Migrations of Alkyl and Aryl Groups from Silicon to Nitrogen in Silylated Aryloxyiminoquinones

Summary This work considers the theoretical aspects of the shift of an alkyl (or aryl) group from a coordinated phosphine to the 16-electron d8 transition metal to which it is coordinated. This rearrangement at first seems to resemble an anionic [1,2]-sigmatropic shift. But a detailed analysis of the relatively low barrier calculated for the reaction shows how all traces of the forbiddeness of the reaction have vanished. Phenyl migration should be still more facile. We also analyze in some detail the possible pathways of alkyl halide addition to a phenyl-Pt complex. An alkyl migration, now from metal to coordinated C-atom, could possibly intervene in this process.

In calix[4]arenes when one methylene bridge carries an alkyl or aryl substituent, two diastereomeric cone conformations are possible in which this substituent assumes the equatorial or axial position. Two diastereomers with cis or trans arrangement of the substituents exist for the corresponding compounds with two substituted bridges, and diastereomeric cone conformations have to be considered additionally in most cases. Molecular mechanics calculations predict an energetical preference of the equatorial position of the substituents in both systems. This preference is markedly more pronounced for alkyl groups than for aryl groups. To test these predictions a series of calix[4]arenes in which one (4) or two opposite (5) methylene bridges are substituted by alkyl or aryl groups was synthesized by fragment condensation. For these calixarenes the solution conformations, the equatorial/axial conformational equilibria, and the energy barriers for the cone to cone ring inversion were determined by 1H NMR spectro...

Two series of terpolymers (SPA-A, -B, and -C) composed of perfluoroalkylene, alkylene, and sulfonated phenylene (SP) groups were prepared to investigate the effect of aliphatic groups and their compositions onto the membrane properties for proton exchange membrane fuel cells. The composition of perfluoroalkylene (m) and alkylene groups (n) in SPA terpolymers was controlled to be m/n = 0.5/0.5 for SPA-A, 0.65/0.35 for SPA-B, and 1/0 for SPA-C. SPA terpolymers provided thin and bendable membranes with ion exchange capacity (IEC) ranging from 1.61 to 3.18 mequiv g–1. Introducing alkylene groups into the polymer main chain was effective in achieving high IEC values. SPA-B membranes with lower alkylene group content showed slightly more developed phase-separated morphology than that of SPA-A membranes with higher alkylene group content. The developed phase separation with interconnected ionic channels resulted in high proton conductivity for SPA-B membranes. The alkylene groups in the main chain also contributed to improving mechanical properties as suggested by stress versus strain curves, in which SPA membranes exhibited higher Young’s moduli and higher yield strength than those of copolymer (SPA-C) membranes with no alkylene groups. An H2/O2 (or air) fuel cell with SPA-B membrane exhibited high open circuit voltage (OCV, 0.99 V at 100% RH with O2), low ohmic resistance (0.05 Ω cm2 at 100% RH with O2), and good current/voltage performance, reflecting the properties of SPA-B membrane. However, interfacial compatibility with the catalyst layers was somewhat deteriorated with SPA-B membrane to cause lower mass activity (70 A g–1 at 100% RH) of the cathode compared to that with SPAF membrane (102 A g–1 at 100% RH). SPA-B membrane was durable in OCV hold test for 1000 h with slight degradation in alkylene groups.

The straightforward room temperature synthesis of hybrid polymetallic manganese clusters is investigated, exploiting complementary ligand combinations of p-tert-butylcalix[4]arene and salicylaldoximes. Eight new [MnIII7MnII] clusters have been prepared wherein the simple substitution of alkyl or aryl groups at well-defined positions of the salicylaldoxime scaffold leads to two distinct structure types that, while exhibiting the same general topology, contain the unique MnII ion in different positions. Incorporation of a methyl, ethyl, or isopropyl group at the 3-position of the aromatic skeleton or a phenyl group at the oximic carbon gives structure type A that displays competing weak ferromagnetic and antiferromagnetic interactions. Substitution of a methyl or ethyl group at the oximic carbon atom invokes structure type B, incorporating an additional bulky chloride or nitrate into the metallic core due to the smaller steric imposition and position of the methyl or ethyl group. The distortion of the cluster core is consequently enhanced, switching the magnetic properties and resulting in single-molecule magnet behavior. The presence of tert-butyl groups at the 3- and 5-positions of the salicylaldoxime skeleton leads to a new [MnIV2MnIII2] cluster that is found to be a single-molecule magnet. The bulky tert-butyl group in the 3-position is too large to facilitate Mn8 cluster formation, and thus assembly occurs by an alternative pathway. Characteristic bonding modes of the constituent ligands are retained in every case, and the results presented here give insight into the potential of ligand combinations in future studies, highlighting the importance of steric factors in evaluating their relevant compatibilities.

