Alexander Wittkopp

The First Enantiomerically Pure Triangulane (M)‐Trispiro[2.0.0.2.1.1]nonane Is a σ‐[4]Helicene

A remarkably high specific rotation, even at 589 nm, is shown by (M)-1, the first enantiomerically pure unbranched [4]triangulane, although it has no chromophore that would lead to any significant absorption above 200 nm. This outstanding rotatory power is in line with a helical arrangement of its σ bonds, as confirmed by high-level computations. Thus, it is appropriate to call (M)-1 a “σ-[4]helicene”, the first σ-bond analogue of the aromatic [n]helicenes.

A Valence Bond Study of the Bergman Cyclization: Geometric Features, Resonance Energy, and Nucleus-Independent Chemical Shift (NICS) Values

The Bergman cyclization of (Z)-hex-3-ene-1,5-diynes (1, enediynes), which produces pharmacologically important DNA-cleaving biradicals (1,4-benzyne, 2), was studied by using Hartree-Fock (HF) and density-functional theory (DFT) based valence bond (VB) methods (VB-HF and VB-DFT, respectively). We found that only three VB configurations are needed to arrive at results not too far from complete active space [CASSCF(6 x 6)] computations, while the quality of VB-DTF utilizing the same three configurations improves upon CASSCF(6 x 6) analogous to CASPT2. The dominant VB configuration in 1 contributes little to 2, while the most important biradical configuration in 2 plays a negligible role in 1. The avoided crossing of the energy curves of these two configurations along the reaction coordinate leads to the transition state (TS). As a consequence of the shape and position of the crossing section, the changes in geometry and in the electronic wavefunction along the reaction coordinate are non-synchronous; the TS is geometrically approximately 80% product-like and electronically approximately 70% reactant-like. While the pi resonance in the TS is very small, it is large (64.4 kcal mol(-1)) for 2 (cf. benzene=61.5 kcal mol(-1)). As a consequence, substituents operating on the sigma electrons should be much more effective in changing the Bergman reaction cyclization barrier. Furthermore, additional sigma resonance in 2 results in unusually high values for the nucleus-independent chemical shift (NICS, a direct measure for aromaticity). Similarly, the high NICS value of the TS is due mostly to sigma resonance to which the NICS procedure is relatively sensitive.

Substituent effects on the Bergman cyclization of (Z)-1,5-hexadiyne-3-enes: a systematic computational study

The effects of several substituents (BH2, BF2, AlH2, CH3, C6H5, CN, COCH3, CF3, SiH3, NH2, NH3+, NO2, PH2, OH, OH2+, SH, F, Cl, Br) on the Bergman cyclization of (Z)‐1,5‐hexadiyne‐3‐ene (enediyne, 3) were investigated at the Becke–Lee–Yang–Parr (BLYP) density functional (DFT) level employing a 6‐31G* basis set. Some of the substituents (NH3+, NO2, OH, OH2+, F, Cl, Br) are able to lower the barrier (up to a minimum of 16.9 kcal mol−1 for difluoro‐enediyne 7rr) and the reaction enthalpy (the cyclization is predicted to be exergonic for OH2+ and F) compared to the parent system giving rise to substituted 1,4‐dehydrobenzenes at physiological temperatures.

Selective Radical Reactions in Multiphase Systems: Phase-Transfer Halogenations of Alkanes

The present paper shows that selective radical reactions can be initiated and carried out in multiphase systems. This concept is applied to the selective functionalization of unactivated aliphatic hydrocarbons, which may be linear, branched, and (poly)cyclic, strained as well as unstrained. The phase-transfer system avoids overfunctionalization of the products and simplifies the workup; the selectivities are excellent and the yields are good. This is the only method for direct preparative iodination of alkanes applicable to large scale as well. We demonstrate that the reaction systems are indeed phase-transfer catalyzed through a systematic study of variations of the reactants, solvents, catalysts, and by measuring as well as computing the H/D kinetic isotope effects for the rate-limiting C-H abstraction step by *CHal3 radicals which are held responsible for the observed radical reactions. In the case of *CBr3, this key intermediate could also be trapped under otherwise very similar reaction conditions. To stimulate further work, the tolerance of some functional groups was tested as well.

