Computational studies of complexation of nitrous oxide by borane-phosphine frustrated Lewis pairs

Computational studies of complexes Ar(3)B-ONN-PR(3) derived from reactions between borane-phosphine frustrated Lewis pairs and N(2)O reveal several interesting facets. Natural resonance theory calculations support a change in the preferred resonance structure as the Lewis acidity of the borane increases. Potential constitutional isomers where phosphorus binds to oxygen and boron to nitrogen are predicted to be unstable with respect to loss of phosphine oxide and free N(2). Other constitutional isomers represent stationary points on the potential energy surface; most are considerably less stable than the observed complexes, but one is predicted to be as stable. This arises because the dominant resonance form combines alternating charge with the presence of a stabilizing NO double bond. The relationship between Lewis acidity and complex formation for a variety of boranes was explored; the results are consistent with the idea that greater Lewis acidity stabilizes both classical and frustrated Lewis acid-base pairs, but to differing degrees such that both types can entrap N(2)O. Calculations addressing the mechanism of complex formation suggest that N(2)O binds first through the nitrogen to the phosphine phosphorus of the FLP, whereupon boron coordinates the oxygen atom. Studies of the mechanism of the degenerate exchange reaction between (4-F-H(4)C(6))(3)B-ONN-P(t-Bu)(3) and B(C(6)H(4)-4-F)(3), involves a "transition state", with relatively short B-O distances, and so resembles a classical I(a) process. The process involves two barriers, one associated with bringing the incoming borane into proximity with the oxygen, and the other associated with isomerising from a ladle-shaped cis-trans ct conformer to the observed trans-trans tt-type structure. The overall barrier for degenerate exchange was predicted to be between 65 and 110 kJ mol(-1), in fair agreement with experiment. Similar studies of the reaction between (4-F-H(4)C(6))(3)B-ONN-P(t-Bu)(3) and B(C(6)F(5))(3) indicate that this process more closely resembles a classical I(d) process, in that the "transition state" involves long B-O distances. Derivatization of the complexed NNO fragment appears possible; interaction between (F(5)C(6))(3)B-ONN-P(t-Bu)(3) and MeLi suggests stability for the ion pairs (F(5)C(6))(3)B-ON(Me)N-P(t-Bu)(3)(-)/Li(+) and (F(5)C(6))(3)B-ONN(Me)-P(t-Bu)(3)(-)/Li(+).     

Catalytic hydrogenation with frustrated Lewis pairs: selectivity achieved by size-exclusion design of Lewis acids

Catalytic hydrogenation that utilizes frustrated Lewis pair (FLP) catalysts is a subject of growing interest because such catalysts offer a unique opportunity for the development of transition-metal-free hydrogenations. The aim of our recent efforts is to further increase the functional-group tolerance and chemoselectivity of FLP catalysts by means of size-exclusion catalyst design. Given that hydrogen molecule is the smallest molecule, our modified Lewis acids feature a highly shielded boron center that still allows the cleavage of the hydrogen but avoids undesirable FLP reactivity by simple physical constraint. As a result, greater latitude in substrate scope can be achieved, as exemplified by the chemoselective reduction of α,β-unsaturated imines, ketones, and quinolines. In addition to synthetic aspects, detailed NMR spectroscopic, DFT, and (2)H isotopic labeling studies were performed to gain further mechanistic insight into FLP hydrogenation.     

Dihydrogen activation by frustrated carbene-borane Lewis pairs: an experimental and theoretical study of carbene variation

