1,3-diene complexes

The results of the reductions of some steroidal a,)3-unsaturated ketones have been summarized by Brown. " The carbonyl group is usually reduced to the hydrocarbon, but the behavior of the double bond depends on the structure of the compound undergoing the reduction. Cholest-4-en-3-one gives chol-est-4-ene. Addition of aluminum chloride to a solution of a 4-ene-3,6-dione followed by treatment with LiAIH4 gives the 4-ene-6-one. Steroid 4,6-dien-3-ones yield mixtures of dienes. When the ketone and double bond are in different rings the results become even more complex dienes as well as mono-enes are obtained. 

Diels-Alder reaction of fullerenes with complex dienes type 52 (Figure 2.6) which have a 2,3-bis-(methylene) bicyclo[2.2.2]octane unit [49]. 

Entry 9 uses the oxaborazolidine catalysts discussed on p. 505 with 2-bromopropenal as the dienophile. The aldehyde adopts the exo position in each case, which is consistent with the proposed TS model. Entry 10 illustrates the use of a cationic oxaborazolidine catalyst. The chirality is derived from trans-1,2-diaminocyclohcxanc. Entry 12 shows the use of a TADDOL catalyst in the construction of the steroid skeleton. Entry 13 is an intramolecular D-A reaction catalyzed by a Cu-Ws-oxazoline. Entries 14 and 15 show the use of the oxazaborolidinone catalyst with more complex dienes. 

The nickel-catalyzed [4 + 4]-cycloaddition of butadiene to form cyclooctadiene was first reported by Reed in 1954.90 Pioneering mechanistic and synthetic studies largely derived from the Wilke group advanced this process to an industrially important route to cyclodimers, trimers, and other molecules of interest.91-94,943 95,96 While successful with simple dienes, this process is not useful thus far with substitutionally complex dienes as needed in complex molecule synthesis. In 1986, Wender and Ihle reported the first intramolecular nickel-catalyzed [4 + 4]-reaction of... 

The 13C NMR signals of a diene are shifted far upheld upon complexation to a transition metal33. The terminal carbons (C1/C4) for a complexed diene are ca 50 to 80 ppm more shielded than the free ligand, while the internal carbons (C2/C3) are ca 20 to 60 ppm more shielded (Tables 3 and 4). The a2,jt bonding mode (2b) which is found for Zr, Hf and Nb s-cis diene complexes is revealed in the diminished coupling to... 

Dienes form very stable complexes with a variety of metal caibonyls, particularly Fe(CO)s, and the neutral V-diene metal carbonyl complexes are quite resistant to normal reactions of dienes (e.g. hydrogenation, Diels-Alder). However, they are subject to nucleophilic attack by a variety of nonstabilized carbanions. Treatment of -cyclohexadiene iron tricarbonyl with nonstabilized carbanions, followed by protonolysis of the resulting complex, produced isomeric mixtures of alkylated cyclohexenes (Scheme 15).24 With acyclic dienes, this alkylation was shown to be reversible, with kinetic alkylation occurring at an internal position of the complexed dienes but rearranging to the terminal position under thermodynamic conditions (Scheme 16).2S By trapping the kinetic product with an electrophile, overall carbo-... 

In addition to protection, a change of diene reactivity is effected by coordination to carbonyl. Butadiene forms the very stable complex 56 and its reactions are different from those of free butadiene. Electrophiles attack C(l) or C(4) of the complexed dienes, and reactions that are impossible with uncomplexed dienes now become possible. 

Similarly, in the tricarbonyliron cyclobutadiene complex 285 (78AJC1607), the metal carbonyl groups stabilize the positive charge in the a-positions to the complexed diene system (H-2 and H-6 8 8.29 ppm). 

Other imino derivatives arise, as by-products or in side reactions, on heterocyclization. Thus, the treatment of cinnamoyltropolones 75 with hy-droxylamine (Scheme 19) yields, in the case of the 5-nitro derivative, the corresponding isoxazolotroponeoxime (89JHC371). The formation of oximes and several hydrazones from 3-acetyltropolone or its derivatives has also been mentioned (Section II,A,3,c). Moreover, an azine was obtained in addition to quinoxalotropone 213 (Section II,B,2,c) a tropone immonium salt was isolated after an extremely complex diene reaction of an 6-amino-2-azaazuIene (93CB441). 

Contrary to what is observed during tandem addition reactions to [Os]-toluene (vide supra), electrophilic additions to [Os]-bound ortho- and meta-xylenes result in regioselective attack at C6 (Table 3). A coordination isomer having the metal across C4-C5 (19) is the only isomer observed for both ortho- and meta-xylene. Electrophilic addition of HOTf (entry 1) or dime-thoxymethane (entries 2 and 3) at C6 generates the complexed allyl cation 20, which can be trapped with MMTP to form the complexed diene 21. Demetalation using AgOTf releases the free diene 22, which potentially possesses two adjacent quaternary centers (entry 3) [15]. 

