Olefin complexes intermolecular

Examples of catalytic formation of C-C bonds from sp C-H bonds are even more scarce than from sp C-H bonds and, in general, are limited to C-H bonds adjacent to heteroatoms. A remarkable iridium-catalyzed example was reported by the group of Lin [116] the intermolecular oxidative coupling of methyl ethers with TBE to form olefin complexes in the presence of (P Pr3)2lrH5 (29). In their proposed mechanism, the reactive 14e species 38 undergoes oxidative addition of the methyl C-H bond in methyl ethers followed by olefin insertion to generate the intermediate 39. p-hydride elimination affords 35, which can isomerize to products 36 and 37 (Scheme 10). The reaction proceeds under mild condition (50°C) but suffers from poor selectivity as well as low yield (TON of 12 after 24 h). 

As was noted in the discussion of olefin complexes, a twisting of the C=C bond is usually observed in acetylene complexes. This twisting is manifested in a nonzero R—C=C—R torsion angle. Angles y and 5, defined the same way as for olefin complexes, should ideally be 0° and 180°, respectively. Values of y up to 9° have been observed, and the two 8 angles are not necessarily equal (Table I). As for olefin complexes, the differences in the 5 s can be attributed to nonbonded interactions, both intra- and intermolecular. 

As previously discussed, the copolymers produced in the zinc chloride-free radical system are not necessarily random copolymers but are probably the result of the copolymerization of the acrylonitrile-complexed acrylonitrile complex with the olefin-complexed acrylonitrile complex. Further, the olefin-alkylaluminum halide complexed acrylonitrile complex only differs from the olefin—zinc chloride complexed acrylonitrile complex in degree rather than in kind—i.e., the former is an unstable charge transfer complex capable of spontaneous uncoupling of the diradical system followed by intermolecular diradical coupling, while the latter is a stable charge transfer complex requiring radical attack to uncouple the diradical system. 

Olefin ligands are less common in complexes of metals in higher oxidation states, with charges greater than +1, and ivith d electron coimts. Yet, olefin complexes of d metals are important intermediates in olefin polymerization. Because of the importance of these d olefin complexes, the direct observation of such complexes has been sought, and such olefin complexes have recently been identified, usually at low temperatures. Two examples of d olefin complexes generated from the intermolecular coordination of ethylene are shown in Figure 2.21. 

The oxidative functionalization of olefins through ir-olefin complexes of palladium also has a long history, including the industrial production of acetaldehyde and vinyl acetate. Related reactions, including the conversion of olefins to vinyl ethers and enamines, have been studied in more recent times for fine chemical synthesis. These oxidative C-0 and C-N bond formations have been conducted with a variety of oxidants, including Oj, and have been studied as both intermolecular and intramolecular processes. 

Acyclic diene molecules are capable of undergoing intramolecular and intermolec-ular reactions in the presence of certain transition metal catalysts molybdenum alkylidene and ruthenium carbene complexes, for example [50, 51]. The intramolecular reaction, called ring-closing olefin metathesis (RCM), affords cyclic compounds, while the intermolecular reaction, called acyclic diene metathesis (ADMET) polymerization, provides oligomers and polymers. Alteration of the dilution of the reaction mixture can to some extent control the intrinsic competition between RCM and ADMET. 

Non-heteroatom-stabilised Fischer carbene complexes also react with alkenes to give mixtures of olefin metathesis products and cyclopropane derivatives which are frequently the minor reaction products [19]. Furthermore, non-heteroatom-stabilised vinylcarbene complexes, generated in situ by reaction of an alkoxy- or aminocarbene complex with an alkyne, are able to react with different types of alkenes in an intramolecular or intermolecular process to produce bicyclic compounds containing a cyclopropane ring [20]. 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

In spite of the numerous spectral observations of complex formation between aromatic and olefinic donors with the dihalogens, the preparations of the corresponding crystalline complexes have been hindered by their enhanced reactivity (as well as the relatively weak bonding). As such, only few examples of the X-ray structural characterization of the corresponding intermolecular associates are reported, the most notable exception being the dibromine complex with benzene. 