A variety of Lewis acid-base pairs consisting of tris(pentafluorophenyl)borane, B(C(6)F(5))(3), in combination with sterically demanding five- and six-membered N-heterocyclic carbenes (NHCs) of the imidazolin-2-ylidene, imidazolidin-2-ylidene, and tetrahydropyrimidin-2-ylidene types were investigated with respect to their potential to act as frustrated Lewis pairs (FLP) by reaction with dihydrogen (H(2)) and tetrahydrofuran (THF). A sufficient degree of "frustration" was usually established by introduction of a 1,3-di-tert-butyl or 1,3-diadamantyl carbene substitution pattern, which allows an unquenched acid-base reactivity and thus leads to heterolytic dihydrogen activation and ring-opening of THF. In contrast, 1,3-bis(2,6-diisopropylphenyl)-substituted carbenes showed ambiguous behavior, and the corresponding five-membered imidazolin-2-ylidene formed a stable carbene-B(C(6)F(5))(3) adduct, whereas fast C-F activation and formation of a zwitterionic pyrimidinium-fluoroborate was observed for the six-membered tetrahydropyrimidin-2-ylidene. A stable adduct was also isolated for the combination of the acyclic carbene bis(diisopropylamino)methylene with B(C(6)F(5))(3), and consequently no reactivity toward H(2) and THF was observed. To rationalize the reactivity of the carbene-borane Lewis pairs, the thermodynamics of adduct formation with B(C(6)F(5))(3) were calculated for 10 different carbenes; the stability (or instability) of these adducts can be used as a good measure of the degree of "frustration".     

Concerted attack of frustrated Lewis acid-base pairs on olefinic double bonds: a theoretical study

A computational approach reveals cooperative action of the preorganized acidic and basic centers of the frustrated P(t-Bu)(3)/B(C(6)F(5))(3) Lewis pair on olefinic bonds as the key to the observed regioselective addition reaction.     

Lewis pair polymerization by classical and frustrated Lewis pairs: acid, base and monomer scope and polymerization mechanism

Classical and frustrated Lewis pairs (LPs) of the strong Lewis acid (LA) Al(C(6)F(5))(3) with several Lewis base (LB) classes have been found to exhibit exceptional activity in the Lewis pair polymerization (LPP) of conjugated polar alkenes such as methyl methacrylate (MMA) as well as renewable α-methylene-γ-butyrolactone (MBL) and γ-methyl-α-methylene-γ-butyrolactone (γ-MMBL), leading to high molecular weight polymers, often with narrow molecular weight distributions. This study has investigated a large number of LPs, consisting of 11 LAs as well as 10 achiral and 4 chiral LBs, for LPP of 12 monomers of several different types. Although some more common LAs can also be utilized for LPP, Al(C(6)F(5))(3)-based LPs are far more active and effective than other LA-based LPs. On the other hand, several classes of LBs, when paired with Al(C(6)F(5))(3), can render highly active and effective LPP of MMA and γ-MMBL; such LBs include phosphines (e.g., P(t)Bu(3)), chiral chelating diphosphines, N-heterocyclic carbenes (NHCs), and phosphazene superbases (e.g., P(4)-(t)Bu). The P(4)-(t)Bu/Al(C(6)F(5))(3) pair exhibits the highest activity of the LP series, with a remarkably high turn-over frequency of 9.6 × 10(4) h(-1) (0.125 mol% catalyst, 100% MMA conversion in 30 s, M(n) = 2.12 × 10(5) g mol(-1), PDI = 1.34). The polymers produced by LPs at RT are typically atactic (P(γ)MMBL with ～47% mr) or syndio-rich (PMMA with ～70-75% rr), but highly syndiotactic PMMA with rr ～91% can be produced by chiral or achiral LPs at -78 °C. Mechanistic studies have identified and structurally characterized zwitterionic phosphonium and imidazolium enolaluminates as the active species of the current LPP system, which are formed by the reaction of the monomer·Al(C(6)F(5))(3) adduct with P(t)Bu(3) and NHC bases, respectively. Kinetic studies have revealed that the MMA polymerization by the (t)Bu(3)P/Al(C(6)F(5))(3) pair is zero-order in monomer concentration after an initial induction period, and the polymerization is significantly catalyzed by the LA, thus pointing to a bimetallic, activated monomer propagation mechanism. Computational study on the active species formation as well as the chain initiation and propagation events involved in the LPP of MMA with some of the most representative LPs has added our understanding of fundamental steps of LPP. The main difference between NHC and PR(3) bases is in the energetics of zwitterion formation, with the NHC-based zwitterions being remarkably more stable than the PR(3)-based zwitterions. Comparison of the monometallic and bimetallic mechanisms for MMA addition shows a clear preference for the bimetallic mechanism.     