Das erste enantiomerenreine Triangulan: (M)-Trispiro[2.0.0.2.1.1]nonan ist ein σ-[4]Helicen

Eine auserordentlich hohe spezifische optische Drehung selbst bei 589 nm kennzeichnet (M)-1, das erste enantiomerenreine, unverzweigte [4]Triangulan, obwohl es keinen Chromophor enthalt, der zu einer nennenswerten Absorption oberhalb von 200 nm fuhren wurde. Diese hohe spezifische Drehung hat mit der helicalen Anordnung der σ-Bindungen zu tun, was durch Rechnungen auf hohem Niveau bestatigt wird. Damit erscheint es gerechtfertigt, das [4]Triangulan in Analogie zu den aromatischen π-[n]Helicenen als σ-[4]Helicen zu bezeichnen.

The Rearrangement of the Cubane Radical Cation in Solution

The rearrangement of the cubane radical cation (1*+) was examined both experimentally (anodic as well as (photo)chemical oxidation of cubane 1 in acetonitrile) and computationally at coupled cluster, DFT, and MP2 [BCCD(T)/cc-pVDZ//B3LYP/6-31G* ZPVE as well as BCCD(T)/cc-pVDZ//MP2/6-31G* + ZPVE] levels of theory. The interconversion of the twelve C2v degenerate structures of 1*+ is associated with a sizable activation energy of 1.6 kcalmol(-1). The barriers for the isomerization of 1*- to the cuneane radical cation (2*+) and for the C-C bond fragmentation to the secocubane-4,7-diyl radical cation (10*+) are virtually identical (deltaH0++ = 7.8 and 7.9 kcalmol(-1), respectively). The low-barrier rearrangement of 10*+ to the more stable syn-tricyclooctadiene radical cation 3*+ favors the fragmentation pathway that terminates with the cyclooctatetraene radical cation 6*+. Experimental single-electron transfer (SET) oxidation of cubane in acetonitrile with photoexcited 1,2,4,5-tetracyanobenzene, in combination with back electron transfer to the transient radical cation, also shows that 1*+ preferentially follows a multistep rearrangement to 6*+ through 10*+ and 3*+ rather than through 2*+. This was confirmed by the oxidation of syn-tricyclooctadiene (3), which, like 1, also forms 6 in the SET oxidation/rearrangement/electron-recapture process. In contrast, cuneane (2) is oxidized exclusively to semibullvalene (9) under analogous conditions. The rearrangement of 1*+ to 6*+ via 3*+, which was recently observed spectroscopically upon ionization in a hydrocarbon glass matrix, is also favored in solution.

Is SH4, the simplest 10-S-4 sulfurane, observable?

The kinetic stability of SH4 was investigated theoretically with the coupled cluster ansatz. The two possible modes of decomposition into SH2 and H2 through either a C2v or a C1 transition structure (TS) were investigated using intrinsic reaction coordinate (IRC) computations; orbital interactions along the reaction paths were analyzed. The two dissociation modes are due to differences in the electron delocalization in the TSs. While the C2v TS is bonded rather covalently by a three center–four electron (3c–4e) interaction which is lost in a strictly synchronous way (two electrons occupy the same orbital at a time along the reaction coordinate), the bonding orbital in the C1 TS is merely occupied by a single electron. Surprisingly, this highly polarized TS has a lower barrier. Computations at the CCSD(T)/cc-pVQZ level of theory show that the zero-point corrected enthalpy (ΔH0‡) of the C1 TS is 16 kcal mol−1 above the C4v symmetric ground state; the barrier along the C2v path is 40 kcal mol−1. The overall exothermicity for the dissociation into SH2 and H2 was estimated to be ΔH0=−76 kcal mol−1. The fundamental IR absorptions of SH4 (obtained by scaling the computed harmonic vibrational frequencies taken from the CCSD(T)/cc-pVQZ level of theory) are 1432 and 2037 cm−1.