Infrared-Driven Charge-Transfer in Transition Metal-Containing B12X122- (X = H, F) Clusters.

Density functional theory (DFT) calculations and infrared multiple photon dissociation (IRMPD) spectroscopy are employed to probe [TM·(B12H12)]- and [TM·(B12H12)2]2- clusters [TM = Ag(I), Cu(I), Co(II), Ni(II), Zn(II), Cd(II)]. A comparison is made between the charge-transfer properties of the clusters containing the hydrogenated dodecaborate dianions, B12H122-, and the fluorinated analogues, B12F122-, for clusters containing Cd(II), Co(II), Ni(II), and Zn(II). IRMPD of the [TM·(B12H12)]- and [TM·(B12H12)2]2- species yields B12H11- via hydride abstraction and B12H12- in all cases. To further explore the IR-induced charge-transfer properties of the B12X122- (X = H, F) cages, mixed-cage [TM(B12H12)(B12F12)]2- [TM = Co(II), Ni(II), Zn(II), Cd(II)] clusters were investigated. IRMPD of the mixed-cage species yielded appreciable amounts of B12F12- and B12H12- in most cases, indicating that charge-transfer to the central TM cation is a favorable process; formation of B12F12- is the dominant process for the Co(II) and Ni(II) mixed-cage complexes. In contrast, the Zn(II) and Cd(II) mixed-cage complexes preferentially produced fragments of the form B xH yF z-/2-, suggesting that H/F scrambling and/or fusion of the boron cages occurs along the IRMPD pathway.

Mode-Selective Laser Control of Palladium Catalyst Decomposition.

It is generally assumed that molecules behave ergodically during chemical reactions, that is, reactivities depend only on the total energy content and not on the initial state of the molecule. While there are a few examples of nonergodic behavior in small (usually electronically excited) species, to date there have been no reports of such behavior in larger covalently bound species composed of several tens of atoms. Here, we demonstrate vibrational mode-selective behavior in a series of palladium catalysts. When we excite solvent-tagged gas-phase Pd catalysts with an infrared laser that is tuned to be resonant with specific molecular vibrations, depending on which vibration we excite, we can select different reaction pathways. We also demonstrate that this behavior can be "turned off" via chemical substitution.

Interaction of B12F122- with All-cis 1,2,3,4,5,6 Hexafluorocyclohexane in the Gas Phase.

Clusters of all-cis 1,2,3,4,5,6-hexafluorocyclohexane and the dodecafluorododecaboron dianion, [C6F6H6]n[B12F12]2- (n = 0-4), are investigated in a combined experimental and computational study. DFT calculations and IRMPD spectra in the region of 800-2000 cm-1 indicate that C6H6F6 binds to open trigonal faces of B12F122- via a three-point interlocking binding motif. Calculated binding interactions reveal substantial contributions from C-H···F hydrogen bonding and binding energies that are among the strongest observed for a neutral-anion system.

Intramolecular cation-π interactions in protonated phenylalanine derivatives.

The structures and properties of a series of phenylalanine (Phe) derivatives have been investigated in a joint computational and experimental infrared multiple photon dissociation (IRMPD) study. IRMPD spectra in the 1000-2000 cm-1 region show that protonation is localized on the amine group in all cases. Intramolecular cation-π interactions between the ammonium group and the phenyl ring heavily influence molecular geometries and properties such as gas phase basicity and proton affinity. By varying substituents on the phenyl ring, one can sensitively tune the cation-π interaction and, therefore, the molecular structure and properties. Variations in molecular structures and properties as a function of phenyl ring substitution are shown to correlate with substituent Hammett parameters.

B(C6F5)3-Catalyzed transfer 1,4-hydrostannylation of α,ß-unsaturated carbonyls using iPr-tricarbastannatrane.

