Olefins displacement

Scheme 7 a Chain transfer through associative olefin displacement via hydride complex 1.18. b Chain transfer by concerted hydride transfer from alkyl to monomer via transition state 1.24. Note the similarity in the structures of 1.23 and 1.24... 

Mass spectra of OPEOs, their acidic metabolites, OPEC and halogenated derivatives in El and Cl modes have been reported [80-82]. The most prominent ions formed under Cl resulted from alkyl ion displacement and olefin displacement, and those formed under El resulted from benzylic cleavage [82]. 

The cobalt(O) species [Co(N2)(PPh3)3] is a catalyst for the isomerization of a-olefins (284). The isomerization is actually accelerated by dinitrogen, and this is believed to be due to the fact that the dinitrogen, which is probably a stronger ligand for cobalt in these materials than the olefins, displaces a metallation equilibrium as shown in Eq. (11). 

This reaction constitutes an Rh(I)-catalyzed co-oxidation of triphenylphosphine with an olefin. Displacement of coordinated Ph3PO by fresh Ph3P provides for a catalytic cycle. Unfortunately, the use of stoichiometric amounts of triphenylphosphine is rather unattractive in practice. However, this reaction does indicate the important principle that oxygen transfer to typical olefins may be possible when accompanied by the simultaneous transfer of the second oxygen atom to an... 

When ligand L is an olefin, reaction of Nal with derivatives 176 and 173 proceeds in two different ways. Complex 176 gives the neutral monoalkyl osmium(II) complex 160 by olefin insertion, whereas complex 173 gives the neutral methyl osmium(II) compound 160 by olefin displacement (47). 

Reaction with alcohols. Primary alcohols are converted in high yield into alkyl chlorides by reaction with 1 at 80° (4-8 hours). Tertiary alcohols are dehydrated by 1 to olefins, again in high yield. Secondary alcohols on reaction with 1 give a mixture of alkyl chlorides and olefins. Displacement is favored in the case of less sterically hindered alcohols. The product of dehydration is usually the more thermodynamically stable isomer. 

There is one further area in which the properties of olefin-metal complexes and adsorbed olefins show common behavior. The olefin is often readily displaced by diolefins and alkynes many other ligands, including phosphines, amines, nitriles, cyanide ion, and carbon monoxide, can however cause olefin displacement, and these include molecules which are notorious catalyst poisons. Again no quantitative information is available, but a causal connection is strongly suggested. 

The cis coplanar requirement predicted for insertions of metals has been confirmed by the observation that the following intramolecular reaction proceeds by olefin displacement of PPhs ... 

Addition of strong-field ligands such as carbon monoxide or alkynes to zirconium alkene complexes can also result in olefin displacement. Treatment of the monocyclopentadienyl complexes, [775-C5H3-(l,3-(SiMe2CH2PR2)2)]Zr( 72-G2H4)Br (R = Pr1, 87 R = Me, 88), with CO or alkynes results in ethylene loss and the formation of the corresponding dicarbonyl (R = Pf, 166 R = Me, 167) and alkyne (R = Pr 168 R = Me, 169) complexes (Scheme 26). For the alkyne addition, no metallacycle is observed, presumably due to the sterics of the ligand array. 

In this displacement an olefin replaces another olefin from an organoborane. If the olefin added is less volatile than the olefin displaced from the organoborane, the equilibrium may be shifted to produce the more volatile olefin in high yield. The rate of displacement is independent of the concentration of the displacing olefin, in agreement with the dehydroboration-rehydroboration mechanism, in which the dehydroboration is the ratedetermining step . 

Exceptions are styrene (25 % nonterminal) and Et3SiCH=CH2 (70% nonterminal) . The hydroalumination of Mc3SiCH=CH2 with i-BujAlH followed by olefin displacement (see 5.3.3.4.2) leads to a single product ... 

This olefin displacement is the result of elimination and addition . This displacement presents no problem when true homologues of isobutene are used (e.g., 2-ethylhex-l-ene, 2-phenylpropene ). 

