Olefin n-complexes

Also, neutral group 3 and lanthanide series metallocenes form with olefins n complexes having formally a d° 16-electron configuration, which is favourable for the insertion reaction during olefin polymerisation (or oligomerisation) in the presence of these single-component catalysts [187]. 

More recently, however, results with monooelfins indicate that trans addition can occur to both Pd(II) and Pt(II) n complexes. The first demonstration was the trans addition of amines to Pt(II) complexes 215), and more recently the trans attack of acetate on cyclohexene has been demonstrated (Section III, A, 2, a). However, cis attack can also occur such as the addition of phenylpalladium to cyclohexene (Section III, A, 4) or addition of PdCOOR to cyclic olefins (Section III, A, 3). Also, in the exchange studies (Section III, B, 1) the stereochemistry indicates that some nucleophiles can attack cis or trans. Thus chloride ion containing acetic acid can attack the Pd(II) olefin n complex from either inside or outside the coordination sphere. What are the factors involved The most important appears to be the ability of the nucleophile to coordinate to Pd(II). Thus, phenyl is covalently bonded to Pd(II) and is therefore always in the coordination sphere. Chloride is both inside the coordination sphere as well as outside the coordination sphere and can thus attack both cis and trans. Acetate is not complexed to Pd(II) in chloride-containing media, and thus can only attack trans. On the other hand, in chloride-free acetic acid, acetate is both inside and outside the coordination. Stereochemical results indicate that in this system acetate can attack in both a cis and trans fashion. 

All three reactions in Scheme 3 proceed via the transformation of the olefin n-complex to an organopalladium intermediate =Pd-CH2CH2CH20H. The hetero-lytic cleavage of the Pd-C bond in the intermediate Pd organyl, involving hydride... 

Organometallic tr complexes having hydrogen ona. p carbon commonly react with triphenylmethyl (or trityl) cation. While main-group alkyls fragment to alkene, metal cation and triphenylmethane, comparable transition-metal derivatives produce rearranged cationic olefin n complexes and triphenylmethane ... 

Insertion of norbornadiene into 7r-allyl(hexafluoroacetylacetonato)-Pd(II) yields an adduct stabilized by both chelation of the Pd with the allylic double bond and by the lack of a cis-) -hydrogen to the metal needed for jS-hydride elimination . The reaction proceeds by way of the n-allylpalladium-olefin n complex ... 

Relative Stability of Isomeric Alkyl Complexes and the Olefin n-Complexes. . 159... 

For the model catalysts 1 of Scheme 2 the olefin n-complexes with branched alkyls are more stable than with the linear ones [13d]. This electronic preference is strongly affected by the steric bulk on the catalyst due to an interaction b etween the alkyl group and the catalyst substituents [13d]. Thus, for the most bulky real catalysts the n-complexes with linear aUcyl have lower energy. This is true for both ethylene and propylene complexes. A similar effect was observed for both, the diimine and anilinotropone systems [13c,d, 28]. The presence of the steric bulk also affects the olefin complexation energies, and the relative stability of ethylene and propylene complexes. This has been discussed in details in [13d]. 

One of the earliest observations about bromine addition reactions is that mixing bromine and alkenes leads to a new, transient UV-vis absorption band, suggesting the formation of a bromine-olefin n complex (6). The stabilities of these charge transfer complexes depend significantly on the ionization potentials of the alkenes, although the polarizabilities of the alkenes can be a significant factor also. ... 

Nickel(O) forms a n-complex with three butadiene molecules at low temperature. This complex rearranges spontaneously at 0 °C to afford a bisallylic system, from which a large number of interesting olefins can be obtained. The scheme given below and the example of the synthesis of the odorous compound muscone (R. Baker, 1972, 1974 A.P. Kozikowski, 1976) indicate the variability of such rearrangements (P. Heimbach, 1970). Nowadays many rather complicated cycloolefins are synthesized on a large scale by such reactions and should be kept in mind as possible starting materials, e.g. after ozonolysis. 

