Quadrant model

The low ee-values obtained with simple unsaturated acids as compared to the enamides of dehydroamino acid derivatives show that the oxygen atoms of the amide is a key to complex formation with the metal center. Knowles also proposed a quadrant model that has been adapted for many reactions [5, 22]. The mechanism of the reaction has been investigated, and it is known that the addition of the substrate to the metal is regioselective and that competing catalytic cycles can occur [5, 10, 22, 25, 27, 30-46]. 

The enhanced synthetic potential of rhodium-complex-catalyzed enantioselective hydrogenation provided by these advances in ligand design has led to renewed interest in the reaction mechanism, and here we highlight four recent topics (i) the extended base of reactive intermediates (ii) an improved quadrant model for ligand-substrate interactions (iii) computational approaches to mechanism and (iv) (bis)-monophosphine rhodium complexes in enantioselective hydrogenation. These are discussed in turn. 

UPS did not stop with its synchronized commerce initiative, which, in my judgment, was more revolutionary then evolutionary. Chairman Eskew also codified and explained to UPS employees and to Wall Street, UPS s new organic growth strategy. And Eskew named this strategy the Four Quadrant model based on the University of North Carolina basketball team s use of the four-quadrant offense. As he stated, We will call our offense for innovation The Four Quadrants which focuses on innovating existing business operations internally and externally and, likewise, focuses innovation on new entrepreneurial ventures both internally and externally. ... 

Figure 7.3. A quadrant model proposed to accommodate the enantioface-selection by rhodium complexes with chiral diphosphine ligands.

Figure 7.4. Upper Quadrants model for two possible transition states, TS I (left) and TS II (right) for an olefin (R O CR3 4) insertion into the Rh-H bond of RhH(CO)[(R,S)-BINAPHOS]. Lower Sybyl (Cache) representation of the molecular mechanics calculations of transition states, TS I (left) and TS II (right), for styrene (grey) insertion into the Rh-H bond of RhH(CO)[(R,S)-BINAPHOS] (black). Re-face binding of styrene to Rh is calculated to be lower energy than that of si-face binding to both TS I (by 7.1 kcal/mol) and in TS II (by 1.8 kcal/mol). In both structures, the enantioface selection seems to arise from the steric repulsion between the phenyl group of styrene and one of the naphthyls of (R,S)-BINAPHOS (marked with rectangles).

Fig. 3.30 The quadrant model used to predict enantioselectiv-ity in homogeneous hydrogenation.

Apart from a few exceptions [13], the quadrant model can be employed to predict the asymmetric induction of BINAP complexes. For example, the model can be referred to when considering the reduction of j8-dicarbonyl compounds (Scheme 5). In 24, repulsion between the equatorial phenyl moiety and R occurs when the substrate adapts the orientation where the keto function is com-plexed coparallel to the Ru-H bond, which is... 

The quadrant model which has been described for [Ru-BINAP] can normally be applied analogously with [Pd-BINAP] [15] and [Rh-BINAP] complexes. A new example for the latter is the hydroacylation of 4-substituted 4-pentenal 27 to 3-substituted cyclopentenones 28 using the [Rh-BINAP]C104 catalyst (Scheme 6). However, the strong dependence of the selectivity upon the nature of the substituents R suggests that stereoelectronic factors have to be taken into account as well [16]. 

Scheme 13 Andersson s quadrant model developed to rationalize the enantioselectivity...

The models used for rationalizing the enantioselectivities have been discussed in section 5.3.3. The simple four-quadrant model was cited [46], which gave good results for many substrates, but often failed for styrene. Gleich and Herrmann [15,161 have applied computational methods to hydroformylation using a combination of MM and QM. The predictive power of this approach will certainly increase in the near future. 

TABLE 4.5 A General Description of Herrmann s Four-Quadrant Model of Thinking Modes... 

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