(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Additions of enantiopure alpha-sulfinyl carbanions to (S)-N-sulfinimines: asymmetric synthesis of beta-amino sulfoxides and beta-alphamino alcohols

The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.     

A dimeric constrained‐geometry titanium complex linked by double Ti–CH2–Al–CH2 heterocycles

In the structure of bis({N‐[di­methyl(1η⁵‐2,3,4,6‐tetra­methyl­in­den­yl)­silyl]­cyclo­hexyl­amido‐1κN}(methyl‐3κC)‐di‐μ₃‐methyl­ene‐1:2:3κ³C;1:3:3′κ³C‐tris(pentafluorophenyl‐2κC)titanium) benzene disolvate, [Me₂Si(η⁵‐2,3,4,6‐Me₄C₉H₂)(C₆H₁₁N)]Ti[(μ₃‐CH₂)Al(C₆F₅)₃][AlMe(μ₃‐CH₂)]₂ or [Ti₂(C₂₁H₇AlF₁₅)₂(C₂₁H₃₁NSi)₂]·2C₆D₆, the dimer is located on an inversion center, and the two Ti centers are linked by double Ti(μ₃‐CH₂)Al(C₆F₅)₃AlMe(μ₃‐CH₂) heterocycles. The electron‐deficient Ti centers are further stabilized by two α‐agostic interactions between Ti and one H atom of each bridging methyl­ene group.     

Unusual structural types in manganese cluster chemistry from the use of N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine: Mn8, Mn12, and Mn20 Clusters

The syntheses, crystal structures, and magnetochemical characterization are reported for three new mixed-valent Mn clusters [Mn(8)O(3)(OH)(OMe)(O(2)CPh)7(edte)(edteH(2))](2)CPh) (1), [Mn(12)O(4)(OH)(2)(edte)(4)C(l6)(H(2)O)(2)] (2), and [Mn(20)O(8)(OH)(4)(O(2)CMe)(6)(edte)(6)](ClO(4))(2) (3) (edteH(4) = (HOCH(2)CH(2))(2)NCH(2)CH(2)N(CH(2)CH(2)OH)(2) = N,N,N',N'-tetrakis(2-hydroxyethyl)ethylenediamine). The reaction of edteH(4) with Mn(O(2)CPh)(2), MnCl(2), or Mn(O(2)CMe)(2) gives 1, 2, and 3, respectively, which all possess unprecedented core topologies. The core of 1 comprises two edge-sharing [Mn(4)O(4)] cubanes connected to an additional Mn ion by a micro(3)-OH- ion and two alkoxide arms of edteH(22-). The core of 2 consists of a [Mn(12)(micro(4-)O)(4)](24+) unit with S4 symmetry. The core of 3 consists of six fused [Mn(4)O(4)] cubanes in a 3 x 2 arrangement and linked to three additional Mn atoms at both ends. Variable-temperature, solid-state dc and ac magnetization (M) studies were carried out on complexes 1-3 in the 5.0-300 K range. Fitting of the obtained M/Nmicro(B) vs H/T data by matrix diagonalization and including only axial zero-field splitting (ZFS) gave ground-state spin (S) and axial ZFS parameter (D) of S = 8, D = -0.30 cm-1 for 1, S = 7, D = -0.16 cm-1 for 2, and S = 8, D = -0.16 cm-1 for 3. The combined work demonstrates that four hydroxyethyl arms on an ethylenediamine backbone can generate novel Mn structural types not accessible with other alcohol-based ligands.     

Total structure determination of grassypeptolide, a new marine cyanobacterial cytotoxin

A collection of the cyanobacterium Lyngbya confervoides off Grassy Key in Florida yielded grassypeptolide (1), a 31-membered macrocyclic depsipeptide with unusually high D-amino acid content, two thiazolines, and one beta-amino acid. We report the rigorous 3D structure determination and conformational analysis in solution and solid state by NMR, MS, X-ray crystallography, chemical degradation, and molecular modeling involving distance geometry and restrained molecular dynamics. Grassypeptolide (1) inhibited cancer cell growth with IC50 values from 1.0 to 4.2 microM.     

