Formal Dimerization Research Articles

Reductive Retrocyclization of a Mangana(II)cyclopentasilane to Form Manganese(0) Bis(η2-disilene) Complexes.

Ligand-exchange reactions on a mangana(II)cyclopentasilane complex that contains two THF ligands with aryl isocyanides led to the formation of manganese(0) bis(η2-disilene) complexes via a retrocyclization. In stark contrast, ligand-exchange reactions with CNtBu, an N-heterocyclic carbene, or pyridine-based ligands furnished manganese(II) complexes wherein the manganacyclopentasilane framework remained intact. The thermolysis of the obtained bis(η2-disilene) complex in the presence of mesityl isocyanide led to the formation of a cyclotetrasilane via the formal dimerization of the two η2-disilene moieties. The insertion of a mesityl isocyanide into the Mn-Siβ bond results in the formation of a manganese(II) complex supported by a [SiCSi]-type tridentate ligand scaffold.

Reductive Retrocyclization of a Mangana(II)cyclopentasilane to Form Manganese(0) Bis(η2‐disilene) Complexes

AbstractLigand‐exchange reactions on a mangana(II)cyclopentasilane complex that contains two THF ligands with aryl isocyanides led to the formation of manganese(0) bis(η2‐disilene) complexes via a retrocyclization. In stark contrast, ligand‐exchange reactions with CNtBu, an N‐heterocyclic carbene, or pyridine‐based ligands furnished manganese(II) complexes wherein the manganacyclopentasilane framework remained intact. The thermolysis of the obtained bis(η2‐disilene) complex in the presence of mesityl isocyanide led to the formation of a cyclotetrasilane via the formal dimerization of the two η2‐disilene moieties. The insertion of a mesityl isocyanide into the Mn−Siβ bond results in the formation of a manganese(II) complex supported by a [SiCSi]‐type tridentate ligand scaffold.

Steric Influence on Reactions of Benzyl Potassium Species with CO.

Reactions of benzyl potassium species with CO are shown to proceed via transient carbene‐like intermediates that can undergo either dimerization or further CO propagation. In a sterically unhindered case, formal dimerization of the carbene is the dominant reaction pathway, as evidenced by the isolation of ((Ph3SiO)(PhCH2)C)2 2 and PhCH2C(O)CH(OH)CH2Ph 3. Reactions with increasingly sterically encumbered reagents show competitive reaction pathways involving intermolecular dimerization leading to species analogous to 2 and 3 and those containing newly‐formed five‐membered rings tBu2C6H2(C(OSiR3)C(OSiR3)CH2) (R=Me 6, Ph 7). Even further encumbered reagents proceed to either dimerize or react with additional CO to give a ketene‐like intermediates, thus affording a 7‐membered tropolone derivative 14 or the dione (3,5‐tBu2C6H3)3C6H2CH2C(O))2 15.

Synthesis and Biological Evaluation of (2S,2'S)-Lomaiviticin A.

(-)-Lomaiviticin A (1) is a genotoxic C2-symmetric metabolite that arises from the formal dimerization of two bis(glycosylated) diazotetrahydrobenzo[b]fluorenes. Here we present a synthesis of the monomer 17 and its coupling to form (2S,2'S)-lomaiviticin A (4), an unnatural diastereomer of 1. (2S,2'S)-Lomaiviticin A (4) is significantly less genotoxic, a result we attribute to changes in the orientation of the diazofluorene and carbohydrate residues, relative to 1. These data bring the importance of the configuration of the conjoining bond to light and place the total synthesis of 1 itself within reach.

Easy access to medium-sized lactones through metal carbene migratory insertion enabled 1,4-palladium shift

Reactions that efficiently construct medium-sized lactones are significant, as they overcome the unfavorable entropic factor and transannular interactions for ring closure, and the lactones produced are common structural motifs recurring in many biologically active compounds. Herein, we describe a valuable strategy for medium-sized lactone synthesis by accomplishing site-selective C–H bond functionalization via a palladium carbene migratory insertion enabled 1,4-palladium shift. The overall process achieves the formal dimerization of two readily available benzaldehyde derivatives, providing value-added products medium-sized lactones. Our method is amenable to late-stage modification of approved drugs and other complex molecules. Mechanistic studies including deuterium-labeling experiments and DFT calculation shed light on the reaction pathways.