The focus of the research done to date has been directed at the modification of the ligand system, et-ph-P4, (Et2P(CH2)2P(Ph)CH 2P(Ph)(CH2)2PEt2) primarily to replace the phenyl groups on the internal phosphorus atoms with various alkyl groups. This would effectively tailor the electron density donated to the rhodium centers and control access to the metal's olefin binding site during hydroformylation catalysis. By changing the organic substituents on the phosphorus atoms, the donor/acceptor properties of the ligand can also be adjusted to optimize binding to the metal. In order to customize our ligand to meet this requirement many routes towards the alkylation of phosphines were explored before a breakthrough allowed us to obtain almost quantitative alkyl substitutions. We have developed a zinc modified route which provides an effective one pot synthesis for the aryl or alkyl halo- or dihalophosphines. With this procedure the halo- and dihalophosphines can be synthesized quantitatively and with stoichiometric control. The organozinc route is not limited to alkyl or aryl groups with large steric bulk as noted in the literature synthesis of halo and dihalophosphines using other organometallic reagents. We have also directed our efforts towards the synthesis of halogenated bisphosphines. Our efforts have resulted in reproducible success in the synthesis of bis(dichlorophosphino)methane starting with aluminum foil and dichloromethane. However, the isolation of the desired bisphosphine is complicated, tedious, and gives low yields. Attempts at improving the isolatable yields by using POCl3 by itself and in conjunction with other reagents to complex aluminum trichloride have had little or no success. The best result obtained involves the use of Na3PO4 as a complexing agent for AlCl3. The use of the polynuclear aromatic hydrocarbons with convex surfaces as ligands for transition metal complexes has not been accomplished yet. Our organometallic efforts are directed at exploring the coordination chemistry of these aromatics with a variety of mono- and polymetallic transition metal centers. Although a number of metals belonging to the early, middle and late transition series did show modest to excellent coordination with corannulene, the complex itself was never isolated.

Allylcarbinyl tosylate was found to solvolyze in 98% formic acid 3.7 times faster than n-butyl tosylate. Changes in the rate ratio with nucleophilicity of the solvent suggest different mechanisms for these solvolyses. The formolysis products of allylcarbinyl tosylate, when corrected for secondary rearrangements, were found to be virtually identical with those from cyclobutyl tosylate. Deuterium-labeling experiments indicated complete scrambling of the methylene groups in the ring-closed products. These results are interpreted in terms of formation of bicyclobutonium ion intermediates. The rate acceleration for formolysis obtained upon methyl substitution in the 3- and 4-positions and phenyl substitution in the 4-position have been measured and found to be as large as 4500 for the 4,4-dimethyl substituted allylcarbinyl tosylate. The nature of the initial solvolytic transition state is discussed; the results are shown to be inconsistent with formation of classical carbonium ion intermediates. 1,1,4,4-Tetrafluoro-1,3-butadiene has been shown by dipole moment studies to exist in the s-trans conformation. The F19-F19 spin-spin coupling constants for this compound were found to be very similar to those of bis-4,5(difluoromethylene)-cyclohexene and perfluoro-1,2-dimethylenecyclobutane. These results are discussed in terms of the mechanism of fluorine-fluorine spin-spin coupling. The dissociation constants of a series of methyl alkyl ketone cyanohydrins have been measured by nuclear magnetic resonance spectroscopy. The heats of solution of the ketones and the cyanohydrins in methanol have been measured. The effect of alkyl groups on the heats of solution have been shown to be unimportant in determining the observed dissociation constants of the cyanohydrins of these ketones.

