adduct 4 is reported.These data represent well a crystallographically characterized example of a molecular compound with a Lewis acid-base adduct, which found various applications as building block for applications using the traceless Staudinger Ligation.

Figure 1 .
Figure 1.A view of the structure of the title molecules in crystals of 4 showing the atom labeling scheme.Displacement ellipsoids are drawn at the 50% probability level.

Figure 2 .
Figure 2. View of the packing of molecules in crystals of 4 along the crystallographic b axis.

Table 1 .
Crystal data and structure refinement for compound 4.