Development of [3]ferrocenophane-derived N/B frustrated Lewis pairs for the metal-free catalytic hydrogenation of imines

A series of novel [3]ferrocenophane-derived N/B frustrated Lewis pairs (FLPs) were synthesized and successfully applied to the catalytic hydrogenation of imines in 71–93% yields. This approach could be easily conducted on gram scale and provided versatile synthetic route for the key intermediate of sertraline hydrochloride without heavy metal residues.     

Synthesis and reactivity of electron poor allenes: formation of completely organic frustrated Lewis pairs

The synthesis of several electron poor allenes bearing electron withdrawing substituents is described and their use as Lewis acids in the field of frustrated Lewis pair (FLP) chemistry reported. At room temperature the combination of N-heterocyclic carbenes (NHC) with the allenes under study invariably afforded the corresponding Lewis adducts; however, at -78 °C this reaction is in most of the cases inhibited and kinetically induced organic FLPs are formed. Under these conditions the activation of S-S bonds in disulfides has been achieved in excellent yields.     

Tuning Lewis acidity using the reactivity of "frustrated Lewis pairs": facile formation of phosphine-boranes and cationic phosphonium-boranes

The concept of "frustrated Lewis pairs" involves donor and acceptor sites in which steric congestion precludes Lewis acid-base adduct formation. In the case of sterically demanding phosphines and boranes, this lack of self-quenching prompts nucleophilic attack at a carbon para to B followed by fluoride transfer affording zwitterionic phosphonium borates [R(3)P(C(6)F(4))BF(C(6)F(5))(2)] and [R(2)PH(C(6)F(4))BF(C(6)F(5))(2)]. These can be easily transformed into the cationic phosphonium-boranes [R(3)P(C(6)F(4))B(C(6)F(5))(2)](+) and [R(2)PH(C(6)F(4))B(C(6)F(5))(2)](+) or into the neutral phosphino-boranes R(2)P(C(6)F(4))B(C(6)F(5))(2). This new reactivity provides a modular route to a family of boranes in which the steric features about the Lewis acidic center remains constant and yet the variation in substitution provides a facile avenue for the tuning of the Lewis acidity. Employing the Gutmann-Beckett and Childs methods for determining Lewis acid strength, it is demonstrated that the cationic boranes are much more Lewis acidic than B(C(6)F(5))(3), while the acidity of the phosphine-boranes is diminished.     

Heterolytic activation of hydrogen using frustrated Lewis pairs containing tris(2,2',2'-perfluorobiphenyl)borane

The extremely sterically hindered borane tris(2,2',2'-perfluorobiphenyl)borane (PBB) has been structurally characterised. In combination with bulky nitrogen bases, it forms the 'frustrated Lewis pairs' (FLPs) PBB/2,2,6,6-tetramethylpiperidine (TMP) (1), PBB/1,4-diazobicyclo[2.2.2]-octane (DABCO) (2) and PBB/2,6-lutidine (lut) (3). These novel, unquenched acid-base pairs have been shown to effect facile room temperature heterolytic cleavage of dihydrogen to form the ammonium borate salts [2,2,6,6-Me(4)C(5)H(6)NH(2)][HB(C(12)F(9))(3)] (4) and [N(C(2)H(4))(3)NH][HB(C(12)F(9))(3)] (5), and lutidinium borate [2,6-Me(2)C(5)H(3)NH][HB(C(12)F(9))(3)] (6). Although these reactions are equilibria, the reverse reaction and release of hydrogen gas was not apparent at temperatures up to 120 °C. The relative Lewis acidity of PBB has been determined using the Gutmann-Beckett method.     

A computational study on hydrogenation of CO2, catalyzed by a bridged B/N frustrated Lewis pair

Structural Chemistry - A DFT-based computational study has been performed on the hydrogenation of CO2, catalyzed by a bridged FLP. Formic acid might be formed in two possible pathways as revealed...     

N,N'-dimethyl-N,N'-diarylurea anion radicals: an intramolecular reductive elimination

The one-electron reduction of tertiary N,N'-dimethyl-N,N'-diarylureas (aryl = phenyl, beta-naphthyl, alpha-naphthyl), in HMPA, results in anion radicals that undergo novel intramolecular reductive elimination reactions leading to the formation of the anion radicals of the corresponding biaryls. These results are due to face to face pi-pi stacking interactions involving the two aromatic rings in the urea systems. The overlapping p(pi)() orbitals on the ipso carbons of opposing aryl groups evolve into a sigma bond leading to the formation of the biaryl anion radical. In the case of the N,N'-dimethyl-N,N'-di-2-pyrenylurea system, there is a node in the LUMO of the number 2 carbon, and the parent anion radical remains intact.     

