Supporting InformationReferences to metal-mediated/catalyzed cycloaddition reactions leading to seven-membered ring formation:(1) Pentadienyl/alkyne [5 + 2] and [5 + 2 + 2] cycloadditions (various metals): (a) Wilson, A.

Alkyl-substituted η5-pentadienyl complexes of cobalt have been reported to undergo [5 + 2] cycloaddition reactions with alkynes to form substituted η2,η3- and η5-cycloheptadienyl products, providing a new route to the synthesis of substituted cycloheptadienes. A series of cyclopentadienyl and pentamethylcyclopentadienyl cobalt(III) η5-pentadienyl complexes have been prepared, incorporating alkyl and aryl substituents at various positions on the pentadienyl ligand. The crystalline complexes have been completely characterized spectroscopically and, in the solid state, by X-ray crystallography. The alkyl-substituted pentadienyl complexes can be prepared by a range of methodologies, most generally by acid-promoted dehydration of in situ-derived η2- or η4-dienol complexes. Two variations on this classic strategy have been developed, starting from either conjugated (1,3-) or nonconjugated (1,4-) dienyl alcohols. For both cyclopentadienyl and pentamethylcyclopentadienyl ancillary ligands, the substituted η5-pentadienyl complexes are obtained in reasonable to good isolated yields, limited by the extent of substitution on the starting allylic alcohol. The cationic cobalt(III) η5-pentadienyl complexes are indefinitely stable to air and moisture; isolation and purification is accomplished by chromatography on the bench. The substitutional lability of the pentadienyl ligand has been investigated using both neutral and anionic donor ligands (CO, isonitrile, acetonitrile, and halide salts). The results reveal that η5-pentadienyl complexes react by equilibrium dissociation of the most substituted end of the pentadienyl moiety, providing the corresponding η3-coordinated pentadienyl adducts. A comparative study of 11 X-ray crystal structures covering a range of pentadienyl substitution patterns is also reported, revealing long Co−C1 and Co−C5 bonds and, consistently, short Co−C3 bond lengths. The internal bond angles of substituent-bearing carbon atoms are smaller than those observed at unsubstituted carbons, with the compression as much as 10° at the substituted center. As expected, alkyl substituents invariably deviate out of the pentadienyl plane, toward the metal. In unbiased cases, greater deviations are noted for C1 substituents than for C2 substituents, although the deviations at C2 increase significantly for 1,2-disubstituted ligands. In some cases, the carbon−carbon bond distances display distortions toward η2,η3- or η1,η4-hapticity, although no particular correlation between structural distortion and solution reactivity is evident.

The preparation of seven-membered carbocycles via traditional organic synthesis is difficult, yet essential, due to the prevalence of these moieties in bioactive compounds. As we report, the Co-mediated pentadienyl/alkyne [5 + 2] cycloaddition reaction generates kinetically stable η 2 ,η 3 -cycloheptadienyl complexes in high yield at room temperature, which isomerize to the thermodynamically preferred η 5 -cycloheptadienyl complexes upon heating at 60−70°C. Here we describe an extended investigation of this reaction manifold, exploring substituent effects and extending the reaction to tandem cycloaddition/nucleophilic cyclizations, generating fused bicyclic compounds. We also describe a new high-yielding photolytic method for the decomplexation of organic cycloheptadienes from Co(I) complexes. Both C 5 Me 5 (Cp*) and C 5 H 5 (Cp) halfsandwich complexes are active in [5 + 2] cycloaddition with alkynes, with Cp* generally providing higher yields of cycloheptadienyl complexes. Cp cycloheptadienyl complexes, however, are resistant to thermal η 2 ,η 3 → η 5 isomerization. The reaction remains limited to open pentadienyl complexes incorporating substituents in the terminal (1 and 5) positions, except for the unsubstituted CpCo(η 5 -cycloheptadienyl) + complex, which is modestly reactive. Incorporation of tethered latent nucleophiles allows cyclization onto the intermediate cycloheptadienyl cations, producing bicyclo[5.3.0]decadiene and bicyclo[5.4.0]-undecadiene systems with complete diastereocontrol. A selection of intermediate complexes have been crystallographically characterized. Addition of tethered malonate nucleophiles occurs reversibly with equilibration to a thermodynamic elimination product, while enolate nucleophiles cyclize reliably under kinetic control. The resulting bicyclic products are decomplexed in high (>90%) yield by UV photolysis in the presence of allyl bromide to provide the organic bicyclic diene with complete retention of ring fusion geometry and without double-bond isomerization. ■ INTRODUCTIONSince the first report of the Diels−Alder reaction, cycloaddition methodology has become a key component of the synthetic organic chemist's toolbox. This utility is largely due to the formation of multiple carbon−carbon bonds in a single step, often with excellent control of regiochemistry and stereoselectivity. Thermal, photochemical, and metal-mediated cycloaddition reactions for the synthesis of small-and medium-sized carbocycles have been developed, with four-, five-, and sixmembered rings receiving most of the attention (i.e., [2 + 2], [3 + 2], and [4 + 2] cycloaddition reactions). In contrast, the preparation of seven-membered rings by cycloaddition remains less developed, 1 despite the almost continuous discovery of bioactive natural products with seven-membered-ring structures in the core. 2 One important pathway to access the sevenmembered ring is the formal [5 + 2] cycloaddition reaction, which has a rich and varied history. 3 Among unsolved problems in [5 + 2] cycloaddition is the cyclization...

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