Ethylene complexes insertion

Negishi et al. reported the regioselective synthesis of diisoalkyl derivatives from monosubstituted alkenes in yields ranging from 58-95%, Scheme 8, from the in situ prepared ethylene complex Cp2Zr(C2H4).35 The zirconocene-ethylene complex presumably undergoes alkene insertion to furnish a zirconacyclopentane which further reacts with diethylzinc to yield the diisoalkylzinc compound. 

Figure 4.77 The optimized structure of the transition state II for the ethylene-insertion reaction II III (4.106), with forward activation energy A > = 6.90 kcalmol-1 relative to the metal-ethylene complex II.

Substituted cyclopropanols were also obtained, albeit in moderate yields, upon reaction of esters such as methyl pentanoate with l,4-bis(bromomagnesium)butane (38) in the presence of titanium tetraisopropoxide. This corroborates the formation of a titanacy-clopropane—ethylene complex 40 from an initially formed titanacyclopentane derivative 39 (Scheme 11.12) [103], Apparently, an ester molecule readily displaces the ethylene ligand from 40, and a subsequent insertion of the carbonyl group into the Ti—C bond, a formal [2S + 2J cycloaddition, leads to the oxatitanacyclopentane 42, the precursor to 1-butylcyclopropanol (43). 

Figure 4. Structures resulting from ethylene insertion and chain termination due to the generic catalyst (HN=C(H)-C(H)=NH)PdC3H7+. Ethylene complex (3a) insertion transition state (TS[ 3a-4a]) termination transition state (TS[3a-5a) new olefin product from termination process (5a).

Figure 6. Ethylene complex (Ilia) and insertion transition state TS[IIIa-IVa] for the polymerization process involving la (or Ila)...

The first step is coordination of the ethylene through its n orbital. The ethylene is trans to Cl with the C=C bond in the Cl-Ru-H plane. Facile migratory insertion (AE = 7.6 kcal.mol 1) of the coordinated ethylene in the Ru-H bond leads to an alkyl intermediate 6.2 kcal.mol 1 less stable than the n ethylene complex. The alkyl intermediate has a strong P C-H agostic interaction as illustrated by the unusually long agostic C-H bond (1.221 A) which helps to stabilize the unsaturation in the formally 14-electron alkyl intermediate. 

Beyond this work, the chemistry of neutral r 2-ethylene complexes remains largely unexplored. However, it has been reported that in the presence of B(C6F5)3 at 60 °C, 158 converts to the platinacyclic complex 176 (Scheme 13).67 This conversion, which also occurs in the absence of the borane at 80 °C, results from ethylene insertion into the Pt—Fh linkage, followed by rapid intramolecular orthometallation. A comparable reaction has also been noted with propene, though in this instance the intermediate r 2-complexes are never observed (see also Section III.C.l). 

A mononuclear tantalum-benzyne complex (121) has been prepared by thermolysis of 120 [Eq. (20)].14 An X-ray crystal structure was reported for 121. Bond lengths for the benzyne unit are given in Table III. Complex 121 exhibits a rich insertion chemistry similar to that of Ti, Zr, and Ru benzyne complexes. Insertion reactions of 121 with ethylene, 2-butyne, acetonitrile, and carbon dioxide give 122, 123, 124, and 125, respectively (Scheme 15). Diphenylacetylene does not couple with 121, presumably because of steric constraints. Reagents with acidic protons such as methanol or terminal alkynes cleave the Ta—C bond to give butyl isocyanide and carbon monoxide, but... 

We have already seen in Section 2.2.2 that metal-alkyl compounds are prone to undergo /3-hydride elimination or, in short, /3-elimination reactions (see Fig. 2.5). In fact, hydride abstraction can occur from carbon atoms in other positions also, but elimination from the /8-carbon is more common. As seen earlier, insertion of an alkene into a metal-hydrogen bond and a /8-elimination reaction have a reversible relationship. This is obvious in Reaction 2.8. For certain metal complexes it has been possible to study this reversible equilibrium by NMR spectroscopy. A hydrido-ethylene complex of rhodium, as shown in Fig. 2.8, is an example. In metal-catalyzed alkene polymerization, termination of the polymer chain growth often follows the /8-hydride elimination pathway. This also is schematically shown in Fig. 2.8. 

Anionic acyl-iron complex (8) may be intercepted by ethylene, which inserts into the acyl-iron bond to produce a S-metalloketone that rearranges to an a-metalloketone (Scheme 4). Protonation affords the ethyl ketone. Intramolecular (see Intramolecular) versions of this process are known as well, and provide an approach to cychc ketones. That the cyclization is not always regioselective (see Regioselectivity) is shown in an example used in the total synthesis of ( )-aphidicolin (Scheme 5). 4i... 

Addition of acid to the bis(ethylene) complex produees [HMo(C2H4)2(diphos)j]", whieh exhibits averaging between the hydride and only one of the olefins above -85°. This is the first case where the insertion-deinsertion proeess, fundamental to many eatalytie meeha-nisms, can be observed directly. 

Table 2. Ethylene Complexation and Insertion Activation Energies (kcal/mol) for M—C3H7 Complexes...

Table 6. Ethylene Complexation, Ethyl Insertion Activation, yS-Hydrogen Transfer to Monomer Activation, ...

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