Tris(pentafluorophenyl)borane, B(C6F5)3, has been found to be an effective catalyst to access the hydridoborate anion, [N(CH2CH2CH2)3Sn][HB(C6F5)3], via hydride abstraction from the hypercoordinated tin reagent, iPr-tricarbastannatrane. This process has been applied to the B(C6F5)3-catalyzed transfer 1,4-hydrostannylation of electron-deficient olefins, namely benzylidene barbituric acids. Insights into the mechanism have been obtained via a series of 1H, 2H, 11B, 13C, and 119Sn NMR spectroscopy, mass spectrometry, and labeling experiments.

Changes in Tricarbastannatrane Transannular N-Sn Bonding upon Complexation Reveal Lewis Base Donicities.

Hypercoordinated complexes involving tricarbastannatrane cation [N(CH2CH2CH2)3Sn]+ with various Lewis bases are investigated in the gas and solution phases using a combination of infrared multiple photon dissociation (IRMPD) spectroscopy, NMR spectroscopy, and density functional theory calculations. Coordination is found to occur at the apical position leading to a pentacoordinated Sn center. Strongly electron donating Lewis bases disrupt the N···Sn transannular interaction and induce higher degrees of geometric distortion at the metal center than weakly donating Lewis bases, an effect that manifests as anharmonic shifts in the vibrational spectra. Once characterized in the gas phase, [N(CH2CH2CH2)3Sn(Lewis base)]+ structures were embedded in a dichloroethane polarizable continuum model to investigate solution phase properties. Calculated 119Sn NMR chemical shifts were found to be in good agreement with those measured experimentally, thus suggesting that the bonding properties of [N(CH2CH2CH2)3Sn]+ are essentially the same in the gas and solution phases.

Janus Face Aspect of All-cis 1,2,3,4,5,6-Hexafluorocyclohexane Dictates Remarkable Anion and Cation Interactions In the Gas Phase.

Experiments have been carried out in which electrospray ionization has been used to generate ionic complexes of all-cis 1,2,3,4,5,6 hexafluorocyclohexane. These complexes were subsequently mass isolated in a quadrupole ion trap mass spectrometer and then irradiated by the tunable infrared output of a free electron laser in the 800-1600 cm(-1) range. From the frequency dependence of the fragmentation of the complexes, vibrational signatures of the complexes were obtained. Computational work carried out in parallel reveals that the complexes formed are very strongly bound and are among the most strongly bound complexes of Na(+) and Cl(-) ever observed with molecular species. The dipole moment calculated for the heaxafluorocyclohexane is very large (∼7 D), and it appears that the bonding in each of the complexes has a significant electrostatic contribution.

The structures and properties of proton- and alkali-bound cysteine dimers.

The proton-, lithium-, and sodium-bound cysteine dimers have been investigated in a joint computational and experimental infrared multiple photon dissociation (IRMPD) study. IRMPD spectra in the 1000-2000 cm(-1) region show that protonation is localized on an amine group, and that intermolecular hydrogen bonding occurs between the protonated amine and the carbonyl oxygen of the neutral Cys moiety. Alkali-bound dimers adopt structures reminiscent of those observed for the monomeric Cys·Li(+) and Cys·Na(+) species. Calculations of the heavier Cys2·M(+) (M = K, Rb or Cs) species suggest that these are significantly less strongly bound than the lighter (M = H, Li, or Na) dimers.

Infrared-Driven Charge Transfer in Transition Metal B12F12 Clusters.

A combination of infrared multiple photon dissociation (IRMPD) spectroscopy and density functional theory calculations is used to investigate the structures and charge-transfer properties of clusters containing transition metals (TM = Co(II), Ni(II), Cu(I), Zn(II), Rh(III), Pd(II), Ag(I), Cd(II)) and the dodecafluorododecaboron dianion, B12F12(2-). In all cases, IRMPD resulted in transfer of electron density to the metal center and production of B12F12(-). Metals that exhibit the highest degree of charge transfer are found to induce reaction among the B12F12 cages, leading to production of BnFm (up to n = m = 24).

