Lewis frustrated

Even more effective in hydrogen activation are frustrated Lewis pairs [220] containing NHCs as Lewis bases. In 2008 two research groups reported the activation of hydrogen by frustrated Lewis pairs made up from a suitably Af,A -substituted NHC and B(CgF5)3 (Fig. 30) [221, 222]. The corresponding reaction of the frustrated Lewis pair with primary and secondary alkyl amines resulted in the formation of aminoboranes [222]. 

CT-VPP-REDOR) or the pulse duration fp (CT-VPD-REDOR) then produces CT-REDOR curves, from which the second moment may be evaluated with distinctively superior accuracy as compared to the values obtained from a parabolic fit to the conventional REDOR data. When restricting the experiment to short dipolar evolution times, the two-spin approximation may be applied for the data analysis, which proves to be especially attractive for amorphous solids, for which the exact spin geometry is unknovm. The data presented on the model compoimds illustrate the various facets of CT-REDOR NMR spectroscopy. First application examples, namely, the evaluation of the heteronuclear Li-Ti dipolar couplings within the garnet structure of Li5La3Nb20i2, the determi-nation of the intemuclear B- P distance in frustrated Lewis pairs, the analysis of Na- F dipolar interaction in fluormica or Na- P... 

The discovery happened by accident. Lewis and Anders were frustrated by their failure to find the carrier of anomalous xenon in carbonaceous chondrites. They decided to try an extreme treatment to see if they could dissolve the carrier. They treated a sample of the colloidal fraction of an Allende residue with the harshest chemical oxidant known, hot perchloric acid. The black residue turned white, and to their surprise, when they measured it, the anomalous xenon was still there The residue consisted entirely of carbon, and when they performed electron diffraction measurements on it, they found that it consisted of tiny (nanometer sized) diamonds. After a detailed characterization that included chemical, structural, and isotopic studies, they reported the discovery of presolar diamond in early 1987 (Lewis et al., 1987). The 23-year search for the carrier of CCFXe (Xe-HL) was over, and the study of presolar grains had begun. 

If one attempts lo draw a unique Lewis structure for the (SN)t chain, one is immediately frustrated by the odd number of electrons available. Many resonance structures can be drawn and they contribute to the hybrid structure, but the single structure ... 

In the framework of Frustrated Lewis Pairs, Erker also investigated the hydroboration of a-alkenyl-phosphines with HB(C6F5)2.52/53 Starting from dimesityl(vinyl)phosphine, the CH2CH2-bridged PB 40a was first... 

For example, if the BN bond were to break during the dehydrohalogenation process amine bases could be chosen that selectively capture a proton while the halogenated boron species remains uncoordinated due to steric hindrance. This concept is related to the recently published frustrated Lewis pair concept used for hydrogen activation by sterically demanding borane and amine species [104]. 

Fig. 30 Reaction of the frustrated Lewis acid/Lewis base pair NHC/BlC Fsia with Hi...

Donor-r acceptor (D-rA) interactions. The combination of donor and acceptor sites raises the possibility of D A interactions (intramolecular and intermolecular). In most systems, such D A interactions are prevented by the steric demand of the substituents (steric frustration) and/or the nature of the organic linker (geometric frustration). Noticeable exceptions are illustrated in Scheme 1 (i) the Cj-bridged phosphine-alane 3 adopts a head-to-tail dimeric structure and is usually reacted in the presence of an additional Lewis base to displace the A1 interactions " (ii) the di- and tri-phosphine boranes 7 and 8 equilibrate in solution between open and closed forms which were unambiguously identified in the solid state by X-ray diffraction analyses (iii) the pyridine-borane 2 exists in solution as a mixture of closed monomeric form and head-to-tail dimeric structure, but N B interactions are readily cleaved upon coordination to Ru. ... 

Frustrated Lewis Pairs A Metal-Free Strategy for Hydrogenation Catalysis... 

I 77 Frustrated Lewis Pairs" A Metal-Free Strategffor Hydrogenation Catalysis... 

The unique reactivity of the above system with H2 appears to arise from the unquenched Lewis basicity and acidity of the respective donor P and the acceptor B centers. This inference prompted questions about the nature and reactivity of other phosphine-borane systems and, more broadly, of Lewis acid/base combinations. Is it necessary to have a link between the donor and acceptor sites Could similar H2 activation arise from combinations of donors and acceptors in which steric encumbrance frustrates Lewis acid-base adduct formation If indeed such frustrated Lewis pairs could be uncovered, could one exploit them for the activation of small molecules and applications in catalysis ... 

I 77 Frustrated Lewis Pairs A Metal-Free Strategy for Hydrogenation Catalysis Table 11.1 Catalytic hydrogenation of imines with phosphonium borate catalysts [45]. 

Does the presence of excessive steric bulk when a Lewis acid and a Lewis base attempt to form an adduct automatically render these species inert towards each other The unique behavior of sterically frustrated Lewis pairs (FLPs), pioneered by Stephan, is a vigorous research area with applications for small molecule activation and catalysis." The highly Lewis acidic and sterically bulky tris(pentafluorophenyl)borane" plays a role in many FLP reactions. The great promise of FLP chemistry was revealed by reactions between tris(pentafluorophenyl)borane and tertiary and secondary phosphines, where sterics preclude formation of classic adducts. A seminal example of a frustrated Lewis pair is that of the secondary phosphine di(2,4,6-trimethylphenyl)phosphine that is precluded from forming a classic adduct with tris(pentafluorophenyl)borane. The phosphine Lewis pair is frustrated since it cannot interact with boron to form the adduct. 

Spectacular small molecule activation has been achieved with FLPs, including with 1 2 and N20. The chanistry enabled by frustrated Lewis pairs is being exploited to develop non-transition-metal catalysts that promise to substitute for catalysts that contain toxic and expensive heavy metals. ... 

The ability of frustrated Lewis pairs to capture NO (nitric... 

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