A Rapid Aza-Bicycle Synthesis from Dendralenes and Imines.

The diene-transmissive 2-fold Diels-Alder sequence between carbon-based dienophiles and [3]dendralenes is becoming an established method for polycarbocycle synthesis. Here, we demonstrate for the first time that imines are competent participants in intermolecular formal [4 + 2] cycloadditions with dendralenes. After a second Diels-Alder process with a carbadienophile, hexahydro- and octahydro-isoquinoline structures are formed. The formal aza-Diels-Alder reaction, which requires Lewis acid promotion, proceeds in high regio- and stereoselectivity under optimized conditions. ωB97XD/Def2-TZVP//M06-2X/6-31+G(d,p) calculations reveal a stepwise ionic mechanism for the formal aza-dienophile cycloadditions and also explain an unexpected Z → E olefin isomerization of a non-reacting C═C bond in the first formal cycloaddition.

Copper and Zinc Complexes of 2,7-Bis(6-methyl-2-pyridyl)-1,8-naphthyridine─A Redox-Active, Dinucleating Bis(bipyridine) Ligand.

The ligand 2,7-bis(6-methyl-2-pyridyl)-1,8-naphthyridine (MeL) acts as a dinucleating analogue of ubiquitous 2,2'-bipyridine ligands. Coordination of MeL to [Cu(NCMe)4]PF6 and Zn(OAc)2 led to isolation of monometallic [Zn(OAc)2(MeL)], homobimetallic [Cu2(MeL)2][PF6]2, and heterobimetallic [CuZn(µ-OAc)2(MeL)]PF6 complexes. The redox-active nature of the ligand enables access to four redox states of the complex [Cu2(MeL)2][PF6]2. DFT studies indicate that these comprise a metal-centered oxidative and ligand-centered reductive processes.

Tuning Photoenolization-Driven Cycloadditions Using Theory and Spectroscopy.

The first broad spectrum investigation into the photoenolization/Diels-Alder (PEDA) sequence was carried out using M06-2X/6-31+G(d,p) in conjunction with SMD solvation and supported by experimental UV-vis spectroscopy. A test set of 20 prodienes was chosen to examine the role of the H atom acceptor group (substituted and unsubstituted carbonyl, thiocarbonyl, and imine), the H atom donor group, and bystander ring substituents. As reaction partners for the photogenerated dienes, a diverse test set of 20 dienophiles was examined, comprising electron rich, electron poor, neutral, strain activated, hydrocarbon, and heteroatom-containing molecules including CO2 and CO. A key finding of this work is the demonstration that the PEDA sequence of carbonyl based prodienes is tolerant of most substitution patterns. Another is that thiocarbonyl derivatives should behave analogously to the carbonyls but are likely to do so much more slowly, due to an inefficient intersystem crossing, an endothermic 1,5-hydrogen atom transfer (HAT) step, and a [1,5] sigmatropic H shift to regenerate the starting material that outcompetes the [4 + 2]cycloaddition. In contrast, the T1 state of the ortho-alkyl imines displays the incorrect orbital symmetry for 1,5-HAT and is correspondingly accompanied by higher barriers, even in the excited state. However, provided these barriers can be overcome, the remaining steps in the PEDA sequence are predicted to be facile. The Diels-Alder reaction is predicted to be of much broader scope than reported synthetic literature: while electron poor dienophiles are expected to be the most reactive partners, ethylene and electron rich alkenes should react at a synthetically useful rate. CO is predicted to undergo a facile (4 + 1)cheletropic addition instead of the normal [4 + 2]cycloaddition pathway. This unique photoenolization/cheletropic addition (PECA) sequence could provide metal-free access to benzannelated cyclopentanones.