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P,P-Dimethylformylphosphine: The Phosphorus Analogue of DMF Kevin M. Szkop, Andrew R. Jupp, and Douglas W. Stephan J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b09266 • Publication Date (Web): 01 Oct 2018 Downloaded from http://pubs.acs.org on October 1, 2018
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Journal of the American Chemical Society
Analogue of DMF P,P-Dimethylformylphosphine: The Phosphorus Analogue Kevin M. Szkop, Andrew R. Jupp and Douglas W. Stephan*[a] University of Toronto, Department of Chemistry, University of Toronto, 80 St. George St., Toronto, ON, Canada M5S3H6 Supporting Information Placeholder ABSTRACT: The neutral PCO derivative Ph3GePCO undergoes addition of borohydride to generate the anion [Ph3GePC(H)OBPh3]- A subsequent deprotection strategy provides to the mono-methyl and dimethylformyl phosphines, Ph3GeP(Me)C(H)O and Me2PC(H)O, respectively. The latter species is the phosphorus analogue of DMF was characterized and shown to complex via phosphorus to Ru. A computational study also highlights the similarities and differences between the P- and N-containing molecules.
The recent resurgence of interest in the 2-phosphaethynolate anion and its reactivity has provided chemists with synthons to build inorganic variants of simple organic molecules in which phosphorus replaces nitrogen. For example, the Grützmacher group developed a synthetic route to the phosphorus-containing analogue of cyanuric acid,1 exploiting the reaction of Na[PCO] with B-chlorodiisopinocampheylborane, followed by alcoholysis (Scheme 1). Goicoechea and co-workers employed a fatty acid to protonate Na[PCO] to generate and isolate the phosphoruscontaining analogue of isocyanic acid2 (Scheme 1) while reaction with ammonium salts afforded the phosphorus-containing analogue of urea, H2PC(O)NH2 (Scheme 1).3 Among the simple phosphorus-containing small molecules that have not been prepared to date is the analogue of N,Ndimethylformamide, DMF, in which phosphorus replaces the nitrogen atom. While computational studies have examined the electronic structures of acyl and formylphosphines,4 and acylphosphines and formylphosphonates have been synthesized,5 only one example of a formylphosphine exists in the literature. In 1999, Bertrand and coworkers6 isolated bis(dialkylamido)formylphosphines. Subsequent computational data suggested that the electron-donating properties of the -NR2 amide groups (R = cyclohexyl, isopropyl) on the P center were responsible for the observed stability of such species.4f Bertrand also showed a decrease in stability of formylphosphine oxides compared to the P(III) counterpart with regard to decarbonylation, a feature previously observed in the chemistry of formylphosphonates.6 In a very recent communication, the simplest formylphosphine H2PC(H)O was generated from PH3/CO ice mixtures under interstellar medium -like conditions and observed by mass spectrometric techniques.7 Current synthetic strategies to access acylphosphines requires lengthy syntheses involving the use of pyrophoric and toxic reagents such as PH3, P(SiMe3)3 and volatile acyl chlorides.8 Noting that the previously described phosphorus-containing analogues of small molecules were prepared from salts of [PCO]-,9 we envisioned a strategy in which a protecting group would allow addi-
tion of a nucleophile to the PCO fragment while subsequent deprotection would allow for facile installation of a range of alkyl groups. To this end, we report a synthetic protocol exploiting the species Ph3GePCO to effect the synthesis of P,Pdimethylformylphosphine, Me2PC(H)O, the phosphorus analogue of DMF, while a computational study explores the structural and electronic differences between the nitrogen and phosphorus congeners.