C-H activation and functionalization are on the forefront of modern synthetic chemistry. Imagine if any C-H bond of a molecule could be converted to a C-X bond, where X is the target functionality-this would alter the synthetic blueprints for complex target molecules since it would provide novel disconnections in retrosynthetic analysis. Collaborations between many experimental and computational groups have led to rapid developments of new C-H functionalization methods. Our groups represent an example of this; we were brought together as part of the NSF-supported Center for Selective C-H Functionalization. Many examples of experimental-computational synergy for selective Pd(II)-catalyzed C-H activation of aryl and alkyl groups are described in this Account. We describe computations by the Houk group made in response to experimental stimuli by the Yu group. The first section discusses the experimental and computational investigations of oxazoline-directed stereoselective Pd(II)-catalyzed C(sp3)-H bond activation that occurs through the concerted metalation-deprotonation (CMD) pathway involving a monomeric Pd(II) complex. The second section involves two types of bidentate ligands, mono-N-protected amino acid (MPAA) and acetyl-protected aminoethyl quinoline (APAQ) ligands that promote the C-H activation reactions with the ligand as the internal base. In the MPAA-assisted remote C-H bond activation, the basic dianionic amidate ligand participates in the deprotonation of a specific C-H bond. This mechanism accounts for the improved reactivity and selectivity in C-H activation reactions with MPAA ligands. The chiral APAQ ligands enable asymmetric palladium insertion into prochiral C-H bonds on a single methylene carbon center. The dianionic amidate of the APAQ ligand acts as an intramolecular base to deprotonate the methylene C-H asymmetrically and facilitate chiral Pd-C bond formation. The origins of the dramatic differences of five-membered (relatively inactive) and six-membered (highly active) chelation in β-methylene C(sp3)-H activation reactions by a Pd(II) catalyst were explained with density functional theory (DFT) calculations. This is mainly due to the steric repulsions between the ArF group of the substrate and the quinoline group of the ligand. The steric repulsion between the ArF group of the substrate and the quinoline group of the APAQ ligand destabilizes the five-membered chelate transition structure, increasing the energy of the transition state. The third section discusses a mechanism involving a Pd-Ag heterodimeric complex intermediate in the template-directed, Pd(II)-catalyzed remote meta functionalization of toluene derivatives and benzoic acid derivatives. The nitrile directing group of the template coordinates with Ag while the Pd is placed adjacent to the meta-C-H bond in the transition state, leading to the observed high meta selectivity. The selective activation of remote meta-C-H bonds at various distances can be achieved by tuning the template. The dual role of AgOAc as both an oxidant and part of the heteronuclear active species in the mechanism involving PdAg(OAc)3 was determined by DFT calculations and is in accord with literature information about complexes. For the systems discussed in these three sections, the similarity is that they all proceed via the CMD mechanism. The differences lie in the proton acceptors and the active Pd species. Common CMD involves a monomeric Pd mechanism with acetate as the proton acceptor. Both MPAA and APAQ ligands react via monomeric Pd mechanisms with a ligand moiety (the amidate oxygen) as the proton acceptor. Nitrile-containing template-mediated meta-C-H activations proceed via a Pd-Ag heterodimeric mechanism, still with acetate as the proton acceptor. The interaction between our two groups, experts in experiment and computation, and the discoveries made possible by that interplay are highlighted in this Account.

