Alan S. Goldman

Alkane metathesis by tandem alkane-dehydrogenation-olefin-metathesis catalysis and related chemistry

Methods for the conversion of both renewable and non-petroleum fossil carbon sources to transportation fuels that are both efficient and economically viable could greatly enhance global security and prosperity. Currently, the major route to convert natural gas and coal to liquids is Fischer-Tropsch catalysis, which is potentially applicable to any source of synthesis gas including biomass and nonconventional fossil carbon sources. The major desired products of Fischer-Tropsch catalysis are n-alkanes that contain 9-19 carbons; they comprise a clean-burning and high combustion quality diesel, jet, and marine fuel. However, Fischer-Tropsch catalysis also results in significant yields of the much less valuable C(3) to C(8)n-alkanes; these are also present in large quantities in oil and gas reserves (natural gas liquids) and can be produced from the direct reduction of carbohydrates. Therefore, methods that could disproportionate medium-weight (C(3)-C(8)) n-alkanes into heavy and light n-alkanes offer great potential value as global demand for fuel increases and petroleum reserves decrease. This Account describes systems that we have developed for alkane metathesis based on the tandem operation of catalysts for alkane dehydrogenation and olefin metathesis. As dehydrogenation catalysts, we used pincer-ligated iridium complexes, and we initially investigated Schrock-type Mo or W alkylidene complexes as olefin metathesis catalysts. The interoperability of the catalysts typically represents a major challenge in tandem catalysis. In our systems, the rate of alkane dehydrogenation generally limits the overall reaction rate, whereas the lifetime of the alkylidene complexes at the relatively high temperatures required to obtain practical dehydrogenation rates (ca. 125 -200 °C) limits the total turnover numbers. Accordingly, we have focused on the development and use of more active dehydrogenation catalysts and more stable olefin-metathesis catalysts. We have used thermally stable solid metal oxides as the olefin-metathesis catalysts. Both the pincer complexes and the alkylidene complexes have been supported on alumina via adsorption through basic para-substituents. This process does not significantly affect catalyst activity, and in some cases it increases both the catalyst lifetime and the compatibility of the co-catalysts. These molecular catalysts are the first systems that effect alkane metathesis with molecular-weight selectivity, particularly for the conversion of C(n)n-alkanes to C(2n-2)n-alkanes plus ethane. This molecular-weight selectivity offers a critical advantage over the few previously reported alkane metathesis systems. We have studied the factors that determine molecular-weight selectivity in depth, including the isomerization of the olefinic intermediates and the regioselectivity of the pincer-iridium catalyst for dehydrogenation at the terminal position of the n-alkane. Our continuing work centers on the development of co-catalysts with improved interoperability, particularly olefin-metathesis catalysts that are more robust at high temperature and dehydrogenation catalysts that are more active at low temperature. We are also designing dehydrogenation catalysts based on metals other than iridium. Our ongoing mechanistic studies are focused on the apparently complex combination of factors that determine molecular-weight selectivity.

Catalytic Cleavage of Ether CO Bonds by Pincer Iridium Complexes

The development of efficient catalytic methods to cleave the relatively unreactive C-O bonds of ethers remains an important challenge in catalysis. Building on our groups recent work, we report the dehydroaryloxylation of aryl alkyl ethers using pincer iridium catalysts. This method represents a rare fully atom-economical method for ether C-O bond cleavage.

Net Oxidative Addition of C(sp3)-F Bonds to Iridium via Initial C-H Bond Activation

An unusual mechanism to cleave carbon-fluorine bonds may facilitate more efficient transformations of fluorocarbons. 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(sp3)-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.

Cleavage of sp3 C-O Bonds via Oxidative Addition of C-H Bonds

(PCP)Ir (PCP = kappa(3)-C(6)H(3)-2,6-[CH(2)P(t-Bu)(2)](2)) is found to undergo oxidative addition of the methyl-oxygen bond of electron-poor methyl aryl ethers, including methoxy-3,5-bis(trifluoromethyl)benzene and methoxypentafluorobenzene, to give the corresponding aryloxide complexes (PCP)Ir(CH(3))(OAr). Although the net reaction is insertion of the Ir center into the C-O bond, density functional theory (DFT) calculations and a significant kinetic isotope effect [k(CH(3))(OAr)/k(CD(3))(OAr) = 4.3(3)] strongly argue against a simple insertion mechanism and in favor of a pathway involving C-H addition and alpha-migration of the OAr group to give a methylene complex followed by hydride-to-methylene migration to give the observed product. Ethoxy aryl ethers, including ethoxybenzene, also undergo C-O bond cleavage by (PCP)Ir, but the net reaction in this case is 1,2-elimination of ArO-H to give (PCP)Ir(H)(OAr) and ethylene. DFT calculations point to a low-barrier pathway for this reaction that proceeds through C-H addition of the ethoxy methyl group followed by beta-aryl oxide elimination and loss of ethylene. Thus, both of these distinct C-O cleavage reactions proceed via initial addition of a C(sp(3))-H bond, despite the fact that such bonds are typically considered inert and are much stronger than C-O bonds.

