Central Eight-membered Ring Research Articles

(1R,2R)-(-)-Bis[mu4-1-[[1,1-dimethyl-2-(trimethylsilyl)amidino]amino]-2-([1,1-dimethyl-2-[1,1-dimethyl-2-(trimethylsilyl)amidino]amidino]amino)cyclohexane]tetralithium(I).

The title compound, [Li(4)(C(21)H(46)N(8)Si(2))(2)], has been prepared and its structure determined. The whole molecule can be regarded as a molecular cage made up of the central eight-membered ring and six contiguous rings of three types. As the first example of a bridged amidinate-dicyanamide framework, this noteworthy structure and its electronic features are presented.

Studies Towards the Total Synthesis of Cycloaraneosene and Ophiobolin M: A General Strategy for the Construction of the 5—8 Bicyclic Ring System.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Studies Towards the Total Synthesis of Cycloaraneosene and Ophiobolin M: A General Strategy for the Construction of the 5−8 Bicyclic Ring System

The synthesis of the core unit of cycloaraneosene and ophiobolin M has been investigated, following a general strategy applicable to both 5−8 bicyclic systems. The synthetic strategy includes a ring-closing metathesis reaction to generate the central eight-membered ring as well as a palladium-mediated coupling of a Grignard reagent to introduce the exocyclic side-chain. The stereochemistry of the ring junction is also discussed and moderate diastereoselectivity has been achieved. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Stereochemistry of caracurine V.

The 3D structure of the Strychnos alkaloid caracurine V was determined by means of NMR spectroscopy and semiempirical calculations. The previously unknown absolute configuration in the central eight-membered ring was assigned as (16R, 16'R, 17R, and 17'R).

Complexes of heterocyclic thiones and Group 12 metals: Part 2: The chemical and electrochemical synthesis of mercury(II) complexes of 1-methylimidazoline-2(3 H)-thionate. The crystal structure of trans-[bis-{(η 1- S-1-methylimidazoline-2(3 H)-thione)(η 1- S-1-methylimidazoline-2(3 H)-thionate)(μ 2- S, N-1-methylimidazoline-2-thionate)mercury(II)}] at 160 K

Two complexes of mercury(II) and the anion of 1-methylimidazoline-2(3 H)-thione (1-meimz2SH) have been prepared and characterised. The addition of an aqueous solution of mercury(II) acetate to an aqueous solution of 1-methylimidazoline-2(3 H)-thione and triethylamine produced [Hg(1-meimz2S) 2], as a colourless and intractable precipitate. The electrochemical oxidation of elemental mercury, in the presence of a saturated acetonitrile solution of 1-methylimidazoline-2(3 H)-thione and a supporting electrolyte, produced two crystalline products, a pale-yellow microcrystalline complex, [Hg(1-meimz2S) 2] and a colourless crystalline complex, [Hg(1-meimz2S) 2(1-meimz2SH)]. An X-ray crystal structure analysis of [Hg(1-meimz2S) 2(1-meimz2SH)] showed that the complex consists of discrete, centrosymmetrically constrained, mercury(II) dimers. Each dimer contains a pair of bifunctional, binuclear bridging, μ 2-S,N(η 1-S;η 1-N), 1-methylimidazoline-2(3 H)-thionate anionic ligands (HgS bridging=2.5196(6), HgN=2.325(2) Å, NCS exo=126.0(2)°). The bridging ligands adopt a ‘head-to-tail’ configuration. This arrangement generates a central eight-membered ring, Hg 2S 2C 2N 2, in the complex. The ring adopts a chair conformation. Distorted, pseudo-tetrahedral four-coordination at each metal is completed by a pair of monodentate, sulfur donating, 1-methyl-imidazoline-2(3 H)-thione neutral and anionic ligands (HgS thione=2.5735(7) and HgS thionate=2.4829(7) Å). NH ⋯N hydrogen bonds (N ⋯N=2.701 Å) effectively link the terminal ligands into eight-membered chelates.

Powerful Oxidizing Properties of a Planar [8]Annulene: Tetrakis(perfluorocyclobuta)cyclooctatetraene and Its Radical Anion

The central eight-membered ring of tetrakis(perfluorocyclobuta)cyclooctatetraene (1) is flat both in the crystal and in solution, but tetrakis(perfluorocyclopenta)cyclooctatetraene (2) assumes a tub shape similar to that of parent cyclooctatetraene. This sterically induced structural difference strongly affects the physical properties of compounds 1 and 2. The first and second reduction potentials of 1 in solution, +0.79 and +0.14 V vs. SCE, are considerably more positive than the corresponding values, +0.20 and −0.21 V, of 2. Accordingly, mere contact with Hg metal in N,N-dimethylformamide converts 1 to an unusually stable radical anion 1.− that is persistent in air. The temperature-independent coupling constant, aF=+0.320 mT, of the sixteen equivalent β-19F-nuclei in 1.− is much smaller than the expected value, and the g factor of 1.− is markedly lower than those generally found for structurally related radical anions. In contrast, the characteristic data for 2.−, generated from 2 with K metal in 1,2-dimethoxyethane, comply with the expected data. The coupling constants aF=+0.976 and +0.935 mT, each of eight β-19F-nuclei in 2.−, average +0.951 mT upon raising the temperature from 220 to 260 K. The gas-phase electron affinity of 1, A=3.4±0.2 eV, which was estimated from the reduction potential in solution and from charge-transfer absorption spectra obtained from a series of methylated benzenes and naphthalenes, is among the highest known for neutral organic molecules.