The combination of the direct synthesis of i-BujAlH or i-BujAl (see 5.3.3.2.1) with the olefin displacement reaction opens a useful route to new organoaluminums. 

The alumination of cyclopentadiene by R3AI proceeds only under forcing conditions giving a small yield (e.g., i-BujAlCjHj-h, 10-20%) and many products from olefin displacement . The polymeric compound [Me2Al(/x-C5H5)] is obtained pure ... 

M = Pd, Pt) have enabled a new homoleptic olefin complexes of Pt(0) and Pd(0) to be prepared by olefin displacement reactions (Scheme 1) . The dibenzylideneacetone (DBA) complexes (DBA = PhCH=CHCOCH=CHPh) of Pt(0) and Pd(0), which are formed by treating Na2MCl4 with DBA in methanol in the presence of sodium acetate , readily lose dibenzylideneacetone and so form valuable starting materials for Pt(0) and Pd(0) complexes of weakly bonded ligands, such as olefins ... 

This growth reaction is hindered by concurrently occurring olefin displacement reactions [Eq. (32)]. In essence, however, it presents a method of building long carbon chains from ethylene by using the initial aluminum alkyl as a framework. The exciting aspect is that introduction of certain... 

These experiments would exclude the conventional mechanism were it not for one qualification, that the conventional mechanism would seemingly not be excluded if the rate-determining step were the olefin displacement, step 2 in Eq. (7), because, as shown in Scheme 1, the alkylidene groups would then be R,CH=CHR, R,CH CHR,... 

As Fig. 2 shows, [Ciz]/[Ci4] and [C,6]/[Ci4] at zero time are not zero, excluding the conventional mechanism if the metathesis step were rate determining, but not if the olefin-displacement reaction [step 2 in Eq. (7)] were. Notice that the values of [Ci2]/[Ci4] and [Ci6]/[Ci4] are not the 0.5 expected according to the assumptions made above. This is considered later below. 

Consider now what the product r, x /-z would be if the conventional mechanism were correct, but the olefin displacement reaction were rate determining. The kinetics of a mechanistic scheme much like that in Scheme 4 shows (66) that ri x at zero time is a function of two ratios, that of the concentrations of butene and octene and that of their reactivities. However, for all reasonable values of these ratios, rj x rz is never greater than 2.94. Accordingly, since in the double cross experiment the product was determined to be about 4 at zero time, the conventional mechanism is excluded no matter which step is ratedetermining. The implication is that the carbene chain mechanism is correct. 

Addition of trimethyl phosphite to benzene solutions of the chloro-bridged dimer [Rh(cod)Cl]a in the stoicheiometric amount 3 1 causes simultaneous bridge-splitting and olefin displacement, the final product being the monomeric species [Rh P(OMe)3 3Cl]. In the reaction with [Rh(CO)2Cl]2, trimethyl phosphite preferentially displaces one carbon monoxide at each rhodium atom and then proceeds to cleave the dimer. Further addition of phosphite causes displacement of chloride rather than carbon monoxide from /ra 5-[Rh(CO)- P(OMe)3 3Cl]. Both reactions can be followed using P H n.m.r. and i.r. spectroscopy, which enables a characterization of most of the species formed in solution. It is noteworthy that for the ratio P(OMe)3 Rh = 4 1 the spectra suggest the presence of five-co-ordinate [Rh(CO) P(OMe)3 4] which probably has a trigonal-bipyramidal structure with axial CO, ... 

The case of inverse-electron-demand can, perhaps, be recognized in the reaction of Scheme 16, where dative Pd(0)-Mo(II) bonds exist, which are replaced by Pd(0)-olefin bonds/ The first step of the reaction is quantitative and, formally, the weaker acceptor dba olefin is replaced by the stronger acceptor Mo in the dinuclear complex to form the tetranuclear cluster. In the second step (moderate yields), stronger acceptor olefins displace the Mo as... 

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