Diene complexes formation

However, upon thermolysis of 178.d or 178.h, isomeric mixtures of the butadiene complexes 183.a and 183.b were formed. Since intramolecular hydrogen transfer within (3,3-dime thyl-773 r -allylacy iron complexes is well precedented101 (see Section VI,B), it seems likely that this process is responsible for diene complex formation. Note that only the Z-diene complex was isolated from the reaction mixture, a surprisingly stereoselective result. 

Exocyclic Diene Complexation. Formation of iron carbonyl complexes of (6) illustrates the mild nature of the complexation conditions (eq 7). ... 

In a somewhat different area of organozirconium chemistry, Erker s group has recently reported the formation of Cp2Zr(CO)2 (2) in very good yield via the carbonylation of the zirconocene diene complexes 26a and 26b (57). In cases where the diene was structurally less complex (e.g.,... 

A similar ring-opening metallation is observed with bicyclo[3.1.0]hex-2-ene and bieyclo[5.1.0]oct-2-ene. The olefin 49 undergoes ring enlargement with Fe2(CO)9 to form the corresponding rc-allyl and >/4-diene complexes [27]. trans-Palladation of 53 results in the formation of the 7t-allyl complexes 54 [28]. (Scheme 19 and 20)... 

If the unsaturated hydrocarbon is a diene, both double bonds may coordinate to palladium ). (Diene)palladium(II) complexes have been isolated and characterized. For example, 2 and 3 are stable complexes in which both double bonds are coordinated to the metal10. Conjugated dienes constitute a special case and although /j4-diene complexes, e.g. 4, are postulated as intermediates, they have not yet been isolated. The butadiene complex 4 is in equilibrium with the zr-allyl complex 5 in solution, and attempts to isolate the diene complex from this mixture lead to formation of a yellow crystalline complex 511. 

This novel electroreductive cyclocoupling corresponds to a 1,4-addition of a one-carbon unit to the 1,3-diene, and does not take place without using magnesium electrode. The first step in this coupling reaction is the cathodic reduction of 1,3-diene to an anion radical, and the second step is the formation of a Mg-diene complex, which thereafter reacts with the ester to yield the coupling product as shown in equation 23b. 

The intermediary formation of the Mg-diene complex is confirmed by a two-step reaction method, namely in the first step a solution of 1,3-diene is electrochemically reduced with magnesium electrode in the absence of the ester. After a sufficient amount of electricity is passed, the current is terminated and the ester is added to the solution. The fact that the coupling product is also formed by this two-step method strongly supports the formation of the intermediate Mg-diene complex. 

In complexes where the carbomethoxy or acyl group is adjacent to the metal-complexed acyl group (e.g., 145.a), photolysis affords a 1 1 mixture of isoprenic diene complexes (E-152 and Z-152) directly. The formation of an intermediate allylketene complex (153) may be demonstrated by methanol trapping, followed by aerial oxidation to quantitatively yield the diester 154. 

As before, when the carbomethoxy group is bonded to the central carbon of the allyl portion of the vinylketene complex, the increased lability of a methyl proton allows the formation of a second product, in this case the diene complex 150. In the cases where R = PF(146.e and 148.a), decomplexation of the tricarbonyliron moiety allows the thermodynamically favored... 

The method has been extended to bimetallic complexes (equation 87) and to butadiene type compounds (equation 88).309 These dienes may be used in complex formation producing complexes analogous to butadiene, as shown by equation (89). The structure of (59b) and the diene [Cp(CO)2 Mo=P(R)CH=P(R)] (59c) have been proved by X-ray crystallography. A similar series of dienyl complexes have been synthesized by Huttner et a/.281 (see Section 14.5.1.3.i). 

Curiously, no ij4-diene complexes have been obtained using perfluoro-cyclopentadiene, although a number of rj2 complexes such as 38 (32), 39 (83a), and [Co(f/5-C5H5)(CO)(f/2-C5F6)] (Si) have been prepared. Once again, the propensity for formal oxidative addition is revealed by the formation of complex 31 (see Section II1,D) in the reaction of perfluorocyclopentadiene with... 

The complementary approach, activation of unsaturated hydrocarbons toward electrophilic attack by complexation with electron-rich metal fragments, has seen limited investigation. Although there are certainly opportunities in this area which have not been exploited, the electrophilic reactions present a more complex problem relative to nucleophilic addition. For example, consider the nucleophilic versus electrophilic addition to a terminal carbon of a saturated 18-electron metal-diene complex. Nucleophilic addition generates a stable 18-electron saturated ir-allyl complex. In contrast, electrophilic addition at carbon results in removal of two valence electrons from the metal and formation of an unstable ir-allyl unsaturated 16-electron complex (Scheme 1). 

This chapter illustrates that electron-rich transition metal-diene complexes can couple with carbon electrophiles and, thereby, provide unusual methods for carbon-carbon bond formation. These procedures are of interest from a synthetic viewpoint since normally uncomplexed dienes or polyenes are not reactive toward weak carbon electrophiles or, with strong electrophiles, undesirable reactions such as polymerization occur. Furthermore, the metal-mediated route often results in desirable regio- and/or stereo-selectivity. Important to the utility of these methods is the ability to free the organic ligand from the metal. In most instances efficient oxidative procedures have been developed for such cleavage reactions. 

