Olefins rr-bonded

The spectra and calculations all led to the conclusion that there is an usually large interaction between both the 7T and lone-pair orbitals in the carbonyl portion of the molecule with the n and a orbitals of the olefin portion. The first ionization potential (9.57 eV) involves ionization of an electron from the oxygen lone pair, whereas the second (11.19 eV) involves ionization of an electron from the olefin rr-bond. The most vertical ionization is from the 7 ax MO (16.11 eV), the second lone-pair orbital on oxygen. 

The pentacyclic alkene (172) is photochemically reactive and is readily transformed (0°C, quartz filter, MeCN solution) into the hexacyclic alkane (173) in high yield. The reaction is a conventional [2+2] cycloaddition between the olefinic rr-bond and the cr-bond of the cyclopropane ring. Compound (174) reacts analogously, yielding (175). 

Figure 2.19 illustrates Cossee s mechanism for polymerization with coordination catalysts. The active site is depicted as having a coordination vacancy that attracts the electrons in the olefin rr-bond. Coordination is followed by insertion into the polymer chain (R) and the re-establishment of the coordination vacancy for further monomer insertion. This figure also shows an important characteristic of coordination polymerization that makes it very different from free-radical polymerization the monomer is inserted between the carbon-metal bond. As a consequence, the electronic and steric environment surrounding the transition metal has a huge influence on the kinetics of polymerization. This is why... 

The dissociation energy of alkyllithium is very large. In the case of MeLi (dimer, trimer and tetramer) they are —42, —82 and — 128kcaI/mole, respectively [42]. Organolithium compounds are non-transition metal compounds but they can form t-bond structures. The elements of non-transition metal compounds which can form the 7t-bond, are Na, Be, Mg, Ca, B, Al, Ga, In, Tl, Ge, Sn, Pb, P, As, Sb, S, Se, Te, etc., besides Li [44]. The olefinic rr-bond with transition metals is well-known the coordination of the Tt-bond is such that the electrons of the olefinic n bond are donated to the vacant d orbitals, and the backdonation of the rr-bond is such that the electrons of the metal d-orbitals are donated to the antibonding n orbital of the olefin. However, as non-transition metals have no vacant d orbitals, the r-electrons of olefins only partially move to the s- or p-orbital of the metal. Then, the electrons largely remain in a non-bonding orbital, and the backdonation is therefore almost none [44]. 

As in the case of the reaction of hydroperoxide with the rr-bond of the olefin, the reaction of ROOH with the rr-bond of the aromatic ring occurs more rapidly than the attack of ROOH on the C—H bond of alkylaromatic hydrocarbon. 

In addition to the [2-I-2-I-1] carbocyclization, an analogous [3-I-2-I-1] version has been reported [23]. The high 7r-character of the rr-bond of the cyclopropane provides reactivity similar to that of an olefin thus when the 4-pentynyl cyclopropanes 42 were subjected to the PK reaction conditions, the bicyclo[4.3.0]nonenone 43, in addition to a small amount of the oxidized product 44, was afforded in modest yield (Eq. 7). The analogous transformation with the vicinally disubstituted cyclopropane 45 furnished regioisomers 46 and 47, in which cleavage at the less substituted bond is favored (Eq. 8). 

The only way of using exchange with deuterium to solve Problem B is to study compounds that have eclipsed vicinal pairs of hydrogens but with only the remotest chance of forming rr-bonded olefinic complexes. Caged compounds are necessary and a very suitable choice is the heptacyclotetradecane shown in Fig. 9... 

M—has also been reported for olefins and acetylenes ir-bonded to rhodium and to platinum (6, 21, 46, 87). In the case of rhodium, iy(i°3Rh—is between 10 and 16 Hz for a 7r-bonded olefin (see Table XXVII), while for the cr-bonded carbon in [(C5H5)Rh(ff-C3Hs)-(w-CsHb)], 7( ° Rh—is 26 Hz. It was suggested the bonding of the olefin results from a 60% contribution from a dsp -vnet X orbital and sp -carbon orbital 21). For the olefins and acetylenes w-bonded to platinum 7( Pt—is between 18 and 195 Hz (see Table XXIX) compared to the range of 360 to 1000 Hz reported for carbon cr-bonded to platinum. It was found that 7( Pt— C) is less for a 7r-bonded acetylene than for a rr-bonded ethylene. This was considered as evidence for the Chatt-Dewar-Duncanson molecular orbital model 39, 63) of TT-bonding (XIV), rather than the formally equivalent valence-bond treatment, (XV) and (XVI) 46). However, no allowance appears to have been made for the effect on the hybridization at the carbon of the pseudo-... 

All the photopolymerizable monomers discussed in this review contain two conjugated olefinic double bonds. Since the rr-electron conjugation of the monomer (A) is interrupted by the formation of a cyclobutane ring to produce a molecule larger than a dimer (B), the n-n electronic transition of B is shifted to a higher energy level than that of A. 

Even though the X-ray data suggest the presence of two a bonds and one 7T bond between metal and olefin as was postulated for the butadiene-iron carbonyls, the UV spectra of the triphenyltropone complexes closely resemble that of the free olefin (80) suggesting that the bonding of the complex may indeed be intermediate between that of structure (96) and simple rr bonding to two olefinic double bonds (80). This concept is more fully discussed in the section on cobalt (Section VII, A). 

Chiefly through the work of Jonas and co-workers (55) mixed-metal organometallic complexes also are known that involve interactions of lithium atoms with unsaturated rr-bonded hydrocarbon ligands (olefin, cyclopentadienyl, arene, etc.). While reviews already are available (55, 89), we include examples for comparison purposes. The lithium complexes in this section show increasing complexity and diversity both in the geometries around the lithium atoms and in the degree and type of interactions involved. The common feature in these compounds is the interaction of a lithium atom with a hydrocarbon ligand which is tt... 