Scheme 1. Phosphorus-containing analogue of simple molecules derived from [PCO]-. The reactions of K[HBPh3] with Ph3GePCO at ambient temperature in THF generated [K][Ph3GePC(H)OBPh3] ([K][1]), involving nucleophilic hydride addition to the carbon and B-O bond formation (Scheme 1). While this species gives rise to one broad resonance in the 11B{1H} NMR spectrum at 6.4 ppm, the 31P{1H} NMR spectrum displays resonances at δ 23.9 and 0.6 ppm in an intensity ratio of 1:0.8, attributable to (E) and (Z) isomers, respectively. These spectroscopic features are similar to those seen for (E/Z)-[Na(HP=CHO)(DME)x].10 1H-1H exchange spectroscopy (EXSY) experiments confirmed that these species are in equilibrium and interconvert on the NMR time scale with a rate constant of 0.6 sec-1 (see ESI). Addition of 18-crown-6 to the above reaction gave [K(18-crown-6)][Ph3GePC(H)OBPh3] ([K(18-crown6)][1]) (Scheme 2). X-ray diffraction studies of single crystals of the (E)-([K(18-crown-6)][1] obtained from a saturated benzene/pentane solution (Figure 1) confirmed the formulation of anion [1]. The structural data revealed Ge-P, P-C, C-O and O-B distances of 2.274(4), 1.711(9), 1.32(1) and 1.71(1) Å, respectively, as well as the (E)-orientation about the C=P double bond. Treatment of [K][1] with one equivalent of CH3I yielded multiple products as evidenced by 31P{1H} NMR spectroscopy, however, treatment of [K][1] with CH3I in the presence of the fluoride source, [NBu4][Ph3SiF2], afforded a single major product, as evidenced by the 31P{1H} NMR doublet resonance at -46.3 ppm (2JPH = 83 Hz). The corresponding 1H NMR spectrum showed a doublet resonance at 11.0 ppm (2JPH = 83 Hz), which collapsed into a singlet upon 31P decoupling. The 19F{1H} and 11B{1H} NMR
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spectra showed the formation of the anion [Ph3BF]- and Ph3SiF. These data were consistent with quantitative borane abstraction, and methylation at P generating Ph3GeP(Me)C(H)O 2 (Scheme 2). All efforts to isolate 2 from the complex product mixture were unsuccessful. Nonetheless, the generation of 2 reveals that the presence of the oxophilic BPh3 fragment in [K][1] allows for methylation at phosphorus, with subsequent borane capture by fluoride.
Figure 1. POV-ray depiction of the anion of (E)-([K(18-crown6)][1]. Hydrogen atoms except the formyl CH and the cation are omitted for clarity. H: white, C: black, P: orange, O: red, B: light green, Ge: dark green.
Ph3GePCO
KHBPh 3 THF
CH3I [NBu4][Ph3SiF 2] THF
Ph3B
O
H
P
GePh3 [K]
[K][1]
THF 18-crown-6
O Ph3B
GePh3 P 2 CH3
H
H
CH3I [NBu4 ][Ph3SiF 2] THF O H 3
O P
GePh 3
[K(18-crown-6)] ([K(18-crown-6)][1]
H3C CH3 [(p-cymene)RuCl2]2 H C 3 P P CH3 Ru 4
O H Cl Cl
containing product 3 as Me2PC(H)O, the phosphorus analogue of N,N-dimethylformamide. Separation of compound 3 from the by-products was accomplished by a trap-to-trap distillation at ambient temperature and ca. 10-3 Torr. NMR spectroscopic analysis confirmed the colorless liquid obtaining from distillation contained a clean THF solution of compound 3. T1 relaxation time of compound 3 were determined to be 5.77(2) seconds via inverse-gated recovery 31P NMR experiments. This allowed the determination of isolated yield of 3 via integration of 31P NMR spectra where a known amount of PPh3 was added as an internal standard. In this fashion, the yield of 3 following distillation was calculated to be 50%. The synthesis of 3 is amenable to scale up and can be repeated starting with several hundred milligrams of [K][1]. Solutions of 3 in THF show no signs of decomposition after weeks in the freezer or after refluxing for 8 hours. In contrast to the observation of Bertrand6 for (R2N)2PC(H)O, photolysis of 3 does not lead to clean decarbonylation. Instead, multiple unidentified products are formed after