In a similar manner, the diffusion of hexane into dichloromethane solutions containing mixtures of the alkylammonium salts of bromide and the olefinic acceptors o-CA and TCNE result in the formation of brown-red crystals [23]. X-ray analysis reveals the (1 1) complex of bromide with o-CA, in which the anion is located over the center of the C - C bond of the acceptor moiety (Fig. 15b) and Br - C contacts are shortened by as much as 0.6 A relative to the sum of van der Waals radii (Table 3). In bromide complexes with TCNE, the location of the anion relative to the acceptor is variable. In fact, a 2 1 complex [(Br )2,TCNE] is isolated in which both anions reside over the olefinic bond when the tetraethylammonium salt of bromide is used. In comparison, if the tetrapropyl- or tetrabutylammonium salts of the same anion are employed, the (1 1) complexes [Br ,TCNE] are formed in which the bromide donors are shifted toward the cyano substituents (Fig. 15a). In both cases however, the short intermolecular separations that are characteris-... 

Pd(OAc)2 works well with strained double bonds as well as with styrene and its ring-substituted derivatives. Basic substituents cannot be tolerated, however, as the failures with 4-(dimethylamino)styrene, 4-vinylpyridine and 1 -vinylimidazole show. In contrast to Rh2(OAc)4, Pd(OAe)2 causes preferential cyclopropanation of the terminal or less hindered double bond in intermolecular competition experiments. These facts are in agreement with a mechanism in which olefin coordination to the metal is a determining factor but the reluctance or complete failure of Pd(II)-diene complexes to react with diazoesters sheds some doubt on the hypothesis of Pd-olefin-carbene complexes (see Sect. 11). 

Intermolecular cyclopropanation of olefins poses two stereochemical problems enantioface selection and diastereoselection (trans-cis selection). In general, for stereochemical reasons, the formation of /ra ,v-cyclopropane is kinetically more favored than that of cis-cyclopropane, and the asymmetric cyclopropanation so far developed is mostly /ram-selective, except for a few examples. Copper, rhodium, ruthenium, and cobalt complexes have mainly been used as the catalysts for asymmetric intermolecular cyclopropanation. 

The Heck reaction, a palladium-catalyzed vinylic substitution, is conducted with olefins and organohalides or pseudohalides are frequently used as reactants [15, 16], One of the strengths of the method is that it enables the direct monofunctionalization of a vinylic carbon, which is difficult to achieve by other means. Numerous elegant transformations based on Heck chemistry have been developed in natural and non-natural product synthesis. Intermolecular reactions with cyclic and acyclic al-kenes, and intramolecular cyclization procedures, have led to the assembly of a variety of complex and sterically congested molecules. 

Diastereoselective intermolecular nitrile oxide—olefin cycloaddition has been used in an enantioselective synthesis of the C(7)-C(24) segment 433 of the 24-membered natural lactone, macrolactin A 434 (471, 472). Two (carbonyl)iron moieties are instrumental for the stereoselective preparation of the C(8)-C(ii) E,Z-diene and the C(i5) and C(24) sp3 stereocenters. Also it is important to note that the (carbonyl)iron complexation serves to protect the C(8)-C(ii) and C(i6)-C(i9) diene groups during the reductive hydrolysis of an isoxazoline ring. 

Grubbs has reported a similar tandem olefin metathesis-carbonyl olelination process for the preparation of cyclic olefins [31]. In this case, treatment of a keto-olefin with the molybdenum alkylidene 1 at 20°C generates an intermediate alkylidene complex. Under these conditions, competing intermolecular olelination does not occur. However, intramolecular carbonyl olelination of the initially formed alkylidene complex can occur and this results in the formation of a cyclic olefin. This tandem sequence is illustrated by the transformation of keto-olefins... 

Referring to the ADMET mechanism discussed previously in this chapter, it is evident that both intramolecular complexation as well as intermolecular re-bond formation can occur with respect to the metal carbene present on the monomer unit. If intramolecular complexation is favored, then a chelated complex, 12, can be formed that serves as a thermodynamic well in this reaction process. If this complex is sufficiently stable, then no further reaction occurs, and ADMET polymer condensation chemistry is obviated. If in fact the chelate complex is present in equilibrium with re complexation leading to a polycondensation route, then the net result is a reduction in the rate of polymerization as will be discussed later in this chapter. Finally, if 12 is not kinetically favored because of the distant nature of the metathesizing olefin bond, then its effect is minimal, and condensation polymerization proceeds efficiently. Keeping this in perspective, it becomes evident that a wide variety of functionalized polyolefins can be synthesized by using controlled monomer design, some of which are illustrated in Fig. 2. 

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