These systems were extensively studied with the goal of finding even better, more efficient catalysts. Based on these studies it appears that the key attributes for high activity appear to be three-fold (1) the transition metal center is complexed only by metal olefin Tt-bonds and a metal-carbon rr-bond (which can be part of an allylic system), (2) the metal center is cationic, and (3) the counter-ion is a weakly coordinating anion. 

With species such as CO, olefins, acetylenes, etc., whose coordination to metal ions depends on rr-bonding through the donation of d, el ctrons from the metal to the ligand, e.g. ... 

Several mechanisms can be envisaged for heme alkylation that are consistent with the experimental data, none of which involves a concerted transfer of the oxygen to the rr-bond. Subsequent to possible formation of a charge transfer complex between the ferryl species and the olefin -ir-bond, addition of the oxygen to the Tr-bond could give a... 

The hydroboration reaction is also very predictable with regard to the stereochemistry of addition. The addition occurs stereospecifically syn through a four-center transition state with essentially simultaneous bonding to boron and hydrogen. Both the new C-B and C-H bonds are therefore formed from the same side of the multiple bond. In molecular orbital terms, the addition reaction is viewed as taking place by interaction of the olefin rr-orbital with the empty p-orbital on trivalent boron. Formation of the carbon-boron bond is accompanied by concerted rupture of a B-H bond ... 

Some simpler ligands can also serve as ir-donors. We saw earlier that acetylenes can act as CT-donors and ir-acceptors in the same way as olefins. However, the rr-bonding orbitals that are perpendicular to those oriented toward the metal can also serve as ir-donors, as shown in Figure 1.29. Acetylene ligands in complexes that possess less than 18 electrons without such TT-donation are sometimes considered to be "four-electron donors." This is the origin of the listing of alkynes as 2-electron or 4-electrori ligands in Table 1.1. 

Cl-symmetric catalysts such as 9 and 10 bear a Cp ring substituent pointing in the same direction as one of the chlorine atoms on the Zr center. In the active catalyst species, the chlorine is replaced by the growing polymer chain or a n-complex-bonded olefin. The coordination site on the same side as the Cp substituent (site B) is more sterically hindered than the other coordination site (site A). After insertion of a monomer coordinating at site B, the polymer chain is walked back to less sterically hindered site A by the next monomer insertion (alternating/chain migratory mechanism). But in some cases, the polymer chain can back skip to site A before the next monomer insertion (site epimerization). Norbornene, as a bulky olefin, can be rr-bonded only at site (see Chapter 2 for a further discussion of the alternating and site epimerization mechanisms with catalyst 9). 

In this complex, the molecule of coordinated ethylene occupies a single coordination site and is oriented perpendicularly to the plane of the square anion. In both complexes mentioned above (i.e., the complexes with two-center ligands and the olefin rr complexes), the bonding of ligands to the central atom occurs via the dihapto type. Moreover, the rotation of olefin about the metal-ligand bond was observed both in Zeise s salt and in some other ir complexes. Thus, the free energies for the activation of olefin (L) rotation in complexes (PR3)PtCl2 -L and PtCl(acac) - L depend on the nature of R and L, and vary within 44.4-67 kJ mol (10.6 16 kcal mol ) [80]. 

We considered the structure and dynamic behavior of Group 5 and 6 d transition metal fluoro complexes with n-donor 0,N- and N,N-two-center ligands in nonaqueous solutions, which allowed us to suggest a model of bonding of n-donor two-center ligands in d transition metal complexes. The survey of available data allowed us to draw an analogy between the structure and dynamic behavior of these complexes and those of olefin rr complexes. 

Pd(II) compounds coordinate to alkenes to form rr-complexes. Roughly, a decrease in the electron density of alkenes by coordination to electrophilic Pd(II) permits attack by various nucleophiles on the coordinated alkenes. In contrast, electrophilic attack is commonly observed with uncomplexed alkenes. The attack of nucleophiles with concomitant formation of a carbon-palladium r-bond 1 is called the palladation of alkenes. This reaction is similar to the mercuration reaction. However, unlike the mercuration products, which are stable and isolable, the product 1 of the palladation is usually unstable and undergoes rapid decomposition. The palladation reaction is followed by two reactions. The elimination of H—Pd—Cl from 1 to form vinyl compounds 2 is one reaction path, resulting in nucleophilic substitution of the olefinic proton. When the displacement of the Pd in 1 with another nucleophile takes place, the nucleophilic addition of alkenes occurs to give 3. Depending on the reactants and conditions, either nucleophilic substitution of alkenes or nucleophilic addition to alkenes takes place. 

Facile reaction of a carbon nucleophile with an olefinic bond of COD is the first example of carbon-carbon bond formation by means of Pd. COD forms a stable complex with PdCl2. When this complex 192 is treated with malonate or acetoacetate in ether under heterogeneous conditions at room temperature in the presence of Na2C03, a facile carbopalladation takes place to give the new complex 193, formed by the introduction of malonate to COD. The complex has TT-olefin and cr-Pd bonds. By the treatment of the new complex 193 with a base, the malonate carbanion attacks the cr-Pd—C bond, affording the bicy-clo[6.1,0]-nonane 194. The complex also reacts with another molecule of malonate which attacks the rr-olefin bond to give the bicyclo[3.3.0]octane 195 by a transannulation reaction[l2.191]. The formation of 194 involves the novel cyclopropanation reaction of alkenes by nucleophilic attack of two carbanions. 

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