Anion radicals of di-trans-[12]annulene and heptalene in a one-pot synthesis from a common fire retardant

[reaction: see text] Low temperature (-100 degrees C) dehydrohalogenation of 1,2,5,6,9,10-hexabromocyclododecane (a common fire retardant) with potassium tert-butoxide in THF followed by one-electron reduction yields the anion radical of the di-trans form of [12]annulene. This system yields a well-resolved EPR signal that reveals that most of the spin density resides on one side (the planar side) of the anion radical. Five of the carbons in this [12]annulene system are twisted from the plane of the remaining seven carbons, and the rate of rearrangement between the degenerate conformations is on the EPR time scale (k = 10(6)-10(7) s(-1)). Warming of the solution results in the formation of a sigma-bond between the two internal carbons, loss of molecular hydrogen, and consequent generation of the anion radical of heptalene. Tractable quantities of neutral heptalene can be obtained via the reoxidation of this anion radical with iodine.     

The second cyclopropannulene: cycloprop-[8]annulene

Reacting (at 0 degrees C) a mixture of CH2Cl2 and monobromo[8]annulene (C8H7Br) with potassium tert-butoxide in hexamethylphosphoramide (HMPA) and following with exposure to potassium metal led to the formation of the anion radical of an HMPA-[6.1.0]bicyclononatetraene condensation product, in which two HMPA fragments are geminal and attached to the number 9 carbon. When the reaction sequence is carried out in THF, the dianion of cycloprop[8]annulene is predominantly formed. Neutral cycloprop[8]annulene can be isolated via the I2 oxidation of the THF solution. The NMR analysis reveals that the eight-membered ring is nearly planar, and the three-membered ring is more like a dimethylenecyclopropane than it is like a cyclopropene. Further, the chemical shifts due to the protons on the eight-membered ring are nearly 2 ppm further upfield than are those for [8]annulene itself, suggesting a paratropic ring current.     

12]Annulene radical anions revisited: evaluation of structure assignments based on computed energetic and electron spin resonance data

We report density functional and coupled cluster calculations on numerous monocyclic and bicyclic (CH)12(*-) isomers. At the RCCSD(T)/cc-pVDZ//UB3LYP/6-31+G* level, a nearly planar, bond-equalized radical anion of 1,7-di-trans-[12]annulene (4a(*-)) is lowest in energy; several other isomers and conformations lie within 3 kcal/mol of 4a(*-). RCCSD(T)/AUG-cc-pVDZ//UB3LYP/6-31+G* results place the all-cis isomer 3(*-) slightly below 4a(*-) in energy. Validation studies on the heptalene radical anion, [16]annulene radical anion, and tri-trans-[12]annulene radical anion indicate that electron spin resonance (ESR) hyperfine coupling constants (aH values) computed at the BLYP/EPR-III level on DFT geometries give much better agreement with experimental values than those computed using B3LYP/6-31G*. We were unable to locate any C12H12(*-) isomer that could account for the ESR spectrum previously attributed to a highly twisted structure for the 1,7-di-trans-[12]annulene radical anion. Our computed energetic and ESR data for [12]annulene radical anions and their valence isomers suggest that 4a(*-) may have been made, yet its ESR spectrum was incorrectly assigned to the bicyclic isomer 6b(*-). Finally, the computed (1)H NMR shift values of the dianion of 4 reveal a distinct diatropic ring current that should aid in its characterization.     

The capture of sym-[8]annuldiyne: the cyclooctadienyne-eta2-ynyl potassium zwitterionic radical

[reaction: see text] Reaction of 1,4-dibromo-[8]annulene (C(8)H(6)Br(2)) with potassium tert-butoxide in THF followed by exposure to potassium metal leads to the formation of the anion radical of sym-[8]annuldiyne. The rapid interchange of Jahn-Teller-induced alternating bond angle conformers of sym-[8]annuldiyne is halted by ion association with a metal-crown ether complex forming the cyclooctadienyne-eta(2)-ynyl potassium zwitterionic radical, rendering all four protons nonequivalent. Neutral sym-[8]annuldiyne can form the [2 + 2] polymer, which is not soluble in the THF solution.     