Synthesis and characterization of tricarbastannatranes and their reactivity in B(C6F5)3-promoted conjugate additions.

The synthesis and characterization of a series of tricarbastannatranes, in the solid state and in solution, are described. The structures of the complexes [N(CH2 CH2 CH2 )3 Sn](BF4 ), [N(CH2 CH2 CH2 )3 Sn](SbF6 ), [N(CH2 CH2 CH2 )3 Sn]4 [(SbF6 )3 Cl], and [(N(CH2 CH2 CH2 )3 Sn)2 OH][MeB(C6 F5 )3 ] were determined by X-ray crystallography. Furthermore, the B(C6 F5 )3 -promoted conjugate addition of alkyl-tricarbastannatranes to benzylidene derivatives of Meldrum's acid was investigated, and detailed mechanistic studies are presented.

Expedient synthesis of complex γ-butyrolactones from 5-(1-arylalkylidene) meldrum's acids via sequential conjugate alkynylation/Ag(I)-catalyzed lactonization.

The conjugate alkynylation of alkylidene Meldrum's acids with alkynylalanes and alkynyl Grignards allows for the formation of propargylic all-carbon quaternary stereocenters in high yields. Ag2CO3-catalyzed intramolecular cyclization of propargylic Meldrum's acid derivatives offers a two-step entry into complex γ-alkylidene butyrolactones containing an all-carbon quaternary center at the C-4 position.

Persistent Intramolecular C-H···X (X = O or S) Hydrogen-Bonding in Benzyl Meldrum's Acid Derivatives.

C-H···X (where X = O or S) intramolecular hydrogen bonding is investigated in three benzyl Meldrum's acid derivatives using a combination of solution phase NMR spectroscopy, gas phase infrared multiple photon dissociation spectroscopy, and density functional theory calculations. In one compound, an abnormally large C-H···S hydrogen bond energy of 30.4 kJ mol-1 is calculated with a natural bond orbital analysis. Intramolecular C-H···O hydrogen bonding is found to persist in the gas phase. Gibbs energy decomposition pathways are calculated.

Carbon-based leaving group in substitution reactions: functionalization of sp3-hybridized quaternary and tertiary benzylic carbon centers.

Lewis acid promoted substitution reactions employing Meldrum's acid and 5-methyl Meldrum's acid as carbon-based leaving groups are described which transform unstrained quaternary and tertiary benzylic C(sp(3))-C(sp(3)) bonds into C(sp(3))-X bonds (X = C, H, N). Importantly, this reaction has a broad scope in terms of both suitable substrates and nucleophiles with good to excellent yields obtained (typically >90%).

Meldrum's acids and 5-alkylidene Meldrum's acids in catalytic carbon-carbon bond-forming processes.

Meldrum's acid (2,2-dimethyl-1,3-dioxane-4,6-dione) is a molecule with a unique history, owing to its originally misassigned structure, as well as a unique place among acylating agents, owing to its high acidity and remarkable electrophilicity. In this Account, we outline the work of our group and others toward harnessing the reactivity of Meldrum's acid derivatives in catalytic C-C bond-forming reactions. Taking advantage of the ability of Meldrum's acid to decompose to CO(2) and acetone following acyl substitution, we have shown that intramolecular Friedel-Crafts acylations can be performed under mild Lewis acidic conditions to yield a variety of benzocyclic ketones. In a further expansion of this method, a domino Friedel-Crafts acylation/alpha-tert alkylation reaction was used to complete the first total synthesis of (+/-)-taiwaniaquinol B. The unique characteristics of Meldrum's acid extend to its alkylidene derivatives, which have also proven exceptionally useful for the development of new reactions not readily accessible from other unsaturated carbonyl electrophiles. By combining the electrophilicity and dienophilicity of alkylidene Meldrum's acid with our Friedel-Crafts chemistry, we have demonstrated new domino syntheses of coumarin derivatives and tetrahydrofluorenones by conjugate additions, Diels-Alder cycloadditions, and C-H functionalizations. Additionally, we have used these powerful acceptors to allow conjugate alkenylation with functionalized organostannanes, and conjugate allylation under very mild conditions. We have also shown that these molecules permit the asymmetric formation of all-carbon quaternary stereocenters via enantioselective conjugate additions. These reactions employ dialkylzinc nucleophiles, maximizing functional group compatibility, while the presence of a Meldrum's acid moiety in the product allows a variety of postaddition modifications. A full investigation of this reaction has determined the structural factors of the alkylidene that contribute to optimal enantioselectivity. We have also used these acceptors to form tertiary propargylic stereocenters in very high enantiomeric excess by an extremely mild, Rh(I)-catalyzed addition of TMS-acetylene. Overall, we demonstrate that Meldrum's acid and its derivatives provide access to a broad range of reactivities that, combined with their ease of handling and preparation, make them ideal electrophiles.

