Amidophosphine Ligand Research Articles

Small Molecule Activation on a Base-stabilized Phosphinidene.

Reduction of (NP)PCl2 (NP=phosphinoamidinate [PhC(NAr)(=NPPri 2 )]- ) with KC8 affords the phosphinoamidinato-supported phosphinidene (NP)P (9). Reaction of 9 with a N-heterocyclic carbene (MeC(NMe))2 C: results in the NHC-adduct NHC→P-P(Pri 2 )=NC(Ph)=NAr featuring an iminophosphinyl group. Reactions of 9 with HBpin and H3 SiPh led to the metathesis products (NP)Bpin and (NP)SiH2 Ph, respectively, whereas with HPPh2 a base-stabilized phosphido-phosphinidene, the product of N-P and H-P bond metathesis, was obtained. Reaction of 9 with tetrachlorobenzaquinone results in oxidation of P(I) to P(III), accompanied by oxidation of the amidophosphine ligand into P(V). Addition of benzaldehyde to 9 results in a phospha-Wittig reaction affording a product of P=P and C=O bond metathesis. Related reaction with phenylisocyanate results in a product of N-P(=O)Pri 2 addition to the C=N bond of an intermediate iminophosphaalkene to produce a phosphinidene intramolecularly stabilized by a diaminocarbene.

Reversible Reductive Elimination in Aluminum(II) Dihydrides

AbstractOxidative addition and reductive elimination are defining reactions of transition‐metal organometallic chemistry. In main‐group chemistry, oxidative addition is now well‐established but reductive elimination reactions are not yet general in the same way. Herein, we report dihydrodialanes supported by amidophosphine ligands. The ligand serves as a stereochemical reporter for reversible reductive elimination/oxidative addition chemistry involving AlIand AlIIIintermediates.

Reversible Reductive Elimination in Aluminum(II) Dihydrides.

Oxidative addition and reductive elimination are defining reactions of transition‐metal organometallic chemistry. In main‐group chemistry, oxidative addition is now well‐established but reductive elimination reactions are not yet general in the same way. Herein, we report dihydrodialanes supported by amidophosphine ligands. The ligand serves as a stereochemical reporter for reversible reductive elimination/oxidative addition chemistry involving AlI and AlIII intermediates.

Flexible Coordination of N,P-Donor Ligands in Aluminum Dimethyl and Dihydride Complexes.

Aluminum hydrides, once a simple class of stoichiometric reductants, are now emerging as powerful catalysts for organic transformations such as the hydroboration or hydrogenation of unsaturated bonds. The coordination chemistry of aluminum hydrides supported by P donors is relatively underexplored. Here, we report aluminum dihydride and dimethyl complexes supported by amidophosphine ligands and study their coordination behavior in solution and in the solid state. All complexes exist as κ2-N,P complexes in the solid state. However, we find that for amidophosphine ligands bearing bulky aminophosphine donors, aluminum dihydride and dimethyl complexes undergo a “ligand-slip” rearrangement in solution to generate κ2-N,N complexes. Thus, importantly for catalytic activity, we find that the coordination behavior of the P donor can be modulated by controlling its steric bulk. We show that the reported aluminum hydrides catalyze the hydroboration of alkynes by HBPin and that the variable coordination mode exhibited by the amidophosphine ligand modulates the catalytic activity.

Copper-catalyzed enantioselective 1,4-conjugate addition of dialkylzinc reagents to α,β- and α,β,γ,δ-unsaturated ketones

An enantioselective Cu(II)-catalyzed conjugate addition of dialkylzinc reagents to α,β- or α,β,γ,δ-unsaturated ketones with chiral cyclohexane-based amidophosphine ligands was developed. With 2 mol% of Cu(OAc)2·H2O/L5, the conjugate addition of diethylzinc to α,β-unsaturated ketones was achieved in good-to-excellent yields (up to 98%) and high enantioselectivities (up to 92% ee). This catalytic system was shown to be efficient for the 1,4-conjugate addition of Et2Zn to (2E,4E)-1,5-diphenylpenta-2,4-dien-1-one with 85% yield and 90% ee. Moreover, with 1 mol% of Cu(OTf)2/L11, the conjugate addition of α,β,γ,δ-unsaturated ketones was accomplished with 1,4-regioselectivity, good yields (79–86%) and excellent enantioselectivities (up to 97% ee).

