Phosphenium Ion Research Articles

Gas-phase fragmentation of di- and tripeptides via ion–molecule reactions with ClPCl +

The reactivity of the phosphenium ion ClPCl + toward gaseous neutral di- and tripeptides was examined by Fourier transform ion cyclotron resonance mass spectrometry. ClPCl + was found to react with the peptides at collision rate. Extensive fragmentation of the peptides leads to only one (dipeptides) or two (tripeptides) dominant product ions. For each dipeptide, the major product ion has a molecular weight that is 34 u less than that of the dipeptide, i.e. the side chains are retained in the fragment ion. A substantial amount of effort was given to resolve the structure of this product ion, with a focus on the ion ( m/z 98) generated from reaction of Gly–Gly (MW 132) with ClPCl +. The molecular formula of the ion of m/z 98 was determined to be C 4H 6NO by exact mass measurements. The relative energies of a variety of isomeric ions with this elemental composition were examined by molecular orbital calculations in order to identify the most likely structures. Collision-activated dissociation experiments, mechanistic considerations, and comparison of calculated adiabatic recombination energies of isomeric ions to that determined experimentally for the unknown ion, led to the assignment of a six-membered, highly unsaturated ring structure to the product ion of m/z 98 of Gly–Gly. This assignment is supported by the finding that the experimentally determined acidity of the unknown ion is in excellent agreement with that calculated for the proposed six-membered structure. The most likely pathway for the formation of this product ion, as well as those obtained for the other dipeptides, involves an electrophilic attack of ClPCl + at the amide carbonyl oxygen of the dipeptide, followed by a hydride shift, proton transfer, and loss of H 2O and Cl 2PO ·. The very high exothermicity of the initial addition (69 kcal mol −1, AM1) allows the adduct to form high-energy intermediates (highest estimated to lie at 34 kcal mol −1 above the adduct) during the reaction that is exothermic by about 36 kcal mol −1 overall. An analogous mechanism readily rationalizes the formation of the two major product ions observed for tripeptides. These reactions involve the cleavage of each peptide bond, yielding product ions that contain the intact side chains of two adjacent amino acids. Similar observations were made for the BrPBr + reagent ion. The results presented here suggest that ClPCl + (and BrPBr +) may provide useful sequence information for peptides.

A G2 Ab Initio Investigation of Ligand-Exchange Reactions Involving Mono- and Bis-Adducts of the Phosphenium Ion.

Ab initio calculations at the G2 level have been employed to investigate the ligand-exchange reactions between mono-adducts of the phosphenium ion (e.g., [H(3)N-PH(2)](+)) and simple first- or second-row Lewis bases (e.g., NH(3)). We have found virtually all the reactions to proceed without an intermediate barrier via a bis-adduct of the phosphenium ion with two Lewis bases (e.g., [H(3)N-PH(2)-NH(3)](+)). The ligand-exchange reactions are predicted to be experimentally feasible and the bis-adducts of the phosphenium ion to be experimentally observable in appropriate cases. The energetics of the ligand-exchange reactions may be rationalized using qualitative orbital interaction arguments in terms of the electronegativity of the ligand in the mono-adducts and the donor ability of the reacting Lewis base. Comparisons with previous investigations of corresponding ligand-exchange reactions involving pi-ligands reveal that the electronic characteristics of the bis-adduct resemble those of the transition structures for pi-ligand exchange.

Strong P=P pi Bonds: The First Synthesis of a Stable Phosphanyl Phosphenium Ion.

A 35-fold excess of methyl triflate (2) is required to quantitively prepare 3, the first phosphanyl phosphenium ion, from diphosphene 1. Experimental data and calculations indicate that the P=P bond becomes stronger upon alkylation.

Cation Stabilities, Electrophilicities, and “Carbene Analogue” Character of Low Coordinate Phosphorus Cations