Transfer-dehydrogenation of alkanes catalyzed by rhodium(I) phosphine complexes

Abstract Complexes of the form Rh(PMe3)2ClL′ (L′ = CO or trisubstituted phosphine) and [Rh(PMe3)2Cl]2 have previously been reported to catalyze the transfer-dehydrogenation of alkanes, using olefinic hydrogen acceptors under a dihydrogen atmosphere. Such complexes are herein reported to effect transfer-dehydrogenation in the absence of H2 but with much lower rates and total catalytic turnovers, even at much greater temperatures. Analogs with halides other than chloride (Br, I), or with pseudo-halides (OCN, N3), are found to exhibit generally similar behavior: high catalytic activity under H2 and measurable but much lower activity in the absence of H2. Thermolysis (under argon) of complexes [RhL2Cl]n (n = 1, 2; L is a phosphine bulkier than PMe3) in cyclooctane in the absence of hydrogen acceptors yielded cyclooctene. However, transfer-dehydrogenation was plagued by ligand decomposition. Under a hydrogen atmosphere complexes containing ligands much bulkier than PMe3 do not effect dehydrogenation. Complexes with tridentate ligands, (η3-PXP)RhL′ (PXP = (Me2PCH2Me2Si)2N, Me2PCH2(2,6-C6H3)CH2PMe2; L′ = CO, C2H4), were also found to catalyze thermal or photochemical dehydrogenation of cyclooctane with limited reactivity. The structure of [Rh(PMe3)2Cl]2 was determined by single-crystal diffraction. The Rh(μ-Cl)2Rh bridge of 1 is folded like that of [Rh(CO)2Cl]2, unlike that of the planar PPh3 and PiPr3 analogs.

Mechanism of the photochemical dehydrogenation and transfer-dehydrogenation of alkanes catalyzed by trans-Rh(PMe3)2(CO)Cl

Abstract Rh(PMe 3 ) 2 (CO)Cl catalyzes the photochemical dehydrogenation of alkanes to yield alkenes. H 2 is evolved or, in the presence of potential hydrogen acceptors such as t-butylethylene and styrene, hydrogen can be transferred to the olefins. The photokinetics and selectivity of the dehydrogenation reactions have been investigated. Both reactions proceed via a single photochemical step, photoextrusion of carbon monoxide from Rh(PMe 3 ) 2 (CO)Cl. Study of the transfer-dehydrogenation, particularly of the product ratio (H 2 : hydrogenated acceptor), reveals aspects of the dehydrogenation mechanism including steps subsequent to the turnover-limiting step.

Ir-Catalyzed Functionalization of C–H Bonds

The ability to selectively functionalize C–H bonds holds enormous potential value in virtually every sphere of organic chemistry, from fuels to pharmaceuticals. Transition metal complexes have shown great promise in this context. Iridium provided the first examples of oxidative addition of C–H bonds; this addition is key to iridium’s leading role in alkane dehydrogenation and related reactions. Catalysts based on iridium have also proven highly effective for valuable borylations of C–H bonds and, to a lesser extent, for C–Si coupling. Compared with other platinum group metals, iridium chemistry has not been developed as extensively for the elaboration of C–C bonds from C–H bonds, but significant promise is indicated, particularly for coupling with simple hydrocarbons which lack functionalities that can act as directing groups.

Catalytic ring expansion, contraction, and metathesis-polymerization of cycloalkanes

Tandem dehydrogenation-olefin-metathesis catalyst systems, comprising a pincer-ligated iridium-based alkane dehydrogenation catalyst and a molybdenum-based olefin-metathesis catalyst, are reported to effect the metathesis-cyclooligomerization of cyclooctane and cyclodecane to give cycloalkanes with various carbon numbers, predominantly multiples of the substrate carbon number, and polymers.

Thermochemical alkane dehydrogenation catalyzed in solution without the use of a hydrogen acceptor

(PCP)IrH2 [PCP = η3-C6H3(PBut2)2-1,3] catalyzes the efficient (several hundred mol product/mol catalyst) dehydrogenation of alkanes under reflux to give the corresponding alkenes and dihydrogen.

Catalytic Synthesis of n-Alkyl Arenes through Alkyl Group Cross-Metathesis

n-Alkyl arenes were prepared in a one-pot tandem dehydrogenation/olefin metathesis/hydrogenation sequence directly from alkanes and ethylbenzene. Excellent selectivity was observed when ((tBu)PCP)IrH2 was paired with tungsten monoaryloxide pyrrolide complexes such as W(NAr)(C3H6)(pyr)(OHIPT) (1a) [Ar = 2,6-i-Pr2C6H3; pyr = pyrrolide; OHIPT = 2,6-(2,4,6-i-Pr3C6H2)2C6H3O]. Complex 1a was also especially active in n-octane self-metathesis, providing the highest product concentrations reported to date. The thermal stability of selected olefin metathesis catalysts allowed elevated temperatures and extended reaction times to be employed.