Mononuclear and Binuclear Complexes of Platinum(0) Containing (Alkynyl)phenylsilanes

Reaction of bis(cycloocta-1,5-diene)platinum(0) with the (alkynyl)phenylsilanes Ph3SiC2But, Ph2Si(C2But)2 and PhSi(C2But)3 gives, respectively, [Pt (Ph3SiC2But)2] (1b), [Pt {Ph2Si(C2But)}]2 (2b), and [Pt {PhSi(C2But)3}]2 (4b), which contain zerovalent platinum atoms coordinated by two alkyne units. Spectroscopic data indicate that (2b) and (4b) contain two PtC4 and two SiC4 tetrahedra joined at the corners. X-Ray crystallography shows that complex (4b) is isostructural and isomorphous with the known nickel analogue, two of the alkyne units being uncoordinated; the central eight-membered ring comprising two silicon, four alkyne carbon and two platinum atoms has an approximate chair conformation. In contrast, the monomer (1b) is isostructural but not isomorphous with the analogous nickel compound (1a); in the crystal there is evidence for a weak intramolecular phenyl-phenyl interaction.

Reactions of 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene with 1,2-C6H4(SiMe2)2Pt(PPh3)2 and Pt(CH2-CH2) (PPh3)2. x-ray structure determination of resulting double silylated compounds and platinum-alkyne complexes

The platinum complex of 5,6,11,12-tetradehydrodibenzo[a,e]cyclooctene (1), [(Ph3P)2Pt]2[1], was synthesized quantitatively by the reaction of 1 with Pt(CH2=CH2)(PPh3)2. The reaction of 1 with 1,2-C6H4(SiMe2)2Pt(PPh3)2 gave 5,5,10,10,15,15,20,20-octamethyl-5,10,15,20-tetrahydro-5,10,15,20-tetrasiladibenzo[b,n]tetraphenylene ([C6H4(SiMe)2]2(C16H8)) and [(Ph3P)2Pt][C6H4(SiMe2)2-C16H8] C6H4(SiMe2)2C16H8=5,5,16,16-tetramethyl-5,16-dihydro-10,11-didehydro-5,16- disiladibenzo[a,e]naphtho[c]cyclooctene) as well as [(Ph3P)2Pt]2[1]. The central eight-membered ring of [C6H4(SiMe2)2]2(C16H8) has a typical tub-shaped structure, while that of [(Ph3P)2Pt]2[1] is much less folded. The structure of [(Ph3P)2Pt][C6H4(SiMe2)2C16H8] lies between those of [C6H4(SiMe2)2]2(C16H8) and [(Ph3P)2Pt]2[1]. Crystal data: [(Ph3P)2Pt]2[1]·2toluene, C88H68P4Pt2·2C7H8, monoclinic, P21/n, a = 16.369(2), b = 30.104(6), c = 17.120(3) Å, β = 102.69(1)°, V = 8230(2) Å3, Z = 4. [C6H4(SiMe2)2]2(C16H8), C36H40Si4, monoclinic, P21c, a = 18.421(2), b = 11.131(1), c = 16.444(1) Å, β = 94.540(4)°, V = 3361(4) Å3, Z = 4. [(Ph3P)2Pt][C6H4(SiMe2)2C16H8], C62H54P2PtSi2, triclinic, P̄1, a = 13.538(2), b = 17.244(4), c = 12.510(3) Å, α = 111.10(2), β = 104.93(1), γ = 80.06(2)°, V = 2622(1) Å3, Z = 2.

An Al4(OH)4 Eight‐Membered Ring in a Molecular Aluminopolysiloxane and Its Behavior with Bases

Only three of the four diethyl ether molecules are coordinated to the OH groups in the crystal structure of the aluminopolysiloxane 1. Compound 1 is obtained from tert-butoxyaluminum dihydride and diphenylsilanediol in Et2O. The fourth coordination site is sterically shielded by two oppositely placed OSi(Ph2)OSi(Ph2)O bridges. Replacement of Et2O with Et3N results in twofold deprotonation of the polycycle; now only two donor molecules can coordinate to the central eight-membered ring.

