Alkene complexes substitution

Similar correlations of orbital interactions with substituent effects were also found in additions of alkenes to substituted carbenes and of N2 to transition metal complexes (see Zollinger, 1983 b, 1990). 

Since the early work dealing with Zeise s salt, many complexes have been prepared with the formula [PtL(C2H4)X2], where L = quinoline, pyridine, or ammonia and X=C1 , Br , I, or N()2. Similar compounds have been prepared that contain other alkenes than C2H4. Many of the complexes containing dienes, trienes, and tetraenes as ligands also contain carbonyl ligands. In fact, metal carbonyls are frequently starting complexes from which alkene complexes are obtained by substitution reactions. 

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... 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

The high levels of enantioselectivity obtained in the asymmetric catalytic carbomagnesa-tion reactions (Tables 6.1 and 6.2) imply an organized (ebthi)Zr—alkene complex interaction with the heterocyclic alkene substrates. When chiral unsaturated pyrans or furans are employed, the resident center of asymmetry may induce differential rates of reaction, such that after -50 % conversion one enantiomer of the chiral alkene can be recovered in high enantiomeric purity. As an example, molecular models indicate that with a 2-substituted pyran, as shown in Fig. 6.2, the mode of addition labeled as I should be significantly favored over II or III, where unfavorable steric interactions between the (ebthi)Zr complex and the olefmic substrate would lead to significant catalyst—substrate complex destabilization. 

Terminal RCH—CH2 1-Hexene C4H9CH=CH2 is isomerized by complex 1 in accordance with the factors influencing the thermodynamic stability of cis- and trans-2 -hexene [15], At the end of the reaction, the alkyne complex 1 was recovered almost quantitatively. No alkene complexes or coupling products were obtained. The corresponding zirconocene complex 2a did not show any isomerization activity. Propene CH3CH=CH2 reacts with complex 6 with substitution of the alkyne and the formation of zirconacydopentanes as coupling products, the structures of which are non-uniform [16]. 

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. 

Step (1) is reminiscent of electrophilic addition to an alkene. Aromatic substitution differs in that the intermediate carbocation (a benzenonium ion) loses a cation (most often to give the substitution product, rather than adding a nucleophile to give the addition product. The benzenonium ion is a specific example of an arenonium ion, formed by electrophilic attack on an arene (Section 11.4). It is also called a sigma complex, because it arises by formation of a o-bond between E and the ring. See Fig. 11-1 for a typical enthalpy-reaction curve for the nitration of an arene. 

Palladium-catalyzed allylic oxidations, in contrast, are synthetically useful reactions. Palladium compounds are known to give rise to carbonyl compounds or products of vinylic oxidation via nucleophilic attack on a palladium alkene complex followed by p-hydride elimination (Scheme 9.16, path a see also Section 9.2.4). Allylic oxidation, however, can be expected if C—H bond cleavage precedes nucleophilic attack 694 A poorly coordinating weak base, for instance, may remove a proton, allowing the formation of a palladium rr-allyl complex intermediate (89, path by694-696 Under such conditions, oxidative allylic substitution can compete... 

The reactivity of alkenes increases with their nucleophilic nature in the order tetra-substituted>trisubstituted>disubstituted>monosubstituted. Further, the epoxidation rate V = /c2X[alkene][complex]/(l + J [alkene]) shows that decomposition of the alkene-metal complex represents the rate determining step in this reaction. 

The reactivity of -Tr-allylpalladium-phosphine complexes generated stoichiometrically or from alkenes allylically substituted with a leaving group, is essentially identical and, as a result, allyl species will be generally considered in this section without distinction as to the origin of the palladium complex. 

Much work has gone into the optimization of results with functionalized alkenes. In the reaction of cyclic alkenes, ds-decomposition of organopalladium halide-alkene complex gives -palladium complex 41. This subsequently undergoes syn-p-hydride elimination since only one such hydrogen is available, deconjugation to 3-substituted alkenes should... 

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