Iron Dihydride Complexes: Synthesis,Reactivity, and Catalytic Applications

Iron dihydride complexes often play important roles in catalytic reactions that employ reducing agents such as H₂, boranes, and silanes. Development of more efficient and selective iron-based catalysts for these processes requires effective synthetic strategies and a deeper understanding of the reactivity of the dihydride complexes. The purpose of this review is to provide the readers with an overview of different ligand systems that have been used to support iron dihydride complexes.     

[2+2+2]‐Cyclotrimerization of 1‐Cyclopropyl‐1,6‐diynes with Alkynes: Formation of Cyclopropylarenes.

Cyclotrimerization of 1‐cyclopropyl‐1,6‐diynes with various terminal alkynes was tested under catalytic conditions using rhodium and ruthenium catalysts. We observed that the regioselectivity of the reaction, that is, formation of 1,2‐ or 1,3‐regioisomers, was opposite for the two metals. For the ruthenium complex [Cp*Ru(cod)Cl]‐catalyzed reactions the yields were in many cases high with a strong preference for the formation of 1,3‐substituted regioisomers. In the case of catalysis by the rhodium complex [RhCl(PPh₃)₃], 1,2‐substituted products were generally preferred, albeit the selectivity was often modest. However, by changing the ligand environment around the central rhodium atom the regioselectivity as well as yields of the products were significantly improved. For example, by using a combination of the rhodium complex [Rh(cod)₂BF₄] and 1,4‐bis(diphenylphosphino)butane the regioselectivity was changed from 1:1 to 1:12 in favor of the 1,2‐regioisomer. This catalytic system was also applied for synthesis of a substituted 4‐cyclopropyl‐3‐hydroisobenzofuran‐1‐one that could serve as a potential intermediate for preparation of antihypertensive agents.

     

Iron‐Catalyzed Oppenauer‐Type Oxidation of Alcohols

A (hydroxycyclopentadienyl)iron dicarbonyl hydride catalyzes the Oppenauer‐type oxidation of alcohols with acetone as the hydrogen acceptor. Many functional groups are tolerant to the oxidation conditions. The same complex also catalyzes the dehydrogenation of diols to lactones. A mechanism involving the formation of iron‐alcohol complexes and their rapid ligand exchange with free alcohols is proposed. The trimethylsilyl groups on the cyclopentadienyl ligand of the catalyst play a critical role in stabilizing the iron hydride and increasing the catalyst lifetime.     

Hexakis(PCP‐Platinum and ‐Ruthenium) Complexes by the Transcyclometalation Reaction and Their Use in Catalysis

Hexakis(PCP‐pincer) complexes [C₆{PtBr(PCP)}₆] ( 5d ) and [C₆{RuCl(PCP)(PPh₃)}₆] ( 5e ) were synthesized via the transcyclometalation (TCM) procedure. Mixing the hexakis(PCHP‐arene) ligand 7 with six equivalents of [PtBr(NCN)] ( 1a ) or [RuCl(NCN)(PPh₃)] ( 1b ), respectively, resulted in the selective metalation of all PCP‐ligand sites and the concomitant formation of six equivalents of the NCHN‐arene ligand. This procedure was found to be superior over existing metalation procedures. In addition, hexakisruthenium complex 5e was applied as homogeneous catalyst in the hydrogen transfer reactions of cyclohexanone, acetophenone and benzophenone to the corresponding alcohols. In these reactions, the activity per ruthenium center of 5e was found to be of the same order of magnitude as that of the mononuclear analogue [RuCl(PCP)(PPh₃)] 3b , indicating that all ruthenium centers act as independent catalytic sites.     