This step has been shown to be stereospecific in the case of hydrogenation (deuteration) catalyzed by dicyclopentadienylmolybdenumdihydride (dideuteride) (62) with cis addition of the metal hydride to the double bond (70). In these systems there is spectral evidence for the initial formation of cr- and 7r-electron donor-acceptor complexes between the catalyst and substrate prior to 7r-complex formation and n-a- rearrangement (70, 71). This catalyst (62) has also been used for the selective homogeneous hydrogenation of 1,4- or 1,3-dienes to monoenes, for example, cyclo-pentadiene (63). The reactions are run at elevated temperature (72). 

Iron carbonyls have been used in stoichiometric and catalytic amounts for a variety of transformations in organic synthesis. For example, the isomerization of 1,4-dienes to 1,3-dienes by formation of tricarbonyl(ri4-l,3-diene)iron complexes and subsequent oxidative demetallation has been applied to the synthesis of 12-prostaglandin PGC2 [10], The photochemically induced double bond isomerization of allyl alcohols to aldehydes [11] and allylamines to enamines [12,13] can be carried out with catalytic amounts of iron carbonyls (see Section 1.4.3). 

Ohashi et al. [128] found that the yields of ortho photoaddition of acrylonitrile and methacrylonitrile to benzene and that of acrylonitrile to toluene are considerable increased when zinc(II) chloride is present in the solution. This was ascribed to increased electron affinity of (meth)acrylonitrile by complex formation with ZnCl2 and it confirmed the occurrence of charge transfer during ortho photocycloaddition. This was further explored by investigating solvent effects on ortho additions of acceptor olefins and donor arenes [136,139], Irradiation of anisole and acrylonitrile in acetonitrile at 254 nm yielded a mixture of stereoisomers of l-methoxy-8-cyanobicyclo[4.2.0]octa-2,4-diene as a major product. A similar reaction occurred in ethyl acetate. However, irradiation of a mixture of anisole and acrylonitrile in methanol under similar conditions gave the substitution products 4-methoxy-a-methylbenzeneacetonitrile (49%) and 2-methoxy-a-methylbenzeneacetonitrile (10%) solely (Scheme 43). 

Thus activation and functionalization of alkenes, enones and conjugated dienes are possible based on the 7i-allylpalladium complex formation from these unsaturated compounds. 

Unconjugated dienes form the 1,3-diene complexes after isomerization to conjugated dienes. Formation of the stable conjugated diene complexe is the driving force of the isomerization. For example, the 1,4-diene in the synthetic intermediate 29 of prostaglandin A can be protected as the diene complex 30 after isomerization to the conjugated diene when it is treated with Fe2(CO)9. This method was applied to the synthesis of prostaglandin C (13). The diene complex 30 is stable for the oxidation of the lactol and introduction of the a-chain [7]. 

Anhydrous iron(III) halides catalyse coupling of alkynes and aldehydes.211 Simple terminal alkynes, R CH, react with aldehydes, R2CHO, to give ( ,Z)-1,5-dihalo-1,4-dienes (55). In contrast, non-terminal arylalkynes give ( ,)-o, /3-unsaturated ketones. The catalysts also promote standard Prins cyclization of homoallylic alcohols. Studies of intermediates and of alkyne hydration - together with calculations - all support FeX3 complex formation with alkyne as the activating step. 

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

This reaction elucidates the mechanism of the photoreaction of 73 with dienes. In the first step, 73 loses one carbonyl ligand with formation of the reactive 16-electron species [0/5-C5Hj)Mo(CO)2CH3] (87) (109-113), which adds a diene molecule. jj2-Diene complexes [( 5-C5H5)Mo(CO)2CH3-( 72-diene)] (88) are quite likely as intermediates. Coordination of the free C=C double bond of the r 2-diene ligand causes insertion of CO into the Mo—C [Pg.338]

Coordination polymerisation via re complexes comprises polymerisation and copolymerisation processes with transition metal-based catalysts of unsaturated hydrocarbon monomers such as olefins [11-19], vinylaromatic monomers such as styrene [13, 20, 21], conjugated dienes [22-29], cycloolefins [30-39] and alkynes [39-45]. The coordination polymerisation of olefins concerns mostly ethylene, propylene and higher a-olefins [46], although polymerisation of cumulated diolefins (allenes) [47, 48], isomerisation 2, co-polymerisation of a-olefins [49], isomerisation 1,2-polymerisation of /i-olcfins [50, 51] and cyclopolymerisation of non-conjugated a, eo-diolefins [52, 53] are also included among coordination polymerisations involving re complex formation. 

Studies on the thermodynamics of lanthanide complex formation with ligands like ethylenediamine (en) and diethylenetriamine (dien) in acetonitrile to form [M(en) ]31... 

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