Conjugated diene complexes protonation

In a proton NMR experiment in which 1,4-pentadiene was added to a solution of HNi[P(OMe)3]4, it was possible to watch the isomerization of 1,4- to 1,3-pentadiene, followed by formation of l,3-dimethyl-7t-allyl complexes (53). The observation of 7t-allyl products in the reaction of the hydride with the conjugated diene, but not in the ff-alkyl intermediates involved in isomerization, illustrates the much greater stability of zr-allyl complexes of nickel compared to tr-alkyls, a feature which is also observed in the hydrocyanation reactions. 

Although complexes with C—H—metal three-center, two-electron bonds were first observed several years ago (40-42), they have received increasing attention recently as model systems for C—H activation by transition metal complexes (43). A general route to such compounds involves the protonation of diene (35,44-51) or olefin complexes (52-56). The resulting 16-electron species are stabilized by the formation of C—H—metal bridges. Irradiation of the complexes [Cr(CO)s L] [L = CO, P(CH3)3, P(OCH 3)3 jin presence of conjugated dienes having certain substituents provides a photochemical route to electron-deficient >/4 CH-diene complexes. 

Since the initial report in 1962 by Merten and Muller of the cycloaddition of an -acyl imine with a conjugated diene,a number of examples of this type of reaction have appeared. In general, IV-acyl im> ines are highly reactive, unstable species which are rarely isolated, but rather are generated in situ from stable precursors. Depending upon the method of formation of the particular dienophile and the reaction conditions, a neutral A -acyl imine or a protonated (or Lewis acid complexed) IV-acyl immonium ion may be involved in the Diels-Alder reaction. 

An olefinic bonds conjugated with the 1 position of a diene complex is highly reactive toward electrophilic reagents because of the stability of the resulting complexed cation. These reagents include not only protons but the trityl cation and bromine. ... 

Equations 3.64-3.66 illustrate routes to allyl complexes from dienes, diene complexes, and olefins. Allyl complexes have been prepared by the insertion of a conjugated diene into a metal hydride, alkyl, or acyl linkage, as illustrated for the cobalt complexes in Equation 3.64. ° Alternatively, allyl complexes have been prepared by nucleophilic or electrophilic attack on a coordinated diene. Equation 3.65 shows the formation of allyl complexes by the addition of carbanions to a cationic diene complex, and Equation 3.66 shows the formation of a cationic diene complex by the protonation of a neutral 1,3-diene complex. Allyl complexes have also been formed by the abstraction of an allylic proton from a metal-olefin complex, either by a base or by the metal itself. This reaction has been proposed as a step in the isomerization of olefins (Equation 3.67) and in the allylic oxidation of olefins (Equation 3.68). - ... 

On protonation, the latter complex would produce the stereodefined conjugated diene as a single regioisomer. On the basis of this proposal, the authors checked the importance of the metallated alkoxide in a related experiment, showing that the reaction of an internal alkyne bearing a tert-butyldimethylsilyl (TBS)-protected hydroxy group with a preformed titanium alkyne complex led exclusively to a complex mixture of products [125]. 

When the cationic i7 -allylpalladiuin(II) complex is formed in the absence of nucleophiles, elimination of proton and conjugate diene formation ensues (see Sect V.2.5.1 for a specific coverage of eliminations). Formally this is a dehydration of allylic alcohols in neutral medium by prior conversion into allylic carbonate however, in some cases a tertiary base is added to accelerate proton elimination. Tsuji s group has reported the elimination in steroidal and related compounds. Different configurations at C-3 (A ring) afford different diene systems (Scheme 50). The combination of Pd(0Ac)2 and tributylphosphine is the precatalytic combination preferred by this group, which uses NMR to control the quality of the catalytic mixture. ... 

Isomerization of conjugated dienes is possible by protonation of the corresponding tricarbonyliron complexes [Eqs. (147) (Birch and Williamson, 1973) and (148) (Birch et ai, 1975)]. These isomerizations presumably also involve transfer of allylic hydrogen to the transition metal (cf. Scheme 11). 

Loss of a proton is relatively rare but may take place in cases where there is a positive charge on the complex (e.g. IrH2(diene)L in equation 8) or the anionic conjugate base is stabilized (e.g. HCo(CO)4 in equation 9). 

However, 1,2-diamino-l,2-di-(cr(-butylethane (3) holds particular interest because of its increased steric bulk and the absence of benzylic protons. Its recent ready availability should render it as attractive as the frequently used vicinal diamines 7 and 8. To our knowledge, only one application of this diamine has been previously described in the literature (eq 5), where the regio- and enantioselective epoxidation of conjugated aliphatic dienes were studied using the chiral manganese salen complex (9). 

Moving to palladium complexes, related early studies on [PdX(Et4-dien)]+ also indicated the possible operation of a CB mechanism. Hydroxide ions reacted differently to other nucleophiles at these steri-cally hindered molecules in showing a distinct bimolecular (A2) dependence as well as a solvolytic pathway (85). This was unexpected because OH" is known to be a poor nucleophile in these systems, and it led to the proposal of the conjugate base contribution. The ions [PdX-(Et4Me-dien)]+, which have no acidic protons, did not show this behavior. The CBs reacted some 30 times faster than their precursors (a reactivity enhancement somewhat less than their gold(III) counterparts with the same ligands), again probably by an associative route. 

Full stereocontrol is achieved for the addition of protons at the exocyclic double bond of (Ti -5-alkylidenecyclohexa-l,3-diene)tricarbonyliron complexes to afford tri-carbonyl(ri -cyclohexadienylium)iron complexes. This demonstrates that the stereodirecting influence of the complex moiety in this conjugate system is even effective for the position P to the diene ligand with the proton entering anti to the tricarbonyliron fragment (Scheme 4-149). ... 

Formation of cationic T -dienyliron complexes by proton addition to (Ti -diene)iron complexes with an additional conjugated double bond can also be applied to acyclic systems. Monocyclization and polycyclization reactions proceed by nucleophilic addition of pendent double bonds to the complexes. " - " ... 

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