Towards enantiopure macrocyclic trans-dinucleating hemilabile P-Alkene ligands: Syntheses, structures, and Chiral Pd-Complexes

Dibenzazepinyl dichlorophosphine 2 reacts with (R,R)-2,3-O-isopropylidene-threitol (3) in Et2O solution to afford gram-quantities of the enantiopure macrocylic phosphoramidite (all-R)-6, which may be seen as a formal dimer of classic phosphoramidite P-alkene hybrid ligands. Complexation experiments with PdCl2 reveal highly selective formation of the trans-dinuclear complex (all-R)-11. The corresponding bulkier and rigidly trans-eclipsed 1,4-diol (S,S)-bis-hydroximethyl-9,10-dihydro-9,10-ethaneanthracene (4) does not favor macrocycle formation and exclusively leads to the new dibenzazepinyl phsophormaidite P-alkene ligand 7, which in Pd-catalyzed asymmetric allylic amination comes the well-known ‘privileged’ binol-derived P-alkene analogue 1 close in terms of enantioselection.

Chemoselective Photoredox Synthesis of Unprotected Primary Amines Using Ammonia.

Unprotected α-amino carbon radicals are produced as novel intermediates via a transformation that merges acid-promoted N-H imine generation and chemoselective photocatalytic single-electron reduction. Coupling ammonia and aldehydes/ketones allows the generation of primary amines under mild conditions without the need for protecting groups. The key intermediate can be efficiently transformed into primary (di)amines by a formal dimerization, reductive amination via hydrogen atom transfer, and arylation through radical-radical coupling.

Porphyrin Co(III)-Nitrene Radical Mediated Pathway for Synthesis of o-Aminoazobenzenes.

Azobenzenes are versatile compounds with a range of applications, including dyes and pigments, food additives, indicators, radical reaction initiators, molecular switches, etc. In this context, we report a general method for synthesizing o-aminoazobenzenes using the commercially available cobalt(II) tetraphenyl porphyrin [CoII(TPP)]. The net reaction is a formal dimerization of two phenyl azides with concomitant loss of two molecules of dinitrogen. The most commonly used methodology to synthesize azobenzenes is based on the initial diazotization of an aromatic primary amine at low temperatures, which then reacts with an electron rich aromatic nucleophile. As such, this limits the synthesis of azobenzenes with an amine functionality. In contrast, the method we report here relies heavily on the o-amine moiety and retains it in the product. The reaction is metal catalyzed and proceeds through a porphyrin Co(III)-nitrene radical intermediate, which is known to form on activation of organic azides at the cobalt center. The synthesized o-aminoazobenzenes are bathochromatically shifted, as compared to azobenzenes without amine substituents. Based on the crystal structure of one of the products, strong H-bonding between the N-atom of the azo functionality and the H of the NH2 substituent is shown to stabilize the trans isomeric form of the product. The NH2 substituents offers possibilities for further functionalization of the synthesized azo compounds.

Ring Closure Reactions of 2,6-Diazaheptatrienyl Metal Compounds: Synthesis of 3-Aminoindole Derivatives and 14-Membered Macrocyclic Dimers

2,6-Diazaheptatrienyl metal compounds 6(-)K(+) are easily accessible from the corresponding diimines 6 by deprotonation using KO-t-Bu as base. According to quantum chemical calculations, they are, in comparison to other isomeric species with nitrogen atoms in other positions, highly reactive intermediates, which undergo in dilute solution at 50 °C ring closure reactions to form 3-aminoindole derivatives 8/10. In contrast, in more concentrated solution at room temperature, the formation of 14-membered macrocyclces 13 as a result of formal dimerization is observed. The 3-aminoindole derivatives obtained in this work possess rare substitution patterns, and the macrocyclic compounds are essentially unknown. Two-fold vinylogous derivatives 7 give rise to tricyclic systems with δ-carboline backbone 12. The experimental results are interpreted using high-level DFT calculations with regard to the possible reaction mechanism and the nature of the transition state of the five-membered ring formation. The molecular structures in the solid state of all types of compounds were elucidated by X-ray diffraction.

