Catalytic dealkylation of phosphates with binuclear boron compounds

Computational exploration of Cu(II)-en chelate-catalyzed hydrolysis of O-isopropyl methylphosphonofluoridate

CONTEXT

In contrast to un-catalyzed hydrolysis of organophosphorus (OP) compounds, metal ions or/and their complexes with chelating ligands show catalytic effects in several ways depending upon the nature of the metal, ligand, substrate, and medium. It is known that Cu(II)-en chelate containing copper complexes accelerate the hydrolysis of OP compounds. However, the mechanism for this rate enhancement in the Cu(II)-en chelate catalytic hydrolysis reaction of sarin remains unexplored. We have examined possible mechanisms involving a Cu(II)-en with hydroxide nucleophile for the reaction pathway of the hydrolysis of O-isopropyl methylphosphonofluoridate (sarin) computationally. The density functional (B3LYP) employed in this study has reproduced the experimental Gibb's free energy of activation value 15.5 kcal/mol for alkaline hydrolysis of sarin. Earlier proposal of push-pull mechanism for metal ion chelate-catalyzed hydrolysis of OP compounds has been found to be unfavorable in the present study. The role of water molecules in catalyzing the hydrolysis of sarin with Cu(II)-en chelate is crucial. The catalytic process involving Cu(II)-en chelate with one water molecule is the more plausible pathway to achieve the hydrolysis of sarin with Cu(II)-en chelate complexes.

METHODS

The most popular B3LYP method was used for optimization of given geometries. Except LANL2DZ for Cu atom, all the atoms are described using the 6-31 + G(d) basis set. The stability test has been performed for the wave functions as we are dealing with the open-shell molecules in order to ensure stable electronic configuration form, and the stable wavefunction is used as the initial configuration for the subsequent optimization. Harmonic frequency calculations and thermodynamic corrections were performed at the same level of theory. PCM method has been used for solvation effects. In order to ensure that each saddle point is linked to a minimum, IRC calculations were performed in forward and reverse directions to ensure the eigenvectors associated with the unique negative eigenvalues of the Hessian matrix. All energies discussed are solvated Gibbs free energies corrected to 298.15 K for the relative stability of the chemical structure. All calculations were performed using the Gaussian 09 code.

Synthesis, Characterization, and Stability of Dealkylated Salen-Supported Aluminum Phosphates

Three salen aluminum bromide compounds salen(^tBu)AlBr (1) (salen = N,N'-ethylenebis(3,5-di-tert-butylsalicylideneimine)), salpen(^tBu)AlBr (2) (salpen = N,N'-propylenebis(3,5-di-tert-butylsalicylideneimine)), and salophen(^tBu)AlBr (3) (salophen = N,N'-o-phenylenenebis(3,5-di-tert-butylsalicylideneimine) were evaluated for their potential use as dealkylation agents with a series of organophosphates. These reactions led to the aluminum phosphate compounds containing six-coordinate aluminum centers and hydrolytically stable P-O-C bonds: 4 = [salen(^tBu)AlOP(O)(OMe)₂]_n, 5 = [salen(^tBu)AlOP(O)(OEt)₂]_n, 6 = [salen(^tBu)AlOP(O)(OPh)₂]_n, 7 = [salophen(^tBu)AlOP(O)(OMe)₂]_n, 8 = [salpen(^tBu)AlOP(O)(OⁱPr)₂]₂, 9 = (salen(^tBu)AlO)₃PO, 10 = (salpen(^tBu)AlO)₃PO, 11 = (salophen(^tBu)AlO)₃PO. All the compounds were characterized by ¹H, ¹³C, ²⁷Al, and ³¹P NMR, IR, and mass spectrometry. Furthermore, compounds 4-8 were structurally characterized by single-crystal X-ray diffraction. The potential hydrolysis of these compounds was modeled with 4 and demonstrated the unique stability of the final product and ease of isolation.