The stabilities of low coordinated phosphorus cations can be expressed in the frame of the HSAB concept by the transferred charge density Δq(N) which a cation receives upon formation of a donor-acceptor adduct with a Lewis base N. This concept allows to differentiate between relative stabilities towards different reaction partners, and to compare the electrophilicities of phosphenium ions to those of isoelectronic carbenes and silylenes. An analysis of substituent influences on Δq(H) in cations [P(R)2]+suggests an increasing stabilizing power of substituents in the series R = Cl < CH3 < OH, SH < NH2. The same ordering was derived from isodesmic hydride transfer reactions. Interpretation of population analyses suggests that the individual substituent contributions to cation stabilities result from a balance between π-donation into the empty p(P) orbital and electrostatic stabilization by polar P–R σ-bonds. A further stabilizing effect, which is of similar magnitude as in isoelectronic carbenes or silylenes, may arise from cyclic π-conjugation between a diaminophosphenium fragment and an adjacent double bond. Substituent effects influence further the nature of the frontier orbitals of phosphenium ions, resulting in orbital sequences which resemble those of carbenes, allyl anions, or phospholides, respectively. The absence of frontier orbital related changes in reactivity patterns suggests that in all reactions, including metal complex formation, phosphenium ions behave as purely electrophilic rather than ambiphilic species.

Gas-phase reactivity of (CH3O)2P+ phosphenium ion towards 2,3-dimethylbuta-1,3-diene in a quadrupole ion trap

Gas-phase reactivity of (CH3O)2P+ phosphenium ion towards 2,3-dimethylbuta-1,3-diene in a quadrupole ion trap

Stereoselective Chemical Ionization Mass Spectrometry: Reactions of CH3OPOCH3+ with Cyclic Vicinal Diols

The phosphenium ion CH3OPOCH3+ readily attacks hydroxyl groups of neutral substrates in the gas phase in a Fourier-transform ion cyclotron resonance mass spectrometer. The electrophilic character of CH3OPOCH3+ is in agreement with molecular orbital calculations (Becke3LYP/6-31G(d) + ZPE) that predict a singlet electronic ground state for this species. The observed reactions provide a convenient synthetic route to various larger phosphenium ions in the gas phase. Most importantly, however, CH3OPOCH3+ was found to be extremely sensitive to the stereochemical structure of the neutral substrate. The dramatically different reaction product distributions obtained for diastereomeric cyclic vicinal diols suggest that CH3OPOCH3+ provides a powerful chemical ionization reagent for the mass spectrometric determination of the stereochemistry of diols.

Synthesis and characterization of functionalized phosphenium ions, stabilized by two intramolecular dative P ← N bonds

Abstract ArPH2 (Ar=[C6H3(CH2NMe2)2-2,6]) 3 undergoes hydride abstraction on treatment with trityl cation to give the stabilized phosphenium ion [ArPH]+BF4−, 4a. The same cation was prepared by reaction of 3 with BrCCl3 (4b) and also by lithiation of 3 followed by treatment with I2 (4c). ArLi reacts with PX3 (X = Cl or Br) to give the stabilized phosphenium ion [ArPX]+X− which affords 3 by LiAlH4 reduction. In contrast, DIBAL-H reduction of [ArPX]+X− gives [ArPH]+X−. This last reaction constitutes a transformation of a stabilized and functionalized phosphenium ion into a different ion. Confirmation of the structure of these salts was given by single-crystal X-ray diffraction analysis of [ArPH]+PF6− 4e.

Gas-phase Ion-Molecule Reactions of Trimethyl Phosphite with the Phosphonium Ion OP(OCH3)2+ in a Quadrupole Ion Trap

The gas-phase ion–molecule reactions of the phosphonium ion OP(OCH3)2+ formed by electron impact with neutral trimethyl phosphite were studied with the use of a quadrupole ion trap (QIT) mass spectrometer. Observed reactions include competing methoxy transfers between OP(OCH3)2+ and the neutral species to yield a phosphenium ion :P(OCH3)2+ or a tetracoordinate phosphorus ion P(OCH3)4+. Background water in the QIT gives an adduct with OP(OCH3)2+ which protonates neutral trimethyl phosphite, as shown by triple mass spectrometric experiments. High-level ab initio calculations and chemical ionization reactions were performed to investigate the unimolecular loss of methanol from protonated trimethyl phosphite. © 1997 by John Wiley & Sons, Ltd.

Gas‐phase Ion–Molecule Reactions of Trimethyl Phosphite with the Phosphonium Ion OP(OCH3)2 in a Quadrupole Ion Trap

The gas-phase ion–molecule reactions of the phosphonium ion OP(OCH3)2+ formed by electron impact with neutral trimethyl phosphite were studied with the use of a quadrupole ion trap (QIT) mass spectrometer. Observed reactions include competing methoxy transfers between OP(OCH3)2+ and the neutral species to yield a phosphenium ion :P(OCH3)2+ or a tetracoordinate phosphorus ion P(OCH3)4+. Background water in the QIT gives an adduct with OP(OCH3)2+ which protonates neutral trimethyl phosphite, as shown by triple mass spectrometric experiments. High-level ab initio calculations and chemical ionization reactions were performed to investigate the unimolecular loss of methanol from protonated trimethyl phosphite. © 1997 by John Wiley & Sons, Ltd.

