Unraveling C-Selective Ring-Opening of Phosphiranes with Carboxylic Acids and Other Nucleophiles: A Mechanistically-Driven Approach

Phosphiranes are weak Lewis bases reacting with only a limited number of electrophiles to produce the corresponding phosphiranium ions. These salts are recognized for their propensity to undergo reactions with oxygen pronucleophiles at the phosphorus site, leading to the formation of phosphine oxide adducts. Building on a thorough mechanistic understanding, we have developed an unprecedented approach that enables the selective reaction of carboxylic acids, and other nucleophiles, at the carbon site of phosphiranes. This method involves the photochemical generation of highly reactive carbenes, which react with 1-mesitylphosphirane to yield ylides. The latter undergoes a stepwise reaction with carboxylic acids, resulting in the production of the desired phosphines. In addition to DFT calculations, we have successfully isolated and fully characterized the key intermediates involved in the reaction.

Protonation of P-Stereogenic Phosphiranes: Phospholane Formation via Ring Opening and C-H Activation

Protonation of cyclopropanes and aziridines is well-studied, but reactions of phosphiranes with acids are rare and have not been reported to result in ring opening. Treatment of syn-Mes*PCH₂CHR (Mes* = 2,4,6-(t-Bu)₃C₆H₂, R = Me or Ph, syn-1-2) or anti-Mes*PCH₂CHPh (anti-2) with triflic acid resulted in regiospecific anti-Markovnikov C-protonation with ring opening and cyclophosphination of a Mes* ortho-t-Bu group to yield the phospholanium cations [PH(CH₂CH₂R)(4,6-(t-Bu)₂-2-CMe₂CH₂C₆H₂)][OTf] (R = Me or Ph, 3-4), which were deprotonated with NEt₃ to give phospholanes 5-6. Enantioenriched or racemic syn-1 both gave racemic 3. The byproduct [Mes*PH(CH₂CH₂Me)(OH)][OTf] (7) was formed from syn-1 and HOTf in the presence of water. Density functional theory calculations suggested that P-protonation followed by ring opening and hydride migration to C yields the phosphenium ion, [Mes*P(CH₂CH₂Me)][OTf], which undergoes C-H oxidative addition of an o-t-Bu methyl group. This work established a new reactivity pattern for phosphiranes.

Configurational Lability at Tetrahedral Phosphorus: syn/anti-Isomerization of a P-Stereogenic Phosphiranium Cation by Intramolecular Epimerization at Phosphorus

Tetrahedral main-group compounds are normally configurationally stable, but P-epimerization of the chiral phosphiranium cations syn- or anti-[Mes*P(Me)CH₂ CHPh][OTf] (Mes*=2,4,6-(t-Bu)₃ C₆ H₂ ) occurred under mild conditions at 60 °C in CD₂ Cl₂ , resulting in isomerization to give a syn-enriched equilibrium mixture. Ion exchange with excess [NBu₄ ][Δ-TRISPHAT] (Δ-TRISPHAT=Δ-P(o-C₆ Cl₄ O₂ )₃ ) followed by chromatography on silica removed [NBu₄ ][OTf] and gave mixtures of syn- and anti-[Mes*P(Me)CH₂ CHPh][Δ-TRISPHAT]⋅x[NBu₄ ][Δ-TRISPHAT]. NMR spectroscopy showed that isomerization proceeded with epimerization at P and retention at C. DFT calculations are consistent with a mechanism involving P-C cleavage to yield a hyperconjugation-stabilized carbocation, pyramidal inversion promoted by σ-interaction of the P lone pair with the neighboring β-carbocation, and ring closure with inversion of configuration at P.

Asymmetric 1,3-dipolar cycloaddition reaction of chiral 1-alkyl-1,2-diphospholes with diphenyldiazomethane

The 1,3-dipolar cycloaddition of chiral 1-alkyl-1,2-diphosphacyclopenta-2,4-dienes ((1-(−)-menthyl)oxymethyl-1,2-diphosphole and 1-(+)-neomenthyl-1,2-diphosphole) with diphenyldiazomethane leads to novel P-chiral bicyclic phosphiranes having six chiral centers. The degree of diastereoselectivity depends on the substituent at phosphorus, and dramatically increases in the case of (+)-neomenthyl group (de up to 71%). DFT calculations indicate that the cycloaddition is thermodynamically controlled.

The 1,3-dipolar cycloaddition of chiral 1-alkyl-1,2-diphosphacyclopenta-2,4-dienes with diphenyldiazomethane leads to novel P-chiral bicyclic phosphiranes having six chiral centers.

Organoiron- and Fluoride-Catalyzed Phosphinidene Transfer to Styrenic Olefins in a Stereoselective Synthesis of Unprotected Phosphiranes

Catalytic phosphiranation has been achieved, allowing preparation of trans-1-R-2-phenylphosphiranes (R = t-Bu: 1-t-Bu; i-Pr: 1-i-Pr) from the corresponding dibenzo-7-(R)-7-phospha-norbornadiene (RPA, A = C₁₄H₁₀, anthracene) and styrene in 73% and 57% isolated yields, respectively. The cocatalyst system requires tetramethylammonium fluoride (TMAF) and [Fp(THF)][BF₄] (Fp = Fe(η⁵-C₅H₅)(CO)₂). In the case of the t-Bu derivative, the reaction mechanism was probed using stoichiometric reaction studies, a Hammett analysis, and a deuterium labeling experiment. Together, these suggest the intermediacy of iron-phosphido FpP(F)(t-Bu) (2), generated independently from the stoichiometric reaction of [Fp(t-BuPA)][BF₄] with TMAF. Two other plausible reaction intermediates, [Fp(t-BuPA)][BF₄] and [Fp(1-t-Bu)][BF₄], were prepared independently and structurally characterized.

Zirconium-catalyzed intermolecular hydrophosphination using a chiral, air-stable primary phosphine

Catalytic hydrophosphination of alkenes using a chiral, air-stable primary phosphine, (R)-MeO-MOPH₂, proceeds under mild conditions with a zirconium catalyst to selectively furnish anti-Markovnikov, air-stable secondary phosphine or tertiary phosphine prodcuts with slight modification of the protocol.

Organomonophosphines in PtP2Cl2 derivatives: structural aspects

In this review, 260 monomeric Pt(II) complexes are summarized and analyzed in which the inner coordination sphere is built up by a pair of organomonophosphines and two chlorine atoms (PtP2Cl2). These complexes are crystallized in four crystal systems: tetragonal (x17)<orthorhombic (x31)<triclinic (x90)<monoclinic (x122). The square-planar environments have cis- and trans-configurations. The former by far prevails. The mean Pt-L bond distances (cis- vs. trans-configurations) are 2.347 Å (Cl, trans to P) and 2.230 Å (P, trans to Cl) vs. 2.307 Å (Cl, trans to Cl) and 2.320 Å (P, trans to P). There are two types of isomerisms: cis-, trans- and distortion. In general, the derivatives with a cis-configuration are somewhat more distorted than that with a trans-configuration.