Bond insertion mechanism

Previously, trifluorosilyl groups have been bound to phosphorus (40) and silicon via the SiF (g), fluorine-bond insertion-mechanism (41). The new compound HgCSiFs) is readily hydrolyzed, but it can be stored for long periods of time in an inert atmosphere. It is a volatile, white solid that is stable up to at least 80°C. The preparation of bis(trifluoro-silyDmercury, of course, raises the possibility of (a) synthesis of the complete series of trifluorosilyl, "silametallic compounds, as had previously been done for bis(trifluoromethyl)mercury by using conventional syntheses, and (b) transfer reactions similar to those in Section II, as well as (c) further exploration of the metal-vapor approach. The compound Hg(SiF.,)j appears also to be a convenient source of difluoro-silane upon thermal decomposition, analogous to bis(trifluoromethyl)-mercury ... 

Figure 4.17 Schematic diagram of the bond insertion mechanism, showing how bulk interstitials should react relatively easily with surface dangling bond sites, but less easily with sites saturated by a strongly bonded adsorbate.

Degradation of the labeled allene is being carried out by MacKay et al. (1963). A large fraction of activity in the central carbon atom would support the direct double bond insertion mechanism. The interesting possibility that spiropentane might be formed in hquid or sohd ethylene if the electrons on the intermediate in (52) are paired has not been investigated as yet. 

In 2009, Prof. Yu s group from Peking University concluded the generally accepted O-H bond insertion mechanism of metal carbene [8], as shown in Fig. 3.25. 

Prof. Yu summarized that the metal carbene O-H bond insertion mechanism includes three steps. Step a is that transition metal catalyst decomposes the a-diazo carbonyl compound A and releases nitrogen to form the metal carbene product B Step b is that metal carbene product B and the substrate R2OH form metal-associated oxonium ylide C Step c is that metal-associated oxonium ylide (C or... 

Fig. 3.25 The general O-H bond insertion mechanism (Reprinted with the permission from Ref. [8]. Copyright 2009 American Chemical Society)...

Another aspect of stereochemistry of the CO insertion which has received attention concerns the actual process of formation of the acyl moiety from the coordinated CO and R. Three possible pathways may be envisaged. First, the alkyl moves from the metal onto an adjacent CO. This is known as the alkyl migration mechanism. Second, a coordinated CO moves to insert into the M—R bond—a CO insertion mechanism. Third, both CO and R move in a cooperative manner. These three pathways are represented schematically in Eq. (46). 

During the insertion mechanism, the metal is inserted into the carbon-oxygen bond. The insertion is promoted by a strong metal—oxygen interaction. It is thought that unreduced metal ions may play an important role in the insertion mechanism (electrophilic catalysis). The type of the catalyst, the method of preparation, and the additives can influence the concentration and stability of these ions. 

On Pt and Pd, cleavage of the C-O bond results from a hydrogenolytic cleavage, whereas on Ni and Cu, an insertion mechanism occurs. The regiose-lectivity of the two mechanism is different. The less sterically hindered bond (b) is cleaved on Pt and Pd, whereas the more hindered bond (a) is cleaved on Ni and Cu (Scheme 4.62). 

It was found in the case of O-benzyl systems that palladium oxide is much more effective than palladium metal. No such effect was observed with the N-benzyl system.8 It is possible that the N-compounds can poison the electrophile metal ions, and the hydrogenolysis of the N-benzyl bond can take place only by the hydrogenolytic cleavage instead of the insertion mechanism. This is supported by the experimental finding that the product amine can inhibit the catalyst, and this can be minimized by buffering at a pH less than 4. 

The Cossee mechanism has been demonstrated by direct observation of organometallic complexes where a C = C bond inserts itself into an M-C bond as shown in Eq. (7)-(9). A labeling experiment on a cationic platinum complex 111 indicated the reversible insertion of the coordinated alkene into the Pt-C bond as shown in Eq. (7) [142]. 