12]Annulynes

Only one isomer of o-benzyne ([6]annulyne or 1,2-didehydrobenzene) exists, but the dehydro analogue of the "ring-opened double benzene", [12]annulyne, was generated in several isomeric forms. 1,5-Hexadiyne undergoes self-condensation in the presence of potassium tert-butoxide to yield two isomers of [12]annulyne (3,11-di-trans-[12]annulyne and 5,9-di-trans-[12]annulyne), both of which exhibit a weak paratropic ring current in their 1H NMR spectra and are oxygen sensitive. They can be reduced to their respective dianions, which are diatropic. A third isomer (3,9-di-trans-[12]annulyne) was generated via the complete dehydrohalogenation of hexabromocyclododecene and found to be much less stable but can be tamed via one- or two-electron reduction. A tight association of the cation (K+) with the p(y)-orbitals within the alkyne moiety results in an unusually low-field resonance for an adjacent external proton.     

The second triannulenylene: tri-[8]annulenylene

Room-temperature dehydrohalogenation of bromocyclooctatetraene (BrC8H7) with potassium tert-butoxide followed (after a couple of minutes) by alkali metal reduction was used to generate the anion radical of tri-[8]annulenylene [(C8H6*-)3] in HMPA. EPR analysis reveals that the odd electron is primarily located in one of the three eight-membered ring systems, which is rendered planar. Excellent agreement was obtained between spin densities predicted by B3LYP/6-31G* calculations and those observed. The neutral tri-[8]annulenylene system has a propensity toward polymerization, but it can be isolated for NMR and mass spectral analysis via the I2 oxidation of the anion radical. The NMR analysis reveals that two of the eight-membered rings are bent above the plane of the benzene ring and the other is bent below. Tri-[6]annulenylene (triphenylene) is the only other known member of the triannulenylenes.     

The Radical Anions of Heptalene and 1, 7-Methano-[12]annulene

The radical anions of heptalene and of 1, 7-methano-[12]annulene are generated by metal reduction and characterized by means of their ESR-spectra. Whereas the neutral hydrocarbons are π-bond localized their corresponding radical anions turn out to be π-bond delocalized. This could be deduced from an interpretation of the different hyperfine splittings using a simple MO-model.     

Single-electron entrapment of [8]annulyne, biannulenylenes, and an annulenoannulene

The low-temperature (-100 degrees C) dehydrohalogenation of bromocyclooctatetraene followed by immediate electron-transfer yields a stable solution of the [8]annulyne anion radical. If the unstable [8]annulyne is reacted with itself, cyclobutadiene, or benzyne, the respective bi-[8]annulenylene, [6]annuleno[8]annulene, or [6]-[8]annulenylene can be trapped as their anion radicals via one-electron transfer. These condensation products were all obtained from simple [2 + 2] cycloaddition reactions. B3LYP/6-31G geometry optimizations were carried out, and the calculated spin densities were compared to the EPR spectral results obtained for the anion radicals of [6]annuleno[8]annulene, [8]annulyne, bi[8]annulenylene, and [6]-[8]annulenylene, and excellent agreement has been realized. This simple "one-pot" approach should be applicable to a wide range of such systems.     

Competing isomerizations of [12]annulenes: Diels-Alder reaction versus electrocyclization

Experimentally, tri-trans-[12]annulene and tris(cyclohexeno)[12]annulene exhibit differing reactivities. Whereas the former, after isomerizing to its di-trans isomer, undergoes sequential electrocyclizations, the latter follows a Diels-Alder pathway after initial electrocyclization. B3PW91/6-31+G*//B3LYP/6-31G* calculations indicate that cyclohexenofusion simultaneously hinders the second electrocyclization and facilitates Diels-Alder reaction, primarily by inducing greater puckering in the intermediate eight-membered ring.     

Computation of the ESR. Coupling Constants of the [16]Annulene Radical Anion; Theoretical and Experimental Reinvestigation of the Excited States of [16]Annulene and its Mono- and Dinegative Ions

The problem of assigning the two small ESR. coupling constants of the [16]annulene radical anion to the two sets of four equivalent outer and inner ring protons is treated by three different semiempirical MO methods. All three methods indicate that the smaller coupling constant (0.743 Gauss) should be assigned to the inner protons, and the larger one (0.963 Gauss) to the outer ones. The electronic spectra of [16]annulene, its radical anion, and its dianion have been remeasured. Especially interesting was the detection of a symmetry-forbidden band of the radical anion at 1473 nm (6789 cm^?1, logε 2.57). The spectra are discussed in terms of different MO models.     

