Ligands -diop, structure

Unfortunately in hydrocarbonylation in the presence of chiral ligands other than DIOP, only 2-phenyl-1-propene was used as the substrate and therefore the model cannot be applied because the relative positions of the substituents L, S, H, and Z are unknown. However, the prevailing chirality and isomer observed in hydrocarbalkoxylation of 2-phenyl-l-propene with five other ligands, having structures similar to DIOP (24), indicate (see Table III) that in four cases out of five, the relative position of the substituents in the transition state should be the same as in the model of the transition state containing DIOP. 

Table 15. Influence of the structure of the substrate on the prevailing chirality and on the maximum optical yield obtained in asymmetric hydrocarbonylation with different metallic components of the catalyst in the presence of the same asymmetric ligand [(—)-DIOP]... 

Figure 6.13 The DIOP ligand a structure b molecular graph c adjacency matrix. The broken linesindicatetheshortestand longest P-P connectivity paths, Dpi p2 and AP1 P2, respectively. The adjacency matrix of a molecular graph is a matrix with rows and columns labeled by graph vertices v (i.e., the atoms), with a 1 or 0 in position (vp Vj) according to whether Vj or Vj are adjacent or not.

The isolation of a series of iridium(I) complexes with the optically-active diop ligand (46) [diop = 4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-l,3-dioxolane] has been reported. Equivalent portions of diop and [Ir(Cl)(cod)]2 in ethanol yield [Ir(Cl)(diop)(cod)] EtOH.126 The X-ray crystal structure of this complex shows a distorted trigonal bipyramid in which diop serves as an apical-equatorial bidentate ligand. The structure is maintained in solution, as evidenced by 31P NMR data in CDC13 at 223 K.127 In acetone solution, reaction between [Ir(Cl)(cod)]2 and two or more equivalents of diop affords [Ir(diop)2](BPh4), which reacts with CO to yield [Ir2(CO)2(diop)3](BPh4)2.126... 

Asymmetric hydroformylations with the chiral ligand DIOP (1) and variations on this structure have also been studied. " Although optical... 

Figure 1. Structures of dppe, SHOP ligand, DIOP, and DIPAMP...

Axially chiral spirosilane 85 was efficiently prepared by double intramolecular hydrosilylation of bis(alkenyl)dihy-drosilane 84. By use of SILOP ligand, a -symmetric spirosilane which is almost enantiomerically pure was obtained with high diastereoselectivity (Scheme 24).80 SILOP ligand is much more stereoselective for this asymmetric hydrosilylation than DIOP 58 though they have similar structures. 

The development of the next major class of ligands occurred during the 1990s, with Burks DuPhos (42) family of phospholane ligands [222, 223]. (An individual member of the family is named after the substituent R in Me-DuPhos, R=Me.) This structure could be considered an improvement on the DIOP-derived ligands, where the stereogenic centers are now closer to phosphorus. In addition to the aromatic spacer of DuPhos, there is also the related BPE (43) family, where the spacer between the two phosphorus atoms is less rigid. In both series the phosphorus is... 

Furthermore, the above change in the structure of the ligand causes in the case of 1-butene a great increase (about 5 times) and with (Z)- and (E)-2-butene an even larger decrease of the optical yield. Part of this effect might be connected with the different extents of isomerization of the substrates in the two cases in fact, with DIOP-DBP, a three times longer reaction time is required to reach the same conversion of olefin to aldehyde 48). 

The introduction of chirafity into NHCs will therefore follow different strategies than those that have proved to be successful in phosphine-based asymmetric catalysis. For example, N-heterocydic carbene units will not create an edge-to-face arrangement of their aryl substituents, a structural feature common to many chiral diphosphines, such as the derivatives of Diop, Binap, Josiphos, Chiraphos and others. Results obtained in asymmetric catalysis, using chiral phosphine ligands, are therefore not directly transferable to the respective NHC-analogues. 

The chiral environment around the reaction center, which is apparently the nickel atom, might control the absolute structure of the reaction intermediate, to afford optically active coupling products. It was very fortunate that this type of a chiral environment was easy to introduce onto the nickel by a complexation with a chiral ligand. This was realized by Consiglio et al. in 1973 [Eq. (152) 287] and by Kumada et al. in 1974 [Eq. (153) 352] soon after the first report on achiral, nickel-catalyzed coupling reactions had been made. The first, chiral nickel catalyst they used was ((— )-diop)NiCl2, which, in fact, afforded an optically active coupling product, yet with low e.e.s. 

The hydrogenation reaction is also sensitive to the structure of the alcohol comonomer, with the primary alcohols of the hydroxyethyl methacrylate polymer interacting with the catalyst to give results more closely resembling those found when ethanol is used as the solvent. Similar results were found with DIOP-type ligands (Figure 4). 

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