Friedel Crafts acylation of complexed dienes can be accomplished on the nonsubstituted terminus withont isomerization. An example of a complexation-acylation can be seen in Scheme 151. The cis trans complex is often obtained as the major prodnct Ifom Friedel Crafts acylations (Scheme 152). Acid-catalyzed isomerization ftir-nishes the thermodynamically more stable trans trans complexes. 

Double bonds adjacent to complexed dienes can be cyclopropanated using diazomethane, methyl diazoacetate, or sulfur-based ylids. Cycloheptatriene iron tricarbonyl undergo a [2 -F 2] cycloaddition with chloroketene derived from trichloroacetyl chloride (Scheme 161). 

Although butadiene reacts with Co2(CO)8 to yield the diene complexes (diene)C02(CO)o and (diene)2Co2(CO)4 (268), with alkyl- or acylcobalt tetracarbonyls it produces only the Tr-allylic species, 1-alkyl- or 1-acylmethyl-TT-allylcobalt tricarbonyls (281). These will react, in turn, with P(C3Hb)3 which displaces one CO ligand to form monotriphenyl-phosphine derivatives (281). 

An NMR study (597) of ligand exchange in the system (diene)MCl(L) (diene = norbornadiene or 1,5-cyclooctadiene M = Rh or Ir L = tertiary phosphine, arsine, or stibene) shows a first-order dependence of the rate upon both L and the olefin complex in the temperature range from —70° to —10°C. The exchange involves an 8 2 mechanism with the five-coordinate complex (diene)MCl(L)2 as intermediate. The intermediate iridium complexes (l,5-CgHi2)IrCl(L)2 can be isolated from ethanolic solution. The activation energy for the process ranges from 4 to 10 kcal/mole (597). 

By analogy with the cobalt analogs, the complexes (diene)Rh(C5Hs)... 

At complex-diene ratios of 1 1, 2 3, and 1 2 the reaction is second order, and first order in each component. An associative mechanism has been suggested. It was shown that with a monoolefin iron complex the substitution reaction follows first-order kinetics with a rate independent of diene concentration. 

Reviews have appeared of the photophysics of molybdenum complexes, primary and secondary processes in organometallic chemistry, flash photolysis of Pe(CO)5 and Cr(CO)g, dinuclear manganese carbonyl compounds, the photochemistry of metal complexes isolated in low temperature matrices, cluster complexes, diene complexes, photoproduction of coordinativeiy unsaturated species containing rhodium or iridium, and redox chemiluminescence of organometallic compounds.Synthetic and metal organic photochemistry in industry has also been reviewed. 

Siloxy-rhodium(I) complexes of general formula [(diene)Rh(//-OSiMe3)]2, where diene = cod, nbd, showed much higher catalytic activity in the hydrosilylation of 1-alkenes [50] and allyl ethers [51] by triethoxy silane than the respective chloro-rhodium(I) complexes [(diene)RhCw-Cl)2] suggesting a possible application of dimeric bridged siloxy-metal complexes as potent precursors of a variety of hydrosilylation reactions. 

Generally the reaction of unsaturated aldehydes (aromatic, olefmic and acetylenic) with chiral boronates has provided homoallylic alcohols in low to moderate enantioselectivity [124]. However, the enantioselectivity of the allyl- and 2-bu-tenylborations of benzaldehyde and unsaturated aldehydes is significantly improved when a metal carbonyl complex is utilized as the substrate [131]. For example, the reaction of iron carbonyl-complexed diene 225, chromium carbonyl-complexed benzaldehyde 226 and dicobalt hexacarbonyl-complexed acetylene 227 all give significantly increa.sed allyl and 2-butenylboration selectivities compared to the parent aldehydes (Fig. 10-6). In the case of chiral substrates 225 and 226, these species can be obtained in enantioenriched form by kinetic resolution by use of the asymmetric allylboration reaction. 

Cyclopropanation of noncomplexed double bonds in the presence of complexed diene units within the substrate and decomplexation of the resulting acyclic or cyclic diene ligands, especially from tricarbonyliron complexes, is used in the synthesis of unsaturated cyclopropanes. The tricarbonyliron unit serves as a removable protecting group which allows regioselective cyclopropanation of the uncomplexed double bond. ... 

Liberation of a complexed diene ligand may be accomplished under oxidizing conditions. (Diene)ZrCp2 complexes, (diene)TiCp X complexes , and (diene)Mn(CO)3 anions i are all relatively sensitive and undergo oxidative decomplexation upon exposure to air to afford the free ligand. The majority of other diene-metal complexes are somewhat stable in air. In the case of the neutral complexes (diene)Mn(CO)2NO f g, (diene)Fe(CO)2L (L = CO, and (diene)CoCp " " " - . or cationic (diene)Mo... 

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