D. Christopher Braddock

A recyclable ‘boomerang’ polymer-supported ruthenium catalyst for olefin metathesis

Abstract Polymer supported ruthenium catalyst 8 was found to be an effective catalyst for ring closing metathesis. The rate of reaction and activity was found to be comparable to that of homogeneous catalyst 1 . In addition, the catalyst could be recycled and re-used up to three times by simple filtration. The residual ruthenium/catalyst content in the product mixtures using catalyst 8 were found to be much reduced compared with the homogeneous catalyst 1 .

Bromonium Ion Induced Transannular Oxonium Ion Formation-Fragmentation in Model Obtusallene Systems and Structural Reassignment of Obtusallenes V-VII

Ring-closing metathesis was used to construct the strained 11-membered ring of obtusallenes II (and IV). Bromonium ion induced transannular oxonium ion formation-fragmentation gave the macrocyclic carbon skeleton of obtusallene VII with a bromine atom at C-13, in line with a previously published hypothesis. An additional brominated [5.5.1]bicyclotridecane adduct that must arise from a bromonium ion induced transannular oxonium ion formation-fragmentation could also be isolated, suggesting that this adduct represents the core of an as yet undiscovered natural product. An authentic sample of obtusallene V was studied by NMR spectroscopy, and the position of the halogens at C-7 and C-13 was reassigned on the basis of a (13)C NMR chlorine induced isotopic shift. This revised structure was subsequently confirmed by X-ray crystallography. These findings allow us to confidently conclude that the structures of obtusallenes VII and VI should also be reassigned.

Improving the permeance of hybrid polymer/metal–organic framework (MOF) membranes for organic solvent nanofiltration (OSN) – development of MOF thin films via interfacial synthesis

In situ growth (ISG) of metal–organic frameworks (MOFs) within the pores of integrally skinned asymmetric polymer membrane supports has previously been shown to outperform mixed matrix membranes (MMMs) for organic solvent nanofiltration (OSN) applications. However these membranes have the disadvantage of low flux performances. In order to improve solvent permeance through hybrid MOF/polymer membranes a fabrication methodology to produce MOF thin film composite membranes (MOF-TFCs) has been developed. Interfacial synthesis was utilised to produce a thin layer of HKUST-1 on Polyimide P84 ultrafiltration supports. Two different fabrication methodologies were employed; methodology A resulted in the HKUST-1 layer growing above the polymer membrane surface and methodology B resulted in HKUST-1 growth embedded in the surface of the polymer support membrane. The MOF-TFCs produced via methodology A were shown to have similar solute retentions as ISG membranes; however the permeance values achieved were over 3 times higher than that of the ISG membranes.

Structural reassignment of obtusallenes V, VI, and VII by GIAO-based density functional prediction.

Revised structures proposed previously for obtusallenes V-VII ( 5- 7) have been confirmed on the basis of computed GIAO-DFT 13C NMR chemical shifts.

Hafnium(IV) and zirconium(IV) triflates as superior recyclable catalysts for the atom economic nitration of o-nitrotoluene

Abstract The hydrated group 4 metal triflates, Hf(OTf)4 and Zr(OTf)4, were found to be excellent catalysts (10 mol%) for the mononitration of o-nitrotoluene using a single equivalent of concentrated (69%) nitric acid. The only side product is water and the catalysts are readily recycled from the aqueous phase and re-used.

A tripartite asymmetric allylboration — Silicon tethered alkene ring closing metathesis — in situ ring opening protocol for the regiospecific generation of functionalized (E)-disubstituted homoallylic alcohols

Molybdenum carbene 6 catalyzed ring closing metathesis of (alkenyl)silyl ethers of homochiral allylic alcohols to afford 1,2-oxasilines which were elaborated in situ to give (E)-4-alkyl-1,2-disubstituted 3-buten-1-ol and (Z)-4-alkyl-4-silyl-1,2-disubstituted 3-buten-1-ol derivatives as single geometric isomers.

In situ tandem allylic acetate isomerisation-ring closing metathesis: 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ruthenium benzylidenes and palladium(0)–phosphine combinations

Abstract The use of 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene ruthenium benzylidenes as olefin metathesis pre-catalysts in conjunction with palladium(0)–phosphine combinations allows tandem in situ allylic acetate isomerisation–ring closing metathesis reactions to occur. The tandem reaction can either be performed on separate substrates in the same pot, or on substrates containing both allylic acetate and diene functionality.

On the use of tandem allylic acetate isomerisation and ring-closing metathesis with palladium(0) phosphine complexes and ruthenium benzylidenes as orthogonal catalysts

Abstract Attempted tandem allylic acetate isomerisation and ring-closing metathesis using combinations of palladium(0)/Grubbs’ catalyst/PPh 3 led to either allylic isomerisation or ring-closing metathesis but not both in tandem. The effect was traced to deactivation of Grubbs’ catalyst by added PPh 3 and poisoning of the Pd(0) by the necessarily released PCy 3 (Cy=cyclohexyl) from Grubbs’ catalyst. Successful tandem isomerisation/ring-closing metathesis was achieved by the use of palladium(0)/PPh 3 systems in conjunction with a PCy 3 free olefin metathesis catalyst.

Clarification of the Stereochemical Course of Nucleophilic Substitution of Arylsulfonate-Based Nucleophile Assisting Leaving Groups

Secondary alcohols modified as tosylates, PEG-sulfonates, or quisylates undergo inversion of configuration at the reacting center when treated with lithium halide in acetone at reflux, where the PEG-sulfonates and quisylates are substantially more reactive. In sterically hindered cases, elimination is a competing process. In contrast, when treated with TiCl(4), simple secondary sulfonates give chloride products with partial inversion of configuration. Any observed retention of configuration in a given alkyl sulfonate substrate under these conditions is likely due to neighboring group participation or diastereoselective attack on a carbocation (or ion pair) rather than an S(N)i mechanism.

Intramolecular Bromonium Ion Assisted Epoxide Ring-Opening: Capture of the Oxonium Ion with an Added External Nucleophile

9-Oxabicyclo[6.1.0]non-4-ene (1) undergoes intramolecular bromonium ion-assisted epoxide ring-opening using N-bromosuccinimide via a presumed oxonium ion that is subject to stereospecific, nonregioselective capture with added external nucleophiles producing novel bicyclo[4.2.1] and bicyclo[3.3.1] ethers. Carboxylic acids (as catalyzed by tetramethylguanidine), alcohols, water, and halides can all function as effective nucleophiles. Stereospecific direct opening of the bromonium ion with carboxylic acids was found to be a competing process where high dilution disfavors this pathway. Halogen-induced isotopic (13)C NMR shifts (Δδ CBr 1.3-1.9 ppb; Δδ CCl 8.6-8.7 ppb) were found to be most useful in unambiguously identifying halogen-bearing carbons, and correlation of these (13)C NMR shifts allowed ready assignment of diastereomeric structures. The structure of adducts 6b, 6c, 7b, 7c, 7d, and 8a-d were all elucidated by X-ray crystallography.