Binding of molecular magnesium hydrides to a zirconocene-enyne template

An enyne-zirconium complex stabilizes molecular magnesium hydride ('MG''H IND. 2') and even a molecular hydride, n'C IND. 4''H IND. 9''MG'H. These systems feature magnesium olefin "pi" complexation.Deutsche ForschungsgemeinschaftNRW Forschungsschule “Molecules and Materials

Trisubstituted Boroles by 1,1-Carboboration

A 1,3,4-trisubstituted borole is readily obtained by a 1,1-carboboration sequence starting from a bis­(ethynyl)­borane and B­(C₆F₅)₃. Subsequent photolysis converts it to the 1,2,5-trisubstituted borole by means of a di-π-borane rearrangement

Comparative phenology of epiphytic and tree-phase strangler figs in a Venezuelan palm savanna

<i>N</i>-Propargyltetramethylpiperidine reacts with a series of <i>trans</i>-alkenyl-B­(C₆F₅)₂ compounds to give the substituted alkenyl-bridged frustrated N/B Lewis pairs <b>5</b>. Their structures and spectroscopic features indicate a pronounced participation of the mesomeric <i>s-trans</i>-iminium/borata-alkene resonance form. The compounds are thought to be formed in a stepwise addition/rearrangement process which is initiated by a <i>trans</i>-1,2-amine/borane FLP addition to the carbon–carbon triple bond to generate a reactive zwitterionic aziridinium/alkenylborate intermediate. Subsequent alkenylborate attack leads to opening of the activated three-membered heterocycle with clean formation of the products <b>5a</b>–<b>c</b>. Treatment of the propargyl-TMP substrate with B­(C₆F₅)₃ gave a stable example of such an aziridinium/borate betaine, which was isolated and amply characterized. The products <b>5a</b>–<b>c</b> are active N/B FLPs. They split dihydrogen heterolytically under mild conditions to give the respective NH⁺/BH^– products <b>9a</b>–<b>c</b>. These contain <i>Z</i>-configurated core CC double bonds, which indicates rotational equilibration around the central C–C bond of <b>5a</b>–<b>c</b> during this reaction. Structural and chemical features of the <b>5c</b> system were analyzed by DFT calculations

Remarkable Behavior of a Bifunctional Alkynylborane Zirconocene Complex toward Donor Ligands and Acetylenes

Treatment of a [(trimethylsilylethynyl)­alkenyl]­ZrCp₂ complex with Piers’ borane [HB­(C₆F₅)₂] resulted in the clean formation of the Zr/B complex [η²-(C₆F₅)₂B-CC-SiMe₃]­ZrCp₂ (<b>3</b>). This compound shows some unique reaction patterns. With a variety of typical donor ligands (carbon monoxide, an isonitrile, a nitrile, THF-<i>d</i>₈,) it forms the respective adducts <b>5</b> (four examples characterized by X-ray diffraction). Toward terminal alkynes (RCCH) compound <b>3</b> behaves as a Zr­(II) metal Lewis base/boron Lewis acid frustrated Lewis pair (FLP) and undergoes typical regioselective 1,2-Zr/B addition reactions to yield the metallaheterocyclic products <b>7</b> (four examples characterized by X-ray diffraction). Compound <b>3</b> also undergoes characteristic 1,4-Zr/B FLP addition to a silyl-substituted conjugated enyne (to give <b>9</b>) and also to 1,4-bis­(trimethylsilyl)­butadiyne to yield a metallacyclic butatriene derivative <b>10</b> (both <b>9</b> and <b>10</b> were characterized by X-ray diffraction). Eventually, compound <b>3</b> reacted with mes₂P–CCH to give the metallacycloallenoid product <b>15</b>, which was probably formed by means of 1,2-Zr/B FLP addition followed by alkynyl shift from boron to zirconium and reductive coupling. Treatment of <b>3</b> with tolane gave the closely related zirconacycloallenoid complex <b>16</b> by carbon carbon coupling. Both the products <b>15</b> and <b>16</b> were characterized by X-ray diffraction