Reversible Transformation of a Stable Monomeric Silicon(II) Compound into a Stable Disilene by Phase Transfer: Experimental and Theoretical Studies of the System {[(Me3Si)2N](Me5C5)Si }n with n = 1,2

The salt (eta(5)-pentamethylcyclopentadienyl)silicon(II) tetrakis(pentafluorophenyl)borate (5) reacts at -78 degrees C with lithium bis(trimethylsilyl)amide in dimethoxyethane (DME) as solvent to give quantitatively the compound [bis(trimethylsilyl)amino][pentamethylcyclopentadienyl]silicon(II) 6A in the form of a colorless viscous oil. The reaction performed at -40 degrees C leads to the silicon(IV) compound 7, the formal oxidative addition product of 6A with DME. Cycloaddition is observed in the reaction of 6A with 2,3-dimethylbutadiene to give the silicon(IV) compound 8. Upon attempts to crystallize 6A from organic solvents such as hexane, THF, or toluene, the deep yellow compound trans-1,2-bis[bis(trimethylsilyl)amino]-1,2-bis(pentamethylcyclopentadienyl)disilene (6B), the formal dimer of 6A, crystallizes from the colorless solution, but only after several days or even weeks. Upon attempts to dissolve the disilene 6B in the described organic solvents, a colorless solution is obtained after prolonged vigorous shaking or ultrasound treatment. From this solution, pure 6A can be recovered after solvent evaporation. This transformation process can be repeated several times. In a mass spectroscopic investigation of 6B, Si=Si bond cleavage is observed to give the molecular ion with the composition of 6A as the fragment with the highest mass. The X-ray crystal structure analysis of the disilene 6B supports a molecule with a short Si=Si bond (2.168 A) with efficiently packed, rigid sigma-bonded cyclopentadienyl substituents and silylamino groups. The conformation of the latter does not allow electron donation to the central silicon atom. Theoretical calculations at the density functional level (RI-BP86 and B3LYP, TZVP basis set) confirm the structure of 6B and reveal for silylene 6A the presence of an eta(2)-bonded cyclopentadienyl ligand and of a silylamino group in a conformation that prevents electron back-donation. Further theoretical calculations for the silicon(II) compound 6A, the disilene 6B, and the two species 11 and 11* derived from 6A (which derive from Si=Si bond cleavage) support the experimental findings. The reversible phase-dependent transformation between 6A and 6B is caused by (a) different stereoelectronic and steric effects exerted by the pentamethylcyclopentadienyl group in 6A and 6B, (b) some energy storage in the solid state structure of 6B (molecular jack in the box), (c) a small energy difference between 6A and 6B, (d) a low activation barrier for the equilibration process, and (e) the gain in entropy upon monomer formation.

A Zwitterionic Silylene as Reactive Intermediate and Its Unusual Dimerization to an N‐Heterobicyclic Disilane

A way to synthesize the transient zwitterionic silylene L'Si: 8 {L'=CH[(C=CH(2))CMe(N(tBu))(2)]} and achieve its facile dimerization to the remarkable N-heterobicyclic disilane 8(2) is described. At first, employing the beta-diketiminate ligand L [L=CH(CMeN(tBu))(2)], both starting materials LH (2) and its N-lithium salt LLi (3) can react with SiBr(4) to yield the silylene precursor L'SiBr(2) (4) by silicon-induced C-H activation at an exocyclic methyl group on the backbone of the ligand. Compound 4 reacts with SiBr(4) above room temperature to afford the unexpected terminal CH(SiBr(3))-substituted dibromosilane 6 along with the unique tricyclic trisilane 7. Reduction of 4 with KC(8) at 0 degrees C furnishes the novel N-heterobicyclic disilane 8(2), which is a formal dimer of the desired zwitterionic silylene L'Si: (8). It has been reasoned that compound 8(2) may results from [4+1] cycloaddition of two molecules of 8 to give the transient dimer 8(2)', which subsequently undergoes hydrogen transfer from a terminal methyl group on the backbone of the C(3)N(2)Si ligand to the low-coordinate Si atom. The latter dimerization can be rationalized by the intrinsic zwitterionic character of 8 and insufficient steric protection through the tBu groups at the nitrogen atoms. The novel compounds 3, 4, 6, 7, and 8(2) have been characterized by (1)H, (13)C, and (29)Si NMR spectroscopy, mass spectrometry, and elemental analysis. Additionally, the structures of 3, 6, 7, and 8(2) were also established by single-crystal X-ray diffraction analyses.

