Mechanistic study of beta-substituent effects on the mechanism of ketone reduction by SmI(2)

Recent Advances in Dearomative Partial Reduction of Benzenoid Arenes

Dearomative partial reduction is an extraordinary approach for transforming benzenoid arenes and has been well-known for many decades, as exemplified by the dehydrogenation of Birch reduction and the hydroarylation of Crich addition. Despite its remarkable importance in synthesis, this field has experienced slow progress over the last half-century. However, a revival has been observed with the recent introduction of electrochemical and photochemical methods. In this Minireview, we summarize the recent advancements in dearomative partial reduction of benzenoid arenes, including dihydrogenation, hydroalkylation, arylation, alkenylation, amination, borylation and others. Further, the intriguing utilization of dearomative partial reduction in the synthesis of natural products is also emphasized. It is anticipated that this Minireview will stimulate further progress in arene dearomative transformations.

Strategies and Lessons Learned from Total Synthesis of Taxol

Taxol (paclitaxel), the most well-known taxane diterpenoid, is the best-selling natural-source anticancer drug ever produced and one of the most common prescriptions in the treatment of breast, lung, and ovarian cancers, saving countless lives around the world. Structurally, Taxol possesses a highly oxygenated [6-8-6-4] core bearing 11 stereocenters, seven of which are contiguous chiral centers. Moreover, the extremely strained bicyclo[5.3.1] undecane ring system with a bridgehead double bond is a unique structural feature. All these features make Taxol a highly challenging synthetic target. Tremendous synthetic efforts from more than 60 research groups around the world have already culminated in ten total syntheses and three formal syntheses, as well as more than 60 synthetic model studies of Taxol. This review is intended to provide a long-overdue appraisal of the great achievements in the total syntheses of Taxol reported in the last few decades. In doing so, we summarize the development of synthesis toward Taxol from 1994 to 2022, including the evolution of synthetic strategy for accessing this complex molecular scaffold and key lessons learned from such endeavors. Finally, we briefly discuss the future of the research in this area.

Ammonia Solvation vs Aqueous Solvation of Samarium Diiodide. A Theoretical and Experimental Approach to Understanding Bond Activation Upon Coordination to Sm(II)

Coordination-induced desolvation or ligand displacement by cosolvents and additives is a key feature responsible for the reactivity of Sm(II)-based reagent systems. High-affinity proton donor cosolvents such as water and glycols also demonstrate coordination-induced bond weakening of the O-H bond, facilitating reduction of a broad range of substrates. In the present work, the coordination of ammonia to SmI₂ was examined using Born-Oppenheimer molecular dynamics simulations and mechanistic studies, and the SmI₂-ammonia system is compared to the SmI₂-water system. The coordination number and reactivity of the SmI₂-ammonia solvent system were found to be similar to those of SmI₂-water but exhibited an order of magnitude greater rate of arene reduction by SmI₂-ammonia than by SmI₂-water at the same concentrations of cosolvent. In addition, upon coordination of ammonia to SmI₂, the Sm(II)-ammonia solvate demonstrates one of the largest degrees of N-H bond weakening reported in the literature compared to known low-valent transition metal ammonia complexes.

Mechanism of SmI2 Reduction of 5-Bromo-6-oxo-6-phenylhexyl Methanesulfonate Studied by Spin Trapping with 2-Methyl-2-nitrosopropane

The radical formed by reduction of 5-bromo-6-oxo-6-phenylhexyl methanesulfonate, an α-bromoketone, with SmI₂ was spin trapped with 2-methyl-2-nitrosopropane. Electron paramagnetic resonance spectra of the spin adduct and the adduct formed in the analogous reaction with selectively deuterated substrate identify the radical intermediate in this SmI₂ reduction as a carbon-centered radical. This result supports the proposal that the formation of reactive Sm-enolates arises from reduction of the carbon-bromine bond rather than a ketyl radical anion.

Total synthesis and structural revision of vannusals A and B: synthesis of the originally assigned structure of vannusal B

The total synthesis of the originally assigned structure of vannusal B (2) and its diastereomer (d-2) are described. Initial forays into these structures with model systems revealed the viability of a metathesis-based approach and a SmI(2)-mediated strategy for the key cyclization to forge the central region of the molecule, ring C. The former approach was abandoned in favor of the latter when more functionalized substrates failed to enter the cyclization process. The successful, devised convergent strategy based on the SmI(2)-mediated ring closure utilized vinyl iodide (-)-26 and aldehyde fragment (+/-)-86 as key building blocks, whose lithium-mediated coupling led to isomeric coupling products (+)-87 and (-)-88 (as shown in Scheme 17 in the article). Intermediate (-)-88 was converted, via (-)-89 and (-)-90/(+)-91, to vannusal B structure 2 (as shown in Scheme 18 in the article), whose spectroscopic data did not match those reported for the natural product. Similarly, intermediate (+)-25, obtained through coupling of vinyl iodide (-)-26 and aldehyde (+/-)-27 (as shown in Scheme 13 in the article) was transformed via intermediates (-)-97 and (+)-98 (as shown in Scheme 19 in the article) to diastereomeric vannusal B structure (+)-d-2 (as shown in Scheme 19 in the article) which was also proven spectroscopically to be non-identical to the naturally occurring substance. These investigations led to the discovery and development of a number of new synthetic technologies that set the stage for the solution of the vannusal structural conundrum.