Synthesis of 1,3-diazepines and ring contraction to cyanopyrroles

Several tetrazolo[1,5-a]pyridines/2-azidopyridines undergo photochemical nitrogen elimination and ring expansion to 1,3-diazacyclohepta-1,2,4,6-tetraenes, as well as ring cleavage to cyanovinylketenimines, in low temperature Ar matrices. 6,8-Dichlorotetrazolo[1,5-a]pyridine/2-azido-3,5-dichloropridine undergoes ready exchange of the chlorine in position 8 (3) with ROH/RONa. 8-Chloro-6-trifluoromethyltetrazolo[1,5-a]pyridine undergoes solvolysis of the CF(3) group to afford 8-chloro-6-methoxycarbonyltetrazolo[1,5-a]pyridine. Several tetrazolopyridines/2-azidopyridines afford 1H- or 5H-1,3-diazepines in good yields on photolysis in the presence of alcohols or amines. 5-Chlorotetrazolo[1,5-a]pyridines/2-azido-6-chloropyridines and undergo a rearrangement to 1H- and 3H-3-cyanopyrroles and, respectively. The mechanism of this rearrangement was investigated by (15)N-labelling and takes place via transient 1,3-diazepines. The structures of 6,8-dichloro-tetrazolo[1,5-a]pyridine, 6-chloro-8-ethoxytetrazolo[1,5-a]pyridine, dipyrrolylmethane, and 2-isopropoxy-4-dimethylamino-5H-1,3-diazepine were determined by X-ray crystallography. In the latter case, this represents the first reported X-ray crystal structure of a 5H-1,3-diazepine.     

Chemical reduction of 2,4,6-tricyano-1,3,5-triazine and 1,3,5-tricyanobenzene. Formation of novel 4,4',6,6'-Tetracyano-2,2'-bitriazine and its radical anion

Chemical reduction of 2,4,6-tricyano-1,3,5-triazine, TCT, results in the formation of an unstable radical anion that undergoes immediate dimerization at a ring carbon to form [C(12)N(12)](2-), [TCT](2)(2-), characterized by a long 1.570 (4) A central C[bond]C. [TCT](2)(2-) can decompose into the radical anion of 4,4',6,6'-tetracyano-2,2'-bitriazine, [TCBT]*-, the one-electron reduced form of planar (D(2h)) TCBT, which is also structurally characterized as the [TMPD][TCBT] charge-transfer complex (TMPD = N,N,N',N'-tetramethyl-p-phenylenediamine) with a 1.492 (2) A central sp(2)[bond]sp(2) C[bond]C. Although crystals could not be obtained for the radical anion [TCBT]*-, the electrochemistry (E degrees = +0.03 V), EPR (g = 2.003, (2)A((14)N) = 3.347 G, and (4)A((14)N) = 0.765 G and a line width of 0.24 G), and theoretical calculations support the formation of [TCBT]*-. In addition, thermolysis of [TCT](2)(2-) yields [TCBT]*-. Chemical reduction of 2,4,6-tricyanobenzene, TCB, forms an unstable radical anion that immediately undergoes dimerization at a ring carbon to form [C(12)H(6)N(6)](2-), [TCB](2)(2-), which has a long 1.560 (5) A central C[bond]C. Reaction of TCT with tetrathiafulvalene (TTF) forms structurally characterized [TTF][TCT], and in the presence of water, TCT hydrolyzes to 2,4-dicyano-6-hydroxy-s-triazine, DCTOH. In contrast, the reaction of TCT with TMPD forms [TMPD][TCT], which in the presence of water forms structurally characterized [HTMPD](+)[DCTO](-).     

A Nonaromatic Thiophene‐Fused Heptalene and Its Aromatic Dianion

Heptalene, a nonaromatic, bicyclic 12 π‐electron system with a twisted structure, is of great interest with regard to its potential Hückel aromaticity in the two‐electron oxidized or reduced forms. The synthesis of thiophene‐fused heptalene 5 from the reductive transannular cyclization of bisdehydro[12]annulene 4 , and its solid‐state structure, which was confirmed by X‐ray crystallographic analysis, is presented. Chemical reduction of 5 readily generated the corresponding dianion, which was successfully isolated as [(K[2.2.2]cryptand)⁺]₂ 5 ^2?. The X‐ray crystallographic analysis of the dianion revealed a shallower saddle structure for the heptalene moiety and a lesser degree of bond alternation relative to 5 . ¹H NMR spectroscopy exposed the effect of a diamagnetic ring current on dianion 5 ^2?, which was corroborated by nucleus‐independent chemical shift (NICS) calculations. These results demonstrate that the heptalene dianion, containing 14 π‐electrons, does indeed exhibit pronounced degrees of Hückel aromaticity.     