Asymmetric synthesis of all-carbon benzylic quaternary stereocenters via conjugate addition to alkylidene and indenylidene Meldrum's acids.

A systematic study outlining the enantioselective 1,4-addition of dialkylzinc reagents to 5-(1-arylalkylidene) and indenylidene Meldrum's acids is presented. Variation of the aryl and alkyl groups present on the alkylidene was thoroughly explored. The 1,4-addition displayed compatibility with a wide range of heteroaromatics and functional groups, and the arene pattern of substitution affected enantioselection, with a para-substituted aryl group consistently leading to high enantioselectivities. The nature of the organozinc reagent on the efficiency and selectivity of the conjugate addition was also investigated. The solid-state conformation was determined for a number of alkylidene Meldrum's acids and correlated with the observed enantioselectivity in relation to the arene pattern of substitution.

Enantioselective rhodium-catalyzed conjugate alkynylation of 5-benzylidene Meldrum's acids with TMS-acetylene.

The enantioselective alkynylation of benzylidene Meldrum's acids has been successfully achieved through rhodium-catalyzed addition of TMS-acetylene in the presence of bisphosphine ligand 3,5-Xylyl-MeOBIPHEP. The resulting Meldrum's acids were obtained in good yields and up to 99% enantiomeric excess. The alkynylation method is carried out under mild reactions conditions and is compatible with an array of functional groups.

Hydrogenolysis of unstrained carbon-carbon sigma bonds: stereoselective entry into benzylic tertiary centers.

The modification of sp(3)-hybridized carbon centers through Pd-catalyzed reductive cleavage of unstrained carbon-carbon sigma bonds is described. From the hydrogenolysis of benzyl Meldrum's acids bearing an all-carbon benzylic quaternary center, Meldrum's acid and aromatics substituted with a tertiary benzylic stereocenter were obtained in good to excellent yields. Mechanistic studies showed that the reductive cleavage of enantioenriched benzylic quaternary centers proceeded with inversion of configuration, supporting a "loose" S(N)2 pathway.

Asymmetric addition of alkenylstannanes to alkylidene Meldrum's acids.

Herein, we describe enantioselective addition of alkenyltin reagents possessing a reactive and sensitive allylic functionality not readily available to other classes of alkenyl metals. This method is enabled by the use of highly electrophilic alkylidene Meldrum's acids as acceptors and a cationic Rh(I)-diene complex as catalyst.

Sc(OTf)3-catalyzed conjugate allylation of alkylidene Meldrum's acids.

Alkylidene Meldrum's acids are allylated in a conjugate fashion by allyltin nucleophiles under mild Sc(OTf)(3)-catalyzed conditions. The addition is functional group tolerant and reactions with nonracemic alkylidenes are highly diastereoselective. Allylation of alkylidenes derived from alpha-ketoesters yield all-carbon quaternary stereocenters.