| Summary: | In the pursuit of synthesising novel heteroatom-rich phosphorus analogues of ubiquitous ureas, thioureas and guanidines in a sustainable manner, an atom efficient hydrofunctionalisation approach was undertaken. Either by catalysis or catalyst-free (where viable), testing the addition of different P-H bond containing substrates to multiple bond containing heterocumulenes; isocyanates (RN=C=O), isothiocyanates (RN=C=S) and carbodiimides (RN=C=NR).
A catalyst-free low solvent approach for the hydrophosphinylation of isocyanates and isothiocyanates is applicable to a range of phosphorus nucleophiles; including secondary phosphine oxides HP(O)R2 (R = Ph, iPr), phosphites HP(O)(OR)2 (R = Me, Et), and methyl phenylphosphinate. The procedure tolerated isocyanates and isothiocyanates featuring a wide range of substituents in moderate to excellent yields, and by using 4 eq. of 2-MeTHF solvent, solid substrates can be utilised. Facile scale-up and simple reaction conditions make this a straightforward and practical methodology.
Low-coordinate Iron-based m-terphenyl precatalyst, 40 performed double hydrophosphination of isocyanates with primary phosphines, RPH2 (R = Fc, Ad) and additionally formed novel diinsertion-cyclisation products (52 or 55). These are formed when less bulky aryl isocyanates are utilised with alkyl isocyanates only forming mono-insertion hydrophosphination products. Selectivity for diinsertion-cyclisation products can be achieved by decreasing solvent (C6D6) quantity, increasing isocyanate concentration and increasing temperature.
Additionally, 40 is an efficient precatalyst for single (with HPPh2) and double (with FcPH2) hydrophosphination of carbodiimides. Under moderate conditions hydrophosphination reactions undergo near quantitative (HPPh2) and moderate to excellent (FcPH2) conversions for a range of carbodiimides, with only 1 of 8 possible isomers selectively being formed for single hydrophosphination.
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