Alkenes trans effect

It has long been known that substitution at the anion of Zeise s salt, [Pt(CH=CH2)Cl3], is, thanks to the high trans effect of the coordinated ethene, very fast. Recent developments in low-temperature stopped-flow apparatus have now permitted the study of the kinetics of substitution at Zeise s and other [Pt(alkene)Cl3] anions in methanol solution. These substitutions obey the customary two-term rate law (i.e. with kohs = ki+ /s3[nucleophile]), with large negative AS values for the k2 term as expected for Sn2 processes (196). 

An alkene complexed to platinum(II) is only slightly modified on coordination, but complexation to platinum(O) causes major changes. Platinum(O) alkene complexes show both weakening and lengthening of the carbon-carbon bond, as well as distortion of the plane of the double bond away from the platinum. In platinum(ll) alkene complexes the double bond lies approximately perpendicular to the square plane of platinum(II), but in platinum(O) complexes there is only a small dihedral angle between the platinum and alkenic planes. For platinum(II) the energy barrier to free rotation of the alkene about the platinum(D)-alkene bond is only about 40-65 kJ mol-1, whereas no rotation is observed with platinum(O) alkene complexes. Alkenes bonded to platinum(ll) exert a large trans effect but only have a small trans influence. 

Identify reaction steps in Fig. 8.2 that illustrate the concepts of (a) the trans effect, (b) nucleophilic attack on a coordinated ligand, (c) insertion of alkene into an M-H bond, and (d) intramolecular electron transfer. 

These results rationalize, in MO terms, the greater reactivity of alkynes than al-kenes toward nucleophiles, and the trans-stereoselectivity of nucleophilic additions to alkynes. It is now clear why alkynes behave as if the LUMO energies of these molecules were lower than those of the corresponding alkenes the effective EAs of alkynes are higher than those of alkenes. There are further consequences. 

Another class of mthenium aUcene complexes contains those derived from the hexaaqua ion [Ru(H20)6] +. The thermodynamically stable complex [(cod)Ru(H20)4] + (74) forms directly from [Ru(H20)6] + and cod in alcohol at ambient temperature (equation 14). In (74), the redox potential of Ru has shifted more positive for the oxidation to Ru and more negative for the reduction to Ru or Ru°, so as to impose a high stability towards disproportionation see Disproportionation) (in contrast to the readily disproportionating aqua ion [Ru(H20)6] +). The X-ray crystal structure see X-ray Crystallography) of the Tosylate (Ots) salt disclosed quite different R-OH2 distances of 2.095(2) and 2.156(2) A for water gauche or trans to the alkene double bond, showing the structural trans effect see Trans Effect) of the latter on a a- (and tv-) donor ligand trans... 

Oxidation of Alkenes and Other Unsaturated Hydrocarbons. The epoxidation of double bonds has been the major area for the application of DDO methodology and a wide range of alkenes are effectively converted to epoxides by solutions of DDO. Epoxidation is stereospecific with retention of afkene stereochemistry, as shown by the reactions of geometrical isomers for example, ( -1-phenylpropene gives the cw-epoxide cleanly (eq 2), whereas the (E) isomer yields the corresponding trans-epoxide. Rate studies indicate that this reagent is electrophilic in nature and that alkyl substitution on the double bond enhances reactivity Interestingly, cw-disubstituted alkenes react 7-9 times faster than the trans isomers, an observation that has been interpreted in terms of a spiro transition state. ... 

If the counterion (X) in the oxidative addition complex is iodide or bromide (and no thallium or silver salts are present) the dissociation of one of the phosphorus atoms in the bidentate ligand from the metal is probably attributed to the relatively high trans effect exerted by the halidesJ This reversible displacement facilitates formation of a neutral rr-complex, in which the rr-system of the electron-rich alkene is only weakly polarized. Therefore, after insertion and hydridopalladium halide elimination, a larger fraction of /3-arylated product is formed, since steric factors always favor terminal ary-lation. 

Reactions in [PtCl4] -ethylene-[SnCl3] systems are dominated by the large 7r-acceptor and trans effect properties of the [SnClg]" ligand. This is also true for analogous systems containing other alkenes. A preliminary... 

Figure 6. NMR spectra of polymers made by ROMP of cyclic alkenes showing effects due to geometrical isomerism (cis/trans disposition of double bonds).

Dilute solutions ( 10" moldm" ) of Wilkinson s complex in organic solvents such as ethanol/benzene actively catalyse the hydrogenation of alkynes and alkenes at ambient temperature and pressure (1 atm, H2). Traces of oxygen must be excluded. Hydrogenation rates are dependent on steric effects. The following general order has been established 1-alkenes > cis-2-alkenes > trans-2-alkenes > trans-3-alkenes. An exception is ethene itself, which forms a rather stable complex (Fig. 5.19) so that higher temperatures are required. Arenes, esters, ketones, carboxylic acids, amides and nitro compounds are unaffected, but aldehydes are slowly decarbonylated. 

The initiation event is a key step in the alkene metathesis mechanism, and considerable effort has been expended in attempts to understand it (vide supra). Much of the interest in this step was driven by what was seen as the counterintuitive order of initiation rates for G1 and G2 the NHC present in G2 would be expected to accelerate dissociation of the phosphine due to the trans effect, yet it was found that G2 initiated orders of magnitude... 

Heat of hydrogenation values (AH°) for several alkenes. The effect of alkyl substitution is evident. Also, the stability of trans double bonds remains relatively constant, but significant destabilization of the cis alkenes is seen as the R group size increases. Derived from data in Turner, R. B., Jarrett, A. D., Goebel, R, and Mallon, B. J. "Heats of Hydrogenation. IX. Cyclic Acetylenes and Some Miscellaneous Olefins." /. Am. Chem. Sac, 95, 790(1973). 

Perhaps the most important group employs oxidative addition, of which RhCKPPhjis (9.4, Willdnson s cataiyst) is the best known. A catalytic cycle that is important under certain conditions is shown in Fig. 9.3. Hydrogen addition to give a dihydride leads to labilization of one of the PPhs ligands (high trans effect of H) to give a site at which the alkene binds. 

Measurements of 2o /205 j ijj spin-spin couplings for the organothallium compounds formed in oxythallation reactions provides the first direct evidence for trans addition in the oxythallation of acyclic olefins. Products from the reactions of styrene, u-alkylphenols, propylene, and oct-l-ene were studied. The rate constants for the oxythallation of alkenes shows a reasonable correlation with the first ionization energy of the alkenes. Inductive effects were found to be most important in the oxythallation of RCH=CH2 and R R C=CHa alkenes. ... 

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