PHOSPHORYLATION OF ENAMINEHYDRAZONES AS AN EFFICIENT ROUTE TO DIAZAPHOSPHOLINES AND DIAZAPHOSPHOLES

1,2,3-diazaphospholines of new type containing an enamine residue, conjugated with the diazaphospholine cycle, were synthesized by phosphorylation of N,N-dimethyl- and N-substituted hydrazones of ω-fomyl-1,3,3-trimethyl-2-methyleneindolines using phosphorus(III) halides. Reversible ionization of the P-Br bond was observed for a 2-N-methyl-3-P-bromodiazaphospholine in polar solvents. The ionization is caused by stabilization of the phosphenium ion by electrondonating methyl and enamine group. Analogously, 2-N-phenyl-3-P-bromo- as well as 3-P-chloro-1,2,3-diazaphospholines give stable phosphenium ions only after treatment of sodium tetraphenylborate or trimethylsilyl trifluoromethanesulfonate.

Nitrilimines: Evidence for the Allenic Structure in Solution, Experimental andAb InitioStudies of the Barrier to Racemization, and First Diastereoselective [3 + 2]-Cycloaddition

The lithium salts of [bis(diisopropylamino)thioxophosphoranyl]diazomethane (2) and [(diisopropylamino)(dicyclohexylamino)thioxophosphoranyl]diazomethane diazomethane (5) react with the (diisopropylamino)(dicyclohexylamino)chlorophosphine (3) and bis(diisopropylamino)chlorophosphine (6), leading to the nitrilimines 4 (63% yield) and 7 (69% yield), respectively. The lithium salt of 2 also reacts with the bis(dicyclohexylamino)phosphenium ion, affording nitrilimine 9 in 51% yield. Using the presence of a chiral substituent either at the carbon or at the nitrogen terminus of the CNN skeleton, variable-temperature solution NMR studies of nitrilimines 4 and 7 demonstrate that they possess a bent allenic structure. The free energy of activation for racemization is ca. 30 kJ mol(-1). Ab initio/DFT studies performed on nitrilimine with C-P(S)H-2 and N-PH2 substituents 10 show that the most likely pathway for the racemization process is inversion at the carbon atom, followed by rotation of the PSR(2) fragment. Bis(trityl)nitrilimine 17 reacts with (R)-alpha-(acryloxy)-beta,beta-dimethyl-gamma-butyrolactone, leading to diastereomeric pyrazolines 18a,b (3/1 ratio) in 60% total yield; this is the first example of a diastereoselective [3 + 2]-cycloaddition reaction involving a nitrilimine.

N → P intramolecular stabilization of phosphenium ions and preparation of hypercoordinated phosphanes with unusual properties

The reaction of ArLi 1 (Ar = [C 6H 3(CH 2NMe 2) 2-2,6]) with Ph 2PCl affords the phosphane 2 which, on treatment with HCl, gives rise to the monoprotonated ammonium salt 3, even in the presence of an excess of HCl. 2 reacts with MeI to give exclusively the phosphonium salt 4. Reaction of 1 with PhPCl 2 gives the stabilized phosphenium ion [ArPPh] +Cl − 6. This ion is reduced by LiAlH 4 in ArPHPh 9, which undergoes hydride abstraction on treatment with trityl cation, giving the same ionic phosphorus species. This unusual reaction is an example of increased reactivity of a P H bond resulting from hypercoordination at the phosphorus atom.

The cycloaddition of diethyl chlorophosphite with norbornadiene: Synthesis and crystal structure of the cycloadduct.

AbstractIn the presence of Lewis acids, diethyl phosphorochloridite reacts with norbornadiene to afford the tetracyclic phosphinate ester 3. The reaction is believed to involve formation of phosphenium ions stabilized only by alkoxy substituents. The cycloadduct of the reaction was characterized by its spectral data, by single crystal diffraction analysis of the parent acid, and 31P and 27Al nmr experiments.