Phospha-Claisen Type Reactions at Frustrated Lewis Pair Frameworks

The C₄-bridged unsaturated phosphane/borane frustrated Lewis pairs (P/B FLPs) <b>4</b> undergo borane induced phosphane addition to a variety of acetylenic esters or ketones to generate heterocyclic 10-membered intermediates that contain pairs of allyl phosphonium/allenic enolate functionalities. These subsequently undergo phospha-Claisen type rearrangement reactions to give the respective substituted phosphanyl pentadiene products. In two exceptional cases subsequent reactions leading to anomalous phospha-Claisen products were found. One example involved cyclopropane ring formation, and the other carbon−carbon bond activation. Potential mechanistic schemes leading to these products are discussed. Essential examples were characterized by X-ray diffraction

Remarkable Behavior of a Bifunctional Alkynylborane Zirconocene Complex toward Donor Ligands and Acetylenes

Treatment of a [(trimethylsilylethynyl)­alkenyl]­ZrCp₂ complex with Piers’ borane [HB­(C₆F₅)₂] resulted in the clean formation of the Zr/B complex [η²-(C₆F₅)₂B-CC-SiMe₃]­ZrCp₂ (<b>3</b>). This compound shows some unique reaction patterns. With a variety of typical donor ligands (carbon monoxide, an isonitrile, a nitrile, THF-<i>d</i>₈,) it forms the respective adducts <b>5</b> (four examples characterized by X-ray diffraction). Toward terminal alkynes (RCCH) compound <b>3</b> behaves as a Zr­(II) metal Lewis base/boron Lewis acid frustrated Lewis pair (FLP) and undergoes typical regioselective 1,2-Zr/B addition reactions to yield the metallaheterocyclic products <b>7</b> (four examples characterized by X-ray diffraction). Compound <b>3</b> also undergoes characteristic 1,4-Zr/B FLP addition to a silyl-substituted conjugated enyne (to give <b>9</b>) and also to 1,4-bis­(trimethylsilyl)­butadiyne to yield a metallacyclic butatriene derivative <b>10</b> (both <b>9</b> and <b>10</b> were characterized by X-ray diffraction). Eventually, compound <b>3</b> reacted with mes₂P–CCH to give the metallacycloallenoid product <b>15</b>, which was probably formed by means of 1,2-Zr/B FLP addition followed by alkynyl shift from boron to zirconium and reductive coupling. Treatment of <b>3</b> with tolane gave the closely related zirconacycloallenoid complex <b>16</b> by carbon carbon coupling. Both the products <b>15</b> and <b>16</b> were characterized by X-ray diffraction

Noninteracting, vicinal frustrated P/B-Lewis pair at the norbornane framework: synthesis, characterization, and reactions

Hydroboration of dimesitylnorbornenylphosphane with Piers' borane [HB(C6F5)2] gave the frustrated Lewis pair (FLP) 4 in good yield. It has the -PMes2 Lewis base attached at the 2-endo position and the -B(C6F5)2 group 3-exo oriented at the norbornane framework. The vicinal FLP 4 was shown by X-ray diffraction and by spectroscopy to be a rare example of an intramolecular noninteracting pair of a Lewis acid and Lewis base functionality. The FLP 4 rapidly splits dihydrogen heterolytically at ambient temperature to yield the phosphonium/hydrido borate zwitterion 5. It adds to the carbonyl group of benzaldehyde and to carbon dioxide to yield the adducts 6 and 7, respectively. Compounds 5-7 were characterized by X-ray diffraction. Compound 4 adds to the S=O function of sulfur dioxide to give a pair of diastereomeric heterocyclic six-membered ring products due to the newly formed sulfur chirality center, annulated with the norbornane skeleton, which were investigated by 31P/11B single and double resonance solid state NMR experiments. Compound 8 was also characterized by X-ray diffraction. The FLP 4 undergoes a clean N,N-addition to nitric oxide (NO) to give a norbornane annulated five-membered heterocyclic persistent FLP-NO aminoxyl radical 12 (characterized, e.g., by X-ray diffraction and EPR spectroscopy). Additionally, the FLP radical was characterized by 1H solid state NMR spectroscopy. The radical 12 undergoes a H-atom abstraction reaction with 1,4-cyclohexadiene to yield the respective diamagnetic FLP-NOH product 13, which was also characterized by X-ray diffraction and solid state NMR spectroscopy.European Research CouncilDFGNRW Forschungsschul