Mechanistic Evidence for Intermolecular Radical Carbonyl Additions Promoted by Samarium Diiodide

In this work, mechanistic studies were performed to understand the SmI2/H2O-mediated coupling of N-acyl oxazolidinones with acrylates and acrylamides, providing gamma-keto esters and amides, respectively. Our results provide experimental evidence that C-C bond formation via intermolecular radical addition reactions to carbonyl substrates can be promoted by samarium diiodide. Coupling reactions with N-cyclopropylcarbonyl-2-oxazolidinone suggest the alpha,beta-unsaturated esters/amides are reduced by the low-valent lanthanide reagent and not the N-acyl oxazolidinones, as originially proposed (J. Am. Chem. Soc. 2005, 127, 6544). Rate measurements support the preferred reduction of an acrylate or acrylamide by SmI2/H2O in the presence of an N-acyl oxazolidinone. In the absence of the N-acyl oxazolidinone, SmI2/H2O promotes dimerization of the acrylates, whereas the C=C bond of the acrylamides is reduced. In addition, coupling of the Pfp ester of Cbz-protected phenylalanine with an acrylamide leads only to reduction of the acrylamide and recovered ester, whereas the same coupling with the N-acyl oxazolidinone derivative provides the gamma-keto amides. These results imply that a pathway involving nucleophilic acyl substitution cannot take place and that a radical mechanism must be invoked to explain the C-C bond formation. We propose that the acrylate/acrylamide is reduced to a conjugated ketyl radical that adds to the exocyclic carbonyl group of the N-acyl oxazolidinone, activated through bidentate coordination to a lanthanide ion.

Photoinduced Electron Transfer Reactions by SmI2 in THF: Luminescence Quenching Studies and Mechanistic Investigations

Photoluminescence quenching studies of SmI2 in dry THF were carried out in the presence of five different classes of compounds: ketone, alkyl chloride, nitrile, alkene and imine. The free energy change (DeltaG0) of the photoinduced electron transfer (PET) reactions was calculated from the redox potentials of the donor (SmI2) and acceptors. The bimolecular quenching constants (k(q)) derived from the Stern-Volmer experiments parallel the free energy changes of the PET processes. The observed quenching constants were compared with the theoretically derived electron transfer rate constants (k(et)) from Marcus theory and found to be in good agreement when a value of lambda = 167 kJ mol(-1) (40 kcal mol(-1)) was used for the reorganization energy of the system. A careful comparison of the excited state dynamics of SmII in the solid state to the results obtained in solution (THF) provides new insight in to the excited states of SmII in THF. The activation parameters determined for the PET reactions in SmI2/1-chlorobutane system are consistent with a less ordered transition state and high degree of bond reorganization in the activated complex compared to similar ground state reactions. Irradiation studies clearly show that SmI2 acts as a better reductant in the excited state and provides an alternative pathway for rate enhancement in known and novel functional group reductions.

Reduction of β-Hydroxyketones by SmI2/H2O/Et3N

Reduction of a series of beta-hydroxyketones by SmI2/H2O/Et3N provided 1,3-diols in quantitative yields. The reactions were exceedingly clean with no byproduct formation, negating the need for further purification. Most reactions provided moderate to excellent diastereoselectivity with syn-diols as the major isomer in most instances.

Investigation of the [Sm(N(SiMe(3))(2))(2)] reducing system in THF. Rate and mechanistic studies

The reductant [Sm(N(SiMe(3))(2))(2)] was examined by cyclic voltammetry and UV-vis spectroscopy. Rate constants and activation parameters for the reduction of 1-iodobutane, 2-butanone, and methylacetoacetate by [Sm(N(SiMe(3))(2))(2)] were measured in THF by stopped-flow absorption decay experiments. Comparison with SmI(2) and SmI(2)-HMPA shows that the redox potential of [Sm(N(SiMe(3))(2))(2)] is intermediate between the SmI(2)-based reductants, yet it reduces alkyl iodides and ketones at a faster rate than the powerful combination of SmI(2) and HMPA. The activation data for reduction of alkyl iodides and ketones by [Sm(N(SiMe(3))(2))(2)] are consistent with highly ordered transition states having low activation barriers. All of these results taken together suggest that the mechanism of reduction of alkyl iodides and ketones by [Sm(N(SiMe(3))(2))(2)] has more inner-sphere character than reduction by SmI(2) or Sm-(HMPA) complexes. The change in the ET mechanism is attributed to the unique structure of the [Sm(N(SiMe(3))(2))(2)] complex.