Formation of Metallophosphenium Cluster Complexes: Synthesis and Structure of Cp(Ph)[N(SiMe3)2

AbstractThe reaction of [(Me3Si)2N](Ph)PW(CO)2Cp with Fe2(CO)9 produces in good yield the bimetallic compound Cp(Ph)[N(SiMe3)2] that was characterized by mass spectrometry, IR, 1H‐, and 31P‐NMR spectroscopy, and single crystal X‐ray diffraction analysis. The molecular structure consists of Fe(CO)3 and CpW(CO)2 fragments joined by a bridging CO ligand and a phosphenium ion fragment which comprise a four‐membered bicyclic structure.

N + 2]‐Cycloadditionsreaktionen des (Arylimino)phosphenium‐Ions, [P≡NAr]+–kationische PN‐Heterocyclen mit ungewöhnlicher Struktur und Bindungssituation

[n + 2] Cycloaddition Reactions of the (Arylimino)phosphenium Ion, [P≡NAr]+–Kationic PN Heterocycles with Unusual Structure and Bonding SituationThe iminophosphenium ion [P≡NAr]+ (Ar=2,4,6‐tBu3C6H2) (1) reacts with aminoiminophosphanes R2N–P=NtBu (R=iPr, Me3Si) (2) and alkyl azides RN3 (R=tBu, Et3C) (6) to yield the corresponding [n + 2] cycloadducts [R2NP (NAr)P(NtBu)+ (3) and [PN(Ar)NNN(R)]+ (7), respectively. Single‐crystal X‐ray diffraction studies show that 3a can be considered as an intramolecular donor‐acceptor complex while 7 can be regarded as a cyclic diaminophosphenium cation coupled to a diimine fragment, which is in accordance with results of ab initio calculations.

Multifunctional Neutral Phosphenium Ion Complexes

Abstract The synthesis, structural features and reactivity of the neutral phosphenium ion complexes (R)(R′)P[dbnd]MLn (MLn [dbnd] Co(CO)3, Mn(CO)4, MoCp(CO)2; R[dbnd] 2,4,6-tBu3C6H2O; R′ [dbnd] C[tbnd]CPh, CH[dbnd]CHPh, CH2C[tbnd]CH, CH[dbnd]C[dbnd]CH2) will be discussed.

P-Functionalized Phosphaalkenes and Iminophosphines: New Results

Abstract Synthesis, structures, spectroscopic data, and novel reactions of P-[bis(dialkylamino)methylene-amino]- and P-[bis(dialkylamino)methylenephosphino]-substituted phosphaalkenes, iminophosphines and phosphenium ions are discussed.

Preparation of (2S∗, 5S∗) -2,5-Dibenzyl Phospholanic Acid

Abstract Phosphenium ion 1 underwent cheletropic cycloaddition with E-1-benzylbutadiene 2b to afford cyclic phosphonium salts 3b and 4b. In situ hydrolysis of the phosphonium salts produced phosphinamides 6a and 6b in a 3:1 ratio. Three chemical steps coverted phosphinamide 6a to the title compound 12b.

Rings Compounds of Boron and Phosphorus

Abstract The synthesis and characterization of some ring systems of phosphorus and boron are described. Ring systems of boron such as boroxines, silylboranes, and diborabenzenes are considered. The structural and spectroscopic data for the ring derivatives support varying degrees of electron delocalization, depending on the size of the ring. Ring systems of phosphorus such as phosphenium ions and diphosphorus cations are outlined. The importance of P-C π-bonding interactions and the heterocyclic frameworks in the stabilization of these compounds are highlighted. Finally, new heterocyclic organoboranes are reported, which result from reductions of RBX2 and with alkalai metals. All the rings and their respective derivatives have been characterized spectroscopically and the solid-state structures of some representative members were determined by X-ray crystallography. The degree of delocalization in the pertinent ring complexes are compared with theoretical models.

2-Propynyl isomerization in [(R)(HCCCH 2)]PMoCp(CO) 2: Synthesis of 1-molybda-2-phospha-1,3-dienes. X-ray crystal structure of (2,4,6- tBu 3C 6H 2O)[(NEt 2)(Me) CCH]PMoCp(CO) 2

2-Propynyl isomerization (CH 2CCH → CH 2 → CCCH 3 of the bifunctionalized terminal phosphenium ion complex [(2,4,6- tBu 3C 6H 2O) (HCCCH 2)]PMoCp(CO) 2 and its transformation into 1-molybda-2-phospha-1,3-dienes is discussed; the X-ray structure of the heterobutadiene compound {(2,4,6- tBu 3C 6H 2O) [(NEt 2)(Me)CC(H)]}PMoCp(CO) 2 ( 6) is reported.