A new synthetic route to (CpR) 2 -ligated titanaaziridines and titanaoxiranes from stable Ti(II) precursors has been developed, and the enantiomer interconversion rate constants for chiral titanaaziridines and titanaoxiranes have been measured for the first time. The titanaaziridines (CpR) 2 Ti(η 2 -N(R 1 )CHPh)(L) (R = H (10), Me (12); R 1 = Ph (a), o-anisyl (b), SiMe 3 (c); L = PMe 3 (a, c), −OMe (b)) and titanaoxiranes Cp 2 Ti(η 2 -Ph(R)CO)(L) (R = Ph ( 14), H (15); L = PMe 3 ) have been synthesized and characterized spectroscopically; titanaaziridine 10a and titanaoxirane 14 have been characterized by X-ray crystallography. The enantiomer interconversion rate constants for the chiral titanaaziridines and titanaoxiranes have been measured by variable-temperature NMR; k inv for 10b is the fastest enantiomer interconversion rate constant reported for any metallaaziridine or metallaoxirane to date. Titanaaziridines 10 and 12 undergo exchange reactions with CC and CX bonds, whereas the titanaoxiranes 14 and 15 undergo insertions.

The kinetics of insulin-based amyloid gel formation has been studied using extinction and fluorescence detection. The process is treated as autocatalytic, and the kinetic profiles are fit using a nonconventional analysis involving a time-dependent rate constant (factor): k(t) = k(o) + k(c)(k(c)t)(n). The dependence of the kinetic parameters on initial solution conditions of concentration, pH, and ionic strength has been investigated. A mechanism is proposed in which the rate-determining step involves the activation of insulin solute species into partially unfolded, structurally modified monomers, which then aggregate. The influence of added metalloporphyrins on the rate and extent of gel formation is described. Metal derivatives of tetrakis(4-sulfonatophenyl)porphine prove effective at inhibiting the aggregation of insulin via pathways that depend on concentration and identity of the incorporated metal.

The "constrained geometry" (cg) zirconaaziridines Me 4 C 5 SiMe 2 N(tBu)Zr-(η 2 )-[N(Ph)CH(Ph)](PMe 2 R) (R ) Me, Ph) have been synthesized, and the R ) Ph derivative has been structurally characterized by X-ray crystallography. Treatment of these zirconaaziridines with unsaturated electrophiles such as diphenylacetylene results in insertion. Kinetic data for these irreversible reactions indicate that PMe 3 dissociation must occur prior to insertion and that PMe 3 dissociation from the cg zirconaaziridine 4a is faster (k 1 ) 0.204(7) s -1 ) than from the Cp 2 zirconaaziridine 5a (k 1 ′ ) 0.0013(1) s -1 ). The reaction of 4a with diphenylacetylene is several orders of magnitude faster than the reaction of 5a with diphenylacetylene.

The reaction between NO and L(2)(H(2)O)Rh(2+) (L(2) = meso-Me(6)-1,4,8,11-tetraazacyclotetradecane) generates a sky-blue L(2)(H(2)O)RhNO(2+), a {RhNO}(8) complex. The crystal structure of the perchlorate salt features a bent Rh-N-O moiety (122.1(11)(o)), short axial Rh-NO bond (1.998(12) A) and a strongly elongated Rh-OH(2) (2.366(6) A) trans to NO. Acidic aqueous solutions of L(2)(H(2)O)RhNO(2+) are stable for weeks, and are inert toward oxygen. The complex is oxidized rapidly and reversibly with Ru(bpy)(3)(3+), k(f) = (1.9 +/- 0.1) x 10(5) M(-1) s(-1), to an intermediate believed to be L(2)(H(2)O)RhNO(3+). This unprecedented {RhNO}(7) species has a lifetime of about 90 s at room temperature at pH 0. The reverse reaction between L(2)(H(2)O)RhNO(3+) and Ru(bpy)(3)(2+) has k(r) = (1.5 +/- 0.4) x 10(6) M(-1) s(-1). The kinetic data define the equilibrium constant for the L(2)(H(2)O)RhNO(2+)/Ru(bpy)(3)(3+) reaction, K = k(f)/k(r) = 0.13, and yield a reduction potential for the L(2)(H(2)O)RhNO(3+/2+) couple of 1.31 V. Both the redox thermodynamics of L(2)(H(2)O)RhNO(3+/2+) and the kinetics of the reactions with Ru(bpy)(3)(3+/2+) are quite similar to those of uncoordinated NO(+)/NO.