The 16-electron electrocyclisation of vinylogous heptafulvalene

The vinylogous heptafulvalene (“heptafulvadiene”) (5) cleanly undergoes the antarafacial 16-electron electrocyclisation to (11) upon thermal activation (benzene, E_a = 22.1 kcal/mol; log A = 10.8).     

Synthesis and 20π-electrocyclisation of heptahendecafulvadiene - An unusual sequence of pericyclic processes

Upon thermal activation the newly synthesised heptahendecafulvadiene E ( 3) is isomerised into the pentacyclic hydrocarbons 9/11 (xylene,t_1/2(150°C) ca.5min), the formation of which is explained by initial conrotatory 20-electron electrocyclisation followed by a cascade of 10π/6π-pericyclic processes.     

Binuclear vanadium carbonyls: the limits of the 18-electron rule

The fact that the stable mononuclear vanadium carbonyl V(CO)6 fails to satisfy the 18-electron rule has led to an investigation of the binuclear vanadium carbonyls V2(CO)n (n = 10-12) using methods from density functional theory. There are several important experimental studies of these homoleptic binuclear vanadium carbonyls. The global minimum for V2(CO)12 is a singlet structure having two V(CO)6 units linked by a long V-V single bond (3.48 A by B3LYP or 3.33 A by BP86) without any bridging CO groups. For V2(CO)11 the global minimum is a singlet structure V2(CO)10(eta2-mu-CO) with a four-electron pi-donor bridging CO group. For V2(CO)10 the global minimum is an unsymmetrical singlet (OC)4VV(CO)6 structure with three semibridging CO groups and a V-V distance of 2.54 A (B3LYP) or 2.51 A (BP86), suggesting a VV triple bond. The theoretical nu(CO) frequencies of this V2(CO)10 isomer agree approximately with those assigned by Ishikawa et al. (J. Am. Chem. Soc. 1987, 109, 6644) to a V2(CO)10 isomer produced in the photolysis of gas-phase V(CO)6. In contrast, the laboratory bridging nu(CO) frequency assigned to V2(CO)12 by Ford et al. (Inorg. Chem. 1976, 15, 1666) seems more likely to arise from the lowest-lying triplet isomer of V2(CO)11.     

Modeling rate-controlling solvent effects. The pericyclic meisenheimer rearrangement of N-propargylmorpholine N-oxide

The activation parameters of the pericyclic Meisenheimer rearrangement and a competitive rearrangement of N-propargylmorpholine N-oxide were determined by experimental and computational methods. A number of aprotic and protic solvents of different polarities and hydrogen bond-forming abilities and the roles of electron-pair acceptor additives were investigated. The reaction kinetics were followed by means of NMR. In protic solvents, isotope-labeling experiments revealed a novel inverse secondary kinetic isotope effect (k(H)/k(D) about 0.8) for the rate-determining cyclization step, probably occurring because of a C(sp) --> C(sp(2)) change in hybridization at the reaction center. In molecular computations at the B3LYP/6-31++G(d,p) level of theory, implicit, explicit, and joint explicit-implicit solvent models were used. The explicit-implicit model and molecular dynamic simulations gave the most accurate results. The components of the rate-controlling solvent effect are discussed, and general equations are proposed for accurate prediction of the solvent-dependent activation parameters.     

Dimethylamine borane dehydrogenation chemistry: syntheses, X-ray and neutron diffraction studies of 18-electron aminoborane and 14-electron aminoboryl complexes

The reactions of Me(2)NH·BH(3) with cationic Rh(III) and Ir(III) complexes have been shown to generate the 18-electron aminoborane adduct [Ir(IMes)(2)(H)(2){κ(2)-H(2)BNMe(2))](+) and the remarkable 14-electron aminoboryl complex [Rh(IMes)(2)(H)-{B(H)NMe(2))](+). Neutron diffraction studies have been used for the first time to define H-atom locations in metal complexes of this type formed under catalytic conditions.     

Construction of 18-electron arene-ruthenium complexes based on ortho-carborane-1,2-diselenolate ligand

Treatment of ortho-carborane, n-butyl lithium, selenium and [(p-cymene)RuCl₂]₂ under argon leads to complexes (p-cymene)Ru(Se₂C₂B₁₀H₁₀) (I) and (p-cymene)₂Ru₂(μ₂-Se₂C₂B₁₀H₁₀) (II). The further reaction of 16-electron complex I with RC≡CCO₂Me affords addition complexes (p-cymene)Ru(Se₂C₂B₁₀H₁₀)(RC=C-CO₂Me) (III) (R = H (IIIa); CO₂Me (IIIb)). These complexes were characterized by elemental analysis, mass, and NMR spectroscopy. X-ray structural analyses were performed on II and IIIa.     

Electron transfer processes involving ferrocenes. Electrogeneration and reactivity of an unusual species,the monocyclopentadienyliron radical

The very reactive cyclopentadienyliron species, (RC₅H₄)Fe, are relatively easily generated by electrochemical reduction of appropriately substituted and readily available ferrocenes. Thus (MeCOC₅H₄)Fe has been generated by monoelectronic reduction of (MeCOC₅H₄)₂Fe; it reacts with CO leading, after further reduction, to the acetyl cyclopentadienyldicarbonyliron anion. Reactions of this anion give ring-substituted cyclopentadienyldicarbonyliron derivatives containing the electron-withdrawing MeCO substituent.     