Cytotoxic Gold(I) Complexes with Amidophosphine Ligands Containing Thiophene Moieties

A new phosphine ligand bearing a thiophene moiety, C4H3SNHCOCH2CH2PPh2 (L), has been prepared by reaction of the aminophosphine Ph2PCH2CH2NH2 with thiophenecarbonylchloride in the presence of triethylamine. The coordination behavior towards gold(I), gold(III) and silver(I) species has been studied and several metal compounds of different stoichiometry have been achieved, such as [AuL2]OTf, [AuXL] (X = Cl, C6F5), [Au(C6F5)3L], [AgL2]OTf or [Ag(OTf)L]. Additionally, the reactivity of the chloride gold(I) species with biologically relevant thiolates was explored, thus obtaining the neutral thiolate compounds [AuL(SR)] (SR = 2-thiocitosine, 2-thiolpyridine, 2-thiouracil, 2-thionicotinic acid, 2,3,4,6-tetra-6-acetyl-1-thiol-β-d-glucopyranosato or thiopurine). The antitumor activity of the compounds was measured by the MTT method in several cancer cells and the complexes exhibit excellent cytotoxic activity.

Regioselective and Enantioselective Cu(II)-Catalyzed 1,4-Conjugate Addition of Diethylzinc Reagent to Nitrodienes.

The regioselective and enantioselective Cu(II)-catalyzed 1,4-conjugate addition of diethylzinc reagent to nitrodienes is described. With 0.25 mol % of Cu(OAc)2 and chiral amidophosphine ligand L3, the 1,4-addition products were produced with a complete regioselectivity, high yields (81-98%), and excellent enantioselectivities (87-97% ee) in a short reaction time.

Diiron Dithiolate Hydrides Complemented with Proton-Responsive Phosphine-Amine Ligands.

The reaction of Fe2(pdt)(CO)6 with two equivalents of Ph2PC6H4NH2 (PNH2) affords the amido hydride HFe2(pdt)(CO)2(PNH2)(PNH) {[H1H]0, pdt2- = CH2(CH2S-)2}. Isolated intermediates in this conversion include Fe2(pdt)(CO)5-(κ1-PNH2) and Fe2(pdt)(CO)4(κ2-PNH2). X-ray crystallographic analysis of [H1H]0 shows that the chelating amino/amido-phosphine ligands occupy trans-dibasal positions. The 31P NMR spectrum indicates that [H1H]0 undergoes rapid proton exchange between the amido and amine centers. No exchange was observed for the hydride. Protonation of [H1H]0 gives [HFe2(pdt)(CO)2(PNH2)2]+ ([H21H]+), which contains two equivalent amino-phosphine ligands. Single-crystal X-ray crystallographic analysis of [H21H]+ also reveals hydrogen bonds between the exo amine protons with a THF molecule and BF4. Deprotonation of [H1H]0 with potassium tert-butoxide gave [HFe2(pdt)(CO)2(PNH)2]- ([1H]-), which was characterized spectroscopically. The complex has time-averaged C2 symmetry with two amido-phosphine ligands. FTIR spectroscopic measurements show that υCO shifts by approximately 20 cm-1 in the series [1H]-, [H1H]0, and [H21H]+. These shifts are comparable to those seen for the S-protonation of the (NC)2(CO)Fe-(μ-Scys)2Ni(Scys)2 site in the [NiFe]-hydrogenases.[1].

ChemInform Abstract: Enantioselective Cu(II)‐Catalyzed Henry Reactions with Chiral Cyclohexane‐Based Amidophosphine Ligands.

