Phosphinic acid dimerization

The polarity of the Al-C bond allows easy derivatization of the five-coordinate aluminum alkyls by alkane elimination (Figure 9). For example, Salen aluminum alkyls LAlMe could be converted to dimeric or polymeric Salen aluminum phosphinates [LAl 02P(H)Ph ] 98 (n = 2 or oo, depending on the Salen ligand backbone)98,99 by reaction with phenyl phosphinic acid, Salen aluminum siloxides LA10SiPh3 by reaction with triphenyl silanol,96 or Salen aluminum alkoxides LAIOR by reaction with an alcohol.100... 

This rearranges by proton shift to the 2-hydro-phosphinic acid 85b, two molecules of which associate by hydrogen bridging to the dimer. The long-wave maximum at 275 nm (e = 4075), (Amax2 = 237 nm, e = 2925) confirms the phospha-cyclohexadiene (2,4) system. 

The dimers of phosphinic acid derivatives are one of the strongest HB complexes found in gas phase [134—137]. The chirality of the phosphinic acids relies on the presence of two nonidentical substituents on the phosphorus atom and the position of the hydrogen bonded to one of the two oxygens linked to the phosphorus. The chiral recognition in the minimum and proton transfer transition state structures of fifteen pairs of chiral phosphinic acid dimers (Scheme 3.24) has been carried out using DFT and MP2 methods, up to MP2/6-311++G(3df,2p) level [30]. 

Scheme 3.25 Model of congested phosphinic acid dimers,...

G.S. Denisov, K.G. Tokhadze, Ultrastrong hydrogen bond in gas phase. Dimer of dimethyl -phosphinic acid. Dokl. Phys. Chem. 337, 117-119 (1994)... 

L. Gonzalez et al., Very strong hydrogen bonds in neutral molecules The phosphinic acid dimers. J. Chem. Phys. 109, 2685-2693 (1998)... 

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... 

G. A. Kumar and M. A. McAllister, Theoretical investigation of the relationship between proton NMR chemical shift and hydrogen bond strength, J. Org. Chem. 63, 6968-6972 (1998). L. Gonzalez, O. Mo, M. Yanez, and J. Elguero, Very strong hydrogen bonds in neutral molecules the phosphinic acid dimers, J. Chem. Phys. 109, 2685-2693 (1998). 

As an example of hydrogen-bonded molecules which play the role of a bath in relaxation of the excited i (A-H) vibrational level can be the dimers of phosphinic acids studied in the gas phase at high temperatures [43]. In Fig. 8 the results of investigations of profiles of broad IR bands ascribed to i (OH) vibrations with characteristic Hadzi s ABC trio are shown. These spectra are very similar to those recorded for strong hydrogen bonds in condensed phases [44]. 

A complex of formula [Cr(AEP)0H]2,4H20 (AEP = 2-aminoethyl phosphinic acid) has been prepared and characterized by spectroscopic and magnetic studies. A dimeric stereochemistry with two OH bridges is confirmed, with /i = 3.37 BM per Cr atom supporting this stereochemical assignment. The reaction of Cr(CO) with pyridine-2-carboxylic acid (picH) in boiling MeOH affords Cr(pic)3. ... 

A further type of degradation occurs during the photolysis or thermolysis of derivatives of the 2,3-oxaphosphabicyclo[2.2.2]octene system, formally derivatives of cyclic phos-phonic or phosphinic acids. The compounds 42 are obtained by Baeyer-Villiger oxidations, with 3-chloroperoxybenzoic acid, of the 1 1 adducts from monomeric l//-phospho-les and iV-phenylmaleimide, and 43 by the identical oxidation of the products of dimerization of monomeric but unstable li/-phospholes. The simpler substrates 44, similarly obtained by the oxidation of the Diels-Alder adducts prepared from acetylene dicarboxylic ester, have also been examined. The thermolysis (at 80-110 °C in toluene) or... 

