Polyene complexes

2-shifts with a=71 10 kJ mol-S Iog/l = 13.1. [Fe( -CvH8)(CO)3] (35 R=H) has been now shown to be fluxional with AG ca. 23 kcal mol by application of the Forsen-HofFman spin-saturation n.m.r. method. The related silyl and germyl complexes (35 R=Mc3Si) and (35 R=Me3Ge) also undergo a similar oscillatory process. The effect of these processes is that of a 1,3-shift and it is possible that this may occur directly but an alternative pathway could be two sequential 1,2-shifts with the norcaradiene intermediate (36). 

---

Isolated double bonds can be oxidatively cleaved in systems containing a conjugated diene moiety if it is protected as a tricarbonyl(diene)iron complex44. Dienal 39 was acquired in 49% yield by a two-step osmylation-periodate cleavage sequence (equation 27). In contrast, ozonolysis of the polyene complexes is reported to lead to destruction of the complex. 

A significantly higher barrier (ca 32 kcalmol-1) is observed for iron migration in linear polyene complexes, e.g. 118 - 118 (Scheme 22). This isomerization is believed to proceed via rf -> rj1 coordination (118 - 119) followed by migration of the iron in the r]2 coordination mode (119 -> 119 ) (Scheme 22)122. While racemization of acyclic (diene)Fe(CO)3 does not occur at ambient temperatures, it is observed at elevated temperatures. This process is also proposed to occur via r/2 coordination (Scheme 23) however the rate for racemization (ca 2.3-2.7 x 104 at 119 °C) is approximately half the rate of polyene migration122. 

In comparison to the above diastereospecific reactions of cyclic polyene complexes, the reaction of acyclic (diene)Fe(CO)3 complexes (221) with pendant unsaturated functionality has been found to occur in a diastereoselective fashion. The diastereoselectivity in... 

Chen S, Huang X, Zhou X et al (2003) Organizational and mutational analysis of a complete ER-008/candicidin gene cluster encoding a structurally related polyene complex. Chem Biol... 

The [Ni(alkene)(PF3)3] complexes (alkene = C1CH=CH2, CH3CH= CH2, CF3CH=CH2, and FCH=CH2) made by metal vapor synthesis are much less thermally stable than the polyene complexes (28) and decompose readily to give [Ni(PF3)J, alkene, and nickel metal. 

The organometalhc chemistry of Ceo is dictated by its spherical geometry and localized polyalkene tt-electronic structure. All the derivatives reported to date are rf- complexes see Hapticity) in which the metal coordinates at a six-six ring fusion (formal double bond) no analogous jj" -diene or jj -friene complexes have been prepared to date see Alkene Complexes). The instability of these polyene complexes is postulated to be due to the curvature of the Ceo molecule and the corresponding divergence of the surface p-orbitals, which results in poor overlap with the metal orbitals. ... 

Alkene, alkyne, and polyene complexes D. M. P. Mingos, Bonding of Unsaturated Organic Molecules to Transition Metals, In Comprehensive Organometallic Chemistry, G. Wilkinson, F. G. A. Stone, and E. W. Abel, Eds., Pergamon Press Oxford, 1982, Vol. 3, pp. 47-67. 

A significantly higher barrier (ca 32 kcalmol ) is observed for iron migration in linear polyene complexes, e.g. 118 -> 118 (Scheme 22). This isomerization is believed to proceed via zj -> r/- coordination (118 119) followed by migration of the iron in... 

Diene and polyene complexes with t) -coordination are known for nearly all the transition metals from group IVA to VIII. Table I offers examples for Cr—Rh. Ligand... 

Various polyene—ion complexes have been developed. Water-soluble complexes of amphotericin B with calcium and oxalic or succinic acids have been described [382]. Stable water-soluble borate complexes of amphotericin B, candidin, candicidin, pimaricin have been developed [383]. An advantage of producing stable water-soluble iron(Fe(II) and Fe(III))—polyene complexes was that complexes retained the in vitro antifungal properties of the parent compound [384] while water-soluble calcium complexes showed enhanced antifungal action [385] and it has been suggested that amphotericin B—meglumine complexes were less toxic when used intravenously than desoxycholate complexes [386]. 

Significant reduction in serum cholesterol levels can also be demonstrated in animals simultaneously fed polyenes and high sterol diets. Dogs appeared more able to maintain their serum cholesterol levels when fed sterols and filipin, amphotericin B or candicidin [145,535] but nystatin had no effect. The anti-hypercholesterolemic effect was not simply due to the prevention of sterol uptake [538,539]. A consequence of the excretion of sterol-polyene complexes in the faeces is the loss of bile salts (which are steroids) and this tends to remove sterol from the body. This resulted in increased hepatic synthesis, increased utilization of adipose fat and reduced sebum levels. Candicidin has been suggested as an antihypercholesterolemic agent in man [546]. 

Jayaprakash et al. [198] reported the series of push-pull bimetallic polyene complexes [(CO)5M=C(OCH3)(-CH=CH-) (t C5H4)Fe(r C5H5)] (M = W, Cr n = 1-4) with ferrocene as donor and a carbene complex of Cr or W as acceptor. The jSi o6(HRS) values increase with increasing the conjugation of the n linker for the same n linker, the chromophore based on W exhibits higher second-order NLO responses than that based on Cr, as expected for a more efficient n backdonation from the W atom. The quadratic hyperpolarizability of these bimetallic push-pull... 

Associative and dissociative paths have also been established from studies of the kinetics of reactions of [Fe(CO)3( y -dienone)] with tertiary phosphines and now for polyenes/ which is a very useful route to [Fe(CO)g(polyene)] complexes. Both paths as shown in Scheme 3 lead to an initial dissociation of the organic carbonyl, and the rate constants for the unimolecular path increase with increasing donor ability of group R (Ph < H < Me) where the incoming polyene is cyclohepta-l,3,5-triene (L). 

---