No abstract

Carbon-fluorine bonds are the strongest known single bonds to carbon and as a consequence can prove very hard to cleave. Alhough vinyl and aryl C-F bonds can undergo oxidative addition to transition metal complexes, this reaction has appeared inoperable with aliphatic substrates. We report the addition of C(sp(3))-F bonds (including alkyl-F) to an iridium center via the initial, reversible cleavage of a C-H bond. These results suggest a distinct strategy for the development of catalysts and promoters to make and break C-F bonds, which are of strong interest in the context of both pharmaceutical and environmental chemistry.

The V-H bonds in HV(CO)4(P-P) are unusually weak (55 to 58 kcal/mol). They transfer H* to styrene more rapidly than CpCr(CO)3H does and are effective stoichiometric reagents for the cyclization of appropriate 1,6-dienes.

Treating (etha(5)-C(5)H(5))Cr(CO)3H (1) or (etha(5)-C(5)H(5))Cr(CO)3D (1-d(1)) with an excess of olefin containing the opposite isotope generally leads to H/D exchange, although hydrogenation is also observed in some cases. Application of an appropriate statistical correction to the observed exchange rate gives kH and kD, the rate constants for H* (D*) transfer from (etha(5)-C(5)H(5))Cr(CO)(3)H (D) to various olefins. The values of kH and kD vary appreciably with the substituents on the double bond. Phenyl-substituted olefins accept H* more readily than do carbomethoxy-substituted olefins, although the latter accept H* more readily than do alkyl-substituted olefins. A methyl substituent on the incipient radical site increases k(H) at 323 K by a factor between 5 and 50. A methyl substituent on the carbon to which the H* is being transferred decreases kH substantially. On the whole, the rate constants for H* transfer reflect steric effects as well as the stability of the resulting carbon-centered radicals.

A pincer-ligated iridium complex, (PCP)Ir (PCP = κ(3)-C6H3-2,6-[CH2P(t-Bu)2]2), is found to undergo oxidative addition of C(sp(3))-O bonds of methyl esters (CH3-O2CR'), methyl tosylate (CH3-OTs), and certain electron-poor methyl aryl ethers (CH3-OAr). DFT calculations and mechanistic studies indicate that the reactions proceed via oxidative addition of C-H bonds followed by oxygenate migration, rather than by direct C-O addition. Thus, methyl aryl ethers react via addition of the methoxy C-H bond, followed by α-aryloxide migration to give cis-(PCP)Ir(H)(CH2)(OAr), followed by iridium-to-methylidene hydride migration to give (PCP)Ir(CH3)(OAr). Methyl acetate undergoes C-H bond addition at the carbomethoxy group to give (PCP)Ir(H)[κ(2)-CH2OC(O)Me] which then affords (PCP-CH2)Ir(H)(κ(2)-O2CMe) (6-Me) in which the methoxy C-O bond has been cleaved, and the methylene derived from the methoxy group has migrated into the PCP Cipso-Ir bond. Thermolysis of 6-Me ultimately gives (PCP)Ir(CH3)(κ(2)-O2CR), the net product of methoxy group C-O oxidative addition. Reaction of (PCP)Ir with species of the type ROAr, RO2CMe or ROTs, where R possesses β-C-H bonds (e.g., R = ethyl or isopropyl), results in formation of (PCP)Ir(H)(OAr), (PCP)Ir(H)(O2CMe), or (PCP)Ir(H)(OTs), respectively, along with the corresponding olefin or (PCP)Ir(olefin) complex. Like the C-O bond oxidative additions, these reactions also proceed via initial activation of a C-H bond; in this case, C-H addition at the β-position is followed by β-migration of the aryloxide, carboxylate, or tosylate group. Calculations indicate that the β-migration of the carboxylate group proceeds via an unusual six-membered cyclic transition state in which the alkoxy C-O bond is cleaved with no direct participation by the iridium center.