A Study of Polarization and Directing Effects of Unsymmetrical Alkynes Using Regioselective Pd-Catalyzed Bromoallylation: A Study of Polarization and Directing Effects of Unsymmetrical Alkynes Using Regioselective Pd-Catalyzed Bromoallylation

Origins of Stereospecificity and Divergent Reactivity of Pd-Catalyzed Cross Coupling with α,α-Disubstituted Alkenyl Hydrazones

This article presents an exploration of stereospecificity and divergent reactivity of Pd-catalyzed α,α-disubstituted alkenyl hydrazones to synthesize 1,4-dienes in the Z configuration and vinylcyclopropane. We calculated the energy profiles of four α,α-disubstituted alkenyl hydrazones. The results show that the energy profiles of the whole catalytic cycle are basically the same before the syn-carbopalladation step. Subsequent syn-β-C elimination yields skipping dienes, or direct β-H elimination yields vinylcyclopropane. Current theoretical calculations reveal that the stereospecificity and the divergent reactivity of reactions result from the competition between syn-β-C elimination and β-H elimination. The C-C bond rotation and subsequent syn-β-C elimination step control the stereospecificity of the reaction by changing the olefin stereostructure from E to Z configuration. The steric factor of α-substituted groups mediates the transformation between syn-β-C elimination and β-H elimination. The results are of great significance for the scientific design of substrates to achieve accurate synthesis of target products.

P-Alkynyl functionalized benzazaphospholes as transmetalating agents

Exposure of 10π-electron benzazaphosphole 1 to HCl, followed by nucleophilic substitution with the Grignard reagent BrMgCCPh afforded alkynyl functionalized 3 featuring an exocyclic -C[triple bond, length as m-dash]C-Ph group with an elongated P-C bond (1.7932(19) Å). Stoichiometric experiments revealed that treatment of trans-Pd(PEt3)2(Ar)(i) (Ar = p-Me (C) or p-F (D)) with 3 generated trans-Pd(PEt3)2(Ar)(CCPh) (Ar = p-Me (E) or p-F (F)), 5, which is the result of ligand exchange between P-I byproduct 4 and C/D, and the reductively eliminated product (Ar-C[triple bond, length as m-dash]C-Ph). Cyclic voltammetry studies showed and independent investigations confirmed 4 is also susceptible to redox processes including bimetallic oxidative addition to Pd(0) to give Pd(i) dimer 6-Pd2-(P(t-Bu)3)2 and reduction to diphosphine 7. During catalysis, we hypothesized that this unwanted reactivity could be circumvented by employing a source of fluoride as an additive. This was demonstrated by conducting a Sonogashira-type reaction between 1-iodotoluene and 3 in the presence of 10 mol% Na2PdCl4, 20 mol% P(t-Bu)Cy2, and 5 equiv. of tetramethylammonium fluoride (TMAF), resulting in turnover and the isolation of Ph-C[triple bond, length as m-dash]C-(o-Tol) as the major product.

Oxidative Cyclization of o-(1-Hydroxy-2-alkynyl)-N-tosylanilides for the Synthesis of 4-Quinolones

The treatment of easily accessible o-(1-hydroxy-2-alkynyl)-N-tosylanilides 1 with excess manganese(IV) oxide in the presence of substoichiometric tetrabutylammonium iodide (TBAI) in chloroform (or in the absence of TBAI in dimethylformamide, DMF) promoted a sequential oxidation/intramolecular hydroamination to give 4-quinolones 3 and/or (Z)-2-alkylidene-3-oxindoles (Z)-4 in good yields. Possibly, MnO₂ played dual roles as an oxidant and as a Lewis acidic activator of intermediary ynones 2. The product distributions between 3 and (Z)-4 could be controlled by the choice of solvents.