At scan rates above 10 V s-l, the waves complementary to those for oxidation a t 1.1 V, and the reduction a t -0.06 V, appear. By application of the method of Nicholson and Shain,6 the specific rates for the S to 0 isomerization of the metastable 3+ species and the 0 to S isomerization of the metastable reduced species were determined as (5.0 f 0.5) X 10 s-I and (2.0 f 0.2) X 10 s-l, respectively. At low scan rates, <20 mV s-l, the reduction wave at E , = -0.06 V decreases in amplitude, as is seen by comparing Figure I C with Figure 1 b. Intramolecular electron transfer can cause such a decrease, but other processes, for example, intermolecular electron transfer and loss of the electroactive species from the diffusion layer, can also contribute, so that, in the absence' of a more complete study, we can only set the upper limit, 4.1 X s-I, on the rate of intramolecular electron transfer. The intervalence band for the stable form of the mixed-valence species, [S3+/(SO)2+], measured in acetone, has a maximum at -640 nm with c -4 X 10 M-l cm-I. The shift to high energies, as compared to substitutionally symmetric species, is a reflection of the difference in the redox potentials for the two sites. There is a second much stronger absorption at 452 nm which we attribute to ligand to metal charge transfer at R u ( I I I ) .~The more usual redox couples conform to the Marcus relat i~n s h i p ,~ a necessary condition for its validity being that the distortions that bring energy matching at the two sites are nearly harmonic. Those in which the potential profile for at least one oxidation state has two minima-in our case, this is true in both oxidation states-comprise an important class which has not been investigated systematically. Devices of the kind we have described provide a means of studying electron transfer for them in the intramolecular mode. They may also provide a means of locking in charge transfer brought about by light absorption and, as such, may find application in high-density storage of memory. The excited state [Sz+/(SO)3+]* which arises on absorption of light at -640 nm is expected to be quenched rapidly, and if intramolecular electron transfer is slow enough, isomerization to [Sz+/(OS)3+] will occur.Many examples of geometrical isomerization accompanying a change in oxidation states have been reported,'O particularly by Bond and co-workers." The appearance of linkage isomerizations, which depends on a change in back-bonding capacity attending a change in oxidation state, is predictable for many yet untested systems with a reasonable degree of certainty. The field has been greatly extended in the study of molecules in which the metal ion is bound by q2 to organic ligands,I2 including aromatic molecules, and additional chromophores can easily be built into mixed-valence molecules. It needs to be acknowledged that there is precedent for the "double square" potential diagram featured in Figure 2.13 The system that we describe is to be regarded as a prototype of others which can be devised to show "molecul...
A new class of chiral Cz-symmetric bis(phospho1ane) ligands has been prepared and used in rhodium-catalyzed asymmetric hydrogenation reactions. We describe a practical, one-pot procedure which utilizes enantiomerically pure 1,4-diol cyclic sulfates 4 for the preparation of a homochiral series of 1,2-bis(phospholano)ethanes 1 and 1,2-bis-(phospho1ano)benzenes (DuPHOS) 2. Cationic rhodium complexes bearing these new ligands behave as very efficient catalyst precursors for the asymmetric hydrogenation of a broad range of a-(N-acy1amino)acrylate (enamide) substrates 5. Significantly, a variety of unnatural and nonproteinaceous a-amino acid derivatives 6 were obtained directly with enantioselectivities approaching 100% ee when using the DuPHOS ligands 2. Substrate-to-catalyst ratios of 10 000 were routinely used, and ratios as high as 50 000 were demonstrated in these reactions. Details of the DuPHOS-Rh-catalyzed hydrogenations are discussed.Asymmetric phosphine ligands have played a dominant role in the development of novel transition-metal-catalyzed enantioselective syntheses.' Notwithstanding the high selectivities observed in certain applications using some of the more successful asymmetric phosphines such as DIPAMP,Z CHIRAPHOS,3 and BINAP: there are many reactions of interest where catalysts bearing these phosphines perform rather poorly in terms of efficiency and enantioselectivity. Such failure indicates the need for alternative asymmetric ligands and/or catalysts.Toward this goal, our research has been focused on the design of new chiral ligands for use in transition-metal-based asymmetric catalysis. We recently outlined our approach and described the synthesis of new electron-rich Cz-symmetric bis(phospho1ane) and C3-symmetric tris(phospho1ane) ligands-' These studies indicated the potential utility of Cz-symmetric 1 ,2-bis(phospholano)ethane ligands of type 1. The reported ligand preparation, however, was inefficient for cases other than the dimethylsubstituted diphospholane (Le., la, R = Me). We herein report details of an improved synthetic procedure that allows facile preparation of the homochiral series of Cz-symmetric 1,2-bis-(phospho1ano)ethanes 1 (BPE, R = Me, Et, Pr, and i-Pr). The evolutionary nature of our ligand design subsequently led us to prepare the analogous homochiral series of 1 ,2-bis(phospholano)benzene ligands 2 (DuPHOS, R = Me, Et, Pr, and i-Pr).sWe have found that the new ligands 1 and 2 afford efficient catalysts for the highly enantioselective hydr0genation5-~ and t Present address: hydrosilylation1° of various unsaturated substrates. In particular, enantioselectivities approaching 100% ee have been observed in the Rh-catalyzed hydrogenation of a wide range of a-(N-acy1amino)acrylates. Importantly, the high efficiency, selectivity, and substrate generality exhibited by these catalysts provide a practical route to a variety of enantiomerically pure unnatural and nonproteinaceous a-amino acid derivatives. The details of these hydrogenation studies are described herein. Results and Disc...
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