Reaction of organocobalt(III) porphyrins with a cobalt(II) complex of a distinguishable porphyrin or tetrapyrrole resulted in the reversible exchange of the organic axial ligand. The exchange reaction was facile in such solvents as benzene, toluene, dichloromethane, chloroform, and pyridine; was unaffected by total exclusion of light; was faster than would be expected for a homolytic process given known Co-C bond dissociation energies; and was of broad scope with respect to the organic ligand. Methyl, benzyl, primary alkyl, secondary alkyl, and acyl groups exchanged, but phenyl groups did not. The position of the exchange equilibrium was independent of the direction of approach and was nonstatistical. The relaxation to equilibrium appeared to be consistent with that of a second-order process. The rate of the reaction varied with the identity of the R group in the order Bzl > or = Me > Et approximately n-Pr > i-Pr > i-Bu > acetyl approximately neopentyl approximately 2-adamantyl. However, the total variation in reaction rates was remarkably small. Attempts to find evidence of free-radical intermediates by trapping with TEMPO or CO or by alkyl group interchange with an excess of an alkyl halide of a distinct alkyl group were unsuccessful over a time scale comparable to multiple half-lives of the exchange reaction. In addition, no rearrangement products were detected in exchange reactions of the 5-hexenyl group. Use of cobalt porphyrin reactants that were sterically encumbered on both faces with groups large enough to prevent formation of a bridged, Co-C-Co structure resulted in a 5 or more order of magnitude decrease in the rate of methyl exchange, if not its outright cessation, when run with total exclusion of light. The decrease in the rate of the thermal exchange process revealed the existence a slow photochemical exchange process that was driven by room lights. All evidence was consistent with a bimolecular S(H)2 mechanism for the thermal exchange mechanism.

Conformational study of some 3t, 5t-Dimethyl -N-nitroso-2r, 6c-diarylpiperidin-4-one oximes using NMR spectra

Cycloarenes are an essential class of polycyclic aromatic hydrocarbons with unique electronic structure, but their synthesis is very challenging. Herein, we report a facile synthetic strategy prima...

In the title compound, C21H25NO2, the piperidine ring adopts a twisted boat conformation characterized by puckering parameters θ = 89.5 (1) and ϕ = 257.5 (2)°. The phenyl groups are located in equatorial and axial positions on the central piperidine ring, while the methyl group is in an equatorial position. The dihedral angle between the phenyl rings is 49.8 (1)°. An intra­molecular C—H⋯O inter­action occurs. The crystal structure features weak inter­molecular C—H⋯O inter­actions and a stabilizing inter­molecular C—H⋯π contact involving the axial phenyl ring.

Reaction of R(Ph)SnCl2(R=Me, Et) with 2,6-diacetylpyridine bis(thiosemicarbazone) (H2DAPTSC): the crystal structures of [Me(Ph)Sn(HDAPTSC)]Cl · 1.25 MeOH and [Et(Ph)Sn(H2DAPTSC)]Cl2· MeOH · H2O