Electronic Structure and Photophysical Properties of Planar Conjugated Hydrocarbons with 4n-Membered Rings. I. Photoelectron Spectra of 1,5,9-Tridehydro[12]-annulene and Related Compounds

The HeI photoelectron spectra of the title compound 1 (1,5,9-cyclododecatriene-3,7,11-triyne), 1,5-didehydro[12]annulene (1,3,5,9-cyclododecatetraene-7, 11-diyne ( 2 )), sym-tri-benzotridehydro[12]annulene (tribenzo[a,e,i]-5,6,11,12,17,18-hexadehydrocyclododecene ( 3 )), and sym-dibenzodidehydro[8]annulene (dibenzo[a,e]-5,6,11,12-tetradehydrocyclooctene ( 4 )) have been recorded and analysed on the basis of various semi-empirical model calculations. Despite the distinct bond length alternation in the parent compounds and, apparently, in the radical cations, the first ionization occurs at low energies in these compounds (7.6 ± 0.2 eV). The spectra yield little information with regard to the transannular interactions of the triple bonds.     

An annulene-fused cyclopentadienide. A photochromic cyclopentadienodimethyldihydropyrene where the fused cyclopentadienide group resembles benzene in its effect on the dihydropyrene-metacyclophanediene valence isomerization

The synthesis of the green cyclopentadiene-fused dimethyldihydropyrene 12 was achieved in 36% yield in 7 steps from the parent dihydropyrene 3. Reaction of 12 with KH or LiCH(2)SiMe(3) gave the [14]annulene-fused cyclopentadienide anion quantitatively. In the (1)H NMR spectra, the internal methyl protons of 12 at delta -3.9, change dramatically on formation of anion 5, becoming deshielded to delta -1.82. This is caused by the reduction in diatropicity of the [14]annulene ring on fusion to the 6pi-cyclopentadienide ring. The anion is also a photochromic switch. Irradiation of the closed form 5 with visible light opens it to the open form 5', which reverts to the closed form 5 either with UV light or thermally. The switching behavior is between that of the parent 3/3' and the benzannelated system 4/4' and suggests that in its effect on the photoswitching, cyclopentadienide is behaving chemically similar to benzene.     

ESR.-Spektren des Radikal-Anions des syn-1, 6; 8, 13-Bis-oxido-[14]annulens

The ESR.-spectra of the radical anion of syn-1, 6; 8, 13-bis-oxido-[14]annulene have been recorded. The hyperfine structure of the electrolytically generated anion (solvent: N, N-dimethylformamide; gegenion: Et₄N^⊕) is that of an unassociated species; on the other hand, evidence of strong ion-pairing can be derived from the spectra of chemically prepared anions (solvent: 1,2-dimethoxyethane; gegenion: K⊕ or Na⊕). The distribution of the n-spin population confirms the conclusion previously drawn for the radical anion of 1,6-oxido-[70]annulene that the overall effect of the oxygen bridging is electron repelling.     

A new entry to oxacycles via base-catalyzed endo mode cyclization of allenyl sulfoxides and sulfones

[reaction: see text]. Treatment of the allenyl sulfoxides and sulfones possessing a proper delta-hydroxy appendage at the C-1 position with potassium tert-butoxide effected endo mode ring closure at the sp-hybridized carbon center of the allenyl moiety to provide the five- to eight-membered oxacycles in high yields.     