Unusual cycloaddition of an unsaturated nitrone to a vinylogous ester

The anomalous addition of α-dichloro-N-cyclohexylpropanaldonitrone (1) to the carbonyl of quinoidal, vinylogous ester 2 is reported.     

Polyfunctionalized cage compounds by pericyclic domino processes of 4,5-dicyanopyridazine with dienes: applications and limits

The title compound 1 was found to behave as an attractive masked bis-diene to give 4-oxatricyclo[4.3.1.0(3,7)]dec-8-ene, 5-aza- and 5-silatricyclo[5.3.1.0(3,8)]undec-9-ene, tricyclo[3.2.1.0(2, 7)]oct-3-ene, and tricyclo[5.3.1.0(3,8)]undec-9-ene derivatives through purely pericyclic, three-step homodomino processes with diverse bis-dienophiles; whereas the reaction with myrcene (21) was characterized by a complete sitoselectivity affording compound 25, treatment of 1 with (R)-(-)-beta-citronellene (26a) gave a 3:1 mixture of the homochiral diastereomers 30a and 31a. Some limits of this methodology, mainly arising from competitive side reactions upon the key cyclohexa-1,3-diene intermediates, are emphasized. The structures of the new compounds were established on the basis of spectral data.     

Sequential pericyclic reaction of ene-diallenes: an efficient approach to the steroid skeleton

[reaction: see text] The one-pot construction of polycyclic aromatic systems from acyclic ene-bis(propargyl alcohols) was achieved through a tandem dual [2,3]-sigmatropic rearrangement/6pi-electrocyclic reaction/intramolecular [4 + 2] cycloaddition sequence. A steroidal compound was conveniently synthesized using the present method.     

Characterization of pericyclic reactions using multicenter electron delocalization analysis.

This work investigates the applicability of multicenter delocalization analysis to the characterization of pericyclic reactions. The results indicate that multicenter delocalization indices are a powerful tool for studying concerted processes, allowing the characterization of aromatic transition states with a significant increase in the electron delocalization. Moreover, an advantage over magnetic-based indices is that multicenter delocalization indices are not influenced by local electron currents but by the electron delocalization along the multiple (n) centers, and provide, in a quantitative sense, more reliable results. A thorough comparison with magnetic-based indices is carried out for the trimerization reaction of acetylene. Tracking the values of multicenter delocalization indices along the reaction path allows investigation of the nature of concerted mechanisms. Six-center electron delocalization displays a maximum at the transition state of the Diels-Alder reaction, whereas a similar maximum of four-center electron delocalization is slightly displaced to butadiene for the ring opening of cyclobutene. The profile of multicenter electron delocalization indices along the reaction path of [2+2] cycloaddition of ketene to ethene shows the presence of the two independent mechanisms that agree with the two HOMO/LUMO orbital interactions previously proposed to dominate this reaction.     

Stereocontrolled synthesis of Z-dienes via an unexpected pericyclic cascade rearrangement of 5-amino-2,4-pentadienals

Donor-acceptor dienes known as Zincke aldehydes, which derive readily from the ring-opening reactions of pyridinium salts with secondary amines, undergo a fascinating thermal rearrangement reaction to afford Z-alpha,beta,gamma,delta-unsaturated amides with excellent stereoselectivity. Efficient, stereocontrolled access to Z-trisubstituted alkenes with two different substitution patterns is possible in three steps beginning with the appropriately substituted pyridine derivative. Preliminary studies have shown that both the amide and the monosubstituted alkene termini can be selectively functionalized. Ease of access, generality of scope, and facile product manipulation render this process attractive for the synthesis of complex polyenes.     

Toward the synthesis of (+)-peloruside A via an intramolecular vinylogous aldol reaction

The use of the intramolecular vinylogous aldol reaction for the preparation of an advanced intermediate for the synthesis of peloruside A is described. The reaction was applied to compound 19 and proceeds in high yield and good levels of diastereoselectivity. Application of the Achmatowicz reaction to this intermediate provided the corresponding pyranone, a late stage intermediate well positioned for conversion to the natural product.     

The pyridine ring of NAD is formed by a nonenzymatic pericyclic reaction

The biosynthesis of quinolinate 3, the precursor to the pyridine ring of NAD, is still poorly understood. Two pathways have been identified, one involving the direct formation of quinolinic acid from aspartate and dihydroxyacetone phosphate, the other requiring a five-step degradation of tryptophan. The final step in this degradation is catalyzed by the non-heme Fe(II)-dependent enzyme 3-hydroxyanthranilate-3,4-dioxygenase (HAD). This enzyme catalyzes the oxidative ring opening of 3-hydroxyanthranilate (1) to 2-amino-3-carboxymuconic semialdehyde (ACMS, 2) which then cyclizes to quinolinate (3). In this communication, we demonstrate the following: (1) cyclization of ACMS to 3 is not HAD catalyzed, (2) the most stable form of ACMS in solution is an all trans isomer which undergoes facile cis to trans isomerization about the C2-C3 and C4-C5 double bonds via transient formation of its enol tautomer (6), (3) a model study on the ring opening of N,N-dimethylcarbamoylpyridinium with hydroxide and methoxide suggests that the cyclization of ACMS occurs by an electrocyclization reaction of its enol tautomer 6. Thus, the biosynthesis of quinolinic acid, by the tryptophan pathway, is likely to be a member of a growing family of natural products whose biosynthesis involves a pericyclic reaction.