Amido Ca and Yb(II) Complexes Coordinated by Amidine-Amidopyridinate Ligands for Catalytic Intermolecular Olefin Hydrophosphination

A series of amido Ca and Yb­(II) complexes LM­[N­(SiMe₃)₂]­(THF) (<b>1Yb</b>, <b>1–4Ca</b>) coordinated by amidine-amidopyridinate ligands <b>L</b>^{<b>1</b>–<b>4</b>} were synthesized via a transamination reaction between proligands <b>L</b>^{<b>1</b>–<b>4</b>}<b>H</b> and bisamido complexes M­[N­(SiMe₃)₂]₂­(THF)₂ (M = Yb, Ca). The reactions of Yb­[N­(SiMe₃)₂]₂­(THF)₂ with proligands <b>L</b>^<b>2</b><b>H</b>-<b>L</b>^<b>4</b><b>H</b> containing CF₃ and C₆H₄F fragments do not allow for preparing the target Yb­(II) complexes, while the Ca analogues were synthesized in good yields. Complexes <b>1Yb</b> and <b>1</b>–<b>4Ca</b> were evaluated as precatalysts for hydrophosphination of styrene, <i>p</i>-substituted styrenes, α-Me-styrene, and 2,3-dimethylbutadiene with various primary and secondary phosphines (PhPH₂, 2,4,6-Me₃C₆H₂PH₂, 2-C₅NH₄PH₂, Ph₂PH, Cy₂PH). Complexes <b>1Yb</b>, <b>1</b>–<b>4Ca</b> performed high catalytic activities in styrene hydrophosphination with PhPH₂ and Ph₂PH and demonstrated high regioselectivity affording exclusively the anti-Markovnikov addition products. For primary PhPH₂ the reactions (1:1 molar ratio of substrates) catalyzed by <b>1Yb</b>, <b>1Ca</b>, and <b>2Ca</b> proved to be highly chemoselective affording the secondary phosphine Ph­(PhCH₂CH₂)­PH; however, complexes <b>3Ca</b> and <b>4Ca</b> led to the formation of both secondary and tertiary phosphines in 80:20 and 86:14 ratios. Styrene hydrophosphinations with 2,4,6-Me₃C₆H₂PH₂ and 2-pyridylphosphine for all complexes <b>1Yb</b> and <b>1–4Ca</b> proceeded much more slowly compared to PhPH₂. Addition of 2-C₅NH₄PH₂ to styrene catalyzed by complex <b>1Yb</b> turned out to be non-regioselective and led to the formation of a mixture of Markovnikov and anti-Markovnikov addition products, while all Ca complexes enabled regioselective anti-Markovnikov addition. Complexes <b>1Ca</b> and <b>1Yb</b> containing catalytic centers featuring similar ionic radii performed different catalytic activity: the ytterbium analogue proved to be a more active catalyst for intermolecular hydrophosphination of styrene with Cy₂PH, 2-C₅NH₄PH₂, and PhPH₂, but less active with sterically demanding 2,4,6-Me₃C₆H₂PH₂. Styrenes containing in <i>p</i>-position electron-donating groups (Me, tBu, OMe) performed with noticeably lower rates in the reactions with PhPH₂ compared to styrene. Complexes <b>1Yb</b>, <b>1Ca</b>, <b>2Ca</b>, <b>3Ca</b>, and <b>4Ca</b> enabled addition of PhPH₂ toward the double CC bond of α-Me-styrene, and the reaction rate for this substrate is noticeably lower; however quantitative conversions were reached in ∼40 h. Complexes <b>1Ca</b> and <b>2Ca</b> promoted 1,2-addition of PhPH₂ to 2,3-dimethyl butadiene with excellent regio- and chemoselectivity to afford linear secondary phosphines. Hydrophosphination of inert 1-nonene with Ph₂PH with 40% conversion becomes possible due to the application of complex <b>2Ca</b> (40 h, 70 °C). The rate law for the hydrophosphination of styrene with Ph₂PH catalyzed by <b>1Ca</b> was found to agree with the idealized equation: <i>v</i> = <i>k</i>[styrene]¹[<b>1Ca</b>]¹