Studies toward the synthesis of alpha-fluorinated phosphonates via tin-mediated cleavage of alpha-fluoro-alpha-(pyrimidin-2-ylsulfonyl)alkylphosphonates. Intramolecular cyclization of the alpha-phosphonyl radicals

Treatment of the alpha carbanions generated from several alpha-(pyrimidin-2-ylsulfonyl)alkylphosphonates with Selectfluor gave high yields of the alpha-fluoro-alpha-(pyrimidin-2-ylsulfonyl)alkylphoshonates, which were desulfonylated [Bu(3)SnH/2,2'-azobisisobutyronitrile (AIBN)/benzene or toluene/Delta] to give alpha-fluoroalkylphosphonates. "Catalytic" tin hydride, generated from tributyltin chloride and excess polymethylhydrosiloxane in the presence of potassium fluoride, also effected removal of the pi-deficient alpha-(pyrimidin-2-ylsulfonyl) group from the phosphonate esters. Substitution of Bu(3)SnD for Bu(3)SnH gave access to alpha-deuterium-labeled phosphonates. Prolonged treatment of alpha-(pyridin-2-ylsulfonyl)alkylphosphonate with excess Bu(3)SnH/AIBN or catalytic tin hydride also effected desulfonylation but in moderate yields. This represents a mild new methodology for removal of the synthetically useful pi-deficient heterocyclic sulfone moiety and an alternative route for the preparation of alpha-fluorinated phosphonates. Desulfonylation is suggested to proceed via attack of tin radical at an oxygen (or sulfur) atom of the sulfonyl group to give a stabilized alpha-phosphonyl radical intermediate. The latter was found to undergo 5-exo-trig ring closure to give the corresponding 2-methylcyclopentylphosphonates. Treatment of diethyl 1-bromohex-6-enylphosphonate with Bu(3)SnH/AIBN produced an analogous mixture of ring-closure products. Treatment of [(2-bromo-5- methoxyphenyl)(fluoro)(pyrimidin-2-ylsulfonyl)]methylphosphonate with Bu(3)SnH resulted in an intramolecular radical [1,5]-ipso substitution reaction and migration of the pyrimidinyl ring to give fluoro[5-methoxy-2-(pyrimidin-2-yl)phenyl]methylphosphonate.     

On the mechanism of the ethyl elimination from the molecular ion of 6-methoxy-1-hexene

It is shown by ¹³C and D labelling that the ethyl radical elimination from the molecular ion of 6-methoxy-1-hexene is a very complex process involving at least two different channels. The major channel (80%) is induced by an initial 1,5-hydrogen shift in the molecular ion from C(5) to C(l) leading via a series of steps to methoxy-cyclohexnne, which then undergoes a ring contraction to 2-methyl-1-methoxycyclopentane, being the key intermediate for the ethyl loss. The same key intermediate is formed in the other, minor channel (20%) by ring closure directly following an initial 1,6-hydrogen shift in the molecular ion of 6-methoxy-1-hexene from C(6) to C(l). Collision-induced dissociation experiments on the [M ? ethyl]⁺ ion from 6-methoxy-1-hexene have further established that it has the unique structure of oxygen methyl cationized 2-methyIpropen-2-al. This ion is also generated by ethyl loss from the molecular ion of 2-methyl-1-methoxycyclopentane itself, as shown by collision-induced dissociation experiments, thus confirming the key role of the intermediate mentioned.     

The cyclooctatriene-eta2-ynyl potassium zwitterionic radical: evidence for a potassium organometallic

Low-temperature (-120 degrees C) dehydrohalogenation of bromocyclooctatetraene (BrC8H7) with either sodium or potassium tert-butoxide followed by alkali metal reduction was used to generate the anion radical of [8]annulyne (C8H6*-) in tetrahydrofuran. EPR analysis at -120 degrees C reveals an extraordinarily large metal splitting when K or Cs (aK of 0.214 G and aCs of 3.26 G) serves as the reducing agent. The large aM is due to the metal cation interacting with the p-orbitals, within the alkyne moiety, that are in the plane of the ring system. The ionic radius of K+ is 1.33 A, which is larger than the B3LYP predicted distance between carbons 1 and 2 (1.23 A). However, the ionic radius of Na+ is only 0.95 A, and it is too small to simultaneously interact with both p-orbitals. Hence, no aM is observed when Na (ordinarily aNa > aK) or Li serves as the reducing agent. After the addition of 18-crown-6 to either the K or the Cs reduced system, two anion radicals are present. One is the system where the 18-crown-6 encapsulated metal complex is normally ion paired over the face of the ring system and aM = 0. The other is the cyclooctatriene-eta2-ynyl 18-crown-6 encapsulated metal zwitterion radical exhibiting a large aM. The ion pair to organometallic equilibrium constant is 1.6 +/- 0.1 and 3.5 +/- 0.1 for the K and Cs systems, respectively.