Design, synthesis, and evaluation of chiral thiophosphorus acids as organocatalysts.

A series of P-stereogenic chiral phosphorus acids (CPAs) were synthesized to determine the requirements for efficient asymmetric organocatalysis. In order to eliminate the need for C 2-symmetry in common CPAs, various scaffolds containing C 1-symmetrical thiophosphorus acids were chosen. These new compounds were synthesized and evaluated in the asymmetric transfer hydrogenation of 2-phenylquinoline. Although the efficacy of the thiophosphorus acids was disappointing for this reaction, the work should be useful for developing structural design elements.

Synthesis of P-Substituted 5- and 6-Membered Benzo-Phostams: 2,3-Dihydro-1H-1,2-benzazaphosphole 2-Oxides and 2,3-Tetrahydro-1H-1,2-benzazaphosphinine 2-Oxides.

Several approaches were developed for the preparation of phosphorus-substituted 5- and 6-membered benzophostams. Carbodiimide-promoted cyclization of zwitterionic aminophosphinates derived from a nitrobenzene precursor accomplished the cyclization in good yields. Alternatively, a novel copper-catalyzed cross-coupling between a phosphonamide and a bromobenzene precursor produced the heterocycles in moderate to good yields. Three different methods are compared for the synthesis of the P-ethoxy-substituted 5-membered benzophostam.

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.

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.

Manganese-Mediated Homolytic Aromatic Substitution with Phosphinylidenes.

Carbon-phosphorus (C-P) bond-forming reactions via homolytic aromatic substitution (HAS) of non-functionalized substrates with phosphinylidene P(O)H compounds is becoming a useful synthetic tool for the preparation of aryl and heteroaryl phosphonates, phosphinates, and phosphine oxides. This review specifically covers recent work using manganese-mediated free radical processes. Several research groups employ expensive Mn(OAc)3 , whereas our own contributions focus on the novel and inexpensive Mn(OAc)2 /MnO2 system.

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.

General synthesis of P-stereogenic compounds: the menthyl phosphinate approach.

Easily prepared menthyl phosphinates of high diastereoisomeric purity provide versatile intermediates for the synthesis of P-stereogenic compounds. Previous efforts starting about fifty years ago have been hampered by a lack of generality so the menthyl route has been nearly abandoned. Herein we provide a general solution to this long-standing problem and describe a general synthesis of menthyl H-phosphinate and disubstituted phosphinate esters. The method to prepare these versatile precursors relies on a simple and inexpensive process avoiding the use of phosphorus trichloride, Grignard reagents, and complicated cryogenic crystallizations. Established protocols can then be employed to synthesize P-stereogenic secondary and tertiary phosphine oxides and therefore P-stereogenic phosphine ligands.

P(═O)H to P-OH Tautomerism: A Theoretical and Experimental Study.

Phosphinylidene compounds R(1)R(2)P(O)H are important functionalities in organophosphorus chemistry and display prototropic tautomerism. Quantifying the tautomerization rate is paramount to understanding these compounds' tautomerization behavior, which may impact their reactivities in various reactions. We report a combined theoretical and experimental study of the initial tautomerization rate of a range of phosphinylidene compounds. Initial tautomerization rates are found to decrease in the order H3PO2 > Ph2P(O)H > (PhO)2P(O)H > PhP(O) (OAlk)H > AlkP(O)(OAlk)H ≈ (AlkO)2P(O)H, where "Alk" denotes an alkyl substituent. The combination of computational investigations with experimental validation establishes a quantitative measure for the reactivity of various phosphorus compounds, as well as an accurate predictive tool.

Development of a new family of chiral auxiliaries.

A new family of chiral auxiliaries designed on a conformationally restricted version of (-)-8-phenylmenthol has been developed. Both enantiomers are available from an inexpensive synthesis conducted on multigram scale. A first application has showed comparable diastereoselectivity between the novel auxiliary and (-)-8-phenylmenthol.

Carbon-hydrogen to carbon-phosphorus transformations.

Literature published between 2008 and 2013 concerning the functionalization of carbon-hydrogen into carbon-phosphorus bonds is surveyed. The chapter is organized by reaction mechanism. The majority of methods still proceed via deprotonation of C-H into C-M (M=Li, Na, etc.) followed by reaction with a phosphorus electrophile P-X, where X is usually chlorine. A few examples of electrophilic aromatic substitution and related processes have also been reported, although this approach has not yet been developed significantly. Over the past 5 years a rapidly growing family of reactions includes transition metal "C-H activation" and formally related radical-based processes has been developed. The latter processes offer exciting prospects for the synthesis of organophosphorus compounds.

Manganese-mediated intermolecular arylation of H-phosphinates and related compounds.

The intermolecular radical functionalization of arenes with aryl and alkyl H-phosphinate esters, as well as diphenylphosphine oxide and H-phosphonate diesters, is described. The novel catalytic Mn(II) /excess Mn(IV) system is a convenient and inexpensive solution to directly convert Csp2 H into CP bonds. The reaction can be employed to functionalize P-stereogenic H-phosphinates since it is stereospecific. With monosubstituted aromatics, the selectivity for para-substitution increases in the order (RO)2 P(O)H

Phosphinate-containing heterocycles: A mini-review.

This review provides an overview of recent efforts towards the synthesis of phosphinate heterocycles R(1)R(2)P(O)(OR). Our laboratory and others' have been involved in this field and as a result new P-C, P-N, and P-O containing heterocyclic motifs are now available through a variety of methods. While developing rapidly, this area is still in its infancy so that biological testing of the compounds has not yet been conducted and applications are rare. The growing availability of synthetic methods will undoubtedly change this situation in the near future.

