Phosphinous acid boranes

The chemistry of secondary phosphine oxides, R2P(H)0 and their phosphi-nous acid tautomers, R2POH, has continued to attract attention. The study of the phosphinous acid tautomers has been aided by the development of stereoselective procedures for direct conversion of secondary phosphine oxides to the phosphinous acid-boranes (83). Treatment of the secondary phosphine oxide with either a base-borane complex or boron trifluoride and sodium borohyd-ride provides the phosphinous acid-borane with predominant inversion of configuration at phosphorus. The phosphinous acid tautomers are usually trapped as ligands in metal complexes and further examples of this behaviour have been noted. Discrimination of enantiomeric forms of chiral phosphinous acids, Ph(R)OH, coordinated to a chiral rhodium complex, has been studied by NMR. °° Palladium complexes of di(t-butyl)phosphinous acid have found application as homogeneous catalysts.A lithium salt of the tellurophos-phinite Ph2PTeH has been prepared and structurally characterised. ... 

The transformation of readily available enantiopure //-menthylphosphinates 2 into chiral phosphinous acid boranes 5 permits the elaboration of bulky P-stereogenic secondary phosphine boranes. Taking advantage of the synthetic potential of these compounds, abroad range of hindered P-chiral tertiary phosphine boranes 6 were prepared with excellent enantiomeric excesses [12,13]. Phosphinous acid 5 can easily be converted into one or the other enantiomer of the secondary phosphines boranes (Sp)- or (/ p)-6 by stereoselective reduction or substitution of the phosphinite borane derivatives, respectively (Scheme 4 and Table 1). 

Moraleda D, Gatineau D, Martin D, Giordano L, Buono G (2008) A simple route to chiral phosphinous acid-boranes. J Chem Soc Chem Commun 3031-3033... 

To resolve phosphine boranes, an additional protic group is required (entries 13-15). Enantiomerically pure phosphinous acid boranes of entries 14 and 15 are versatile compounds that were transformed, often stereoselectively, to many other P-stereogenic compounds. ... 

Scheme 2.18 One-pot synthesis of P-stereogenic phosphinous acid boranes.

Table 2.10 Synthesis of phosphinous acid boranes (Scheme 2.18).

Alkyl spacers thus offer the opportunity to finely tune the interplay between the antagonist sites of ambiphilic compounds. The proximity of the phosphine and borane moieties, and thus the possible formation of intramolecular P-B interaction depend on the number of carbon atoms present in the backbone and on the Lewis acidity of the borane. Conformational considerations may also play an important role as for the ethylene-bridged systems. 

Both methylphosphine (this synthesis) and dimethylphosphine (the following synthesis) are considerably stronger Lewis bases than phosphine, and their reactions with various Lewis acids, including boranes, have been the subject of recent studies. Both methylphosphine and dimethylphosphine are readily oxidized by air, and thus can serve as sources of methylphos-phonous (methylphosphinic) acid, CH3PHO(OH), and dimethyl-phosphinic acid, (CH3)2PO(OH). ... 

Photolysis of (244) in the presence of alcohols likewise results in the formation of esters of methyl(phenyl)phosphinic acid. However, O-isotopic labelling studies provide evidence of, at least, the partial involvement of a pentacovalent phosphorus intermediate in this reaction, and so the previous assumption of the intermediacy of (245) may not be the whole story. The phosphole oxide dimers (246) have been shown to undergo regioselective reduction and complexation on treatment with the dimethylsulfide-borane adduct, to form the bicyclic system (247). Treatment of the propadienylphosphine oxide... 

Hydroboration of coordinated alkenes has been achieved with the phosphine-stabilized olefin complexes, (775-C5H5)2Zr(772-CH2=CHR)(PPh2Me) (R = H, 78 Et, 79 Ph, 80), upon addition of the Lewis-acidic borane, HB(C6F5)2 (Equation (5)).37,65 Solid-state characterization indicates a weak interaction between the formally positively charged zirconium center and the carbon adjacent to the borate anion. This interaction is maintained in solution, as an upheld shifted 13C NMR resonance is observed for this carbon, which is in agreement with previous reports of metal-carbon interactions of this type. 

The lipase-promoted kinetic resolution of a series of P-boranes (854) which might be treated as phosphinic acid derivatives, proceeded with moderate stereoselectivity to give both the unreacted substrates (855) and their O-acetyl derivatives (856) (Scheme 214). Some additional studies were carried out which proved that the borane reduction proceeded with inversion of configuration at the phosphorus center. ... 

