Control of Selectivity through Synergy between Catalysts, Silanes, and Reaction Conditions in Cobalt-Catalyzed Hydrosilylation of Dienes and Terminal Alkenes

Readily accessible (^{<i>i‑</i>Pr}PDI)­CoCl₂ [^{<i>i‑</i>Pr}PDI = 2,6-<i>bis</i>(2,6-diisopropylphenyliminoethyl)­pyridine] reacts with 2 equiv of NaEt₃BH at −78 °C in toluene to generate a catalyst that effects highly selective anti-Markovnikov hydrosilylation of the terminal double bond in 1,3- and 1,4-dienes. Primary and secondary silanes such as PhSiH₃, Ph₂SiH₂, and PhSi­(Me)­H₂ react with a broad spectrum of terminal dienes without affecting the configuration of the other double bond. When dienes conjugated to an aromatic ring are involved, both Markovnikov and anti-Markovnikov products are formed. The reaction is tolerant of various functional groups such as an aryl bromide, aryl iodide, protected alcohol, and even a silyl enol ether. Reactions of 1-alkene under similar conditions cleanly lead to a mixture of Markovnikov and anti-Markovnikov hydrosilylation products, where the ratio of the products increasingly favors the latter, as the size of the 2,6-substituents in the iminoylaryl group becomes larger. The complex (^{<i>i‑</i>Pr}PDI)­CoCl₂ gives exclusively the linear silane for a wide variety of terminal alkenes. Mechanistic studies suggest a pathway that involves a key role for an in situ-generated metal hydride, (<b>L</b>)­Co­(I)-H. Exclusive reduction of the terminal double bond (vis-à-vis hydrosilylation) when (EtO)₂Si­(Me)H is used in the place of PhSiH₃ is explained on the basis of an alternate silane-mediated decomposition path for the linear Co­(I)-alkyl intermediate

Catalytic Enantio­selective Hetero-dimerization of Acrylates and 1,3-Dienes

1,3-Dienes are ubiquitous and easily synthesized starting materials for organic synthesis, and alkyl acrylates are among the most abundant and cheapest feedstock carbon sources. A practical, highly enantio­selective union of these two readily available precursors giving valuable, enantio-pure skipped 1,4-diene esters (with two configurationally defined double bonds) is reported. The process uses commercially available cobalt salts and chiral ligands. As illustrated by the use of 20 different substrates, including 17 prochiral 1,3-dienes and 3 acrylates, this hetero-dimerization reaction is tolerant of a number of common organic functional groups (e.g., aromatic substituents, halides, isolated mono- and di-substituted double bonds, esters, silyl ethers, and silyl enol ethers). The novel results including ligand, counterion, and solvent effects uncovered during the course of these investigations show a unique role of a possible cationic Co­(I) intermediate in these reactions. The rational evolution of a mechanism-based strategy that led to the eventual successful outcome and the attendant support studies may have further implications for the expanding use of low-valent group 9 metal complexes in organic synthesis

Coupling of Propylene Oxide and Lactide at a Porphyrin Chromium(III) Center

5,10,15,20-Tetraphenylporphyrin chromium chloride (TPPCrCl) with added [Ph₃PNPPh₃]⁺Cl^– (PPN⁺Cl^–) selectively polymerizes lactide (l and <i>rac</i>) dissolved in neat propylene oxide (PO) to yield polylactide (PLA) terminated by the −OCHMeCH₂Cl group. At 0 °C and below, <i>rac</i>-LA yields polymers highly enriched in isotactic tetrads (<i>iii</i>). At 25 °C, some stereoselectivity is lost as transesterification becomes significant, and at 60 °C and above, enchainment of PO leads to the formation of 3,6-dimethyl-1,4-dioxan-2-one by a backbiting mechanism. At 0 °C, after the enchainment of l-(<i>S</i>,<i>S</i>)-LA in neat (<i>R</i>)-(+)-PO, the formation of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one occurs, while at higher temperatures the ratio of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one to (3<i>R</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one falls to 3:2

Coupling of Propylene Oxide and Lactide at a Porphyrin Chromium(III) Center

5,10,15,20-Tetraphenylporphyrin chromium chloride (TPPCrCl) with added [Ph₃PNPPh₃]⁺Cl^– (PPN⁺Cl^–) selectively polymerizes lactide (l and <i>rac</i>) dissolved in neat propylene oxide (PO) to yield polylactide (PLA) terminated by the −OCHMeCH₂Cl group. At 0 °C and below, <i>rac</i>-LA yields polymers highly enriched in isotactic tetrads (<i>iii</i>). At 25 °C, some stereoselectivity is lost as transesterification becomes significant, and at 60 °C and above, enchainment of PO leads to the formation of 3,6-dimethyl-1,4-dioxan-2-one by a backbiting mechanism. At 0 °C, after the enchainment of l-(<i>S</i>,<i>S</i>)-LA in neat (<i>R</i>)-(+)-PO, the formation of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one occurs, while at higher temperatures the ratio of (3<i>S</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one to (3<i>R</i>,6<i>R</i>)-3,6-dimethyl-1,4-dioxan-2-one falls to 3:2