Phosphinate chemistry in the 21st century: a viable alternative to the use of phosphorus trichloride in organophosphorus synthesis.

Organophosphorus compounds are important in everyday applications ranging from agriculture to medicine and are used in flame retardants and other materials. Although organophosphorus chemistry is known as a mature and specialized area, researchers would like to develop new methods for synthesizing organophosphorus compounds to improve the safety and sustainability of these chemical processes. The vast majority of compounds that contain a phosphorus-carbon bond are manufactured using phosphorus trichloride (PCl3) as an intermediate. However, these reactions require chlorine, and researchers would like to avoid the use of PCl3 and develop safer chemistry that also decreases energy consumption and minimizes waste. Researchers have already proposed and discussed two primary strategies based on elemental phosphorus (P4 or Pred) or on phosphine (PH3) as alternatives to PCl3. However, phosphinates, an important class of phosphorus compounds defined as any compound with a phosphorus atom attached to two oxygens, R(1)R(2)P(O)(OR) (R(1)/R(2) = hydrogen/carbon), offer another option. This Account discusses the previously neglected potential of these phosphinates as replacements of PCl3 for the preparation of organophosphorus compounds. Because of their strong reductive properties, industry currently uses the simplest members of this class of compounds, hypophosphites, for one major application: electroless plating. In comparison with other proposed PCl3 surrogates, hypophosphorous derivatives can offer improved stability, lower toxicity, higher solubility, and increased atom economy. When their reducing power is harnessed to form phosphorus-carbon or phosphorus-oxygen bonds, these compounds are also rich and versatile precursors to organophosphorus compounds. This Account examines the use of transition metal-catalyzed reactions such as cross-coupling and hydrophosphinylation for phosphorus-carbon bond formation. Because the most important industrial organophosphorus compounds include compounds triply or quadruply bound to oxygen, I also discuss controlled transfer hydrogenation for phosphorus-oxygen bond formation. I hope that this Account will further promote research in this novel and exciting yet much underdeveloped area of phosphinate activation.

A general strategy for the synthesis of P-stereogenic compounds.

A great leap forward toward the general synthesis of P-stereogenic compounds: Heating H3 PO2 with (-)-menthol and paraformaldehyde gives easily crystallized menthyl hydroxymethyl-H-phosphinate (1). From this product, virtually any P-stereogenic compound can be synthesized.

Hydrophosphinylation of unactivated terminal alkenes catalyzed by nickel chloride.

The room-temperature hydrophosphinylation of unactivated monosubstituted alkenes using phosphinates (ROP(O)H2) and catalytic NiCl2 in the presence of dppe is described. The method is competitive with prior palladium-catalyzed reactions and uses a much cheaper catalyst and simple conditions. The scope of the reaction is quite broad in terms of unactivated terminal olefins, proceeds at room temperature, often avoids chromatographic purification, and allows one-pot conversion to various organophosphorus compounds.

Chemistry of the versatile (hydroxymethyl)phosphinyl P(O)CH2OH functional group.

(Hydroxymethyl)phosphorus compounds are well-known and valuable compounds in general; however the use of (hydroxymethyl)phosphinates R(1)P(O)(OR(2))CH(2)OH in particular has been much more limited. The potential of this functionality has not yet been fully realized because the mild unmasking of the hydroxymethyl group was not available. The mild oxidative conversion of R(1)P(O)(OR(2))CH(2)OH into R(1)P(O)(OR(2))H using the Corey-Kim oxidation is described. Other reactions preserving the methylene carbon are also reported.

Palladium-catalyzed cross-coupling of H-phosphinate esters with chloroarenes.

The palladium-catalyzed cross-coupling reaction between H-phosphinate esters and chloroarenes or chloroheteroarenes is described. This reaction is the first general metal-catalyzed phosphorus-carbon bond-forming reaction between a phosphorus nucleophile and chloroarenes.

Synthesis of Z-alkenyl phosphorus compounds through hydroalumination and carbocupration of alkynyl precursors.

The stereocontrolled synthesis of Z-alkenylphosphine-borane complexes is easily accomplished via the hydroalumination or carbocupration of alkynyl precursors. Z/E ratios are generally higher than 95/5. These reactions are stereocomplementary to our olefination approach.

Mixed 1,1-bisphosphorus compounds: synthesis, alkylation, and Horner-Wadsworth-Emmons olefination reactions.

Mixed 1,1-bisphosphorus compounds were prepared by the reaction between a phosphonate diester anion and a P(III) chlorophosphine, or its P(V) borane complex. After deprotonation either in situ or in a separate step, the resulting products can be alkylated or reacted with carbonyl compounds. A variety of olefination products were obtained, generally with high E-stereoselectivity. The reaction is competitive with other methods for the synthesis of alkenyl phosphorus compounds, and in the case of trisubstituted alkenes, regio- and stereocontrolled olefination provides products not easily accessible via any other process. The deprotection of phosphine-borane adducts was also demonstrated. Overall, a variety of novel organophosphorus reagents and products were synthesized easily and in good yields.

Strategies for the asymmetric synthesis of H-phosphinate esters.

Access to P-chiral H-phosphinates via desymmetrization of hypophosphite esters was investigated. The use of chiral auxiliaries, chiral catalysts, and of a bulky prochiral group that could lead to kinetic resolution was explored. A chiral NMR assay for enantiomeric excess determination of H-phosphinates was developed. An asymmetric route to C-chiral H-phosphinates is also examined and an assay was developed.