(1) All the phenomena hitherto ascribed to the characteristic catalytic action of Japanese acid earth can be explained by taking its strong electron-seeking property into consideration. Thus its ability to promote the hydrolysis of cane sugar or starch and the oxidation of pyrogallol and its property to give various oxidase-like colour reactions are all due to its selective adsorptive power for hydroxyl ion. The facts can similarly be explained that the dehydration or deamination of various organic compounds takes place through its catalytic action followed by the intramolecular rearrangement, as observed in the reaction of producing cyclopentane derivatives from cyclohexanol, cyclohexylamine or their methyl derivatives. The initial step of the formation of β,β′-dinaphthyl from naphthalene in the presence of the earth is the polarisation of the naphthalene molecule promoted by its electron-seeking property. (2) Several diarylacetaldehydes have been newly synthesised and found to rearrange themselves to give the corresponding ketones, when heated at 300°C. in the presence of the earth. Thus phenyl p-tolyl and m-tolyl p-tolyl acetaldehydes gave phenyl p-methylbenzyl and m-tolyl p-methylbenzyl ketones respectively, while o-tolyl p-tolyl and p-tolyl p-chlorophenyl acetaldehydes gave the respective mixtures of o-toly p-methylbenzyl and p-tolyl o-methylbenzyl ketones and of p-chlorophenyl p-methylbenzyl and p-tolyl p-chlorobenzyl ketones. (3) By comparing the yields in the isomeric ketones thus produced the order of the facility of separation of the different aryl groups was determined as follows:— Phenyl, m-tolyl>p-chlorophenyl>o-tolyl>p-tolyl. The phenyl group may most probably have the greater migratory tendency than the m-tolyl. (4) The mechanism of the conversion of the aldehydes to the ketones has been discussed. It has been postulated that an electromeric displacement of electrons in the carbonyl group (Remark: Graphics omitted.) is promoted by the action of the earth, and one of the aryl groups migrates with its valency electrons from the adjacent a-carbon atom to the depleted carbonyl carbon. A proton is then splitted from the carbonyl carbon and unites with the carbonyl oxygen to give the enolic form of the ketone, isomeric to the aldehyde. The conclusion has been reached that the more electron-attracting of the two aryl groups must be first separated as anion. (5) It was confirmed that the capacity of the p-tolyl group for electron-release is much larger than those of o-tolyl, m-tolyl, phenyl and p-chlorophenyl groups. By these results a new chemical evidence has been given for the tautomeric effect of the methyl group. (6) Following compounds have been newly synthesised and their constitutions were confirmed:— a-phenyl a-p-tolyl, a-m-tolyl a-p-tolyl, a-o-tolyl a-p-tolyl and a-p-chlorophenyl a-p-tolyl ethylene glycols and their benzoyl derivatives; p-tolyl m-methylbenzyl, m-tolyl p-methylbenzyl, o-tolyl p-methylbenzyl, p-tolyl o-methylbenzyl, p-chlorophenyl p-methyl-benzyl and p-tolyl p-chlorobenzyl ketones and their oximes and semi-carbazones; m-tolyl p-tolyl, o-tolyl p-tolyl and p-tolyl p-chlorophenyl acetic acids; p-methylbenzoyl carbinol, p-chlorobenzoyl methyl acetate and p-chlorobenzoyl carbinol. (7) o-Tolyl p-methylbenzyl and m-tolyl p-methylbenzyl ketones have been found to be oxidised slowly by the action of atmospheric oxygen on long standing, giving the mixtures of o-toluic and p

A mechanism for the thermal decomposition of zinc dialkyldithiophosphate additives has been proposed that accounts for the major degradation products and their dependence on the nature of the alkyl group. The proposed mechanism generally follows well-known rules of phosphorus chemistry. The major points are as follows. (1) The decomposition is initiated by a general migration of alkyl groups from oxygen atoms to sulfur atoms. The rate of this reaction is governed by the ease with which nucleophilic substitution can take place at the α-carbon atom of the alkyl group. The well-established correlation between decreasing number of β-hydrogen atoms and increasing thermal stability is, therefore, explicable in terms of increasing steric hindrance, by the β-subtituents, of nucleophilic attack on the α-carbon atom. (2) Olefin elimination successfully competes with substitution where the alkyl group structure is favorable (i.e. secondary alkyl groups), leading to the formation of phosphorus acids. (3) Nucleophilic substitution of one phosphorus species by another lends to formation of P-O-P type structures and of zinc mercaptide (Zn(SR)2) as reaction intermediate. Reaction of mercaptide with an alkylating agent generates dialkyl sulfide; reaction of mercaptide with dithiophosphate species leads to formation of trithiophosphates and eventually tetrathiophosphates. (4) In acidic media, mercaptide is protonated to give alkyl mercaptan as a major product. (5) The oil-insoluble deposit is a mixture of zinc thiophosphates and zinc pyro- and polypyro-thiophosphates. Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Dayton, OhiO, October 16–18, 1979 Notes Presented as an American Society of Lubrication Engineers paper at the ASLE/ASME Lubrication Conference in Dayton, OhiO, October 16–18, 1979

A series of novel pyrazolo[3,4-d]pyrimidin-4-one derivatives were synthesized and evaluated for their anti-phosphodiesterase-5 (PDE-5) activity. A total of 28 compounds, containing alkyl and aryl groups at the 1-N and 3-C positions on the pyrazole ring, and also bearing different alkyl substituents on the piperazine ring were synthesized. Four compounds (4d, 5d, 6d, and 5o) were found to have better inhibitory activity against PDE-5 (IC50 < 10 nM). All four of the most active compounds contain a phenyl ring at the N1 position. Compounds containing a 3,5-dimethylpiperazinyl group show better activity than others. These results suggest that compound 5o can be used as a lead structure for developing new inhibitors of PDE-5.