The bicyclic guanidine 1,5,7- triazabicyclo[4.4.0]dec-5-ene (TBD) is an effective organocatalyst for the formation of amides from esters and primary amines. Mechanistic and kinetic investigations support a nucleophilic mechanism where TBD reacts reversibly with esters to generate an acyl-TBD intermediate that acylates amines to generate the amides. Comparative investigations of the analogous bicyclic guanidine 1,4,6-triazabicyclo[3.3.0]oct-4-ene (TBO) reveal it to be a much less active acylation catalyst than TBD. Theoretical and mechanistic studies imply that the higher reactivity of TBD is a consequence of both its higher basicity and nucleophilicity than TBO as well as the high reactivity of the acyl-TBD intermediate, which is sterically prevented from adopting a planar amide structure.

Group transfer polymerization (GTP) is an effective method for the synthesis of poly(methyl methacrylate) (pMMA) with controlled molecular weights and narrow polydispersities. Silyl ketene acetals initiate the polymerization of methyl methacrylate in the presence of either nucleophilic or Lewis acid catalysts. We report the use of N-heterocyclic carbenes (NHCs) as neutral nucleophilic catalysts for GTP of methyl methacrylate (MMA) and tert-butyl acrylate (TBA). For MMA, polymer molecular weights increase linearly with conversion and are predictable over a range of [M] 0 /[I] 0 ratios. Polydispersities trend downward with conversion and approach 1.2 at >95% conversion. Significantly, NHCs were also shown to be effective for the controlled GTP of TBA, generating living polymers with predictable molecular weights, narrow molecular weight distributions (M w /M n e 1.2), and living chain ends, as illustrated by chain-extension experiments.

Voltammetric activities are compared for the designated compounds in 0.50 M H 2 SO 4 at the specified film electrodes configured as rotated disks. The effective number of electrons ͑n eff , equiv mol Ϫ1 ͒ calculated from the Koutecky-Levich plot is 3.1 Ϯ 0.2 for toluene oxidation at the Fe-PbO 2 electrode, compared to 2.0 Ϯ 0.05 and 4.1 Ϯ 0.3 at the Bi-PbO 2 and PbO 2 electrodes, respectively. Gas chromatography-mass spectrometry data confirm that the primary product of toluene oxidation is benzyl alcohol ͑2 equiv mol Ϫ1 ͒ at the Fe-PbO 2 electrode with production of small amounts of benzaldehyde ͑4 equiv mol Ϫ1 ͒ and benzoic acid ͑6 equiv mol Ϫ1 ͒. X-ray photoelectron spectroscopy data indicate the ratio of Fe:Pb is ca. 1:100 ͑atom:atom͒ in the Fe-PbO 2 films. By comparison, a Bi:Pb ratio of 33:100 ͑atom:atom͒ is easily attained in Bi-PbO 2 films. Nevertheless, the apparent heterogeneous rate constant (k app , cm s Ϫ1 ) for toluene oxidation is larger at the Fe-PbO 2 electrode (6.5 Ϯ 0.06 ϫ 10 Ϫ3 ) compared to the Bi-PbO 2 electrode (2.0 Ϯ 0.05 ϫ 10 Ϫ3 ). X-ray absorption near-edge structure data confirm speculation that iron exists in the 3ϩ oxidation state with octahedral coordination by O atoms in Fe-PbO 2 films. The large activity of the Fe-PbO 2 electrode is attributed to the benefit of adsorption of aromatic molecules at Fe͑III͒ sites.

An efficient three-step, chemoenzymatic synthesis of unprotected doxorubicin-14-O-esters from doxorubicin hydrochloride salt is described. The key step is a lipase-catalyzed regioselective transesterification/esterification using commercially available acyl donors and doxorubicin reversibly derivatized with N-alloc to improve substrate loadings. The overall yield is ca. 60% and chromatographic purification is not required, thereby making the process more amenable to scale-up.