This article is focused on recent developments in main group mediated hydrogenation chemistry and catalysis using "frustrated Lewis pairs" (FLPs). The broading range of substrates and catalyst systems is reviewed and the advances in catalytic reductions and the development of stereoselective, asymmetric reductions made since 2012 is considered.

The carbene derived from (1R,3S)-camphoric acid was used to prepare the borane adduct with Piers' borane 7. Subsequent hydride abstraction gave the borenium cation 8. Adducts with 9-BBN and the corresponding (1R,3S)-camphoric acid-derived carbene bearing increasingly sterically demanding N-substituents (R = Me 9, Et 10, i-Pr 11) and the corresponding borenium cations 12-14 were also prepared. These cations were not active as catalysts in hydrogenation, although 9-11 were shown to undergo carbene ring expansion reactions at 50 °C to give species 15-17. The IBOX-carbene precursors 18 and 19 derived from amino alcohols (S)-valinol and (S)-tert-leucinol (R = i-Pr, t-Bu) were used to prepare borane adducts 20-23. Reaction of the carbenes 1,3-dimethylimidazol-2-ylidene (IMe), 1,3-di-iso-propylimidazol-2-ylidene (IPr) 1-benzyl-3-methylimidazol-2-ylidene (IBnMe), 1-methyl-3-phenylimidazol-2-ylidene (IPhMe) and 1-tert-butyl-3-methylimidazol-2-ylidene (ItBuMe) with diisopinocampheylborane (IpcBH) gave chiral adducts: (IMe)(IpcBH) 24, (IPr)(IpcBH) 25, (IBnMe)(IpcBH) 26, (IPhMe)(IpcBH) 27, and (ItBuMe)(IpcBH) 28. Triazolylidene-type adducts including the (10)-phenyl-9-borabicyclo [3.3.2]decane adduct of 1,3,4-triphenyl-1H-1,2,3-triazolium, rac-29 and the 9-BBN derivative of (S)-2-amino-2'-methoxy-1,1'-binaphthalene-1,2,3-triazolium 34a/b were also prepared. In catalytic studies of these systems, while several species were competent catalysts for imine reduction, in general, low enantioselectivities, ranging from 1-20% ee, were obtained. The implications for chiral borenium cation catalyst design are considered.

As the first clinically approved gadolinium-based blood-pool MRI contrast agent, gadofosveset was designed to bind to human serum albumin (HSA) reversibly, extending the circulation time in the bloodstream. This valuable pharmacokinetic property required for vasculature imaging, however, raises the risk of release and accumulation of gadolinium in vivo. The binding of gadofosveset to HSA significantly increases the relaxivity at low field, which decreases drastically when the magnetic field increases, limiting the applications of gadofosveset at fields of 3 T and higher. To address those challenges, we evaluated a novel dimeric manganese(III) porphyrin (MnP2) in vitro and in vivo as a potential gadolinium-free blood-pool agent. Through multiple spectroscopic studies, we demonstrated that MnP2 binds to HSA tightly. MnP2 exhibits a moderate relaxivity decrease on HSA binding. Nevertheless, owing to the unique field-dependent relaxation behaviors and the dimeric construct (two Mn(III) ions per complex), MnP2-HSA has a molar relaxivity twice that of the gadofosveset-HSA complex at 3 T. Through intravenous injection in rats, MnP2 exhibits long retention and significant contrast enhancement in the vascular compartment, as tested in a 3-T high-field clinical MRI scanner. Taken together, these data demonstrate that MnP2 represents a new class of gadolinium-free blood-pool agents suitable for both regular and high-field applications.

Cationic phosphines of the form [(L)PPh] are prepared by reaction of PhPCl with carbenes (L) including a chiral bis(oxazoline)-based carbene, a cyclic(alkyl)(amino) carbene (cAAC), and a 1,2,3-triazolium-derived carbene, affording the products, [(IBox-iPr)PPh][OSOCF] 1, [(cAAC)PPh][OSOCF] 2 and [((TripCHN(NMe)CPh)PPh)(AgCl)][Cl]3. Using PhPCl, the related dication [CH(NCHNDipp)PPh]4 was also prepared. Crystallographically-determined metric parameters and computational data indicate that these species are best described as cationic phosphines rather than phosphenium cations. The oxidation of these cations with XeF afforded [(IBox-iPr)PFPh][OSOCF] 5, [(cAAC)PFPh][OSOCF] 6 and [(TripCHN(NMe)CPh)PFPh][Cl] 7; 4 was not oxidized. These observations are understood by a computational assessment of the average local ionisation potentials at valence shell charge concentrations identified via topological analysis of the electron density.