Enantioseparation of P-Stereogenic 1-Adamantyl Arylthiophosphonates and Their Stereospecific Transformation to 1-Adamantyl Aryl-H-phosphinates

A focused library of 1-adamantyl arylthiophosphonates was prepared in racemic form. An enantioseparation method was developed for P-stereogenic thiophosphonates using (S)-1-phenylethylamine as the resolving agent. Under optimized conditions, three out of the five arylthiophosphonates were prepared in enantiopure form (ee > 99%). The subsequent desulfurization of optically active arylthiophosphonates gave the corresponding H-phosphinates without significant erosion of enantiomeric purity (ee = 95-98%). Hence, this reaction sequence can be considered an alternative method for the preparation of 1-adamantyl aryl-H-phopshinates. The absolute configuration of the (S)-1-adamantyl phenylphosphonothioic acid was assigned using single-crystal XRD and it allowed the confirmation that the removal of the P = S group proceeds with retention of configuration. The organocatalytic applicability of (S)-1-adamantyl phenylphosphonothioic acid was also evaluated as a P-stereogenic Brønsted acid.

Stereoselective Transesterification of P-Chirogenic Hydroxybinaphthyl Phosphinates

The substitution reaction of phosphinates with a binaphthyloxy group at the phosphorus atom with lithium alkoxides proceeded with good to high efficiencies to give P-chirogenic phosphinates with a high enantiomeric ratio. As alcohols, primary, secondary, and tertiary alcohols could be used, and the use of tert-butyl alcohol yielded the products with a higher enantiomeric ratio. A substrate with two different alkyl groups on the phosphorus atom could also participate in the substitution reaction to give the corresponding products in good yields with excellent selectivity. The molecular structures of one of the substrates and the corresponding products, determined by X-ray analyses, proved that the substitution reaction at the phosphorus atom proceeded with inversion of the absolute configuration. The usefulness of the reaction was demonstrated by using it to prepare a drug candidate for Duchenne muscular dystrophy. Finally, thionation of the resulting phosphinates was carried out to form P-chirogenic phosphinothioates.

Resolution of aryl-H-phosphinates applied in the synthesis of P-stereogenic compounds including a Brønsted acid NMR solvating agent

An enantioseparation method for the preparation of P-stereogenic H-phosphinates was elaborated. In stereoselective reactions, various chiral P-stereogenic compounds were prepared and their applications as chiral NMR solvating agents were assessed.

Asymmetric synthesis of organophosphorus compounds using H-P reagents derived from chiral alcohols

Chiral organophosphorus compounds, especially those containing C-stereogenic carbons in the proximity of the phosphorus atom, are known for their unique properties and have found wide applications that span from medicinal chemistry to enantioselective catalysis. However, the synthesis of such chiral molecules, especially with the precise control of stereochemistry at chiral carbon atoms, still remains a very challenging task. This review summarizes recent advances in the highly stereoselective formation of C- and, in some cases, also P-stereogenic organophosphorus compounds. The presented synthesis strategy is based on the use of H-P reagents bearing TADDOL, BINOL or a menthol moiety attached to the phosphorus atom and serving as a chiral auxiliary. Reactions of such chiral H-P species with different partners, e.g., alkenes, alkynes, imines, and carbonyl compounds, leading to structurally diverse chiral organophosphorus compounds with up to five chiral centers are comprehensively discussed. In each case, the stereochemical outcome of the reaction is influenced by the presence of the chiral alcohol used; therefore, the content of this review is compiled into sections with respect to the type of chiral alcohol attached to the phosphorus atom in the H-P species applied.

Evaluation and Development of Methodologies for the Synthesis of Thiophosphinic Acids

Thiophosphorus acids R¹R²P(S)OH constitute an important class of organophosphorus compounds, in which the phosphorus atom is intrinsically chiral if R¹ ≠ R². In connection with a project aimed at the preparation of chiral thiophosphorus acids, various available literature methods were considered, but few fit the requirement of odorless reagents. Herein, the results of our studies on the synthesis of thiophosphinic acids are reported. Ultimately, two major approaches were selected: (1) the Stec reaction of phosphorus amides with carbon disulfide; and (2) the one-pot synthesis of thiophosphorus acids from H-phosphinates, an organometallic nucleophile, and quenching with elemental sulfur. An application to the preparation of a potential chiral phosphorus organocatalyst is also reported.

Focusing on the Catal. of the Pd- and Ni-Catalyzed Hirao Reactions

The Hirao reaction involving the phosphinoylation or phosphonation of aryl halides by >P(O)H reagents is a P–C bond forming transformation belonging to the recently very hot topic of cross-couplings. The Pd- or Ni-catalyzed variations take place via the usual cycle including oxidative addition, ligand exchange, and reductive elimination. However, according to the literature, the nature of the transition metal catalysts is not unambiguous. In this feature article, the catalysts described for the Pd(OAc)₂-promoted cases are summarized, and it is concluded that the “(HOY₂P)₂Pd(0)” species (Y = aryl, alkoxy) is the real catalyst. In our model, the excess of the >P(O)H reagent served as the P-ligand. During the less studied Ni(II)-catalyzed instances the “(HOY₂P)(−OY₂P)Ni(II)Cl⁻” form was found to enter the catalytic cycle. The newest conclusions involving the exact structure of the catalysts, and the mechanism for their formation explored by us were supported by our earlier experimental data and theoretical calculations.