The seemingly plausible Scheme shown in 4 is inconsistent with the results of the 13C0 labeling study as are most schemes which do not involve CO insertion for the chain propagation. We believe that ethylene arises from the same sequence of steps as the other hydrocarbon products. The role of the second metal center in the reduction cannot be described. We believe that the iron-iron bond is cleaved early in the reaction since the reduction in the presence of PBu3 produced the unsubstituted species, LiCpFe(C0)2. While there is too little information currently available to assess the importance of Scheme 3, our results on reduction in this iron system are not consistent with the normal CO insertion mechanism or with carbene oligomerization. We suggest Scheme 3 until further research can be accomplished. 

As demonstrated in the preceding Section III.A, configurations of not only the C-H bond of the alkoxide to be inserted but also the carbenoid center present is evidence for the insertion mechanism. In this regard, norcaranylidene carbenoid30 has an unsymmetrical stereochemical environment similar to those of alkylidenemethylene carbenes (Scheme 20) and, therefore, it is expected to be informative of the insertion mechanism. 

The above reactions reinforce the diradical mechanism proposed for the BF3 reaction. Hexafluorobenzene and the various fluorinated ethylenes 73,74 however, react quite differently. The products in these reactions formally correspond to C-F bond insertion by an SiF2 monomer. 

Scheme 1.3 Polymerization scheme showing the migratory insertion mechanism as well as the possible occurrence of the chain back-skip. The possible formation of agostic Mt-H bonds is...

Carbon dioxide is known to readily insert into a metal-hydride bond to give a metal formate [57, 58] this forms the first step in insertion mechanisms of C02 hydrogenation (Scheme 17.2). Both this insertion step and the return path from the formate complex to the hydride, generating formic acid, have a number of possible variations. 

Recently the primary amine 39 was transformed into indolizidine 40 using an ytterbium catalyst. After a regio-specific C=C bond insertion of the internal olefin into the Ln-N bond, a second insertion is accomplished as illustrated in the proposed mechanism <2004JOC1038> (Scheme 15). 

The direct attack of the front-oxygen peroxo center yields the lowest activation barrier for all species considered. Due to repulsion of ethene from the complexes we failed [61] to localize intermediates with the olefin precoordinated to the metal center, proposed as a necessary first step of the epoxidation reaction via the insertion mechanism. Recently, Deubel et al. were able to find a local minimum corresponding to ethene coordinated to the complex MoO(02)2 OPH3 however, the formation of such an intermediate from isolated reagents was calculated to be endothermic [63, 64], The activation barriers for ethene insertion into an M-0 bond leading to the five-membered metallacycle intermediate are at least 5 kcal/mol higher than those of a direct front-side attack [61]. Moreover, the metallacycle intermediate leads to an aldehyde instead of an epoxide [63]. Based on these calculated data, the insertion mechanism of ethene epoxidation by d° TM peroxides can be ruled out. 

Conversion of a Co2(CO)6-alkyne complex into a cyclopentenone is the Pauson-Khand reaction. It proceeds by loss of CO from one Co to make a 16-electron complex, coordination and insertion of the C6=C7 K bond into the C2-Co bond to make the C2-C6 bond and a C7-Co bond, migratory insertion of CO into the C7-Co bond to make the C7-C8 bond, reductive elimination of the C1-C8 bond from Co, and decomplexation of the other Co from the C1=C2 k bond. The mechanism is discussed in the text (Section B.l.f). 

The important difference between the insertion mechanism (2.2) and the migration mechanism (2.3) is the following. In the insertion mechanism carbon monoxide inserts into the metal methyl bond and the acyl bond formed takes... 

Dehydrocyclization, 30 35-43, 31 23 see also Cyclization acyclic alkanes, 30 3 7C-adsorbed olefins, 30 35-36, 38-39 of alkylaromatics, see specific compounds alkyl-substituted benzenes, 30 65 carbene-alkyl insertion mechanism, 30 37 carbon complexes, 32 179-182 catalytic, 26 384 C—C bond formation, 30 210 Q mechanism, 29 279-283 comparison of rates, 28 300-306 dehydrogenation, 30 35-36 of hexanes over platintim films, 23 43-46 hydrogenolysis and, 23 103 -hydrogenolysis mechanism, 25 150-158 iridium supported catalyst, 30 42 mechanisms, 30 38-39, 42-43 metal-catalyzed, 28 293-319 n-hexane, 29 284, 286 palladium, 30 36 pathways, 30 40 platinum, 30 40 rate, 30 36-37, 39... 

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