Like phosphinic acids, phosphonic acids or their acid esters readily form dimers as in (6.287a) or other hydrogen-bonded arrangements. Dialkylphosphonates may also form dimers as in (6.287b) but in this case the H bonding is much weaker (in all phosphorus compounds with P-H bonds the H bonding is generally weak or absent (Chapter 13.1)). 

Phosphine oxide complexes with aluminium or gallium trimethyl can be made by direct addition to the etherate (9.159), while dimeric phosphinate complexes can be made by slowly adding a benzene solution of a phosphinic acid to a trialkyl aluminium (9.160, 9.161) [11,12]. Large cages are obtained in Equations 9.162 and 9.163 [13,14]. Compounds (9.164) have Ga-O-P linkages. 

In some non-polar solvents such as benzene, phosphoric acid (13.1a), phosphonic acids (13.1b) and phosphinic acids (13.1c) form dimers which have stronger H bonds than the corresponding carboxylic dimers (13.Id). 

Trifluoromethylphosphinic acid is dimeric (13.2a), while the corresponding chloro compound exists as a cyclic trimer (13.2b). Phenylphosphinic acid, Ph(H)P(0)0H, is trimeric in naphthalene, and some phosphonic acids RP(OH)2 are hexameric whereas phosphinic acids of type RjPlOlOH are dimeric. The esters (RO)PO(OH)2 and (RO)2PO(OH) (R = 2-ethylhexyl or n-octyl) are, according to molecular weight data, extensively polymeric and dimeric, respectively. In acetic add solution they are monomeric because of stronger solute-solvent interaction which accounts for all the H bonding. 

Simple chains are present in hypophosphorous acid (at -40°C) (13.14a) and in LiH2P03 (13.14b), and spiralling chains in dialkyl and diphenyl phosphinic acids (13.14c). A combination of dimer rings and chain structure is found in the crystalline lattice of Sn(H2P04)2 (13.14d). 

Furthermore, homobimetallic dimers derived from phosphinous acids [134] or phosphine chlorides [239], palladium nanoparticles stabilized by wBu4NBr [142], as well as the catalyst derived from [Pd(C3H5)Cl]2 as well as the tetraphosphine ligand Tedicyp (l,2,3,4-tetrakis(diphenylphosphinomethyl)cyclopentane) [240] are also systems with high catalytic activity that enable the cross-coupling of primary alkylzincs and aryl bromides or iodides with good generality. 

After publishing what we considered the startling fact that a (AlkO)2-PO(OH), where Aik is 2-ethyl hexyl, is an extremely stable dimer in benzene, we discovered that KosolapofT and Powell had, nearly seven years earlier, shown (Alk)2PO(OH), where Aik is n-butyl or n-hexyl, to be dimeric in naphthalene. It is true that the existence of di-alkyl phosphinic acids as dimers doesn t prove that di-alkyl phosphoric acids are dimers, but it would certainly have provided a valuable hint. 

The phosphinous acid-secondary phosphine oxide tautomerism has been revisited in the context of a p-diketiminato hydroxyphosphenium cation The tautomerism of hydrophosphoryl compounds and their features as ligands in metal complexes were studied by DFT calculations Relating to the Mc2POH Mc2P(0)H tautomeric equilibrium, the formation of the possible dimers of each tautomer, and proton transfer between them, was evaluated by calculations Recent developments in the hydrophos-phinylation of unsaturated compounds have been reviewed by Montchamp and Coudray ... 

Synthesis and characterization of a new type of crown ethers (81)-(83) have been deseribed by Huszthy. The electronic circular dichroism (BCD) spectra of the ehiral crown ethers (i ,i )-(81) and (i ,i )-(82) eontaining an alkyl diarylphosphinate moiety, showed a strong exciton splitting in the Bb spectral region of the aromatic chromophores. In the case of the proton-ionizable ehiral derivative (i ,i )-(83) containing the phosphinic acid unit, the BCD speetrum measured in MeCN, suggested molecular dimerization or aggregation. 

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