Abstract Copper-catalyzed asymmetric Henry reactions are described. Using a new chiral amidophosphine ligand, the Henry reaction of nitromethane and various aldehydes proceeded smoothly to provide chiral β-nitroalcohols in reasonable yields (up to 98%) with high enantioselectivity (up to 97% ee).

Enantioselective Cu(II)-catalyzed Henry reactions with chiral cyclohexane-based amidophosphine ligands

Copper-catalyzed asymmetric Henry reactions are described. Using a new chiral amidophosphine ligand, the Henry reaction of nitromethane and various aldehydes proceeded smoothly to provide chiral β-nitroalcohols in reasonable yields (up to 98%) with high enantioselectivity (up to 97% ee).

Synthesis and structural characterisation of Pd(ii) and Pt(ii) complexes with a flexible, ferrocene-based P,S-donor amidophosphine ligand

1'-Diphenylphosphino-1-{[(2-(methylthio)ethyl)amino]carbonyl}ferrocene (1), accessible via amidation of 1'-(diphenylphosphino)ferrocene-1-carboxylic acid (Hdpf) with 2-(methylthio)ethylamine, reacts with [PdCl2(cod)] (cod = cycloocta-1,5-diene) at a 1 : 1 metal-to-ligand ratio to give trans-[PdCl2(1-κ(2)P,S)] (trans-2) as the sole product. A similar reaction with [PtCl2(cod)] affords a mixture of cis- and trans-[PtCl2(1-κ(2)P,S)] (cis- and trans-3), which can be separated by fractional crystallisation. Complexation reactions performed with 2 equiv. of the ligand are less selective, yielding mixtures of the expected bis-phosphine complexes (i.e., trans-[PdCl2(1-κP)2], or a mixture of cis- and trans-[PtCl2(-κP)2]) with the respective monophosphine complexes. The structures of 1, trans-2, cis-3 and trans-3 determined by X-ray diffraction demonstrate the ability of the title ligand to act as a flexible cis- or trans-P,S-chelate donor (the ligand bite angles are 174.03(2)/173.05(2)° for trans-2/3 and 92.86(2)° for cis-3).

Reactivity of iron complexes containing monodentate aminophosphine ligands – Formation of four-membered carboxamido-phospha-metallacycles

Treatment of [FeCp(CO)2Cl] with 1 equiv of the amidophosphine ligands Li[R2PNR′] (R = Ph, iPr, R′ = iPr, tBu, Cy) afforded complexes of the type [FeCp(CO)(κ2(C,P)-(CO)–NiPr-PPh2)] (1a), [FeCp(CO)(κ2(C,P)-(CO)-NtBu-PPh2)] (1b), and [FeCp(CO)(κ2(C,P)-(CO)-NCy-PiPr2)] (1c) in 40–50% yields. Complex 1a was also formed when [FeCp(CO)2(PPh2NHiPr)]+ (2) was reacted with 1 equiv of KOtBu. These complexes feature a four-membered carboxamido-phospha-ferracycle as a result of an intramolecular nucleophilic attack of the amidophosphine ligand on coordinated CO. Upon treatment of 1a with the electrophile [Me3O]BF4 the aminocarbene complex [FeCp(CO)(κ2(C,P)C(OMe)-NiPr-PPh2)]+ (3) was obtained bearing an aza-phospha-carbene moiety. Upon treatment of cis,trans,cis-[Fe(CO)2(Ph2PNHiPr)2(Br)2] (4a) and cis,trans,cis-[Fe(CO)2(Ph2PNHtBu)2(Br)2] (4b) with KOtBu the carboxamido-phospha-ferracycles trans-[Fe(CO)2(κ2(C,P)-(CO)-NiPr-PPh2)(Ph2PNHiPr)Br] (5a) and trans-[Fe(CO)2(κ2(C,P)-(CO)-NtBu-PPh2)(Ph2PNHtBu)Br] (5b) were formed in moderate yield. Finally, representative structures were determined by X-ray crystallography.

