The parent 6-phospha-2,4,8-trioxa-1,3,5,7-tetramethyladamantane (CgPH) is a phosphine with a rather esoteric structure. It was first reported in 1961 by the group of Buckler while investigating the reactions of PH3 with various carbonyl-containing compounds.[1] It is a sterically bulky ligand/ligand-precursor with a similar profile to tBu2PH, coupled with modest sigma-donicity akin to phosphites, (RO)2PH. However, the compound remained a curiosity, its coordination chemistry dormant, until 1999, when CgPR complexes of palladium were reported by Pringle et al.[2] The work of his group was pivotal in bringing forwards the chemistry of the ligand class. In this regard, the turn of the century saw increased use of CgPR as a ligand in catalysis; several patents appeared, claiming applications of CgP-based ligands in carbonylations, hydroformylations, hydrocyanations, and cross coupling reactions.[3,4] Although the synthesis looks deceptively easy, this compound is better purchased from the few commercial vendors (Strem, Sasol) that offer it for a decent price. I would say it is only worth making in house if you need access to CgPH in large quantities (>100g), and are comfortable making and handling phosphine gas (PH3). Follow the one-pot procedure in reference 5.[5]
Notes:
- CgPH is a white solid readily soluble in all commonly used organic solvents. It is insoluble in water. It is decently air and moisture stable in the solid state, but less so in solution.
- Epstein and Buckler patented CgPH while working under the research division of American Cyanamid Company. The patent is long expired.[6] Since then, several other patents have been filed involving the compound in various applications (see reference 3).
- An intermediate en route to CgPH was isolated in 1997, supporting the proposed mechanism of Epstein and Buckler.[7]
- Note that CgPH is formed and isolated as a mixture of enantiomers. Pringle reported propyl-linked bisCgP, synthesized as a mixture of meso and rac diastereomers. However, both were conveniently separated: “Addition of MeOH to a [DCM] solution of [the diastereomers] leads to selective crystallization of the rac isomer; in this way samples of [each diastereomer] in purity of >95% are readily obtained.”[2]
- Although several procedures for the generation of phosphine exist in the literature, the least cumbersome seems to be that of Gokhale and Jolly, which involves thermolysis of phosphorous acid at around 200degC.[8]
- There is a way to circumvent PH3 by reacting primary phosphines such as PhPH2 with acetylacetone under acidic conditions, which affords tertiary CgPPh.[4a]
- For a review on CgPR ligands, see reference 9.[9]
[1] Epstein, M.; Buckler, S. A. A Novel Phosphorus Heterocyclic System from the Reactions of Phosphine and Primary Phosphines with 2,4-Pentanedione. J. Am. Chem. Soc. 1961, 83, 3279-3282.
[2] Gee, V.; Orpen, A. G.; Phetmung, H.; Pringle, P. G.; Pugh, R. I. Bis(phospha-adamantyl)alkanes: a new class of very bulky diphosphines. Chem. Commun. 1999, 10, 901-902.
[3] a) Suykerbuyk, J. C.; Drent, E.; Pringle, P. G. Diphosphines. US Patent US6156934A, 1997. b) Suykerbuyk, J. C.; Drent, E.; Pringle, P. G. Preparation
of diphosphines as cocatalyst. Shell Internationale Research Maatschappij BV, NL Patent, WO 9842717, 1998. c) Ahlers, W. BASF SE, DE Patent DE10023468A1, 2000. d) Shekhar, S.; Franczyk, T. S.; Barnes, D. M.; Dunn, T. B.; Haight, A. R.; Chan, V. S. Phosphine ligands for catalytic reactions. Abbott Laboratories, US Patent, WO2012009698A1, 2011. e) Klinkenberg, J. L.; Briggs, J. R.; Camelio, A. M.; Grigg, R. D.; Tu, S. Methods for forming 1,3,5,7-tetraalkyl-6-(2,4-dimethoxyphenyl)-2,4,8-trioxa-6-phosphaadamantane. Dow Global Technologies LLC, US Patent, US11111260B2, 2018. f) Klinkenberg, J. L.; Briggs, J. R. Butadiene telomerization catalyst preparation and use thereof. Dow Global Technologies LLC, US Patent US11713286B2, 2022.
[4] a) Baber, R. A.; Clarke, M. et al. Phenylphosphatrioxa-adamantanes: bulky, robust, electron-poor ligands that give very efficient rhodium(I) hydroformylation catalysts. Dalton Trans. 2005, 1079-1085. b) Lavoie, C. M.; Macqueen, P. M.; Stradiotto, M. et al. Challenging nickel-catalyzed amine arylations enabled by tailored ancillary ligand design. Nat. Commun. 2016, 7, 11073.
[5] Downing, J. H.; Floure, J.; Heslop, K.; Haddow, M. F. et al. General Routes to Alkyl Phosphatrioxaadamantane Ligands. Organometallics 2008, 27, 13, 3216-3224.
[6] Epstein, M.; Buckler, S. A. Substituted 1,3,5,7-tetraalkyl-2,6,9-trioxa-10-phosphatricyclo[3.3.1.1.3,7]decanes. US Patent US3026327, 1962.
[7] Bekiaris, G.; Lork, E.; Offermann, W.; Roschenthaler, G.-V. Diastereoselective Synthesis and Molecular Structure of a Bicyclic and a Cage Phosphane. Chem. Ber. 1997, 130, 1547-1550.
[8] Gokhale, S. D.; Jolly, W. L. Phosphine. Inorg. Synth. 1967, 9, 56–58.
[9] Pringle, P. G.; Smith, M. B. Phosphatrioxa-adamantane Ligands in Phosphorus(III) Ligands in Homogeneous Catalysis Design and Synthesis, ed. by Kamer, P. C. J.; van Leeuwen, P. W. John Wiley, New York, 2012.