Triene complexes

The insertion reaction between alkenylcarbene complexes and electron-rich alkynes such as 1-alkynylamines (ynamines) leads to mixtures of two regioi-someric cyclopentyl derivatives [78]. Thus, if the insertion occurs on the carbon-metal bond a new aminocarbene complex is produced which evolves to a cyclopentenylmetal derivative. On the other hand, if the insertion reaction occurs on the carbon=carbon double bond of the alkenyl complex, the reaction gives a l-metala-4-amino-l,3,5-triene complex which finally generates a different regioisomer of the cyclopentenylmetal derivative (Scheme 31). 

The metal-mediated and metal-catalyzed [6 + 2]- and [6 + 4]-cycloaddition reactions, pioneered by Pettit and co-workers105 106 and Kreiter and co-workers,107 respectively, involve the cycloaddition of metal-complexed cyclic trienes with 7r-systems such as alkenes, alkynes, and dienes. The [6 + 2]-reactions produce bicyclo[4.2.1]nonadiene derivatives and the [6 + 4]-reactions produce bicyclo[4.4.1]undecatrienes (Scheme 32). Trienes complexed to chromium, which can be prepared on large scale (40 g) as reported by Rigby and co-workers,108 react with 7r-systems upon thermolysis or irradiation.109-111 Chromium and iron-catalyzed [6 + 2]-reactions of cycloheptatrienes and disubstituted alkynes... 

Amino acids coordinated to (en)2Co(III) centers are the most useful in peptide synthesis, and general routes to the preparation of these important A/) O-chelates are described below. Many of the corresponding trien complexes are also available, usually as /32 isomers, but [Co(NH3)4(AAO)]2+ species are less accessible. The [Co(en)2(AAO)]2+ complexes have been reported for 19 of the 20 naturally occurring... 

The first success in sequence-specific peptide cleavage by an antibody was claimed by Iverson (Iverson and Lemer, 1989). He used hapten [43] containing an inert Cora(trien) complexed to the secondary amino acid of a... 

If the profile of the observed or the intrinsic rate constant plotted against pH resembles the profile for an acid-base titration curve, this strongly suggests that one of the reactants is involved in an acid-base equilibrium in that pH range. Such behavior is ftiirly common and is illustrated by the second-order reaction between the Co(II)-trien complex and O2 (Fig. 1.12). The limiting rate constants at the higher and low acidities correspond to the acidic and basic forms of the Co(II) reactant, probably. 

Fig. 1.12 The pH dependence of the first stage of the reaction between Co(II)-trien complex and Oj. The solid line represents Eqn. (1.207) with = 6 X which is a spectrally determined...

Reaction of CrCl3-6H20 with trien gives a blue-violet mixture from which some chromium(III)-trien complexes (electronic spectra given in Table 56) may be isolated as indicated in Scheme 53. While the conversion of cis-ar-[Cr(0H)(trien)H20]Cl2 to the cis-fi isomer in the temperature range 160-225 °C is reversed on cooling, the process is irreversible if the reaction is carried out above 225 °C. 

Table 56 Electronic Spectra of Chromium(in)-trien Complexes...

Although one can envisage dehydrogenation and rearrangement of the coordinated trienes in 149 and 150 to yield 114, no such process has actually been observed to take place. Because both triene complexes are derived from the trans ligand, and not the cis isomer, it is possible that it is the ds-hexa-l,3,5-triene ligand that produces the benzene ring in cluster... 

Scheme 4 Potential ring conformations in square planar and (rons-octahedral trien complexes...

Fully A-alkylated trien complexes are known for Cu11,841 Pd11704 and Li1842 and the synthesis of Me6- and Et6-trien have been described.841... 

A number of complexes of the general type (CoN4(OH)(OH2)]2+ (N4 = a system of four nitrogen donors) stoichiometrically cleave the N-terminal amino acid from di-or tri-peptides. Reactions have been described for N4 = en2,164-165 trien166 167 and tren.168,186 In the case of trien complexes, the proposed mechanism for peptide hydrolysis is shown in Scheme 7. Hydrolysis can occur by two pathways (a) attack by external hydroxide on the O-bonded chelated peptide, and (b) intramolecular attack of coordinated hydroxide on the N-bonded peptide. 

The major limitation on the reactivity of cycloheptadienyliron complexes is the fact that treatment with hard enolates, such as methyl lithioacetate results in deprotonation to give, e.g. the V-triene complex (89). This appears to be less of a problem in the corresponding cyclohexadienyl systems. 

Aclohcpuidiciiyl complexes, with vanadium, 5, 50 Cycloheptadien-l-yne, in Pd(0) it-complexes, 8, 355 tj6-Cyclohepta-l,3,5-triene complex, as catalyst precursor,... 

Pentadienyltricarbonyliron salts can also be prepared by protonation of f/4-triene complexes (180,184,185). The reverse transformation can be carried out by addition of trimethylamine followed by pyrolysis of the resulting quaternary ammonium salt [Eq. (21)] (180). 

Triene complexes of rhodium have been prepared by a crossed aldol condensation with acetophenone (211) or by Wittig reactions [Eq. (29)] (212). 

Fig. 27 Crystal and molecular structures of (a) [Cun(tren)(H20)]2+, displaying a compressed trigonal bipyramidal coordination geometry [60] and (b) [Cun(trien)(H20)]2+, which shows a square pyramidal geometry [61]. The Cun-OH2 distance is distinctly smaller in the tren complex than in the trien complex...

Steric influences on the heats and entropies of chelate formation may arise from the difficulty encountered by the ligand in assuming the conformation most suitable for effective coordination of a metal ion. Examples of this effect may be seen in the data given in Table V for tetra-coordination of Cu Ni+2 and by polyamines (12, I4). Probable structures of these complexes are represented by Formulas XIV-XVII. It is interesting that the main differences between the stabilities of the tren and trien complexes are due to differences in enthalpies, rather than in entropies, of reaction. Of the two sets of chelate compounds, those of Zn(II) represent the more nearly normaP situation because all four donor groups of each ligand can coordinate to the metal ion with little strain. The remarkable difference between the enthalpies of formation of Chelates... 

Such a change preserves the electron count in the coordination sphere when the extra incoming ligand is attached. Studies on similar molybdenum-triene complexes have resulted in the isolation of 18-electron y-(triene)Mo(CO),(PR,) moreover, an T 4-arcnc complex has been isolated and characterized in the solid state.25... 

This method can also be applied to complexes of conjugated trienes. The uncomplexed double bond may be constructed within the complex via Wittig olefination. Cyclopropanation of the uncomplexed double bond of the resulting tricarbonyliron-triene complexes and oxidative decomplexation leads to dienylcyclopropane products. In this manner chiral dienyl-cyclopropanes 4 and 5 were prepared in high enantiomeric excess (> 90% ee) starting from optically active tricarbonyliron-hexatriene complexes 3 obtained from chirally modified sorbic aldehyde complexes. 

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