Lewis acid-assisted detection of nerve agents in water

The five-coordinate compound, Salen((t)Bu)Al(Ac), prepared in situ from Salen((t)Bu)AlBr and NH4Ac, forms Lewis acid-base adducts in aqueous solution with the G-type nerve agents, Sarin and Soman, and the VX hydrolysis product, ethylmethylphosphonate (EMPA). The resulting compounds, [Salen((t)Bu)Al(NA)](+)[Ac] (-) (with NA = Sarin, Soman, and EMPA) are sufficiently stable to be identified by ESI-MS. Molecular ion peaks were detected for every compound with little or no fragmentation. The distinctive MS signatures for the [Salen((t)Bu)Al(NA)](+) compounds provide a new technique for identifying nerve agents from aqueous solution. The energetics of the displacement of Ac(-) by the nerve agents to form [Salen((t)Bu)Al(NA)](+)[Ac](-) were determined computationally.

4,4′,6,6′-Tetra-tert-butyl-2,2′-[butane-1,4-diylbis(nitrilomethanylylidene)]diphenol

The title compound, C₃₄H₅₂N₂O₂, is centrosymmetric, the mid-point of the central C—C bond being located on an inversion centre. Intra­molecular O—H⋯N and weak C—H⋯O hydrogen bonds are observed, but no significant inter­molecular inter­actions occur in the crystal structure.

Phosphonic acid analogs of GABA through reductive dealkylation of phosphonic diesters with lithium trialkylborohydrides

Lithium trialkylborohydrides were found to effect rapid monodealkylation of phosphonic diesters, and this reaction was applied to the synthesis of alkylphosphonic acid 2-aminoethyl esters [H(2)N(CH(2))(2)OP(OH)R, 4], a little-explored class of analogs of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). Compound 4a (R=Me) proved to be a potent antagonist at human rho1 GABA(C) receptors (expressed in Xenopus laevis oocytes), with an IC(50) of 11.1 microM, but is inactive at alpha(1)beta(2)gamma(2) GABA(A) receptors.

Triferrocenes built on a C3-symmetric ligand platform: entry to redox-active pseudo-triphenylenes via chelation-driven stereoselection of triple Schiff bases

An expedient tandem deprotonation-trapping protocol was employed to prepare a tris(difluoroboronyl) complex of a triferrocenyl ligand that is geometrically analogous to substituted triphenylenes. A triple Schiff base condensation reaction between 1,3,5-triformylphloroglucinol and aminoferrocene afforded the tris(N-salicylideneamine) adducts 5a + 5b in ca. 1:1 ratio. The keto-enamine tautomeric core of this isomeric mixture could be converted to a common enolate-imine intermediate. Subsequent trapping with BF3.Et2O cleanly afforded the tris(difluoroboronyl) adduct 6 in essentially quantitative yield. The electronic and structural properties of this new class of ferrocene compounds were investigated using various methods including UV-vis, cyclic voltammetry (CV), differential pulse voltammetry (DPV), and X-ray crystallography. In CH2Cl2-CH3CN, 6 displayed a reversible three-electron oxidation process at E1/2ox = +210 mV (vs Fc/Fc+). Despite the sharing of a common [pi,pi]/[n,pi]-conjugated core, no significant electronic communication was observed among the three ferrocenyl units in 6 under either CV or DPV conditions. On the other hand, the broad oxidation wave of 5a + 5b at E1/2ox = +60 mV in CH2Cl2-CH3CN was comprised of at least two major components at +20 and +90 mV, which collapsed to become a single peak in DMF electrolyte, despite that the ratios between the two isomers 5a,b remained essentially invariant to the change in solvent.

Synthesis and structures of bimetallic and polymeric zinc coordination compounds supported by salicylaldiminato and anilido–aldimine ligands

A series of bimetallic zinc complexes bearing salicylaldiminato (1b-3b) or anilido-aldimine (4c-5c) ligand frameworks, in which the metal centres are separated by aliphatic spacer groups containing 3-6 methylene units, were targeted. X-Ray analysis of salicylaldiminato derivative 2b, with a 4 carbon spacer group, revealed a coordination polymer in the solid state where each zinc centre is ligated by two salicylaldiminato ligands. Contrastingly, the structure of the anilido-aldimine complex 4c, with a 3 carbon methylene spacer group, was found to be a discrete bimetallic complex. These differences are attributed to the differing steric protection at the anilido vs phenoxy donors, the latter more readily facilitating bridges between metal centres.

