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Inorg. Chem. 1992, that in the complex solution. A plot of koM versus [H+] gave a straight line with a zero y-intercept. The values for these rate constants are given in Table 111.A mechanism consistent with these observations is shown in Scheme 11. The two equilibria represented by K, and K, are rapid and are not seen as distinct steps in the complexation reactions. The formation of the chelated phenanthroline complex is consistent with the orange color of the complex and the first-order [H+] dependence on the reverse reaction. Similar results have been obtained for the Ti3+ and V2+ reactions with complex I prior to electron transfer.I1For the mechanism in Scheme 11, the experimentally determined value of k , can be written as follows: k , = k t I / K , K ,Unfortunately, no determinations of K, or K, are available; thus evaluation of k t l is not possible.The kinetic equilibrium constants for formation of the Ni2+ and Co2+ complexes with I are 239 and 169 M-I, respectively. These values are not strictly comparable to those for the free ligand, since we are dealing with complexation of a 2+ ion with a 3+ complex. However, the values for this process are expected to be much smaller than those for uncomplexed ligand. No values are available for the phenanthroline-Zcarboxamide ligand, but for 2-substituted pyridines containing a carboxyl group, the formation (1 1) Johnson, M.; Balahura, R. Unpublished data. 31, 1835-1844 1835constants are in the range 106-1010 M-' l 2 for complexation with Ni2+ whereas, for the complexation of (NH3)sCo(2-NHCOpy)2+ with Ni2+, the kinetically determined equilibrium constant is only 5.6 X lo3 M-l. Presumably, in the case of the phenanthrolinecarboxamide ligand, steric effects and the increased electrostatic repulsion will decrease the formation constants of the pentaammine complex substantially. ConclusionsThe complexation reactions of I, which possesses a pendant phenanthroline with a carbonyl group in the 2-position, occur via the carbonyl oxygen and the adjacent nitrogen on the phenanthroline ring. Subsequent rearrangement reactions may occur which result in a tridentate complex involving the carbonyl oxygen and both phenanthroline nitrogens. The latter behavior has been observed for Fe(II), Ti(III), and V(I1). Further work on the substitution and electron-transfer reactions of complex I are in progress. Acknowledgment. We thank Dr. M. Mlekuz and L. Randall for acquiring the 13C NMR spectra. We also thank the Natural Sciences and Engineering Research Council of Canada for financial support.Supplementary Material Availabk Tables of rate constants for metal ion complexation reactions (3 pages). Ordering information is given on any current masthead page. A binucleating macrocyclic ligand which embodies 6-and 4-coordinate sites has been prepared. Both homobimetallic and site-specific heterobimetallic complexes of Zn(II), Co(II), and Mn(I1) have been isolated and characterized. The dimanganese(I1) species reacts with dioxygen to give a mixed-valence compound containing both Mn(I1) and Mn...

Catalysts of the type [Rhfchiral diphosphine)]+ convert 4-substituted 4-pentenals into the corresponding 3-substituted cyclopentanones with generally high turnover numbers and frequencies at 25°C. The enantioselectivities of various substituted 4-pentenals with two chiral diphosphines have been explored. It was found that with the binap catalyst, almost complete enantioselectivity is observed for 4-pentenal substrates bearing 4-substituted tertiary substituents and for ester groups. Ketonic substituents give very high enantioselectivities. The mechanism of intramolecular hydroacylation has been explored, and it is suggested that an important consideration for obtaining high turnover frequencies is related to the acyl-alkyl reductive elimination mechanism which is inferred to occur by a process similar to ester hydrolysis. The origin of the enantioselection is discussed in terms of the interactions between the phenyl groups of the phosphine and the substituent of the pentenal.Intramolecular hydroacylation is one of the more attractive catalytic processes because it converts a pentenal to a cyclic ketone (eq 1). The process is unusual in that the events leading to the product appear to involve scission of the acyl-hydrogen bond to give a hydrido-acyl intermediate followed by the addition of the two fragments to the double bond. With rhodium-based catalysts, 5-membered ring products are almost always produced.The discovery of Tsuji1 that Wilkinson's catalyst, [Rh(PPh3)3-Cl], was capable of decarbonylating aldehydes to form the alkane and the carbonylated complex, trans-[Rh(PPh3)2(CO)Cl], suggested that oxidative addition across the acyl-hydrogen bond was a step in the decarbonylation. Such a hydrido-acyl intermediate could, in principle, hydroacylate a double bond, provided that the rate of hydroacylation was faster than the decarbonylation rate.The first example of intramolecular hydroacylation of 4-pentenals was reported by Sakai,2 who used Wilkinson's catalyst.Stoichiometric amounts of [RhCPPhs^Cl] and the pentenal were used, and only a 30% yield of the desired cyclopentanone was obtained. The remainder of the material was composed of decarbonylation products. Subsequently, Miller3 showed that the [RhfPPhs^Cl] complex could act as a catalyst under ethylene pressure, and Larock4 explored the catalysis with a variety of substrates and with catalysts incorporating differently substituted unidentate triarylphosphines. In neither study were more than a few turnovers observed before all of the catalyst was converted

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The mechanism of asymmetric catalytic intramolecular hydrosilation using [Rh(diphosphine)]+ catalysts has been investigated by the use of specifically labeled deuterated substrates. It is concluded that oxidative addition of the silicon hydride bonds to the catalysts is rapid and reversible for most systems. Both 5-membered and 6-membered metallacycles are inferred to form rapidly and reversibly. These species are unproductive catalytic intermediates. It was inferred that @-hydride elimination is very rapid and is faster than chelate ring conformational interconversion. Hydrosilation proceeds by silyl olefin insertion. The turnover-limiting and enantioselective steps are believed to be the silyl olefin insertion. It is concluded that the major diastereomeric intermediate, the silyl metal olefin hydride, produces the major enantiomer of the product.

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