Alkene, it complexes

Apparently, in the intermediate rhodium-it-alkene complex, a low electron density promotes migration and insertion of the hydride into the double bond, which is related to the behavior of rhodium complexes with electron-withdrawing ligands, such as phosphites (Scheme 2.172). 

The monosulfonated PPh derivative, Ph2P(m-C6H4S03K) (DPM) and its rhodium complex, HRh(CO)(DPM)3 have been synthesized and characterized by IR and NMR spectroscopic techniques. The data showed that the structure was similar to [HRh(CO)(PPh3)3]. The catalytic activity and selectivity of [HRh(CO)(DPM)3] in styrene hydroformylation were studied in biphasic catalytic systems.420 421 Rh1 complexes [Rh(acac)(CO)(PR3)] with tpa (131), cyep (132), (126), ompp (133), pmpp (134), tmpp (135), PPh2(pyl), PPh(pyl)2, and P(pyl)3 were characterized with NMR and IR spectra. Complexes with (131), (132), and (126) were catalysts for hydrogenation of C—C and C—O bonds, isomerization of alkenes, and hydroformylation of alkenes.422 Asymmetric hydroformylation of styrene was performed using as catalyst precursor [Rh(//-0 Me)(COD)]2 associated with sodium salts of m-sulfonated diarylphosphines.423... 

It is important to realize that there is a great deal of overlap in the topics covered in this chapter. For example, the chemistry of metal carbonyls is intimately related to metal alkene complexes, because both types of ligands are soft bases and many complexes contain both carbonyl and alkene ligands. Also, both areas are closely associated with catalysis by complexes discussed in Chapter 22, because some of the best-known catalysts are metal carbonyls and they involve reactions of alkenes. Therefore, the separation of topics applied is certainly not a clear one. Catalysis by metal complexes embodies much of the chemistry of both metal carbonyls and metal alkene complexes. 

In this case, the bond between the carbon atoms is about the same as it is for a C-C single bond. Moreover, unlike the anion of Zeise s salt, the carbon atoms are in the plane formed by platinum and the other ligands. Clearly, this represents a significant difference from the usual alkene complexes. In essence, a three-membered C-P-C ring is formed. It appears in this case that the ability of the... 

It is, of course, still possible that alkene complexes are formed by 8-elimination. This is most easily investigated using the platinacyclobutane [ PtCl2(CD2CHMeCHMe) n]. The predicted products according to the two opposing mechanisms are shown in equation (4). 

In addition to /3-H elimination, olefin insertion, and protonolysis, the cr-metal intermediate has also proved to be capable of undergoing a reductive elimination to bring about an alkylative alkoxylation. Under Pd catalysis, the reaction of 4-alkenols with aryl halides affords aryl-substituted THF rings instead of the aryl ethers that would be produced by a simple cross-coupling mechanism (Equation (126)).452 It has been suggested that G-O bond formation occurs in this case by yy/z-insertion of a coordinated alcohol rather than anti-attack onto a 7r-alkene complex.453... 

It was concluded that the high selectivity observed in the hydrogenation experiments using 26 b is explained by the relatively strong coordination of the alkyne to the palladium center, which only allows for the presence of small amounts of alkene complexes. Only the latter are responsible for the observed minor amounts of ( )-alkene, which was shown to be a secondary reaction product formed by a subsequent palladium-catalyzed, hydrogen-assisted isomerization reaction. Since no n-octane was detected in the reaction mixture, only a tiny... 

Whatever the route to a rhodium dihydride alkene complex, the hydrogen must be transferred sequentially to the double bond. It had always been assumed that the first C-H bond is formed / to the amido-group, so that the more stable Rh-substrate chelate is formed. This is the alkylhydride isomer observed in stoichiometric NMR studies at low temperatures, and is supported by studies under catalytic turnover conditions, assuming a normal isotope effect... 

The cyclopentadienyl group is another interesting ligand for immobilization. Its titanium complexes can be transformed by reduction with butyl lithium into highly active alkene hydrogenation catalysts having a TOF of about 7000 h 1 at 60 °C [85]. Similar metallocene catalysts have also been extensively studied on polymer supports, as shown in the following section. 

In this situation, it is evident that it is not crucial to determine whether the reaction proceeds via an early or a late transition state, since the outcome would be the same in both cases. This property is a result of the close similarity between the initial and final structures indeed, the allyl moiety undergoes a rotation of only 30° from its idealised initial geometry to form the -coordinated alkene complex. 

Co-ordination of an alkene to an electronegative metal (often it may carry a positive charge) activates the alkene toward attack of nucleophiles. After the nucleophilic attack the alkene complex has been converted into a c-bonded alkyl complex with the nucleophile at the (3-position. With respect to the alkene (in the "organic" terminology) the alkene has undergone anti addition of M and the nucleophile Nu, see Figure 2.25. 

The major intermediate observed in solution is the alkene complex, but it interchanges rapidly with the aldehyde complex. The product formed according to this scheme is allyl alcohol, because the overall barrier 2 is lower than barrier 1 (above we named this Curtin-Hammett conditions). Barrier 2 is also the ratedetermining step in this sequence. 

It should be noted that not the final stabilities, but rather the intermediates and transition states determine which isomer is formed. In the precursor alkene complex calculations show that already the respective a and (3 carbon atoms occupy the positions closest to the plane of coordination and that the respective barriers in both cases are indeed the lowest in the model studied [12],... 

Insertion and -elimination. A catalytic cycle that involves only one type of elementary reaction must be a very facile process. Isomerisation is such a process since only migratory insertion and its counterpart P-elimination are required. Hence the metal complex can be optimised to do exactly this reaction as fast as possible. The actual situation is slightly more complex due to the necessity of vacant sites, which have to be created for alkene complexation and for P-elimination. 

Photolysis ofbenzylchlorodiazirine (3) in the presence of tetramethylethylene (TME) is known to produce ( )- and (Z)-/l-chlorostyrene (4) and the cyclopropane (5). Plots of [5]/[4] vs [TME] are curved, consistent with the existence of two pathways for the formation of the alkenes (4). Benzylchlorocarbene (BnClC ) was generated by laser flash photolysis of the phenanthrene (6) in the presence of TME. In this case, plots of [5]/[4] vs [TME] are linear, mling out the possibility that the second pathway to the alkenes (4) involves reaction of a carbene-alkene complex. Time-resolved IR spectroscopy revealed that diazirine (3) rearranges to the corresponding diazo compound, but this process is too inefficient to account for the curvatures. It is proposed that the second pathway to alkene formation involves the excited state of the diazirine. 

It is clear from these experiments that the presence of ethylene catalyses the fixation of nitrogen in lithium complexes. This assisted complexation was also observed with methyl-substituted ethylene and butadiene. It is a characteristic property of lithium-alkene complexes, as experiments performed with other lithium complexes have so far not yielded such ternary complexes. If one can easily anticipate that the fractional positive charge on the lithium in LiC2H4 and Li(C2H4)2 facilitates the coordination of N2 with, presumably, a a-donation to lithium, and possibly, to a weaker extent, p-donation from the metal, it is difficult to rationalize why LiC2H2 and LiC2H4 behave so differently with respect to nitrogen, for instance. 

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