The [N]phenylenes display marked deformation from planarity in the crystalline state. In order to probe the generality of this phenomenon, several derivatives were synthesized and their single-crystal X-ray structures were obtained. All new compounds displayed some degree of nonplanarity. Thus, for example, the parent triangular [4]phenylene (4 b) has a median bend angle at the ring junction of 1.58 and a range of 0.38 to 3.58, whereas hexakis[triisopropylsilyl(ethynyl)] triangular [4]phenylene (4 c) possesses the bulkiest appendages and the largest median bend angle and range (3.88 and 1.78 ± 5.68, respectively). A detailed analysis of the bending and twisting angles at the ring junctions, however, revealed that the magnitude of deformations were independent of topology, molecular size, and substituent type. In contrast to the phenylenes, a Cambridge Structural Database (CSD) search of unsubstituted and non-peri-substituted naphthalenes and anthracenes shows these molecules to be virtually planar in the solid state. A comparison of the single-point energies (HF/6-31G*) of the phenylenes with the acenes calculated for molecules possessing a fixed bend angle at the ring fusion of 38, 68, 98, and 128 reveals the former to be 26 % to 45 % easier to deform than the latter. Based on these results, the nonplanarity seen for the phenylenes is most likely a consequence of crystalpacking forces deforming particularly flexible molecules.

Abstract:The cobalt mediated [2+2+2]-cycloaddition of enediynes 13 and 18 affords linear annelated polycycles such as the hexahydro naphthalene 14 and the decahydro anthracene 19. This latter compound can be transformed into the enantiomerically pure anthracene derivative 21 in only two reaction steps. 1,9,10-Trihydroxy octahydro anthracene (21) represents the ABC-framework of many anthracycline antibiotics such as the antitumor active daunomycine. Key words:[2+2+2]-cycloaddition, enediynes, annelated polycycles Generally, the cobalt mediated intramolecular [2+2+2]-cycloaddition 2 of enediynes such as 1 and 3 affords angular annelated polycycles such as 2 3 and 4 (Scheme 1). 4 For the synthesis of linear annelated polycycles the intermolecular co-cyclization of diynes with olefines was used until now. 5 This intermolecular approach appears to be problematic due to a lack of chemo-and regioselectivity of the [2+2+2]-cycloaddition. Scheme 1In our hands the reaction of diyne 5 with 1.5 equivalents of olefin 8 in the presence of CpCo(CO) 2 under irradiation afforded exclusively the cyclobutadiene cobalt complex 7 in 76% yield (Scheme 2). 6 None of the regioisomeric tricycles 9a or 9b could be detected. In the first reaction step the cobaltacyclopentadiene 6 is formed. Obviously, the rate of irreversible rearrangement of this complex into the unreactive cyclobutadiene cobalt complex is much faster than a Diels-Alder type cycloaddition. This undesired side reaction might be suppressed by using olefin 8 as a solvent or by adding the diyne slowly to a solution containing an excess of olefin 8 in a suitable solvent. For an application of this reaction in natural product synthesis by using valuable olefins such as 8, both reaction pathways did not seem to be suitable due to economical reasons and from the point of view of regioselectivity and the overall yield. Therefore, the following approach was chosen: prior to cyclization the diyne and the olefin should be linked via a temporary silicon oxygen tether. 7 In this case the [2+2+2]-cycloaddition can be performed in an intramolecular manner which proceeds much faster and with a complete regioselective control. After the [2+2+2]-cycloaddition this tether can be cleaved hydrolytically or oxidatively.Consequently, 1-trimethylsilyl-octa-1,7-diyne (10) was deprotonated by n-BuLi and allowed to react with Me 2 Si(NEt 2 )Cl to form the disilylated octadiyne derivative 11. This was treated in situ with allylic alcohols 12 or 17 8 to form enediynes 13 and 18 (Scheme 3) under liberation of diethyl amine, which was removed from the reaction mixture under a nitrogen stream. After distillation the enediynes 13 and 18 were isolated in 80-96% yield in analytically pure form. Subsequent CpCo(CO) 2 mediated [2+2+2]-cycloaddition followed by oxidative demetallation using FeCl 3 afforded dienes rac-14a-d and 19 in yields up to 81% (Scheme 3, Table 1). The [2+2+2]-cycloaddition was completed within 2.5 h. The cyclization could be scaled up to 3.00 g without any loss of regio-, chemo-and st...