Let's Make White Phosphorus Obsolete

Industrial and laboratory methods for incorporating phosphorus atoms into molecules within the framework of Green Chemistry are in their infancy. Current practice requires large inputs of energy, involves toxic intermediates, and generates substantial waste. Furthermore, a negligible fraction of phosphorus-containing waste is recycled which in turn contributes to negative environmental impacts, such as eutrophication. Methods that begin to address some of these drawbacks are reviewed, and some key opportunities to be realized by pursuing organophosphorus chemistry under the principles of Green Chemistry are highlighted. Methods used by nature, or in the chemistry of other elements such as silicon, are discussed as model processes for the future of phosphorus in chemical synthesis.

Pd(0)-Catalyzed Cyclizative Phosphorylation of (Z)-1-Iodo-1,6-diene: Synthesis of Alkylphosphonate and Alkylthionophosphonate

A palladium-catalyzed cyclizative phosphorylation reaction of (Z)-1-iodo-1,6-diene with diethyl H-phosphonate is developed, which provides facile access to unsaturated six-membered heterocycles bearing an alkylphosphonate group. The key step of this process is the efficient capture of the involved alkyl-Pd(II) species by H-phosphonate with the help of CsF, followed by reductive elimination of alkyl-Pd(II)-P(O)(OEt)₂. Moreover, this reaction is successfully extended to H-thionophosphonate.

Manganese-Catalyzed and Mediated Synthesis of Arylphosphinates and Related Compounds

The free-radical arylation of H-phosphinates and related compounds was examined. A practical catalytic process with the air as the oxidant could not be found. However, an inexpensive and robust methodology was developed, using catalytic Mn(II) as the radical initiator and excess Mn(IV) as the stoichiometric oxidant. Using these conditions, the inter- and intramolecular arylation of phosphinylidene compounds has a broad scope, including application to the synthesis of P-heterocycles. A full account of this methodology is presented including a discussion of its limitations.

Challenges and solutions in phosphinate chemistry

Several major challenges still remain in organophosphorus chemistry. Organophosphorus compounds are currently synthesized from phosphorus trichloride (PCl3), even though the final consumer products (such as pesticides, flame-retardants, extractants) do not contain reactive phosphorus-chlorine bonds. In order to bypass phosphorus trichloride, significant interest has been devoted to functionalizing elemental phosphorus (P4, the precursor to PCl3), red phosphorus (Pred), or phosphine (PH3). Yet, phosphinates (ROP(O)H2) are already available on an industrial scale and are the most environmentally benign, but their use as phosphorus trichloride replacements has been completely overlooked until a few years ago. An overview of some of the methodologies developed in my laboratory for P–C and P–O bond-forming reactions through phosphinate chemistry, as well as some selected applications, are presented. Another significant challenge remains the synthesis of P-stereogenic compounds. My group’s recent progress in this area is also discussed. Based on menthol as an inexpensive chiral auxiliary, various menthyl phosphinates can be synthesized. These phosphinates are precursor to P-stereogenic phosphines through well-established literature transformations.

On the cost of academic methodologies

Synthetic methodologies can be easily compared using the Cost of Academic Methodologies (CAM) parameter, which estimates the cost of making a mole of a product.

New Developments on the Hirao Reactions, Especially from "Green" Point of View

BACKGROUND

The Hirao reaction discovered ca. 35 years ago is an important P-C coupling protocol between dialkyl phosphites and aryl halides in the presence of Pd(PPh3)4 as the catalyst and a base to provide aryl phosphonates. Then, the reaction was extended to other Preagents, such as secondary phosphine oxides and H-phosphinates and to other aryl and hetaryl derivatives to afford also phosphinic esters and tertiary phosphine oxides. Instead of the Pd(PPh3)4 catalyst, Pd(OAc)2 and Ni-salts were also applied as catalyst precursors together with a number of mono- and bidentate P-ligands.

OBJECTIVE

In our review, we undertook to summarize the target reaction with a special stress on the developments attained in the last 6 years, hence this paper is an update of our earlier reviews in a similar topic.

CONCLUSIONS

"Greener" syntheses aimed at utilizing phase transfer catalytic and microwave-assisted approaches, even under "P-ligand-free. or even solvent-free conditions are the up-to date versions of the classical Hirao reaction. The mechanism of the reaction is also in the focus these days.

Palladium-catalyzed dearomative arylphosphorylation of indoles

A palladium-catalyzed dearomative arylphosphorylation reaction of indoles involving C–C/C–P bond formation is developed.

Palladium-Catalyzed Allylation/Benzylation of H-Phosphinate Esters with Alcohols

The Pd-catalyzed direct alkylation of H-phosphinic acids and hypophosphorous acid with allylic/benzylic alcohols has been described previously. Here, the extension of this methodology to H-phosphinate esters is presented. The new reaction appears general, although its scope is narrower than with the acids, and its mechanism is likely different. Various alcohols are examined in their reaction with phosphinylidene compounds R¹R²P(O)H.