Dienes metallation

The crystal structures of a number of s-trans (diene)metal complexes (3) have been determined10-14. The diene ligand in all s-trans complexes is distinctly non-planar the torsional angle between the two olefin groups is between 114° and 127°. In general, the terminal carbon to metal distance is greater than for the internal carbon to metal distance, and the Cl—C2/C3—C4 bonds are shorter than the C2—C3 bond. 

Dynamic intramolecular rearrangements are observed for a variety of diene-metal complexes at, or near, ambient temperature. This stereochemical non-rigidity may be detected by variable temperature NMR experiments40 in which the signals observed for a static structure coalesce into time averaged signals for the fluxional process. For purposes of this section, processes with activation energies > ca 25 kcal mol 1 or which are irreversible will be considered to be isomerization phenomena and will be discussed in Section IV. 

By far the greatest number of diene-metal complexes are of the type (diene)Fe(CO)3. All of these complexes exhibit the s-cis rf -diene coordination mode. 

A number of cationic (diene)metal complexes undergo a-deprotonation. Treatment of (cyclodiene)Mo(CO)2Cp+ cations (e.g. 44) with NEt3 or other non-nucleophilic bases... 

In general, reagents approach a (diene)metal complex on the face opposite to the metal due to the steric bulk of the attached metal-ligand array. Due to the relatively low cost of iron, the vast majority of examples of these type of reactions utilize the Fe(CO)3 fragment. 

Both 1,3- and 1,4-cyclohexadienes undergo rapid and catalytic disproportionation in the presence of transition metal atoms, possibly through the intermediacy of a bis(diene)metal system... 

The iron complexes 35 (78) and 36(79) have been reported. Crystallographic studies on 35 indicate that the (> 4-diene)metal interaction is best represented by the valence bond description shown, in which the perfluorodiene acts as a... 

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-... 

II. The Unique (,v-tra ,v-q4-Conjugatcd Diene)Metal Complex Family. Ill... 

THE UNIQUE (s-frans-r 4-CONJUGATED DIENE)METAL COMPLEX FAMILY... 

It is well known that x-traw-butadiene can serve as a bridging ligand in binuclear [( i-r 2,r 2-C4H6)M2L ]metal complexes.12 However, since the original discovery in 19804 there has been an increasing number of structurally, chemically, and spectroscopically well characterized mononuclear (s-trura-conjugated diene)metal complex systems reported in the literature.11... 

Higher 43-hydrocarbyl derivatives of transition metals can be obtained by partial protolysis of the bis(7i-allyl)metal compounds formed from cyclo-octa-1,5-diene metal complexes and 1,3-butadiene with one equivalent of the Bronsted acid in the presence of the proper ligand in a polar solvent at lowered temperature (e.g. — 40°C). For instance, Ni [43-(CxHn)l and Ni—[43-(Ci2Hig)] derivatives have been yielded by the reaction of Ni(CxH 2)2 [(Ni(Cod)2] with 1,3-butadiene (C4H6), followed by treatment with a Bronsted acid, according to scheme (1) [132] and scheme (2) [37] respectively ... 

The 1,3-diene-metal complexes show a number of characteristic features and will be treated separately. Data for selected compounds, all containing single-c/s-butadiene, are collected in Table VII (50, 53 -60). Early studies (61) concentrated on differentiating between the two valence bond representations 16 and 17, while recent interest has centered on intramolecular rearrangements (62). 

Thus when the >73-l,l-dimethylallyltris(trifluorophosphine) complexes of cobalt or rhodium are gently warmed a rearrangement to the 73-l,2-isomer occurs (56, 295). The postulated mechanism involves a diene-metal hydride intermediate (Scheme 9). The small PF3 ligand can also be added directly to coordinatively unsaturated 3-allylic systems or to chloro-bridged structures (method H). 

E)-l,3-Dienes, (E,E)-conjugated dienes. Metallation and alkylation of 1 results in a single product fonned by exo-attack. Thermolysis of the product results in a terminal (E)-l, 3-diene. An example is the synthesis of 2, a sex pheromone of the red bollworm moth. 

Some years ago we observed that Group IV metallocenes, i.e., the elusive monomeric units bis(i7-cyclopentadienyl)zirconium and -hafnium, readily form 1 1 complexes with conjugated dienes in their 5-trans conformation (22). To our knowledge, these complexes (3) are the first and, at the time, only known examples of mononuclear (i-/ranj-i7 -diene)metal complexes, i.e., compounds in which both double bonds of the i-irans conformer of a conjugated diene are coordinated to the same transition metal center 21). Some examples of this class of transition metal complexes are stable, isolable compounds even at room temperature 23). 

In addition, the experimental observation of (s-/rani-ij -diene)metal-locene complexes is kineticcdly facilitated by the special appearance of the energy profile of the dienemetallocene system. For the parent system there is evidence (27-31) that the stable (butadiene)ZrCp2 isomers [about equal amounts of (s-trans- (3a) and (s-cis-T]i -butadiene)zirconocene (5a) are obtained under equilibrium conditions (22, 23)] are connected through a reactive (T -butadiene)zirconocene intermediate (4a) which rapidly equilibrates with 5-tra/u-diene complex 3a, but is separated from the (s-cis-diene)metallocene isomer 5a by a rather substantial activation barrier... 

Another early discovery relating to diene-metal complexes was described by Reihlen and co-workers in 1930 (210). These workers treated... 

Another notable advance in diene-metal chemistry was made by Chatt and co-workers in 1957 36,37,38), and somewhat later by Pettit 190) and by Wilkinson et al. 23, 110). These groups of investigators established... 

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