Quenching of the same lithiated species with CO2, followed by reduction of the carboxyUc acid functionality obtained with BH3-THF complex, yielded the next higher analogues 78 to these alcohols [94]. Subsequent treatment of the depro-tonated alcohols with TsCl or MsCl afforded (l )-l-boranato[alkyl(methyl)plios-phino] ethanol-2-tosylates or the mesylate phosphine-boranes in over 90% ee and excellent overall yields. 

As mentioned in Sect. 2.2, phosphine oxides are air-stable compounds, making their use in the field of asymmetric catalysis convenient. Moreover, they present electronic properties very different from the corresponding free phosphines and thus may be employed in different types of enantioselective reactions, m-Chloroperbenzoic acid (m-CPBA) has been showed to be a powerful reagent for the stereospecific oxidation of enantiomerically pure P-chirogenic phos-phine-boranes [98], affording R,R)-97 from Ad-BisP 6 (Scheme 18) [99]. The synthesis of R,R)-98 and (S,S)-99, which possess a f-Bu substituent, differs from the precedent in that deboranation precedes oxidation with hydrogen peroxide to yield the corresponding enantiomerically pure diphosphine oxides (Scheme 18) [99]. 

Tetrasubstituted phosphinous amides of the type R2NPPh2 have been successfully arylated at phosphorus by the action of bromobenzene, in a process catalyzed by NiBr2, to give the aminophosphonium bromides [R2NPPh3] Br [109]. Other representative members of this class form phosphane-borane complexes [62], are aminated at phosphorus by chloramine to yield bis(amino)phos-phonium salts [110] and have been claimed to be protonated at phosphorus by ethereal tetrafluoroboric acid, as determined by NMR analysis [111]. 

Similar to the addition of secondary phosphine-borane complexes to alkynes described in Scheme 6.137, the same hydrophosphination agents can also be added to alkenes under broadly similar reaction conditions, leading to alkylarylphosphines (Scheme 6.138) [274], Again, the expected anti-Markovnikov addition products were obtained exclusively. In some cases, the additions also proceeded at room temperature, but required much longer reaction times (2 days). Treatment of the phosphine-borane complexes with a chiral alkene such as (-)-/ -pinene led to chiral cyclohexene derivatives through a radical-initiated ring-opening mechanism. In related work, Ackerman and coworkers described microwave-assisted Lewis acid-mediated inter-molecular hydroamination reactions of norbornene [275]. 

The reaction between a Lewis acid R3M and a Lewis base ER3 is of fundamental interest in main group chemistry. Synthetic and computational chemists have investigated the influence of both the Lewis acid and the base on the solid state structure and the thermodynamic stability of the corresponding adduct, that is usually expressed in terms of the dissociation enthalpy De. This led to a sophisticated understanding of the nature of dative bonding interactions. In particular, reactions of boranes, alanes and gallanes MR3 with amines and phosphines ER3, typically leading to adducts of the type R3M <— ER3, have been studied.10... 

Standard cyclisation methodology was used to access the cyclic monophosphinic acid derivative 78 by reaction of ammonium phosphonate and ethyldiisopropylamine, followed by the addition of chlorotrimethylsilane, with 2,2 -bis (bromomethyl)-l,l -biphenyl. Silane reduction of 78 gave the secondary phosphine. The secondary phosphine borane complex 79 could be used in alkylation or Michael addition reactions. For example the Michael adduct 80 was produced in high yield by treatment of 78 with a NaH suspension in THF followed by the addition of diethylvinylphosphonate . 

In a modified version of the Suzuki reaction arylboronates or boranes are utilized instead of arylboronic acid. Under the action of phosphine-free palladium catalysts NaBPh4 and tra(l-naphtyl)borane were found suitable phenyl-sources for arylation of haloaromatics in fully or partially aqueous solutions at 20-80 °C with good to excellent yields (Scheme 6.12) [32-34]. Aryl halides can be replaced by water-soluble diaryliodonium salts, At2IX (X = HSO4, BF4, CF3COO) in the presence of a base both Ar groups take part in the coupling [35]. 

Ambiphilic derivatives, also called amphoteric derivatives, are polyfunctional compounds combining Lewis bases and Lewis acids (Figure 1). Such donor-acceptor compounds typically combine group 15 and 13 elements featuring, respectively, a lone pair of electrons and a vacant orbital. Among the possible combinations, phosphine-boranes (PB) clearly occupy a forefront position. 

The variety of C2-bridged PBs was further extended by Muhoro via hydroboration of diphenyl(vinyl)phosphine with catechol- and pinacol-boranes (Scheme 29).56 To compensate for the low Lewis acidity of these boronates, the hydroboration reactions were carried out in the presence of 5 mol% of titanocene bis(catecholborane) as catalyst. The desired products 40g and 40h were obtained with complete anti-Markovnikov selectivity. The spectroscopic data and the crystallographic study performed on 40h showed the expected monomeric open structure. 

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