Step (4) has recently been suggested to be rate-determining (204) it involves a rearrangement from an olefin n- to an alkyl cr-complex. The latter is postulated to react with more olefin [step (5)], regenerating (XX) and... 

Complexes of Halide Anions with Aromatic and Olefinic n-Acceptors. . . 142... 

Scheme 11 Catalyst deactivation routes in copolymerizations with polar olefins a P-elimination of a leaving group to afford allylic or dimeric species, b poisoning by N-complexation in the attempted copolymerization of acrylonitrile...

Early attempts at an asymmetric hydroalumination utilized a chiral -butylsalicylidenime complexed to a nickel(n) complex 117.128 When racemic 3,7-dimethyl-1-octene 116 was treated with 0.2mol% of the nickel complex 117 and 0.3 equiv. of TIBA at 0°C, followed by hydrolysis, the alkene 118 with 1.2% ee was obtained. The unreacted olefin 119 was recovered and found to have an ee of 1.8% (Scheme 14). 

The electronics behind the insertion reaction is generally explained in terms of a simple three-orbitals four-electrons scheme. Hoffmann and Lauher early recognized that this is an easy reaction for d° complexes, and the relevant role played by the olefin n orbital in determining the insertion barrier [26], According to them, the empty Jt orbital of the olefin can stabilize high energy occupied d orbitals of the metal in the olefin complex, but this stabilization is lost as the insertion reaction approaches the transition state. The net effect is an energy increase of the metal d orbitals involved in the d-7t back-donation to the olefin n orbital. Since for d° systems this back-donation does not occur, d° systems were predicted to be barrierless, whereas a substantial barrier was predicted for dn (n > 0) systems [26],... 

The coordination of styrene with the pyrazole detached resulted in the formation of a stable n-complex (vide infra). Therefore, throughout the catalytic cycle the pyrazole plays the role of a hemilabile ligand [55], This is an important result because experimental studies [11] suggest that olefin coordination is slow and reversible. Moreover, the tuning of the steric bulk of the pyrazole substituents could be used to enhance the activity of the... 

JCj-values of some olefin-S02 complexes in n-hexane at 25°C (Booth e< al., 1959)... 

The key feature of efficient metathesis catalysts seems to be their ability to form, before the [2 + 2] cycloaddition step, a n complex with the alkene (Figure 1.7). Comparison of catalyst 1 with the iron complex 3 shows that the latter, although cationic, will not be able to bind to an olefin, because this would give rise to a complex with 20 valence electrons. A similar argument can be used... 

A quantitative treatment of tt complex formation is, however, more complicated, since it is generally recognized that all three wave functions are necessary for an accurate description of the bond. For instance, it has been pointed out by Orgel (27) that n complex stability cannot solely be the result of n electron donation into empty metal d orbitals, since d and ions (Cu+, Ag+, Ni , Rh+, Pt , Pd++) form some of the strongest complexes with poor bases such as ethylene, tt Complex stability would thus appear to involve the significant back-donation of metal d electrons into vacant antibonding orbitals of the olefin. Because of the additional complication of back-donation plus the uncertainty of metal surface orbitals, it is only possible to give a qualitative treatment of this interaction at the present time. 

Various authors have suggested that olefins and other unsaturated hydrocarbons may form n complexes by associating with a single atomic center of the catalyst. These structures are assumed to be analogous to the -IT olefin complexes (33,34) of the transition elements which have been the subject of recent intensive investigations (35). 

The reaction of an olefin with hydrogen may be analogous to the related homogeneous reaction (44, 46). The olefin may form a n complex with a single center and combine with hydrogen in the sequence shown in Fig. 22A. Besides the reverse of the preceding sequence, an alternative... 

Fig. 10.7. Orbital interactions between RzCu and substrates in (a) an early stage of interaction of the cuprate with methyl bromide, and (b) n-complexation to acetylene or olefin.

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