Anakuthil Anoop

Steering the Surprisingly Modular π-Acceptor Properties of N-Heterocyclic Carbenes: Implications for Gold Catalysis

Whereas the excellent s-donor qualities of N-heterocyclic carbenes (NHCs) are undisputed and extensively used as an enabling feature for homogeneous catalysis, the p-acceptor properties of such ligands are usually considered weak or even negligible. Only in the last decade has experimental and in silico 4] evidence been acquired that suggests that the p acidity of NHCs deserves more serious consideration, even though this issue is still a matter of debate. 5] The case study outlined herein may help to clarify this aspect. Rather than relying on the interpretation of spectroscopic and structural fingerprints, we present reactivity data which demonstrate that the course of three mechanistically distinct gold-catalyzed processes can be determined solely by changing the pacceptor properties of the ancillary NHC ligand. We previously introduced the cyclophanic NHCs 2–4 as novel analogues of the known imidazopyridine-2-ylidene derivatives 1 (Table 1). These chiral compounds exhibit interesting spectroscopic characteristics and are studied in some detail by our research group. As part of these investigations, the energies of the orbital occupied by the carbene lone pair of electrons (Es) [9] and of the lowest unoccupied orbital with a nonzero coefficient at the carbene center (Ep) [9] were determined by density functional theory (DFT) calculations at the BP86(RI)/TZVP level. The computed data revealed various salient features (Table 1): whereas the Es values of the parent NHC 1a (R = Me) and its cyclophanic counterpart 2 are almost identical, the Ep value drops significantly from 0.63 eV in 1a to 1.14 eV in 2 ; moreover, the energy of the p-type acceptor orbital is highly responsive to the type of substituents present on the top layer; thus, the Ep values of the tetrafluoroand tetramethoxy-substituted carbenes 3 and 4 are almost 0.5 eV apart. These findings are readily explained by the representation of the p acceptor orbital in Figure 1,

Structure and Bonding in Neutral and Cationic 14-Electron Gold Alkyne π Complexes

Cyclododecyne (5) as a prototype unstrained alkyne was coordinated to either the neutral [AuCl] fragment or to two different cationic [Au(NHC)](+) entities (NHC = N-heterocyclic carbene), and the resulting complexes 6, 8, and 10 were characterized by X-ray crystallography and NMR spectroscopy. Since the structure of cyclododecyne in the solid state could also be obtained after in situ crystallization, a comparison was possible that provides insights into structural changes imposed on the alkyne by the different gold fragments. These data are interpreted on the basis of a DFT analysis of the bonding situation in the individual compounds, which provides insights into the very first elementary step common to many gold-catalyzed transformations.

Catalysis via homolytic substitutions with C-O and Ti-O bonds: oxidative additions and reductive eliminations in single electron steps.

In a combined theoretical and experimental study, an efficient catalytic reaction featuring epoxide opening and tetrahydrofuran formation through homolytic substitution reactions at C-O and Ti-O bonds was devised. The performance of these two key steps of the catalytic cycle was studied and could be adjusted by modifying the electronic properties of the catalysts through introduction of electron-donating or -withdrawing substituents to the titanocene catalysts. By regarding both steps as single electron versions of oxidative addition and reductive elimination, a mechanism-based platform for the design of catalysts and reagents for electron transfer reactions evolved that opens broad perspectives for further investigations.

A Local Pair Natural Orbital Coupled Cluster Study of Rh Catalyzed Asymmetric Olefin Hydrogenation.

The recently developed local pair natural orbital coupled cluster theory with single and double excitations (LPNO-CCSD) was used to study the rhodium-catalyzed asymmetric hydrogenation of two prochiral enamides. The method was carefully calibrated with respect to its accuracy. According to calculations on a truncated model system, the effects of perturbative triples (T) on the reaction energetics are very limited, the LPNO approximation is accurate, and complete basis set extrapolation (CBS) causes only minor changes in the relative energies computed with a standard basis set (def2-TZVP). The results for the full system are thus believed to be within 1-2 kcal/mol of the CCSD(T)/CBS limit for the present systems. Relativistic effects were treated by a scalar relativistic Hamiltonian using the zeroth order regular approximation (ZORA). The results of the study were compared to density functional calculations on the same systems and with calculations available in the literature. All calculations predict the correct stereochemical outcome of the reaction that is determined by the relative energies of the transition states in the early stages of the catalytic cycle. In general, DFT calculations using the B3LYP functional are in reasonable agreement with the LPNO-CCSD results, although deviations of 3-5 kcal/mol exist that are also not entirely systematic in the minor and major reaction branches. The present case study thus demonstrates that catalytic reactions, which are well described by single-reference electronic structure theory, can now be routinely studied with confidence in systems with 50-100 atoms applying local correlation methods that are as easy to use as DFT methods.

Selectivity in Garratt−Braverman Cyclization: An Experimental and Computational Study

Bispropargyl sulfones equipped with aromatic rings of dissimilar nature were synthesized. Under basic conditions, these sulfones isomerized to the bisallenic sulfones, creating a competitive scenario between two alternate Garratt-Braverman (GB) cyclization pathways. The observed product distribution ruled out the involvement of any ionic intermediate and supported the diradical mechanism with greater involvement of the electron-rich aromatic ring via the more nucleophilic radical. DFT-based calculations supported the diradical mechanism along with the observed selectivity.

Synthesis, photophysical and photochemical properties of photoacid generators based on N-hydroxyanthracene-1,9-dicarboxyimide and their application toward modification of silicon surfaces.

