Metal-polyene complexes, synthesis

The most generally applicable route to diene or triene metal PF3 complexes is via metal vapor synthesis (method D), in which the metal vapors are cocondensed with PF3 and the polyene at liquid nitrogen temperatures. 

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. 

Despite their obvious similarity to alkenes and alkynes, transition-metal alkylidene and alkylidyne complexes have not been used as building blocks for the synthesis of lowdimensional materials analogous to polyenes and polyynes. We have begun to explore the syntheses, structures, and properties of conjugated complexes and polymers derived from metal-alkylidyne complexes as part of our effort to develop the chemistry of transition-metal analogues of conjugated organic compounds. 

Ketenes and isocyanates also undergo facile [6 + 2]-photocycloaddition with metal complexed cyclic polyenes. Irradiation of 232 in the presence of diphenylketene gave 256 in good yield (Scheme 58)120. This should be contrasted with the normal behavior of ketenes toward alkenes, which typically involves [2 + 2]-cycloaddition. Isocyanates such as 257 work as well. The adducts are produced in high yields and have considerable potential in synthesis. 

How do antibiotics act Some, like penicillin, block specific enzymes. Peptide antibiotics often form complexes with metal ions (Fig. 8-22) and disrupt the control of ion permeability in bacterial membranes. Polyene antibiotics interfere with proton and ion transport in fungal membranes. Tetracyclines and many other antibiotics interfere directly with protein synthesis (Box 29-B). Others intercalate into DNA molecules (Fig. 5-23 Box 28-A). There is no single mode of action. The search for suitable antibiotics for human use consists in finding compounds highly toxic to infective organisms but with low toxicity to human cells. 

Addition of Grignard reagents to (dienone)iron complexes has been reported as part of the synthesis of an advanced intermediate of the antifungal polyene macrolide antibiotic amphotericin B. The (Ti -dienone)iron complex adopts an s-cis conformation. Clean addition of the nucleophile from the less hindered face opposite to the metal is achieved (Scheme 4-143). ... 

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