We have undertaken theoretical investigations of the asymmetric hydroformylation of styrene by the [Rh{(R,S)-BINAPHOS}(CO)(2)H] catalyst, focusing on the origin of the ligand coordination preferences and stereoinduction. We evaluated the different factors governing the preference of the BINAPHOS ligand to coordinate with the phosphane moiety at the equatorial site and the phosphite moiety at the apical site. The donor-acceptor interactions, obtained using a modified version of energy decomposition analysis (EDA) based on orbital deletion, favour the phosphite moiety at the equatorial site. However, the electronic distortion and the steric effects inverse this tendency. Calculations also suggest that the coordination preference was transferred to the selectivity-determining transition state. We propose a stereochemical model based on quantitative quadrant maps obtained from a new molecular descriptor, the distance-weighted volume (V(W)), which is easily computed from ground-state structures. Repulsive interactions between the substrate and the apical phosphite were responsible for the enantiodifferentiation. The axial chirality of the phosphite discriminated one of the competitive equatorial-apical paths, whereas the axial chirality of the backbone discriminated one of the two enantiomers. Transition-state calculations revealed that the placement of phosphane at the apical site would lower enantioselectivity, explaining the poor performance of other phosphane-phosphite ligands. Finally, comparison with previous studies allowed the definition of several prerequisites for diphosphane ligands for high stereoselectivity: 1) specific equatorial-apical coordination bringing chirality to the apical site, 2) combination of two stereogenic centres and 3) rigid structures.

We present a new methodology to predict the enantioselectivity of asymmetric catalysis based on quantitative quadrant-diagram representations of the catalysts and quantitative structure-selectivity relationship (QSSR) modelling. To account for quadrant occupation, we used two types of molecular steric descriptors: the Taft-Charton steric parameter (ν(Charton)) and the distance-weighted volume (V(W) ). By assigning the value of the steric descriptors to each of the positions of the quadrant diagram, we generated the independent variables to build the multidimensional QSSR models. The methodology was applied to predict the enantioselectivity in the cyclopropanation of styrene catalysed by copper complexes. The dataset comprised 30 chiral ligands belonging to four different oxazoline-based ligand families: bis- (Box), azabis- (AzaBox), quinolinyl- (Quinox) and pyridyl-oxazoline (Pyox). In the first-order approximation, we generated QSSR models with good predictive ability (r(2) =0.89 and q(2) =0.88). The derived stereochemical model indicated that placing very large groups at two diagonal quadrants and leaving free the other two might be enough to obtain an enantioselective catalyst. Fitting the data to a higher-order polynomial, which included crossterms between the descriptors of the quadrants, resulted in an improvement of the predicting ability of the QSSR model (r(2) =0.96 and q(2) =0.93). This suggests that the relationship between the steric hindrance and the enantioselectivity is non-linear, and that bulky substituents in diagonal quadrants operate synergistically. We believe that the quantitative quadrant-diagram-based QSSR modelling is a further conceptual tool that can be used to predict the selectivity of chiral catalysts and other aspects of catalytic performance.

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Ammonolysis of the μ(3)-alkylidyne derivatives [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-CR)] [R = H (1), Me (2)] produces a trinuclear oxonitride species, [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ(3)-N)] (3), via methane or ethane elimination, respectively. During the course of the reaction, the intermediates amido μ-alkylidene [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ-CHR)(NH(2))] [(R = H (4), Me (5)] and μ-imido ethyl species [{Ti(η(5)-C(5)Me(5))(μ-O)}(3)(μ-NH)Et] (6) were characterized and/or isolated. This achievement constitutes an example of characterization of the three steps of successive activation of N-H bonds in ammonia within the same transition-metal molecular system. The N-H σ-bond activation of ammonia by the μ(3)-alkylidyne titanium species has been theoretically investigated by DFT method on [{Ti(η(5)-C(5)H(5))(μ-O)}(3)(μ(3)-CH)] model complex. The calculations complement the characterization of the intermediates, showing the multiple bond character of the terminal amido and the bridging nature of imido ligand. They also indicate that the sequential ammonia N-H bonds activation process goes successively downhill in energy and occurs via direct hydron transfer to the alkylidyne group on organometallic oxides 1 and 2. The mechanism can be divided into three stages: (i) coordination of ammonia to a titanium center, in a trans disposition with respect to the alkylidyne group, and then the isomerization to adopt the cis arrangement, allowing the direct hydron migration to the μ(3)-alkylidyne group to yield the amido μ-alkylidene complexes 4 and 5, (ii) hydron migration from the amido moiety to the alkylidene group, and finally (iii) hydron migration from the μ-imido complex to the alkyl group to afford the oxo μ(3)-nitrido titanium complex 3 with alkane elimination.

. (2014) A theoretical study of the activity in Rh-catalyzed hydroformylation: the origin of the enhanced activity of the π-acceptor phosphinine ligand. Catalysis Science and Technology, 4 . pp. 979-987. ISSN 2044-4753 Access from the University of Nottingham repository: http://eprints.nottingham.ac.uk/29468/1/post_print_final.pdf Copyright and reuse:The Nottingham ePrints service makes this work by researchers of the University of Nottingham available open access under the following conditions. This article is made available under the University of Nottingham End User licence and may be reused according to the conditions of the licence. For more details see: http://eprints.nottingham.ac.uk/end_user_agreement.pdf A note on versions:The version presented here may differ from the published version or from the version of record. If you wish to cite this item you are advised to consult the publisher's version. Please see the repository url above for details on accessing the published version and note that access may require a subscription.For more information, please contact eprints@nottingham.ac.uk Journal Name The factors governing the activity in Rh-catalyzed hydroformylation were investigated using a set of computational tools. We performed DFT calculations on the phosphinine-modified Rh catalyst [HRh(CO) 3 (PC 5 H 2 R 3 )] and compared it to the phosphane-modified HRh(CO) 3 (PR 3 ) and HRh(CO) 2 (PR 3 ) 2 complexes. The -acceptor phosphinine ligand coordinates preferentially at the equatorial site of 10 pentacoordinated Rh complex with the heterocycle perpendicular to the equatorial plane, although the ligand freely rotates around the Rh-P bond. The overall energy barrier can be divided into the following contributions: alkene complex formation, alkene rotation and alkene insertion. In the absence of steric effects (model systems), the overall barrier correlates with the computed barrier for alkene rotation. This proves that -acceptor ligands reduce backdonation to the alkene, leading to a lower rotational barrier, 15 and consequently, to a higher activity. The Rh-P donor-acceptor interactions were quantified using a modified version of energy decomposition analysis (EDA). In Rh-phosphinine systems, the efficient directionality of the -backdonation, rather than the overall acceptor ability, is responsible for the high catalytic activity. Introducing steric effects increases the energy required to coordinate the alkene, increasing the overall barrier. The factors governing the activity in Rh-monophosphane catalysts seem to 20 be related to those derived for Rh-diphosphane during the development of a QSAR model (Catal. Sci. Technol. 2012¸ 2, 1694). To investigate whether the findings for mono-can be extrapolated to diphosphane ligands, we re-examine our previous QSAR model using the Topological Maximum Cross Correlation (TMACC) method based on easy-to-interpret 2D-descriptors. The TMACC descriptors highlight heteroatoms close to phosphorus as activity-increasing atoms, whereas highly substituted carbon 2...