Mononuclear Schiff Base Boron Halides: Synthesis, Characterization, and Dealkylation of Trimethyl Phosphate

A series of mononuclear boron halides of the type LBX(2) [LH = N-phenyl-3,5-di-tert-butylsalicylaldimine, X = Cl (2), Br (3)] and LBX [LH2 = N-(2-hydroxyphenyl)-3,5-di-tert-butylsalicylaldimine, X = Cl (7), Br (8); LH2 = N-(2-hydroxyethyl)-3,5-di-tert-butylsalicylaldimine, X = Cl (9), Br (10); and LH2 = N-(3-hydroxypropyl)-3,5-di-tert-butylsalicylaldimine, X = Cl (11), Br (12)] were synthesized from their borate precursors LB(OMe)2 (1) (LH = N-phenyl-3,5-di-tert-butylsalicylaldimine) and LB(OMe) [LH2 = N-(2-hydroxyphenyl)-3,5-di-tert-butylsalicylaldimine (4), N-(2-hydroxyethyl)-3,5-di-tert-butylsalicylaldimine (5), N-(3-hydroxypropyl)-3,5-di-tert-butylsalicylaldimine (6)]. The boron halide compounds were air and moisture sensitive, and upon hydrolysis, compound 7 resulted in the oxo-bridged compound 13 that contained two seven-membered boron heterocycles. The boron halide compounds dealkylated trimethyl phosphate in stoichiometric reactions to produce methyl halide and unidentified phosphate materials. Compounds 8 and 12 were found to be the most effective dealkylating agents. On reaction with tert-butyl diphenyl phosphinate, compound 8 produced a unique boron phosphinate compound LB(O)OPPh2 (14) containing a terminal phosphinate group. Compounds 1-14 were characterized by 1H, 13C, 11B, 31P NMR, IR, MS, EA, and MP. Compounds 5, 6, and 11-14 also were characterized by single-crystal X-ray diffraction.

Ab initio molecular orbital and density functional studies on the solvolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX-like compound

[reaction: see text] Potential energy surfaces for the alkaline hydrolysis of sarin and O,S-dimethyl methylphosphonothiolate, a VX model compound, and the perhydrolysis of the latter have been computed at the MP2/6-31+G(d)//mPW1K/MIDI! level of theory. The effect of aqueous solvation was accounted for via the integral equation formalism polarizable continuum model (IEF-PCM) at the HF/6-31+G(d) level. Excellent agreement with the experimental enthalpy of activation for alkaline hydrolysis of sarin was found. For the alkaline hydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that the P-O and P-S bond cleavage processes are kinetically competitive but that the products of P-S bond cleavage are thermodynamically favored. For the perhydrolysis of O,S-dimethyl methylphosphonothiolate, it was found that P-O bond cleavage is not kinetically competitive with P-S bond cleavage. In both cases, the data support initial formation of trigonal bipyramidal intermediates and demonstrate kinetic selectivity for nucleophilic attack on the face opposite the more apicophilic methoxide ligand.

Aluminum Phosphinate and Phosphates of Salen Ligands

A new dealkylation reaction between organophosphate esters and Salen aluminum bromide compounds has been used to prepare three new aluminum salen compounds salen((t)Bu)AlOP(O)Ph2 (1) (salen = N,N'-ethylenebis(3,5-di-tert-butylsalicylideneimine)), [(MeOH)Alsalen((t)Bu)[OMePO2(O)]Alsalen((t)Bu)[OMePO2(O)]Alsalen((t)Bu)]Br (2), and [salpen((t)Bu)AlO]2[(BuO)2PO]2 (3) (salpen = N,N'-propylenebis(3,5-di-tert-butylsalicylideneimine)). Compounds 1.MeOH, 2, and 3 were characterized by single-crystal X-ray diffraction. Compound 1 is the first example of a monomeric aluminum Schiff base phosphinate. Compound 2 is a cationic Salen aluminum phosphate, and compound 3 contains an aluminophosphate ring. This work is the first example of the intentional use of an aluminum-based dealkylation reaction to form new compounds.