We have introduced a series of nonionic photoacid generators (PAGs) for carboxylic and sulfonic acids based on N-hydroxyanthracene-1,9-dicarboxyimide (HADI). The newly synthesized PAGs exhibited positive solvachromatic emission (λ(max)(hexane) 461 nm, λ(max)(ethanol) 505 nm) as a function of solvent polarity. Irradiation of PAGs in acetonitrile (ACN) using UV light above 410 nm resulted in the cleavage of weak N-O bonds, leading to the generation of carboxylic and sulfonic acids in good quantum and chemical yields. Mechanism for the homolytic N-O bond cleavage for acid generation was supported by time-dependent density functional theory (TD-DFT) calculations. More importantly, using the PAG monomer N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide (VBSADI), we have synthesized N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide-methyl methacrylate (VBSADI-MMA) and N-(p-vinylbenzenesulfonyloxy)anthracene-1,9-dicarboxyimide-ethyl acrylate (VBSADI-EA) copolymer through atom transfer radical polymerization (ATRP). Finally, we have also developed photoresponsive organosilicon surfaces using the aforementioned polymers.

Development of 1-hydroxy-2(1H)-quinolone-based photoacid generators and photoresponsive polymer surfaces.

A new class of carboxylate and sulfonate esters of 1-hydroxy-2(1H)-quinolone has been demonstrated as nonionic photoacid generators (PAGs). Irradiation of carboxylates and sulfonates of 1-hydroxy-2(1H)-quinolone by UV light (λ≥310 nm) resulted in homolysis of weak N-O bond leading to efficient generation of carboxylic and sulfonic acids, respectively. The mechanism for the homolytic N-O bond cleavage was supported by time-dependent DFT calculations. Photoresponsive 1-(p-styrenesulfonyloxy)-2-quinolone-methyl methacrylate (SSQL-MMA) and 1-(p-styrenesulfonyloxy)-2-quinolone-lauryl acrylate (SSQL-LA) copolymers were synthesized from PAG monomer 1-(p-styrenesulfonyloxy)-2-quinolone, and subsequently controlled surface wettability was demonstrated for the above-mentioned photoresponsive polymers.

A tripyrrolylmethane-based macrobicyclic triazacryptand: X-ray structure, size-selective anion binding, and fluoride-ion-mediated proton-deuterium exchange studies.

A new class of tripyrrolylmethane-based triazacryptand with bridgehead carbons and acyclic molecules were synthesized by the Mannich reaction of tripyrrolylmethane with primary or secondary amine hydrochloride and formaldehyde, respectively. The structure of the triazacryptand was determined by X-ray diffraction (XRD) method. The anion binding properties of both the bicyclic and acyclic receptors were studied by (1)H NMR titration method. The binding studies showed that both receptors exhibit very high affinity and bind strongly with the F(-) ion in DMSO-d(6). However, the binding constant of azacryptand with F(-) is much higher than that of the acyclic receptor. This is attributed to the preorganization of the azacryptand having a specific cavity size, and the strength and the number of hydrogen bonds formed by the F(-) ion. This is supported by the crystal structures of F(-), Cl(-), and Br(-) ion complexes of the bicyclic receptor and by DFT calculations. The X-ray structures showed that the azacryptand receptor forms an inclusion complex with only the F(-) ion; other anions bind in the clefts of the macrobicycle, thus supporting a size-selective anion binding behavior. The high affinity and the selectivity of the macrobicycle as a neutral receptor of the F(-) ion in the presence of other competitive anions in DMSO-d(6) were confirmed by (1)H NMR spectroscopy. Furthermore, the F(-)-ion-mediated hydrogen-deuterium exchanges were monitored by (19)F NMR spectroscopy, showing multiplets based on the formation of all possible deuterium-exchanged fluoride complexes in solution.

Complexation Behavior of the Tri-n-butyl Phosphate Ligand with Pu(IV) and Zr(IV): A Computational Study.

Tri-n-butyl phosphate (TBP), used as the extractant in nuclear fuel reprocessing, shows superior extraction abilities for Pu(IV) over a large number of fission products including Zr(IV). We have applied density functional theory (DFT) calculations to explain this selectivity by investigating differences in electronic structures of Pu(NO3)4·2TBP and Zr(NO3)4·2TBP complexes. On the basis of our quantum chemical calculations, we have established the lowest energy electronic states for both complexes; the quintet is the ground state for the former, whereas the latter exists in the singlet spin state. The calculated structural parameters for the optimized geometry of the plutonium complex are in agreement with the experimental results. Atoms in Molecules analysis revealed a considerable amount of ionic character to M-O{TBP} and M-O{NO3} bonds. Additionally, we have also investigated the extraction behavior of TBP for metal nitrates and have estimated the extraction energies to be -73.1 and -57.6 kcal/mol for Pu(IV) and Zr(IV), respectively. The large extraction energy of Pu(IV) system is in agreement with the observed selectivity in the extraction of Pu.

Aromaticity in all-metal annular systems: the counter-ion effect

The effect of counterions on the bonding, stability and aromaticity of trigonal dianion metal clusters has been analyzed through the behavior of various conceptual density functional theory based reactivity descriptors and the nucleus independent chemical shift calculated at different levels of theory, comprising one-determinant approaches and beyond (QCISD, CASSCF(8,8) and NEVPT2), for a proper benchmarking. Although several important insights into the counter-ion effects are obtained, much needs to be done in order to have a transparent idea therein.