Iron Catalyst Alkene reduction

Commercially available and inexpensive y-Fe203 magnetic nanoparticles (particle size 58 nm) also efficiently activate B2pin2 and promote a direct borylation of alkenes. " The mechanism of this unusual nano-Fc203-catalyzed aromatic borylation reaction is not clear. The kinetic isotope effect was measured to be 1.3, indicating that a C—H bond activation by oxidative addition to the iron catalyst is not likely. An electrophUic metalation by Fe—B species, followed by reductive ehmination, seems conceivable. 

An even more effective homogeneous hydrogenation catalyst is the complex [RhClfPPhsfs] which permits rapid reduction of alkenes, alkynes and other unsaturated compounds in benzene solution at 25°C and 1 atm pressure (p. 1134). The Haber process, which uses iron metal catalysts for the direct synthesis of ammonia from nitrogen and hydrogen at high temperatures and pressures, is a further example (p. 421). 

The nse of polysnlfide complexes in catalysis has been discnssed. Two major classes of reactions are apparent (1) hydrogen activation and (2) electron transfers. For example, [CpMo(S)(SH)]2 catalyzes the conversion of nitrobenzene to aniline at room temperature, while (CpMo(S))2S2CH2 catalyzes a number of reactions snch as the conversion of bromoethylbenzene to ethylbenzene and the rednction of acetyl chloride, as well as the rednction of alkynes to the corresponding cw-alkenes. Electron transfer reactions see Electron Transfer in Coordination Compounds) have been studied because of their relevance to biological processes (in, for example, ferrodoxins), and these cluster compounds are dealt with in Iron-Sulfur Proteins. Other studies include the use of metal polysulfide complexes as catalysts for the photolytic reduction of water by THF and copper compounds for the hydration of acetylene to acetaldehyde. ... 

AlcoholK and alkenes are also primary products and are not shown in the simplified Eq. 15.182. The overall reaction is complicated and, as a result, its mechanism has been the subject of considerable debate.The reaction may be viewed as the reductive polymerization of carbon monoxide, with molecular hydrogen as the reduc-ii agent. A variety oT heterogeneous catalysts, such as metallic iron and cobalt on alumina, have been used. It is believed that carbon monoxide di.ssociaies on the catalytic surface to ve carbides and that these are in tura hydrogenated to give sur ce carbenes ... 

It is well established that ultrasmall metal clusters on supports have catalytic properties distinct from those properties of large bulk-like particles, as illustrated by the selective oxidation of propylene to propylene oxide by gold, alkene and arene hydrogenation catalysis,and CO oxidation. In these examples, the catalytic properties improve as the clusters become smaller. On the other hand, a reduction in size of the metal cluster can lead to less desirable catalytic properties as seen for ammonia synthesis on iron. Various explanations have been offered to account for the unique properties of nanoscaled metal catalysts, however, much remains to be understood. Clearly, experimental and theoretical studies will be required to develop an in-depth under-... 

Lewis acid catalyzed versions of [4 4- 2] cycloadditions are restricted to functionalized dieno-philes. Nonfunetionalized alkenes and alkynes cannot be activated with Lewis acids and in thermal [4 + 2] cycloadditions these suhstrates usually show low reactivity. It has been reported that intcrmolecular cycloaddition of unactivated alkynes to dienes can be accelerated with low-va-lent titanium, iron or rhodium catalysts via metal-mediated - -complex formation and subsequent reductive elimination39 44. Usually, however, low product selectivities are observed due to side reactions, such as aromatization, isomerization or oligomerization. More effective are nickel-catalyzed intramolecular [4 4- 2]-dienyne cycloadditions which were developed for the synthesis of polycycles containing 1.4-cyclohexadienes45. Thus, treatment of dienyne 1, derived from sorbic acid, with 10mol% of Ni(cod)2 and 30 mol % of tris(o-biphenyl) phosphite in tetrahydrofuran at room temperature affords bicyclic 1,4-dienes 2, via intramolecular [4 + 2] cycloaddition, with excellent yield and moderate to complete diastereocontrol by substituents attached to the substrate. The reaction is sensitive towards variation in the catalyst and the ligand. 

In 1991,Inanaga et al. described the reduction at room temperature of some dis-ubstituted alkynes by the combination SmI2/proton source in the presence of some transition-metal catalysts (3% equiv) in THF [155]. By a good choice of catalyst (CoCl2,4 PPh3) and proton source (MeOH, z -PrOH or AcOH) it was possible to orientate the reaction towards the exclusive formation of the Z-alkene. When the same reaction was performed in the presence of HMPA then the E-alkene was produced. Iron(III) and Ni(II) catalysts were found to be less efficient. It was assumed that the reactive species were the corresponding transition-metal hydrides obtained by reduction of the initial complexes. 

Electrocatalytic reductive coupling of aryl chlorides to afford biphenyls can be accomplished with dichloro(l,2-bis(di-propylphosphino)benzene)nickel(II) in yields as high as 96% with 2 mol % of the catalyst in polar, coordinating solvents (55). Similar couplings can also be achieved with nickel-2,2 -bipyridyl and Pd(PPh3)2Cl2 as catalysts (56, 57). Indirect electrochemical reduction of vicinal dibromides to alkenes occurs efficiently with iron and cobalt porphyrins as mediators (58). Vitamin B12 is a mediator for the indirect electrochemical reduction of a-halo acids (59). 

Indirect electrochemical oxidative carbonylation with a palladium catalyst converts alkynes, carbon monoxide and methanol to substituted dimethyl maleate esters (81). Indirect electrochemical oxidation of dienes can be accomplished with the palladium-hydroquinone system (82). Olefins, ketones and alkylaromatics have been oxidized electrochemically using a Ru(IV) oxidant (83, 84). Indirect electrooxidation of alkylbenzenes can be carried out with cobalt, iron, cerium or manganese ions as the mediator (85). Metalloporphyrins and metal salen complexes have been used as mediators for the oxidation of alkanes and alkenes by oxygen (86-90). Reduction of oxygen and the metalloporphyrin generates an oxoporphyrin that converts an alkene into an epoxide. 

Electrocatalytic oxidations (mainly epox-idation) of alkenes by manganese porphyrins [77, 78] and a Schiff-base [79] and iron and cobalt porphyrins [78] have been achieved. Hydrogen peroxide or the superoxide ion (O ) was generated electrochem-ically by reduction of dioxygen in solvents containing an acid or acid anhydride, the metal compounds as catalysts, and olefins as substrates, in the presence or absence of an axial base. The reaction was believed to take place through the formation of a high valent metal 0x0 porphyrin, produced... 

The reductive cyclization of N-(w-iodoalkyl)succinimides induced by samarium(II) iodide was disclosed by Ha et al. as a novel method for making pyrrolizidines and indoUzidines (Scheme 46). " In the apphcation of the method to the synthesis of (+)-lentiginosine (127), reaction of N-(4-iodo-butyl)tartarimide (+)-335 with samarium(II) iodide in the presence of the iron(III)—tris(dibenzoylmethane) complex as catalyst produced the unsaturated indolizidin-3-one (+)-336 in 82% yield. Reduction of the bridgehead alkene was accomplished with triethylsilane and trifluoroacetic acid via an intermediate acyliminium ion, giving (+)-337 as the sole product in 93% yield. Routine hydrolysis of the silyl ethers produced the known diol (+)-177, after which reduction of the lactam with Hthium aluminum hydride then completed this short synthesis of (+)-127. 

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