Olefin Metathesis of Vinylgermanium Derivatives as Method for the Synthesis of Functionalized Cubic and Double‐Decker Germasilsesquioxanes

For the first time, olefin cross metathesis has been developed for vinylgermanium compounds. This paper also marks the first case of transition metal‐catalyzed functionalization of heterosilsesquioxanes, which in our case are cubic vinylgermasilsesquioxanes and newly synthesized di(vinylgermyl)substituted double‐decker silsesquioxane. These processes lead to a series of new molecular, unsaturated mono‐ and divinyl‐substituted germasilsesquioxanes, which can be potentially applied as precursors for optoelectronics and for the synthesis of new advanced materials. Additionally, preliminary tests of metathetic copolymerization of divinylsubstituted double‐decker digermasilsesquioxanes (DDSQ‐2ViGe) with selected diolefins proved to be very promising and resulted in the synthesis of novel stereoregular trans‐germasilsesquioxyl‐vinylene‐phenylene macromolecular derivatives. All newly obtained compounds were isolated and characterized by mass and spectroscopic methods.

     

Catalytic Processes Involving Dihydrogen Complexes and Other Sigma-bond Complexes

The discovery of dihydrogen complexes, L_nM(H₂), pointed to direct transfer of hydrogen from coordinated H₂ ligands to substrates as an operable pathway in catalysis both in homogeneous and heterogeneous systems. Sigma complexes, L_nM(η²-H–X) (X=H, Si, C, etc), are indeed relevant in hydrogenation as well as silane alcoholysis and methane conversion.     

A Ruthenium Complex‐Catalyzed Cyclotrimerization of Halodiynes with Nitriles. Synthesis of 2‐ and 3‐Halopyridines

Monohalo‐ and dihalodiynes efficiently undergo [2+2+2] cyclotrimerization with nitriles in the presence of a catalytic amount of the ruthenium complex Cp*RuCl(cod) (10 mol%) to afford the corresponding halopyridines under ambient conditions in good isolated yields (up to 90%). The halopyridines are formed as two separable regioisomers. This is the first example of a direct synthesis of halopyridines from haloalkynes and nitriles.

     

Practical Synthesis of Chiral N,N′‐Bis(2′‐pyridinecarboxamide)‐1,2‐cyclohexane Ligands

A simple and scalable procedure for the preparation of chiral ligands 1 and 2 from trans‐1,2‐diaminocyclohexane and 2‐picolinic acid is described.     

Atom Economic Ruthenium‐Catalyzed Synthesis of Bulky β‐Oxo Esters

Ruthenium complexes with the formulae Ru(CO)₂(PR₃)₂(O₂CPh)₂ [ 6a – h ; R=n‐Bu, p‐MeO‐C₆H₄, p‐Me‐C₆H₄, Ph, p‐Cl‐C₆H₄, m‐Cl‐C₆H₄, p‐CF₃‐C₆H₄, m,m′‐(CF₃)₂C₆H₃] were prepared by treatment of triruthenium dodecacarbonyl [Ru₃(CO)₁₂] with the respective phosphine and benzoic acid or by the conversion of Ru(CO)₃(PR₃)₂ ( 8e – h ) with benzoic acid. During the preparation of 8 , ruthenium hydride complexes of type Ru(CO)(PR₃)₃(H)₂ ( 9g , h ) could be isolated as side products. The molecular structures of the newly synthesized complexes in the solid state are discussed. Compounds 6a – h were found to be highly effective catalysts in the addition of carboxylic acids to propargylic alcohols to give valuable β‐oxo esters. The catalyst screening revealed a considerably influence of the phosphine′s electronic nature on the resulting activities. The best performances were obtained with complexes 6g and 6h , featuring electron‐withdrawing phosphine ligands. Additionally, catalyst 6g is very active in the conversion of sterically demanding substrates, leading to a broad substrate scope. The catalytic preparation of simple as well as challenging substrates succeeds with catalyst 6g in yields that often exceed those of established literature systems. Furthermore, the reactions can be carried out with catalyst loadings down to 0.1 mol% and reaction temperatures down to 50 °C.

