Terpenes constitute the largest class of natural products and serve as an important source for medicinal treatments. Despite constant progress in chemical synthesis, the construction of complex polycyclic sesqui- and diterpene scaffolds remains challenging. Natural cyclase enzymes, however, are able to form the whole variety of terpene structures from just a handful of linear precursors. Man-made catalysts able to mimic such natural enzymes are lacking. Here, we describe the examples of sesquiterpene cyclisations inside an enzyme-mimicking supramolecular catalyst. This strategy allowed the formation of the tricyclic sesquiterpene isolongifolene in only four steps. The mechanism of the catalysed cyclisation reaction was elucidated using C-labelling studies and DFT calculations.

No abstract

Structural, energetic, and magnetic criteria confirm that the silole dianion (CH)4Si2- (7) and its alkali-metal ion pairs, e.g. (CH)4SiLi- (7a), (CH)4SiLi2 (7b), (CH)4SiNa2 (13), and (CH)4SiK2 (14), are highly aromatic. Inverse sandwich structures and strongly delocalized silole rings are prefered by 7b, 13, and 14. The degree of aromaticity in [η5-(CH)4Si]Li- (7a) exceeds that of the isoelectronic third-period heterocycles (CH)4PLi (5a) and (CH)4SLi+ (6a) and even approaches that of (CH)5Li (3a).

No abstract

The short Li-C distances (Li 1 -C 2 ) 2.615(3) Å, Li 1 -C 3 ) 2.644(3) Å) in the X-ray crystal structure of [Li-O-C(Me)-(c-CHCH 2 CH 2 ) 2 ] 6 (7) 6 characterize Li-cyclopropane edge coordinations. The Li-cyclopropane interactions increase the C 2 -C 3 distances (1.519(3) Å) relative to those of the free cyclopropyl edges (C 2 -C 4 ) C 6 -C 7 ) 1.499(2) Å) by 0.02 Å. The bent bonds of cyclopropane give rise to an electrostatic potential pattern, which strongly favors edge coordination as is observed experimentally in (7) 6 , but also permits a metastable Li + face complex. The cyclopropane edge also is the favored site for hydrogen bonding, but not for protonation. The C-C bond length elongations, the coordination energies E coord , and the charge redistributions upon metal cation edge interactions all are related to the distances between the cyclopropane C-C bond centers and the cations. This is evaluated for the alkali metal cation-cyclopropane complexes (cation ) Li + to Cs + ). More generally, the cyclopropane C-C bond length variations can be employed as a structural measure for the magnitudes of electrostatic interactions.

The X-ray crystal structure of [Li−C⋮C−SiMe2−C6H4−OMe]6 (14)6 features nearly symmetric π-interactions between the lithium ions and the acetylide anions (Li1−Cβ = 2.353(9) Å, Li1−Cα = 2.292(9) Å). These π-contacts are facilitated by the chelating o-anisyl methoxy groups (Li1−Cα−Cβ = 77.6(4)°, Li1−O1 = 2.169(9) Å). The Li−Cα distances in the (LiCα)6 core of (14)6 differ significantly (Li1α−Cα = 2.132(9) Å, Li1b−Cα = 2.205(11) Å). This Li−Cα distance differentiation is unique in organolithium hexamers, and is due to Li(C⋮C−R) “side-on-π” and “end-on-σ” contacts, as is shown computationally in H−C⋮C−Li(LiH)2 (20). A second X-ray crystal structure, [Li−O−CMe2−C⋮C−H]6 (22)6, reveals electrostatic π-interactions between the lithiums in the (LiO)6 core and the nonmetalated acetylene groups (Li1−C2 = 2.443(5) Å, Li1−C3 = 2.749(6) Å). These Li−C π-contacts shorten upon acetylene lithiation, as is shown computationally in Li−O−CH2−C⋮C−(H/Li) (24-H/Li). Additional computations reveal that the π-interactions in (HC⋮C)M2H (26-Li-Cs) complexes (modelling oligo- and polymeric M−C⋮C−R) are weak (only 0.7 kcal/mol for Li), but substantial in M+(H−C⋮C−H) (27-Li-Cs) species (20.2 kcal/mol for Li+). In 26-Li-Cs, the π-contacts increase the C⋮C bond lengths slightly (0.005 Å for Li) and lower the C⋮C stretching frequencies (33 cm-1 for Li), they polarize charge density from Cα toward Cβ and hence result in counterion-induced charge delocalizations. The degrees of π-interactions both in (26-Li-Cs) and in (27-Li-Cs) decrease with increasing size of the alkali cations.

The ligand (1R,2R,4S)-2-endo-hydroxy-2-exo-(o-methoxybenzene)-1,3,3-trimethylbicyclo[2.2.1]heptane (1) serves as a “chiral n-butyllithium trap” and precipitates n-butyllithium in complex 2. With dimethylzinc, ligand 1 forms a dimeric zinc chelate complex (3). The X-ray crystal structures of 2 and 3 are discussed.

Modular cyclodiphosph(V)azanes are synthesised and their affinity for chloride and actetate anions were compared to those of a bisaryl urea derivative (1). The diamidocyclodiphosph(V)azanes cis-[{ArNHP(O)(μ-tBu)}2 ] [Ar=Ph (2) and Ar=m-(CF3 )2 Ph (3)] were synthesised by reaction of [{ClP(μ-NtBu)}2 ] (4) with the respective anilines and subsequent oxidation with H2 O2 . Phosphazanes 2 and 3 were obtained as the cis isomers and were characterised by multinuclear NMR spectroscopy, FTIR spectroscopy, HRMS and single-crystal X-ray diffraction. The cyclodiphosphazanes 2 and 3 readily co-crystallise with donor solvents such as MeOH, EtOH and DMSO through bidentate hydrogen bonding, as shown in the X-ray analyses. Cyclodiphosphazane 3 showed a remarkably high affinity (log[K]=5.42) for chloride compared with the bisaryl urea derivative 1 (log[K]=4.25). The affinities for acetate (AcO(-) ) are in the same range (3: log[K]=6.72, 1: log[K]=6.91). Cyclodiphosphazane 2, which does not contain CF3 groups, exhibits weaker binding to chloride (log[K]=3.95) and acetate (log[K]=4.49). DFT computations and X-ray analyses indicate that a squaramide-like hydrogen-bond directionality and Cα H interactions account for the efficiency of 3 as an anion receptor. The Cα H groups stabilise the Z,Z-3 conformation, which is necessary for bidentate hydrogen bonding, as well as coordinating with the anion.