The Ritter Reaction under Truly Catalytic Brønsted Acid Conditions

AbstractSimple organic acids like 2,4‐dinitrobenzenesulfonic acid (DNBSA) catalyze the Ritter reaction of secondary benzylic alcohols giving rise to the corresponding N‐benzylacetamides in usually high yields. Reactions can be conducted without exclusion of oxygen and without the need of dry solvents. With tertiary α,α‐dimethylbenzylic alcohols a different pathway involving a formal dimerization reaction takes place under the acid‐catalytic conditions used. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)

Brønsted Acid-Catalyzed Benzylation of 1,3-Dicarbonyl Derivatives

The direct alkylation of 1,3-dicarbonyl compounds with benzylic alcohols is shown to be efficiently catalyzed by simple Brønsted acids such as triflic acid (TfOH) and p-toluenesulfonic acid (PTS) to give rise to monoalkylated dicarbonyl derivatives in high yields. In the absence of the nucleophile, substituted alkenes, generated through a formal dimerization reaction, are obtained. The reactions are carried out in air using undried solvents, with water being the only side product of the process.

Metal‐mediated Ring Opening Reaction of Two Aziridines to give 1,2‐Aminoaziridinoethane Ligands

AbstractThe reaction of 2,2′‐dimethylaziridine with the carbonyl halogenido complexes [(CO)5MX] (M = Mn; X = Cl, I; M = Re: X = I) results in the formation of the 1,2‐aminoaziridinoethane complexes [(CO)3XM{NH2CMe2CH2N(C2H2Me2)}]. The bis(2,2′‐dimethylaziridine) complexes [(CO)3XM(NHCH2CMe2)2] may act as intermediates. The new asymmetric chelate ligand is the formal dimerization product of 2,2′‐dimethylaziridine, which is formed and stabilized in the coordination sphere of both transition metals. The new complexes have been characterized using elemental analysis, mass, IR and NMR spectra as well as by X‐ray structure analysis. The metal centers exhibit a distorted octahedral configuration with a facial (CO)3M‐arrangement, and the 5‐membered metallacycles show a twist conformation and lie in the trans‐positions with respect to the CO ligands in the equatorial plane. Whereas the Mn‐N bonds to the intact aziridine system are longer (213.3 and 213.8 pm) than those to the NH2 group (206.5 and 207.4 pm), the corresponding Re–N bonds show the opposite trend, whereby the Re–NH2 bonds are shorter (219.0 and 225.8 pm).

On the Existence of Uncharged Molecules with a Pyramidally Coordinated Carbon: The Cases of Pentacyclo[4.3.0.02,9.03,8.07,9]non-4-ene and Heptacyclo- [7.6.0.01,5.05,15.06,14.010,14.010,15]pentadecane

[reaction: see text] B3LYP/6-31G(d) and MP2 calculations predict interactions between the divalent carbon and one double bond in tricyclo[3.2.2.0(2,4)]nona-6,8-dien-3-ylidene (3). Carbene 3 easily forms the more stable 4, a species with a pyramidal geometry. Moreover, the kinetic stability of 4 has been investigated, and some of its decay products are described. Interestingly, 3,3-dibromotricyclo[3.2.2.0(2,4)]nona-6,8-diene (1), the precursor of carbenoid 3, has already been reported and shown to undergo a formal dimerization to give 2. In addition, 15, a compound structurally related to 4, is expected to have a substantially greater stability toward further rearrangement.

Coupled photocatalytic electron-transfers with 4,4´-bipyridinium derivatives of a betaine alkaloid from Punica granatum

Bis-hydroquinone substituted 4,4´-bipyridinium is the formal dimer of an alkaloid isolated from Punica granatum and can exist as dication, cross-conjugated as well as pseudo-cross-conjugated mesomeric betaine, and dianion. It can moreover form persistent radicals in the solid state as well as in solution due to redox reactions of the reducing and oxidizing partial structures. A reversible coupled photocatalytic process with proflavine as the sensitizer and EDTA as the sacrificial donor is presented. We performed ESR and ENDOR spectroscopy as well as DFT calculations to gain additional knowledge about the radical species.

Formation and electrochemical properties of a 1,4-diphosphafulvene including formal dimerization of phosphaallene.

Reactions of sterically protected 2-bromo-3-phenyl-1-phosphapropenes with bases such as tert-butyllithium and potassium tert-butoxide successfully afforded a bulky 1,4-diphosphafulvene (Mes = 2,4,6-tBu(3)C(6)H(2)) through a novel and formal dimerization pathway of 1-phosphaallene, together with small amounts of 3-phenyl-1-phosphaallene and 3,4-diphosphanylidenecyclobutene. The structure of the 1,4-diphosphafulvene was confirmed by X-ray crystallography indicating somewhat flattened phosphorus atoms due to the bulky Mes groups. The electrochemical properties of the 1,4-diphosphafulvene were investigated to show promising suitability as an electron donor, and indeed, it afforded a charge-transfer complex with TCNQ. Preparation and structural elucidation of an alkoxy-functionalized 1-phosphaallene was also performed, and the effect of the alkoxy group on the 1-phosphaallene moiety was characterized.