Coordination Chemistry of New Chiral P,N Ferrocenyl Ligands with Half-Sandwich Ruthenium(II), Rhodium(III), and Iridium(III) Complexes

The new half-sandwich ruthenium(II), rhodium(III), and iridium(III) complexes (p-cymene)RuCl2L and Cp*MCl2L (M = Rh, Ir), where L is a planar chiral ferrocenyl tosylamine-phosphine ligand {1,2-(TosNRCH2)(PPh2)C5H3}FeCp (R = H, Me) coordinated in a monodentate fashion (κ1P), have been synthesized and fully characterized both in solution (multinuclear NMR, mass spectrometry) and in the solid state (X-ray analysis on single crystals). Intra- and intermolecular N–H···Cl bonds were observed. Variable-temperature NMR shows an equilibrium between different structures, which have been discussed with the help of DFT calculations. Addition of triethylamine to the complexes with R = H allows removing the tosylamide acidic hydrogen, giving rise to a ferrocenyl amidophosphine ligand coordinated in a bidentate fashion (κ2P,N). All complexes have been fully characterized by multinuclear NMR and mass spectrometry. The structure of the iridium complex, determined by X-ray diffraction, shows a planar nitrogen atom and a stable metal-centered chirality. Only the most stable diastereoisomer, according to DFT calculations, has been observed. A number of these complexes were assessed in the catalytic transfer hydrogenation and asymmetric transfer hydrogenation of acetophenone.

Nickel complexes incorporating an amido phosphine chelate with a pendant amine arm: Synthesis, structure, and catalytic Kumada coupling

A series of organonickel(II) complexes incorporating an amido phosphine ligand tethered with an amino pendant have been prepared and characterized. Deprotonation of N-(dimethylaminoethyl)-2-diphenylphosphinoaniline (H[PNN]) with one equivalent of n-BuLi in ethereal or hydrocarbon solutions at -35 °C generates cleanly dimeric {Li[PNN]}(2) as yellow crystals. The reaction of NiCl(2)(DME) with {Li[PNN]}(2) in THF at -35 °C affords green crystalline [PNN]NiCl. Treating [PNN]NiCl with NaX in acetone solutions gives [PNN]NiX (X = Br, I). Alkylation or arylation of [PNN]NiCl with appropriate Grignard reagents in THF at -35 °C produces red crystalline [PNN]NiR (R = Me, Et, i-Bu, n-hexyl, CH(2)Ph, Ph). The chloride complex [PNN]NiCl was found to be an active catalyst precursor for Kumada coupling reactions of PhX (X = I, Br, Cl) with aryl or alkyl Grignard reagents, including those containing β-hydrogen atoms. The X-ray structures of {Li[PNN]}(2) and [PNN]NiX (X = Cl, Br, Me, Et, n-hexyl) are reported.

Zirconium complexes containing a diarylamido diphosphine ligand: Structural preferences controlled by ligand electronics and sterics

This work describes the preparation of [PNP]ZrX 3 ([PNP] − = [N( o-C 6H 4P i Pr 2) 2] −; X = Cl, Me, CH 2SiMe 3) whose structural preference is found to be a function of the electronic and steric nature of the monodentate ligand X −. The reaction of ZrCl 4(THF) 2 with [PNP]Li in toluene at room temperature generates [PNP]ZrCl 3 as a red solid in 60% yield. Alkylation of [PNP]ZrCl 3 with three equivalents of Grignard reagents in diethyl ether at −35 °C produces cleanly [PNP]ZrR 3 (R = Me, CH 2SiMe 3) as yellow crystalline materials. An X-ray diffraction study of [PNP]ZrCl 3 showed it to be a chloride-bridged binuclear species {[PNP]ZrCl 2(μ−Cl)} 2 in which both zirconium atoms are 7-coordinate whereas that of [PNP]ZrMe 3 revealed a mononuclear, 6-coordinate core structure. Interestingly, with the incorporation of more sterically demanding alkyls, [PNP]Zr(CH 2SiMe 3) 3 is a 5-coordinate compound wherein the amido phosphine ligand is κ 2-N,P bound to zirconium. The solution structures of these molecules were also assessed by variable-temperature NMR spectroscopy.