K. Moth-Poulsen would like to thank the Danish research council (grant 09-066585/FNU), the Swedish Research Council (grant 2011-3613), and Chalmers Material and Energy areas of advance for financial support. We acknowledge the National Science Foundation (NSF) Center for Scalable and Integrated Nanomanufacturing SINAM fund CMMI-0751621 and CHE-0907800 (K.P.C.V.) for assisting this work, and the UC Berkeley Marvell Nanofabrication Laboratory for facilitating the assembly of the microfluidic devices. Notes and references

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To understand some experimental data at odds with the computed mechanism of the CpCo(L2)-catalyzed [2 + 2 + 2] cyclotrimerization of ethyne, DFT computations were carried out following the fate of methyl- and hydroxycarbonyl-substituted alkynes to give the corresponding arenes. The key intermediate in all cases is a triplet cobaltacyclopentadiene obtained by oxidative coupling of the corresponding CpCo(bisalkyne) complex and subsequent spin change via a minimum energy crossing point (MECP). From that species, two different catalytic cycles lead to an arene product, depending on the nature of the alkyne and other ligands present: either alkyne ligation to furnish a cobaltacyclopentadiene(alkyne) intermediate or trapping by a sigma-donor ligand to generate a coordinatively saturated cobaltacyclopentadiene(PR3) complex. The former leads to the CpCo-complexed arene product via intramolecular cobalt-assisted [4 + 2] cycloaddition, whereas the latter may, in the case of a reactive dienophile (butynedioic acid), undergo direct intermolecular [4 + 2] cycloaddition to generate a cobaltanorbornene. The bridgehead cobalt atom is then reductively eliminated after another change in spin state from singlet to triplet. The necessary conditions for one or the other mechanistic pathway are elaborated.

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Broad-band irradiation (λ max ) 350 nm) of FvRu 2 (CO) 4 (1, Fv ) η 5 :η 5 -bicyclopentadienyl) resulted in rapid isomerization to colorless (µ 2 -η 1 :η 5 -cyclopentadienyl) 2 Ru 2 (CO) 4 (2) in a novel process involving a formal dinuclear oxidative addition to a C-C bond. The product reverted to 1 upon heating in solution or in the solid state, under the latter conditions with an enthalpy change of -29.8 (1.5) kcal mol -1 . Mechanistic studies with a mixture of 1 and 1-d 8 revealed the absence of label scrambling, pointing to intramolecular pathways. The quantum yield (0.15) was unaffected by the presence of CCl 4 , and no chlorination products were observed under these conditions. Irradiation of solutions of 1 or 2 with 300 nm light provided Fv(µ 2 -η 1 :η 5 -cyclopentadienyl) 2 Ru 4 (CO) 6 (6) or, in the presence of alkynes, the adducts FvRu 2 (CO) 3 (RCCR) (8-10, R ) H, C 6 H 5 , CO 2 CH 3 ). Heating 1 and PR 3 (R ) CH 2 CH 3 , CH 3 , or OCH 3 ) yielded FvRu 2 (CO) 3 (PR 3 ) (12-14), in which a fluxional process occurs characterized by intramolecular terminal to bridging carbonyl exchange. While 12 and 13 were inert, compound 14 rapidly and reversibly afforded the P(OCH 3 ) 3 -substituted analog of 2 (15) upon irradiation with UV light. The two diastereomeric 3,3′-di-tert-butyl-substituted fulvalene analogs of 1 (19) underwent the same reaction sequence with complete retention of stereochemistry, via the diastereomeric photoproducts 20. A double regiochemical labeling experiment proceeded with retention of connectivity and stereochemistry. A concerted mechanism for the photoisomerization is consistent with the experimental observations, but a biradical pathway cannot be ruled out. Kinetic data for the isomerizations of 2, 15, 20a, and 20b to their respective metal-metal-bonded Fv precursors were determined. The entropies of activation (+7 to +21 eu) suggested a disordered transition state. A sequence involving reversible CO loss was ruled out through a crossover experiment with 2-13 CO. Kinetic and labeling experiments point to a change in mechanism when the thermal reversion of 2 to 1 was run under CO (∼1 atm). The occurrence of ligand-induced C-C coupling was indicated through studies of the reactivity of 2 with P(CH 3 ) 3 . Photoisomer 2 reacts with excess CCl 4 to give FvRu 2 (CO) 4 Cl 2 ) by yet another mechanism. As in the potoisomerization of 1, the thermal reversion of 2 may follow a concerted pathway, although biradical intermediates cannot be excluded.(51) This problem is intractable by simple analysis, because (1) Ru has seven natural isotopes, generating 16 readily observable peaks in the mass spectra of 1 and 2, (2) 13 CO enrichment of 2-( 13 CO) was not complete, (3) the commercial 13 CO used (Cambridge Isotope Laboratories, Inc.) contains approximately 10% 18 O, and (4) natural abundance 13 C in the samples had to be considered. We thank Mr. D. Peterson and Professor J. Rice, UCB Department of Statistics for their help in solving it.(52) Hoffmann, R.; Minkin, V. I.; Carpenter, B. K.

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