Detailed Mechanistic Studies

Actual electron transfer to the dinitrogen substrate at the MoFe-protein, with electrons first passing through the MoFe-protein s P-cluster. During this process, dinitrogen is most probably bound to the iron-molybdenum cofactor (FeMoco) of the MoFe-protein.6 

The mechanism and sequence of events that control delivery of protons and electrons to the FeMo cofactor during substrate reduction is not well understood in its particulars.8 It is believed that conformational change in MoFe-protein is necessary for electron transfer from the P-cluster to the M center (FeMoco) and that ATP hydrolysis and P release occurring on the Fe-protein drive the process. Hypothetically, P-clusters provide a reservoir of reducing equivalents that are transferred to substrate bound at FeMoco. Electrons are transferred one at a time from Fe-protein but the P-cluster and M center have electron buffering capacity, allowing successive two-electron transfers to, and protonations of, bound substrates.8 Neither component protein will reduce any substrate in the absence of its catalytic partner. Also, apoprotein (with any or all metal-sulfur clusters removed) will not reduce dinitrogen. 

In addition to dinitrogen, nitrogenase can reduce a variety of small, unsaturated molecules some of these are illustrated in reactions 6.5-6.79  

Seefeldt and Dean7 describe the nitrogenase enzyme s so-called Fe cycle in the following steps  

Two moles of MgATP bind to the reduced Fe-protein containing the [4Fe-4S] cluster in the 1+ oxidation state as [Fe4S4]+. During this step, Fe-protein polypeptide conformation changes take place, altering the electronic properties of the [4Fe lS] cluster. 

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Because transition metals even in a finely-divided state do not readily combine with CO, various metal salts have been used to synthesize metal carbonyls. Metal salts almost always contain the metal in a higher oxidation state than the resulting carbonyl complex. Therefore, most metal carbonyls result from the reduction of the metal in the starting material. Such a process has been referred to as reductive carbonylation. Although detailed mechanistic studies ate lacking, the process probably proceeds through stepwise reduction of the metal with simultaneous coordination of CO (90). 

A detailed mechanistic study of the hydrolysis of 2,2-dichloroazirines (260) has been carried out and shown to be compatible with the route shown (77JPS1653). 

Detailed mechanistic studies have been carried out on aminolysis of substituted aryl acetates and aryl carbonates. Aryl esters are considerably more reactive than alkyl esters because the phenoxide ions are better leaving groups than alkoxide ions. The tetrahedral intermediate formed in aminolysis can exist in several forms which differ in extent and site of protonation ... 

According to a detailed mechanistic study, the first step is the abstraction of the relatively acidic hydrazone proton (93- 97). This is followed by hydride attack on the trigonal carbon of the C=N bond, mainly from the a-side at C-3, together with the concomitant loss of the tosylate anion (97 -> 98). Expulsion of nitrogen from the resulting intermediate (98) yields a fairly insoluble anion-metal complex (99) which upon decomposition with water provides the methylene derivative (100). 

Detailed mechanistic studies by Fodor demonstrated the intermediacy of both imidoyl chlorides (6) and nitrilium salts (7) in Bischler-Napieralski reactions promoted by a variety of reagents such as PCI5, POCI3, and SOCh)/ For example, amide 1 reacts with POCI3 to afford imidoyl chloride 6. Upon heating, intermediate 6 is converted to nitrilium salt 7, which undergoes intramolecular electrophilic aromatic substitution to afford the dihydroisoquinoline 2. Fodor s studies showed that the imidoyl chloride and nitrilium salt intermediates could be generated under mild conditions and characterized spectroscopically. Fodor also found that the cyclization of the imidoyl chlorides is accelerated by the addition of Lewis acids (SnCU, ZnCh), which provides further evidence to support the intermediacy of nitrilium salts. ... 

In a detailed mechanistic study of Mg(OH)2 dehydroxylation, Gordon and Kingery [245] precede consideration of their kinetic data with an... 

Based on the above results and previous works [3,9] on the reaction of epoxides and CO2, we tentatively propose the plausible mechanism for the copolymerization of GMA and CO2 (schane 1), Alkyhnethyl imidazolim salt (QX) and epoxide (GMA) r cted to synthesize an active spedes followed by chain propagation involving a < ncerted insertion of th e epoxide. However, more detailed mechanistic studies are needed to clairly understand the polymerization steps. 

For some of these reactions detailed mechanistic studies were carried out. As an example, in Scheme 5 the pathway suggested for 1.1-diphenylethylene oxidation with O2 into benzophenon [23] is shown. 

Aldehydes can be oxidized to carboxylic acids by both Mn(VII) and Cr(VI). Fairly detailed mechanistic studies have been carried out for Cr(VI). A chromate ester of the aldehyde hydrate is believed to be formed, and this species decomposes in the rate-determining step by a mechanism similar to the one that operates in alcohol oxidations.209... 

A proposed simplified mechanism for the conjugate addition/aldol cyclization, as depicted in Scheme 2.25, is based on detailed mechanistic studies performed on related Rh-catalyzed enone conjugate additions [45]. A model accounting for the observed relative stereochemistry invokes the intermediacy of a (Z)-enolate and a Zimmerman-Traxler-type transition state as shown in 2-110 to give 2-111. 

Since group 3 metallocene alkyls are isoelectronic with the cationic alkyls of group 4 catalysts they may be used as olefin polymerization initiators without the need for cocatalysts. The neutral metal center typically results in much lower activities, and detailed mechanistic studies on the insertion process have therefore proved possible.216-220 Among the first group 3 catalysts reported to show moderate activities (42 gmmol-1 h-1bar-1) was the yttrocene complex (77).221... 

N.N. Greenwood We have not yet undertaken detailed mechanistic studies of the ortho-cyclo boronation reactions but the sequence you envisage has also seemed quite plausible to us. 

Several compounds containing Tt-bonds show reactions with 1 which most likely proceed via [2+1] or [4+1] cycloaddition processes, but no detailed mechanistic studies have been performed so far. Not unexpectedly, the electron-rich species 1 preferentially reacts with electron-poor substrates, and ring-strained or dipolar intermediates rearrange or react further to more stable products in a sometimes rather complicated and surprising fashion. In a few cases even the pentamethylcyclopentadienyl substituents at silicon are involved in the reaction pathways. 

A rationale for the heteroatom effect has recently been provided for the reaction of 1,6-enynes based on detailed mechanistic studies,318 314 especially DFT calculations performed by several groups (Scheme 90) 183>319321 From what has been disclosed so far, an interplay of cationic and carbenoid species is obvious in these processes. Nowadays, there is a good consensus to adopt the involvement of cyclopropyl metallacarbene intermediates. 

Although detailed mechanistic studies are not reported, the postulated mechanism for the reductive cyclization of allenic carbonyl compounds involves entry into the catalytic cycle via silane oxidative addition. Allene silylrhodation then provides the cr-allylrhodium hydride A-18, which upon carbometallation of the appendant aldehyde gives rise to rhodium alkoxide B-14. Oxygen-hydrogen reductive elimination furnishes the hydrosilylation-cyclization product... 

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