Five-Coordinate Aluminum Bromides: Synthesis, Structure, Cation Formation, and Cleavage of Phosphate Ester Bonds

The alkane elimination reaction between Salen((t)Bu)H(2) ligands and diethylaluminum bromide was used to prepare three Salen aluminum bromide compounds salen((t)Bu)AlBr (1) (salen = N,N'-ethylenebis(3,5-di-tert-butylsalicylideneimine)), salpen((t)Bu)AlBr (2) (salpen = N,N'-propylenebis(3,5-di-tert-butylsalicylideneimine)), and salophen((t)Bu)AlBr (3) (salophen = N,N'-o-phenylenenebis(3,5-di-tert-butylsalicylideneimine)). The compounds contain five-coordinate aluminum either in a distorted square pyramidal or a trigonal bipyramidal environment. The bromide group in these compounds could be displaced by triphenylphosphine oxide or triphenyl phosphate to produce the six-coordinate cationic aluminum compounds [salen((t)Bu)Al(Ph(3)PO)(2)]Br (4), [salpen((t)Bu)Al(Ph(3)PO)(2)]Br (5), [salophen((t)Bu)Al(Ph(3)PO)(2)]Br (6), and [salophen((t)Bu)Al[(PhO)(3)PO)](2)]Br (7). All the compounds were characterized by (1)H, (13)C, (27)Al, and (31)P NMR, IR, mass spectrometry, and melting point. Furthermore, compounds 1-3 and 5-7 were structurally characterized by single-crystal X-ray diffraction. Compounds 1-3 dealkylated a series of organophosphates in stoichiometric reactions by breaking the ester C-O bond. Also, they were catalytic in the dealkylation reaction between trimethyl phosphate and added boron tribromide.

Preparation and Structural Characterization of Three Types of Homo- and Heterotrinuclear Boron Complexes: Salen{[B−O−B][O2BOH]}, Salen{[B−O−B][O2BPh]}, and Salen{[B−O−B][O2P(O)Ph]}

Three types of homo- and heterotrinuclear boron complexes have been obtained in moderate to good yields from reactions of salen-type ligands with boric acid and combinations of boric acid with phenylboronic and phenylphosphonic acid. The products are air-stable and have relatively high melting points (>290 degrees C) but are poorly soluble or insoluble in common organic solvents. They have been characterized as far as possible by elemental analysis, mass spectrometry, IR, (1)H, (11)B, and (31)P NMR spectroscopy, and X-ray crystallography. Furthermore, theoretical calculations have been performed for representative examples to permit a complete comparison of the different structure types. A detailed analysis of the molecular structures showed that the complexes are constructed around a central B(3)O(3) or B(2)PO(3) ring. The salen ligands are attached to two boron atoms of these rings, which have therefore tetrahedral coordination geometries. The complexes contain seven- and eight-membered heterocycles of the B(2)C(n)ON(2) (n = 2, 3) type with chair or twisted-chair and boat-chair or chair-chair conformations, respectively. In the homotrinuclear complexes one of the three boron atoms is three-coordinate and can therefore still act as Lewis acid, thus making these products interesting for catalytic applications, e.g. in asymmetric synthesis. Depending on the substitutents attached to the boron atoms, these complexes show a relationship with either trimetaboric acid, boroxine, or the tetraborate dianion found in Borax.

Boron halide chelate compounds and their activity towards the demethylation of trimethylphosphate

Salen(t-Bu)H2 (N,N'-ethylenebis(3,5-di-tert-butyl(2-hydroxy)benzylidenimine) and its derivatives were used to prepare boron compounds having the formula L(BCl2)2 (L = salen(t-Bu) (1), salpen(t-Bu) (2), salben(t-Bu) (3), salpten(t-Bu) (4), salhen(t-Bu) (5)). These are formed from the reaction of the corresponding L[B(OMe)2]2 with BCl3. In addition to being a new type of boron compound, they are also potential two-point Lewis acids. Indeed, they demonstrate Lewis acidic behavior in the dealkylation of trimethylphosphate. All of the compounds were characterized by mp, elemental analysis, 1H and 11B NMR, IR, MS, and in the case of 2 by X-ray crystallography.Key words: boron, salen, dealkylation.