     

A Practical and Effective Ruthenium Trichloride‐Based Protocol for the Regio‐ and Stereoselective Catalytic Hydroamidation of Terminal Alkynes

A rational catalyst development based on mechanistic and spectroscopic investigations led to the discovery of a new protocol for catalytic hydroamidation reactions that draws on easily available ruthenium trichloride trihydrate (RuCl₃⋅3 H₂O) as the catalyst precursor instead of the previously employed, expensive bis(2‐methylallyl)(1,5‐cyclooctadiene)ruthenium(II). This practical and easy‐to‐use protocol dramatically improves the synthetic applicability of Ru‐catalyzed hydroamidations. The catalyst, generated in situ from ruthenium(III) chloride hydrate, tri‐n‐butylphosphine, 4‐(dimethylamino)pyridine and potassium carbonate, effectively promotes the addition of secondary amides, lactams and carbamates to terminal alkynes under formation of (E)‐anti‐Markovnikov enamides. The scope of the new protocol is demonstrated by the synthesis of 24 functionalized enamide derivatives, among them valuable intermediates for organic synthesis.     

Ruthenium‐Catalyzed O‐Allylation of Phenols from Allylic Chlorides via Cationic [Cp*(η3‐allyl)(MeCN)RuX][PF6] Complexes

The [Cp*(MeCN)₃Ru(II)][PF₆] complex is an efficient catalyst precursor for the O‐allylation of phenols with allylic chlorides in the presence of K₂CO₃ under mild conditions. This ruthenium precursor affords branched allyl aryl ethers according to a regioselective reaction, which contrasts with the uncatalyzed nucleophilic substitution from the same substrates. Stable (η³‐allyl)Ru(IV) cationic complexes resulting from the reaction of [Cp*(MeCN)₃Ru][PF₆] with allylic halides were identified as intermediate catalytic species. An X‐ray structure determination of the complex [Cp*(MeCHCHCH₂)(MeCN)RuBr][PF₆] disclosed an (endo‐trans‐MeCHCHCH₂) allylic ligand. The structural information obtained from the study of Cp*(allyl)Ru(IV) complexes indicated that electronic effects at the coordinated allylic ligand likely account for the better regioselectivity obtained from cinnamyl chloride as compared to aliphatic allylic chlorides.     

Ruthenium Pincer‐Catalyzed Cross‐Dehydrogenative Coupling of Primary Alcohols with Secondary Alcohols under Neutral Conditions

Cross‐dehydrogenative coupling of primary alcohols with secondary alcohols to obtain mixed esters with the liberation of molecular hydrogen is achieved in high yield and good selectivity under neutral conditions, using a bipyridyl‐based PNN ruthenium(II) pincer catalyst.     

Molybdenum‐Catalyzed Stereospecific Deoxygenation of Epoxides to Alkenes

Mild and simple catalytic systems consisting of molybdenum(VI) dichloride dioxide [MoO₂Cl₂] as a catalyst and a phosphine as reductant have been developed for the stereospecific deoxygenation of epoxides to alkenes. The reactions using 1,2‐bis(diphenylphosphino)ethane (dppe) and triphenylphosphine (PPh₃) proceed with retention and inversion of stereochemistry, respectively. The mild reaction tolerates the presence of various functional groups and affords stereodefined substituted olefins in good yields.

     

Deoxygenation of stearic acid with a novel Ni(CO)‐MoS2/Fe3S4 catalyst

Magnetically recyclable Ni(Co)‐promoted MoS₂ catalysts with greigite (G) core were synthesized and their activity and selectivity in hydrodeoxygenation of stearic acid were investigated. The activity of the catalysts tested at 320 °C and H₂ initial pressure of 3.5 MPa could be ranked as NiMo/G > CoMo/G > Mo/G. Two main products were detected, C18 (through HDO pathway) and C17 hydrocarbons (through DCO pathway). HDO was the dominant pathway for all of the catalysts. As for the C18/C17 ratio, the catalysts were found to be in the order: Mo/G > CoMo/G ≈ NiMo/G. The Paraffin/Olefin ratio was over 1 for all of the catalysts with NiMo/G showing the highest ratio. Stearic acid was found to have an inhibiting effect on the adsorption of intermediates over the active sites. Moreover, the concentrations of intermediates decreased at high conversions of stearic acid. The formation of the intermediate aldehyde is through C–O hydrogenolysis of the fatty acid following the protonation, dehydrogenation, and hydride addition steps. The same steps were suggested to be involved in the transformation of the aldehyde to the alcohol. Formation of C_n‐1 hydrocarbons was found to be via decarbonylation route. The enhancement of the DCO pathway over the promoted catalysts was related to the electron transfer from the promoting atom to an adjacent sulphur atom and reduction in sulphur‐metal bond strength.     