Electrocyclization Reactions of 1-Aza- and 1-Oxapentadienyl and -heptatrienyl Cations: Synthesis of Pyrrole and Furan Derivatives

Quantum chemical DFT calculations (B3LYP/6−31+G*) have been used to gain insight into the conformational and energy properties of the 1-aza- and 1-oxapentadienyl and -heptatrienyl cations 1, 2, 3, and 4. The calculated thermodynamic and kinetic data of the ring-closure reactions giving the cyclic products 5−14 are reported and discussed with respect to the experimental results. Experimentally, synthetic routes to the α,β-unsaturated carbonyl compounds 24 and 27, each with a leaving group in the γ-position, have been developed. These compounds have been investigated with respect to their ability to undergo 1,5-electrocyclization reactions to yield 2,5-disubstituted furans 28 upon heating in the presence of acid, presumably through the intermediate formation of the 1-oxapentadienyl cations 2. From the corresponding imine 29a the pyrrole 30d was obtained after treatment with tetrakis(triphenylphosphane)palladium. In the presence of benzylamine and the Pd0 catalyst, the corresponding pyrroles 30a−c were formed from 24 and 27. The homologous α,β,γ,δ-unsaturated carbonyl compounds 31 afforded 2-vinyl-substituted furans 32 upon heating with acid, and the 2-vinyl-substituted pyrroles 34 on treatment with benzylamine and the Pd catalyst. No seven-membered heterocyclic rings were formed. Surprisingly, the α,β-unsaturated carbonyl compounds with two phenyl substituents at the γ-position also provided pyrrole derivatives 40 through a formal dimerization. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

Tetraferrocenyl[5]cumulene, (Fc)2CCCCCC(Fc)2: Synthesis, Electrochemistry, and Reactivity, Including Nickel(0)-Promoted [3]Ferrocenophane Formation and [2+2] Cycloaddition with Fullerene C60

Tetraferrocenyl[5]cumulene (Fc)2CCCCCC(Fc)2 (Fc = ferrocenyl) is synthesized from 1,1-diferrocenyl-1-methoxypropyne by formal dimerization of diferrocenylethenylidenecarbene generated in situ by deprotonation and elimination of methoxide. The cumulene is an air-stable compound of blue color with an electron-rich cumulene moiety, as indicated by various spectroscopic methods. In contrast to other (shorter) ferrocenyl cumulenes which show no reactivity due to steric hindrance, (Fc)2CCCCCC(Fc)2 is a quite reactive compound, as exemplified by (i) transition metal complex formation with Rh(I), (ii) [3]ferrocenophane generation by Ni(0)-promoted intramolecular electrophilic aromatic substitution, and (iii) [2+2] cycloaddition with olefins and alkynes, and (iv) [2+2] cycloaddition with C60 and C70, yielding the first cumulene derivatives of fullerenes. MS, UV−vis, IR, Raman, NMR, cyclic voltammetry, controlled potential coulometry, Mossbauer spectroscopy, and X-ray single-crystal structure analysis have been per...

Studies in the Cycloproparene Series: Formation of a New Dimer of 1H-Cyclopropa[b]naphthalene

1-(Trimethylsilyl)-1H-cyclopropa[b]naphthalene (10) and its 1-methyl derivative (11) have been isolated as pure compounds from use of a lipophilic size exclusion gel. Acylation of the 1H-cyclopropa[b]naphthalenyl anion (2) is effected with N,N -dimethyl-benzamide and -acetamide to give (5) and (6), respectively. Analogous reactions with the 1-(trimethylsilyl)-1H-cyclopropa[b]naphthalenyl anion (9) do not yield the 1-acyl-1-(trimethylsilyl)-1H-cyclopropa[b]naphthalenes (12) and (13); instead the novel 6-methyl- 7H-dibenzo[b,g]fluorene (15) results from attempted acetylation. Compound (15), a formal dimer of 1H-cyclopropa[b]naphthalene (1), is formed also from anion (9) both in toluene and from addition of hydrocarbon (1); its structure is assigned from spectroscopic data and confirmed by single-crystal X-ray analysis.