New insights into frustrated Lewis pairs: structural investigations of intramolecular phosphane-borane adducts by using modern solid-state NMR techniques and DFT calculations

Covalent bonding interactions between the Lewis acid and Lewis base functionalities have been probed in a series of "frustrated Lewis pairs" (FLPs) (mainly substituted vinylene linked intramolecular phosphane-borane adducts), using solid-state nuclear magnetic resonance techniques and accompanying DFT calculations. Both the (11)B NMR isotropic chemical shifts and nuclear electric quadrupolar coupling parameters turn out to be extremely sensitive experimental probes for such interactions, revealing linear correlations with boron-phosphorus internuclear distances. The principal component V(zz) of the (11)B electric field gradient tensor is tilted slightly away (~20°) from the boron-phosphorus internuclear vector, leading to an improved understanding of the remarkable reactivity of the FLPs. Complementary (31)P{(1)H}-CPMAS experiments reveal significant (31)P-(11)B scalar spin-spin interactions ((1)J ≈ 50 Hz), evidencing covalent bonding interactions between the reaction centers. Finally, (11)B{(31)P} rotational echo double resonance (REDOR) experiments show systematic deviations from calculated curves based on the internuclear distances from X-ray crystallography. These deviations suggest non-zero contributions from anisotropic indirect spin-spin (J anisotropy) interactions, thereby offering additional evidence for covalent bonding.     

Iminium salts of omega-dithiafulvenylpolyenals: an easy entry to the corresponding aldehydes and doubly proaromatic nonlinear optic-phores

A short, high-yielding route to omega-dithiafulvenylpolyenals (1) via the corresponding iminium salts (2) and starting from trimethyl-1,3-dithiolium tetrafluoroborate is reported. The Knoevenagel reactions of either 1 or 2 with isoxazolone-containing acceptors afford merocyanines 7 and 9, in a process that is often accompanied by a vinylene-shortening side reaction. Experimental and theoretical studies reveal that compounds 7 and 9, featuring two proaromatic end groups, are strongly polarized and show good second-order nonlinear optical responses.     

Insight into the relative reactivity of "frustrated Lewis pairs" and stable carbenes in activating H2 and CH4: a comparative computational study

Computational study has been conducted to gain insight into the relative reactivity of stable carbenes (1 and 2) and typical frustrated Lewis pairs (FLPs, 3-6) in activating H(2) and CH(4). For the FLP H(2) activations, despite the quite different basicities of the Lewis base components, they have comparable reactivities. The unexpected relative reactivity can be attributed to the following two factors: (i) the vacant carbene C: p(π) orbital, which is important when carbene works alone but does not participate in the FLP activation; and (ii) the electrostatic interaction between the Lewis base center and the approaching H atom which plays an important role and can either favor or disfavor a reaction. These explanations are also applicable to methane activations. The study brings two messages to the experimentalists for constructing FLPs: (i) it is recommended to use P- and N-centered Lewis bases to construct FLPs for H(2) activation because using more reactive components does not benefit the activation; and (ii) the FLPs are less reactive in activating CH(4) than H(2). In addition, using more reactive carbenes as Lewis bases in FLPs does not necessarily benefit the methane activation.     

Electrophilic cyclization of o-acetoxy- and o-benzyloxyalkynylpyridines: an easy entry into 2,3-disubstituted furopyridines

[reaction: see text] 2,3-Disubstituted furo[3,2-b]pyridines, 2,3-disubstituted furo[2,3-b]pyridines, and 2,3-disubstituted furo[2,3-c]pyridines are readily prepared under mild conditions via the palladium-catalyzed cross-coupling of 1-alkynes with o-iodoacetoxy- or o-iodobenzyloxypyridines, followed by electrophilic cyclization by I(2) or by PdCl(2) under a balloon of carbon monoxide.