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We describe herein computational studies on the unusual ability of Pd(PtBu ) to catalyze formation of highly reactive acid chlorides from aryl halides and carbon monoxide. These show a synergistic role of carbon monoxide in concert with the large cone angle PtBu that dramatically lowers the barrier to reductive elimination. The tertiary structure of the phosphine is found to be critical in allowing CO association and the generation of a high energy, four coordinate (CO)(PR )Pd(COAr)Cl intermediate. The stability of this complex, and the barrier to elimination, is highly dependent upon phosphine structure, with the tertiary steric bulk of PtBu favoring product formation over other ligands. These data suggest that even difficult reductive eliminations can be rapid with CO association and ligand manipulation. This study also represents the first detailed exploration of all the steps involved in palladium-catalyzed carbonylation reactions with simple phosphine ligands, including the key rate-determining steps and palladium(0) catalyst resting state in carbonylations.

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.

A novel efficient NaOtBu-mediated protocol for the synthesis of 3,3-disubstituted aza-oxindoles proceeds via a Truce-Smiles rearrangement-cyclisation pathway.

A new room-temperature chromium tricarbonyl-mediated cycloaromatization of enediynes is reported. The reaction occurs with both cyclic and acyclic enediynes in the presence of [Cr(CO)3(η6-naphthalene)] and both a coordinating solvent and a hydrogen atom source, providing chromium–arene complexes in reasonable yield and good diastereocontrol. The mechanism of the reaction has been probed through DFT computational and spectroscopic methods. These studies suggest that direct C1–C6 bond formation from an η6-enediyne complex is the lowest-energy path, forming a metal-bound p-benzyne biradical. NMR spectroscopy suggests that enediyne alkene coordination occurs in preference to alkyne coordination, forming a THF-stabilized olefin intermediate; subsequent alkyne coordination leads to cyclization. While biradical quenching occurs rapidly and primarily via the singlet biradical, the triplet state biradical is detectable by EPR spectroscopy, suggesting intersystem crossing to a triplet ground state

Alkyl-substituted η(5)-pentadienyl half-sandwich complexes of cobalt have been reported to undergo [5+2] cycloaddition reactions with alkynes to provide η(2),η(3)-cycloheptadienyl complexes under kinetic control. DFT studies have been used to elucidate the mechanism of the cyclization reaction as well as that of the subsequent isomerization to the final η(5)-cycloheptadienyl product. The initial cyclization is a stepwise process of olefin decoordination/alkyne capture, C-C bond formation, olefin arm capture, and a second C-C bond formation; the initial decoordination/capture step is rate-limiting. Once the η(2),η(3)-cycloheptadienyl complex has been formed, isomerization to η(5)-cycloheptadienyl again involves several steps: olefin decoordination, β-hydride elimination, reinsertion, and olefin coordination; also here the initial decoordination step is rate limiting. Substituents strongly affect the ease of reaction. Pentadienyl substituents in the 1- and 5-positions assist pentadienyl opening and hence accelerate the reaction, while substituents at the 3-position have a strongly retarding effect on the same step. Substituents at the alkyne (2-butyne vs. ethyne) result in much faster isomerization due to easier olefin decoordination. Paths involving triplet states do not appear to be competitive.

Pd/PtBu(3)-catalyzed intramolecular C-O bond formation has been used to access aryl- and alkyl-substituted benzoxazinones.

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.