Metal complexes dienyl

As mentioned earlier, metal complexation not only allows isolation of the QM derivatives but can also dramatically modify their reactivity patterns.29o-QMs are important intermediates in numerous synthetic and biological processes, in which the exocyclic carbon exhibits an electrophilic character.30-33 In contrast, a metal-stabilized o-QM can react as a base or nucleophile (Scheme 3.16).29 For instance, protonation of the Ir-T 4-QM complex 24 by one equivalent of HBF4 gave the initial oxo-dienyl complex 25, while in the presence of an excess of acid the dicationic complex 26 was obtained. Reaction of 24 with I2 led to the formation of new oxo-dienyl complex 27, instead of the expected oxidation of the complex and elimination of the free o-QM. Such reactivity of the exocyclic methylene group can be compared with the reactivity of electron-rich enol acetates or enol silyl ethers, which undergo electrophilic iodination.34... 

As noted in the introduction, in contrast to attack by nucleophiles, attack of electrophiles on saturated alkene-, polyene- or polyenyl-metal complexes creates special problems in that normally unstable 16-electron, unsaturated species are formed. To be isolated, these species must be stabilized by intramolecular coordination or via intermolecular addition of a ligand. Nevertheless, as illustrated in this chapter, reactions of significant synthetic utility can be developed with attention to these points. It is likely that this area will see considerable development in the future. In addition to refinement of electrophilic reactions of metal-diene complexes, synthetic applications may evolve from the coupling of carbon electrophiles with electron-rich transition metal complexes of alkenes, alkynes and polyenes, as well as allyl- and dienyl-metal complexes. Sequential addition of electrophiles followed by nucleophiles is also viable to rapidly assemble complex structures. 

The cationic dinitrosyl complexes (146) are apparently much more electrophilic than (143). Studies of the reactivity of (146) have been focused on attack of coordinated organic ligands of other transition metal complexes, such as the cyclooctatetraene ligand in L3M(cot) complexes (M = Fe, Ru) and Cp Co(cot) complexes (Cp = Cp, Cp ), the cyclopenta-dienyl ligand of CpRh(cot) complexes, and the cyanide ligand... 

Allyl and Dienyl Transition-Metal Complexes Containing Cyclopropanes... 

The regioselectivity in palladium-catalyzed alkylations has been attributed to the dynamic behavior of trihapto pentadienyl metal complexes . For example, competing electronic and steric effects influence product formation in dienyl epoxides, but in palladium-catalyzed reactions steric factors were often found to be more important. Thus, alkylation of dienyl epoxide 76 with bulky nucleophiles such as bis(benzenesulfonyl)me-thane in the presence of (Ph3P)4Pd occuned exclusively at the terminal carbon of the dienyl system producing allyl alcohol 77 (equation 39). However, the steric factors could be overcome by electronic effects when one of the terminal vinylic protons was replaced with an electron-withdrawing group. Thus, alkylation of dienyl epoxide 78 affords homoallylic alcohol 79 as the major product (equation 40). 

An analogy may be drawn between the monocyclopentadienyl complexes such as 7r-CBH6M(CO)3R (XXVI), where M = Mo or W, and R = alkyl, and the bis(7r-cyclopentadienyl) metal complexes. The three carbon monoxide groups in the complexes (XXVI) may be compared with a 7r-cyclopenta-dienyl group, in that they occupy three coordination positions of the metal... 

When similar protonation studies were conducted using mixtures of the docosahedral salts 2 and the neutral alkylidyne(cyclopenta-dienyl)metal complexes 3, subtly different species... 

Table 9.1. Structures of Cyclopentadienyl and Dienyl Metal Complexes...

Reactions of reagents 1 and 2 with metal-complexed aromatic, propargylic and dienylic aldehydes provides homoallylic alcohol products with improved selectivity compared to their uncomplexed counterparts. The reaction of benzaldehyde chromium tricarbonyl complex 14 with (R,R)-1 followed by oxidative decomplexation provided ( -15 in 90% yield and 83% ee. The (JE)-crotylboration of 14 with (R,R)-2 provided 16 in 90% yield and 92% ee. Reaction of aldehyde 14 with (. -crotylboronate 3, however, provided adduct 17 in only 41% ee. 

Cassani MC, Gun ko YK, Hitchcock PB, Lappert MF. The first metal complexes containing the 1,4-cyclohexa-2,5-dienyl hgand (benzene 1,4-dianion) synthesis and structures of [K(18-crown-6)][Ln tl -C5H3(SiMe3)2-l,3 2(C6H6)](Ln = La, Ce). Chem Commun. 1996 1987—1988. 

Hydrocarbon ligands which formally contribute five electrons when bonding to a metal are called Dienyl ligands. Five-electron systems are found for cyclic 5-, 6- and 7-membered hydrocarbon ligands and non-cyclic ligands. The 7r-cyclopentadienyl group is the commonest ligand in this class and the chemistry of cyclopentadenyl metal complexes is discussed first. For convenience, the transition metal cyclopentadienide complexes are also discussed in this section. 

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