Synthesis of ferrocene-based amido-phosphine ligands via highly diastereoselective ortho-lithiation and their application in Pd-catalyzed asymmetric allylic alkylations

An efficient synthesis of novel planar chiral ferrocene-based amido-phosphine ligands was accomplished via highly diastereoselective ortho-lithiation and subsequent in situ trapping with chlorodiphenylphosphine. The palladium complexes of mono-phosphino ferrocenecarboxamide, bis-phosphino ferrocenedicarboxamide ligands, and analogues with a ferrocenophane framework were applied in the allylic alkylation of ( E)-1,3-diphenyl-2-propenyl acetate. The reactions proceeded with excellent conversions and ee’s of up to 82%, the most efficient being the camphane based mono-phosphino ferrocenecarboxamide 2.

Amido phosphine complexes of zinc: synthesis, structure, and catalytic ring-opening polymerization of ε-caprolactone

A series of diarylamido phosphine ligands of the type N-(2-dihydrocarbylphosphinophenyl)-2,6-dialkylanilide 1a-d have been prepared and employed to investigate the coordination chemistry of zinc. Protonolysis of ZnMe2 with one equivalent of N-(2-diphenylphosphinophenyl)-2,6-dimethylaniline (H[1a]) produced a mixture of [1a]ZnMe (2a) and Zn[1a]2 (4a), whereas that involving ZnEt2 gave exclusively the three-coordinate [1a]ZnEt (3a). In contrast, treatment of ZnR2 (R = Me, Et) with N-(2-diphenylphosphinophenyl)-2,6-diisopropylaniline (H[1b]), N-(2-diisopropylphosphinophenyl)-2,6-dimethylaniline (H[1c]), or N-(2-diisopropylphosphinophenyl)-2,6-diisopropylaniline (H[1d]) under similar conditions generated quantitatively the corresponding three-coordinate zinc methyl 2b-d and zinc ethyl 3b-d. The bis-ligand complexes 4a,b,d were isolated by either protonolysis of alkyls 2-3 with one equivalent of H[1] or metathesis of ZnX2 (X = Cl, OAc) with the corresponding lithium derivatives 5. Attempts to prepare [1a-d]ZnX (X = Cl, OAc) were not successful regardless of stoichiometry of the starting materials employed. Alcoholysis of zinc alkyls 2-3 led undesirably to protonation on the amido nitrogen donor of 1, highlighting perhaps its higher basicity than alkyls. The reaction of ZnCl2 with H[1c] generated the phosphorus-bound adduct {H[1c]ZnCl(mu-Cl)}2 (6c). Interestingly, attempts to deprotonate 6c with n-BuLi produced unexpectedly the alkylated product [1c]Zn(n-Bu) (7c) instead of [1c]ZnCl; analogous reactions employing NEt3 led to Lewis base substitution to give H[1c] and [ZnCl2(NEt3)]2. Structural characterization of all new compounds was achieved by multi-nuclear NMR spectroscopy (1H, 13C, 31P, and 7Li) and X-ray crystallography (2c-d, 3c, 4d, 5c-d, and 6c) where appropriate. On the basis of the NMR and X-ray data, in combination with the synthetic investigations, the steric nature of these amido phosphine ligands is recognized to follow the order of 1a < 1b < 1c < 1d. Interestingly, zinc alkyls 2-3 are all active initiators for catalytic ring-opening polymerization of ε-caprolactone whereas the bis-ligand complexes 4 are not.