Catalytic Dehydration of 1,2‐Butanediol to n‐Butyraldehyde in Sub‐ and Supercritical Water

The change of the raw material basis to alternative sources, especially biomass, is advisable because of both the continuous shortage of fossile resources and the advancing change in world climate. Carbohydrates have the highest share in biomass. However, as they are overfunctionalized with chemically almost equal hydroxy groups, the high‐scale industrial use of carbohydrates is restrained. In addition, the diversity and the high amount of functional groups of carbohydrates complicate the prediction of possible reaction pathways. This is the reason why monoalcohols and polyols are used as model compounds. One interesting reaction is the dehydration of 1,2‐butanediol to n‐butyraldehyde, an important chemical intermediate. Supercritical water is an appropriate reaction medium for dehydrations because of its special physical and chemical properties. The addition of acids enhances the reaction rate but at the same time intensifies corrosion. Sulfate salts, especially zinc sulfate, can have similar positive effects without increasing the corrosion potential. The experimental results of the catalytic influence of different metal sulfate salts on the dehydration of 1,2‐butanediol to n‐butyraldehyde are presented in this paper.     

Ring Size‐Selective Ring‐Closing Olefin Metathesis: Taking Advantage of the Deleterious Effect of Ethylene Gas

The deleterious effect of ethylene gas on the ring‐closing olefin metathesis (RCM) for the formation of 5‐ to 8‐membered rings was investigated. Significant rate differences caused by ethylene gas were observed among the different ring‐size formation reactions. Nevertheless, the rate differences can be advantageously utilized for chemoselective RCM.     

n‐butane carbonylation to n‐pentanal using a cascade reaction of dehydrogenation and SILP‐catalyzed hydroformylation

A novel gas‐phase process has been developed that allows direct two‐step conversion of butane into pentanals with high activity and selectivity. The process consists of alkane dehydrogenation over a heterogeneous Cr/Al₂O₃ catalyst followed by direct gas‐phase hydroformylation using advanced supported ionic liquid phase (SILP) catalysis. The latter step uses rhodium complexes modified with the diphosphite ligands biphephos (BP) and benzopinacol to convert the butane/butene mixture from the dehydrogenation step efficiently into aldehydes. The use of the BP ligand results in improved yields of linear pentanal because SILP systems with this ligand are active for both isomerization and hydroformylation. © 2014 American Institute of Chemical Engineers AIChE J, 61: 893–897, 2015     

Ruthenium‐Catalyzed Hydroamination of Aminoallenes: an Approach to Vinyl Substituted Heterocycles

Heterosubstituted aminoallenes underwent smooth ruthenium‐catalyzed intramolecular exo‐hydroamination reactions yielding the corresponding five‐, six‐, or seven‐membered 1,3‐diaza‐ or 1,3‐oxaza‐heterocyclic structures. This procedure is a valuable and less expensive alternative to the already known transition metal‐catalyzed hydroamination reactions of aminoallenes.

     

Silver‐Catalyzed One‐Pot Cyclization Reaction of Electron‐ Deficient Alkynes and 2‐Yn‐1‐ols: An Efficient Domino Process to Polysubstituted Furans

Transition metal‐catalyzed domino reactions have been used as powerful tools for the preparation of polysubstituted furans in a one‐pot manner. In this paper, an efficient synthetic method was developed for the construction of tri‐ or tetrasubstituted furans from electron‐deficient alkynes and 2‐yn‐1‐ols by a silver‐catalyzed domino reaction. It is especially noteworthy that a 2,3,5‐trisubstituted 4‐ynyl‐furan was formally obtained in an extremely direct manner without tedious stepwise synthesis. In addition, regio‐isomeric furans were observed when substituted aryl alkynyl ketones were employed. This methodology represents a highly efficient synthetic route to electron‐deficient furans for which catalytic approaches are scarce. The reaction proceeds efficiently under mild conditions with commercially available catalysts and materials.