Influence of Amide Connectivity on the Hydrogen-Bond-Directed Self-Assembly of [n.n]Paracyclophanes

Reported here is the synthesis and self-assembly characterization of [n.n]paracyclophanes ( [n.n]pCps , n=2, 3) equipped with anilide hydrogen bonding units. These molecules differ from previous self-assembling [n.n]paracyclophanes ( [n.n]pCps ) in the connectivity of their amide hydrogen bonding units (C-centered/carboxamide vs. N-centered/anilide). This subtle change results in a ≈30-fold increase in the elongation constant for the [2.2]pCp -4,7,12,15-tetraanilide ( [2.2]pCpNTA ) compared to previously reported [2.2]pCp -4,7,12,15-tetracarboxamide ( [2.2]pCpTA ), and a ≈300-fold increase in the elongation constant for the [3.3]pCp -5,8,14,17-tetraanilide ( [3.3]pCpNTA ) compared to previously reported [3.3]pCp -5,8,14,17-tetracarboxamide ( [3.3]pCpTA ). The [n.n]pCpNTA monomers also represent the reversal of a previously reported trend in solution-phase assembly strength when comparing [2.2]pCpTA and [3.3]pCpTA monomers. The origins of the assembly differences are geometric changes in the association between [n.n]pCpNTA monomers—revealed by computations and X-ray crystallography—resulting in a more favorable slipped stacking of the intermolecular π-surfaces ( [n.n]pCpNTA vs. [n.n]pCpTA ), and a more complementary H-bonding geometry ( [3.3]pCpNTA vs. [2.2]pCpNTA ).     

Ligand-induced distortion of a tetranuclear manganese butterfly complex

The reaction of the pentadentate Schiff-base ligand 1,3-bis(salicylideneamino)-2-propanol (salproH3) with [Mn3O(O2CR)6(py)3] (R = Me, Et, But) gives the corresponding tetranuclear manganese product [Mn4O2O2CR)5(salpro)] (4Mn(III)). The syntheses, structure and magnetochemical characterization of these complexes are reported. The structure of the [Mn4(mu3-O)2]8+ is butterfly-like much more closed than in previous complexes with this core as a result of the alkoxide oxygen of the salpro ligand bridging the two wingtip Mn atoms. Variable-temperature, solid-state magnetic susceptibility studies reveal that these complexes possess S = 0 ground state spins. Fitting of the magnetic susceptibility data to the theoretical chiMT vs. T expression derived for a C2v symmetry complex, assuming an isotropic Heisenberg spin-Hamiltonian and using the Van Vleck equation, revealed that the various exchange parameters are all antiferromagnetic, and the core thus experiences spin frustration effects.     

Self-assembly approach toward chiral bimetallic catalysts: bis-urea-functionalized (salen)cobalt complexes for the hydrolytic kinetic resolution of epoxides

A series of novel bis-urea-functionalized (salen)Co complexes has been developed. The complexes were designed to form self-assembled structures in solution through intermolecular urea-urea hydrogen-bonding interactions. These bis-urea (salen)Co catalysts resulted in rate acceleration (up to 13 times) in the hydrolytic kinetic resolution (HKR) of rac-epichlorohydrin in THF by facilitating cooperative activation, compared to the monomeric catalyst. In addition, one of the bis-urea (salen)Co(III) catalyst efficiently resolves various terminal epoxides even under solvent-free conditions by requiring much shorter reaction time at low catalyst loading (0.03-0.05 mol %). A series of kinetic/mechanistic studies demonstrated that the self-association of two (salen)Co units through urea-urea hydrogen bonds was responsible for the observed rate acceleration. The self-assembly study with the bis-urea (salen)Co by FTIR spectroscopy and with the corresponding (salen)Ni complex by (1)H NMR spectroscopy showed that intermolecular hydrogen-bonding interactions exist between the bis-urea scaffolds in THF. This result demonstrates that self-assembly approach by using non-covalent interactions can be an alternative and useful strategy toward the efficient HKR catalysis.