A facile synthesis and spectral (ir,1H,13C,31P nmr) studies of 2,10-dichloro-6-aryloxy-12H-dibenzo[d,g][1,3,2]dioxaphosphocin 6-sulfides

The synthesis of new 2,10-dichloro-6-aryloxy-12H-dibenzo[d,g][1,3,2]dioxaphosphocin 6-sulfides 4 was achieved in two steps with high yields from the simple materials 5,5′-dichloro-2,2′-dihydroxydiphenyl-methane (1) and thiophosphoryl chloride (2) which produced the key intermediate 2,6,10-trichloro-12H-dibenzo[d,g][1,3,2]dioxaphosphocin 6-sulfide (3). Treatment of 3 with substituted phenols under phase transfer catalytic (PTC) conditions led to members of 4. Long range coupling [5J(P,H) = 3.6 Hz] was observed between phosphorus and one of the bridged methylene protons in 4. A 13C nmr analysis revealed 2J(P,O,C), 3J(P,O,C) 4J(P,O,C) and 5J(P,O,C) couplings. All 31P nmr chemical shifts for thirteen members of these new heterocycles are reported for the first time. The nmr data are not totally definitive to confirm a boat-chair as the major conformer for the central eight-membered dioxaphosphocin ring, but such a conformer is tentatively suggested as favored.

Total Synthesis of (−)-Cotylenol, a Fungal Metabolite Having a Leaf Growth Activity

Abstract Cotylenol, one of the representative fusicoccane diterpenoids having 5-8-5-membered tricyclic carbon framework, is now synthesized for the first time via an eight-membered ring formation by an intramolecular ene reaction followed by an oxidative formation of the 8β,9α-glycol function on the central eight-membered ring.

Crystal and molecular structure of a new eight-membered tetraphospha-macrocycle, (–PCPhCMeCMeC–)4

A new P4C4 eight-membered macrocycle has been obtained in one step from 3,4-dimethyl-1-phenylphosphole. A comparison of the X-ray crystal structure of this P4C4 macrocycle with that of its decacarbonyldimolybdenum complex shows that the P4C4 crown gains a very high flexibility through an easy rotation of the adjacent phospholyl units round the C–C and P–P bonds of the central eight-membered ring. This means that this P4C4 macrocycle can adapt its shape to the stereochemical requirements of the complexed metals. The P–P and C–C bonds of the crown [average 2.191(3) and 1.47(1)A respectively] are significantly shorter than normal P–P and C–C single bonds. Thus some conjugative interactions probably exist between the adjacent phospholyl rings. These interactions are suppressed in the decacarbonyldimolybdenum complex.

ChemInform Abstract: ELECTROCHEMICAL REDUCTION OF SYM‐DIBENZOCYCLOOCTATETRAENE, SYM‐DIBENZO‐1,5‐CYCLOOCTADIENE‐3,7‐DIYNE, AND SYM‐DIBENZO‐1,3,5‐CYCLOOCTATRIEN‐7‐YNE

AbstractDie reversiblen Halbstufenpotentiale der drei Titelverbindungen, die Geschwindigkeiten des heterogenen Elektronentransfers und die Transferkoeffizienten werden mit Hilfe von ′cyclischen Voltammogrammen und Digital‐Simulationen von Voltammogrammen bestimmt, und die Kinetik der chemischen Folgereaktionen wird untersucht.

Electrochemical reduction of sym-dibenzocyclooctatetraene, sym-dibenzo-1,5-cyclooctadiene-3,7-diyne, and sym-dibenzo-1,3,5-cyclooctatrien-7-yne

A study of the electroreduction of sym-dibenzocyclooctatetraene (DBCOT), sym-dibenzo1,5-cyclooctadiene-3,7diyne (DBCOD) and sym-dibenzo-l,3,5-cyclooctatrien-7-yne (DBCOM) in DMF-TBAP solutions was carried out to investigate the effects of differences in the structure of the central eight-membered ring on the electrochemical behavior. The reversible half-wave potentials ( E , 1 2 9 , heterogeneous electron-transfer rate constants ( k s ) , transfer coefficients, and pseudo-firstorder rate constants of the following chemical reactions were determined by cyclic voltammetric-digital simulation techniques. The results are consistent with reduction of tub-shaped DBCOT to a planar radical anion and dianion and of planar or almost planar DBCOM and DBCOD to planar radical anions. Estimates of the energy of the conformational change were obtained by comparison of E 119 to calculated energies of the lowest unoccupied molecular orbital and from the k , values. The electrochemical behavior of cyclooctatetraene (COT) and related compounds and the electron spin resonance (ESR) of the associated radical anions have been the subjects of numerous investigations.2-8 The general picture which emerges is that reduction of the tub-shaped COT produces a planar or nearly planar radical anion (COT.-); the large change in molecular geometry leads to a high activation energy and hence slow electron-transfer rates. The second reduction step to the dianion involves only small changes in geometry and hence more rapid electron transfer. A more detailed explanation of the experimental results requires taking account of solvation changes, ion pairing effects, and following protonation reactions. Recently, Wong, Garratt, and Sondheimerg reported the synthesis of sym-dibenzo-l,5-cyclooctadiene-3,7-diyne (DBCOD) and sym-dibenzo1,3,5-cyclooctatrien-7-yne (DBCOM). The central eight-membered ring in these com-