Oxaphospholes and Bisphospholes from Phosphinophosphonates and α,β-Unsaturated Ketones

The reaction of a {W(CO)5}-stabilized phosphinophosphonate 1, (CO)5WPH(Ph)–P(O)(OEt)2, with ethynyl- (2 a–f) and diethynylketones (7–11, 18, and 19) in the presence of lithium diisopropylamide (LDA) is examined. Lithiated 1 undergoes nucleophilic attack in the Michael position of the acetylenic ketones, as long as this position is not sterically encumbered by bulky (iPr)3Si substituents. Reaction of all other monoacetylenic ketones with lithiated 1 results in the formation of 2,5-dihydro-1,2-oxaphospholes 3 and 4. When diacetylenic ketones are employed in the reaction, two very different product types can be isolated. If at least one (Me)3Si or (Et)3Si acetylene terminus is present, as in 7, 8, and 19, an anionic oxaphosphole intermediate can react further with a second equivalent of ketone to give cumulene-decorated oxaphospholes 14, 15, 24, and 25. Diacetylenic ketones 10 and 11, with two aromatic acetylene substituents, react with lithitated 1 to form exclusively ethenyl-bridged bisphospholes 16 and 17. Mechanisms that rationalize the formation of all heterocycles are presented and are supported by DFT calculations. Computational studies suggest that thermodynamic, as well as kinetic, considerations dictate the observed reactivity. The calculated reaction pathways reveal a number of almost isoenergetic intermediates that follow after ring opening of the initially formed oxadiphosphetane. Bisphosphole formation through a carbene intermediate G is greatly favored in the presence of phenyl substituents, whereas the formation of cumulene-decorated oxaphospholes is more exothermic for the trimethylsilyl-containing substrates. The pathway to the latter compounds contains a 1,3-shift of the group that stems from the acetylene terminus of the ketone substrates. For silyl substituents, the 1,3-shift proceeds along a smooth potential energy surface through a transition state that is characterized by a pentacoordinated silicon center. In contrast, a high-lying transition state TS(E′–F′)R=Ph of 37 kcal mol−1 is found when the substituent is a phenyl group, thus explaining the experimental observation that aryl-terminated diethynylketones 10 and 11 exclusively form bisphospholes 16 and 17.


Experimental Section
General. All reactions were performed under argon using Schlenk techniques. Diethyl ether and THF were freshly distilled from sodium/benzophenone prior to use. 1 H, 13 C and 31 P spectra were recorded on spectrometers operating at proton frequencies of 400 MHz or 300 MHz.
Chemical shifts are reported in ppm and referenced internally to residual solvent signals ( 1 H, 13 C) or externally to 85% H 3 PO 4(aq) ( 31 P).High resolution mass spectral analyses (HRMS) were performed on high resolution and FTMS+pNSI mass spectrometer (OrbitrapXL).
X-ray data. Crystallographic data sets were collected from single crystal samples mounted on a loop fiber and coated with N-paratone oil (Hampton Research). Crystal handling turned out to be different for the two compounds: while the crystals of 4 are stable once obtained and can be kept as solid blocks, the crystals of 3 (grown from slow evaporation of a pentane solution at 243K) melt at temperatures above 270K and required careful mounting at low temperatures (below 260-270K) to limit fast degradation of the crystals. We tried as best as we could: several data sets were collected for 4, all solutions could reveal the same atom connectivity shown here and only the best data set resolution is here reported (one that shows the most intense and define diffraction; however no diffraction at high angles could be observed).
Crystallographic data sets were collected from single crystal samples mounted on a loop fiber and coated with N-paratone oil (Hampton Research). Collection was performed using a Bruker SMART APEX diffractometer equiped with an APEXII CCD detector, a graphite monochromator and a 3-circles goniometer. The crystal-to-detector distance was 5.0 cm, and the data collection was carried out in 512 x 512 pixel mode. The initial unit cell parameters were determined by a least-squares fit of the angular setting of strong reflections, collected by a 10.0 degrees scan in 33 frames over three different parts of the reciprocal space (99 frames total). Cell refinement and data reduction were performed with SAINT V7.68A (Bruker AXS). Absorption correction was done by multi-scan methods using SADABS96 (Sheldrick). The structure was solved by direct methods and refined using SHELXL97 (Sheldrick). All non-H atoms were refined by full-matrix least-squares with anisotropic displacement parameters while hydrogen atoms were placed in idealized positions. Refinement of F2 was performed against all reflections. The weighted R-factor wR and goodness of fit S are based on F2. Full details concerning the data sets and crystal resolutions can be found in the respective CIF files deposited at the Cambridge Crystallographic Data Centre under the allocated deposition numbers CCDC 848398 (3e) and CCDC 933297 (21).

10.
Ketone 10 was prepared following a literature procedure. NMR data for alcohol and ketone are consistent with the literature [10], [11].
11. Ketone 11 was prepared following a literature procedure. NMR data for alcohol and ketone are consistent with the literature [12].

18.
Ketone 18 was prepared following a literature procedure. Oxidation of the corresponding alcohol was done by BaMnO4 (see preparation of 2b). NMR data for alcohol and ketone are consistent with the literature [13].

Computational Section
All density functional theory calculations were performed with the Gaussian03 suite of programs. 14 All structures were optimized at the B3LYP level of theory 15 using the 6-31G(d) basis set 16 for the main group elements and the LANL2DZ ECP approximation 17