Nickel(II) Complexes Containing Bidentate Amido Phosphine Ligands Derived from α-Iminophosphorus Ylides: Synthesis and Structural Characterization

The reactions of prop-2-ynyltriphenylphosphonium bromide with a series of primary aromatic or aliphatic amines in refluxing acetonitrile generated the corresponding 2-hydrocarbylaminoprop-1-enyltriphenylphosphonium bromide [RNHC(Me)=CHPPh(3)]+Br- (R = 2,6-C(6)H(3)iPr(2) (1a), 2,6-C(6)H(3)Me(2) (1b), Ph (1c), t-Bu (1d)) as crystalline solids. Deprotonation of 1a-d with NaH in THF at -35 degrees C afforded the alpha-iminophosphorus ylides RN=C(Me)CH=PPh(3) (2a-d) in high yield. Spectroscopic and crystallographic data of 2 suggest a strong intramolecular interaction between the imino nitrogen and the phosphorus atom. In contrast to N-arylated 2a-c, the N-tert-butyl-derived 2d is extremely moisture-sensitive. Hydrolysis of 2d led to elimination of benzene and generated concomitantly the phosphine oxide 3d that contains an ene-amine functionality. The reactions of 2a-c with Ni(COD)(2) in the presence of an excess amount of pyridine in toluene produced the divalent nickel complexes of the type [kappa(2)-RNC(Me)=CHPPh(2)]Ni(Ph)(Py) (4a-c). The solution and solid-state structures of these new compounds are presented.

Chiral ferrocenyl amidophosphine ligand for highly enantioselective addition of diethylzinc to N-diphenylphosphinoylimines

The asymmetric addition of diethylzinc to N-diphenylphosphinoylimines afforded N-diphenylphosphinoylamides with enantioselectivity of up to 90% ee in the presence of a catalytic amount of chiral ligand ( S)- N-ferrocenoyl-2-[(diphenylphosphino)methyl]-pyrrolidine 13 (7 mol %) and Cu(OTf) 2 (15 mol %). The remarkable improvement of enantioselectivities, as compared with the same type of chiral ligand 6, could be explained by the unique structure of ferrocenyl amidophosphine ligand combining with the reactive intermediate of this addition reaction.

Diligating Tripodal Amido-Phosphine Ligands: the Effect of a Proximal Antipodal Early Transition Metal on Phosphine Donor Ability in a Building Block for Heterometallic Complexes

The ligand precursors P(CH2NH-3,5-(CF3)2C6H3)3 (1a), P(CH2NHPh)3 (1b), and P(CH2NH-3,5-Me2C6H3)3 (1c), react with the reagents Ti(NMe2)4 and tBuN=Ta(NEt2)3 to generate metal complexes of the type P(CH2NAr(R))3TiNMe2 (2a-c) and P(CH2NAr(R))3Ta=NtBu (3a-c) (where Ar(R) = 3,5-(CF3)2C6H3, Ph, and 3,5-Me2C6H3). Due to ring strain, the phosphine lone pair cannot chelate and is available to bind a second metal, and this feature can be utilized to synthesize heterometallic polynuclear complexes. The 31P chemical shifts observed upon complexation of the early transition metals to the amido donors are large and in the opposite direction expected for the increased C-P-C bond angles in these complexes; these unusual shifts are due to P-Ti and P-Ta distances that are significantly shorter than the sum of van der Waals radii. The reaction of 2c with Ni(CO)4 produces at first the bimetallic complex (CO)3Ni[P(CH2N-3,5-Me2C6H3)3TiNMe2] (4c), which gradually converts to the trimetallic complex (CO)2Ni[P(CH2N-3,5-Me2C6H3)3TiNMe2]2 (5c). The effect of the complexation of Ti and Ta fragments on the donor ability of the phosphine ligand was determined by the preparation of the bis-phosphine complexes trans-L(2)Rh(CO)Cl, (where L = 1a-c, 2a-c, and 3a-c) prepared by the reaction of the appropriate phosphine with [Rh(CO)2(mu-Cl)]2, and a measurement of the resultant CO stretching frequencies. Surprisingly, the complexes with the larger C-P-C angles are significantly poorer donors. Density functional theory calculations were performed to determine what factors affect the donor ability of the phosphine and if through-space interactions might play an important role in the observed electronic properties.