Diene insertion

Hydrogenation of 1,3-dienes to terminal olefins is catalyzed by HRh(PPh3)4 and [Rh(CO)2(PPh3)2]2 in the presence of excess phosphine diene insertion into a metal- hydride bond to give a-alkenyl rather than 7r-allyl intermediates was postulated for the initial step (141). Mechanistic studies of the HRh(PPh3)4 catalyst (142) and a more reactive phosphole analog (143) HRh(DBP)4 [5-phenyl-5//-dibenzophosphole (DBP), 7] for... 

An electron-rich metal can deprotonate the dicarbonyl derivative, affording the hydridopalladium intermediate 23, which can undergo a Tr-allyl 24 formation through diene insertion (which can be assimilated to a hydridopalladation of olefin) (Scheme 7). The attack of the enolate to the -jr-allyl species occurs with good enantioselectivity in the presence of the chiral ligand. The final product 21 is released and the palladium(O) complex 22 is regenerated. 

The diene polymerizations catalyzed by organometallic compounds are probably further examples of diene insertion reactions of metal alkyls (25). But there is little evidence on the detailed mechanisms of these polymerizations. 

Metal-Oxygen Compounds and Metal-Metal Compounds. Clear examples of diene insertions with metal-oxygen or metal-metal groups are not known. 

Figure E shows some conjugated diene insertion reactions. As expected, these compounds react similarly to the olefins—the same reagents add. Manganese...

The regioselectivity of the insertion of 1,3-dienes into ir-allylpalladium complexes possesses two components. First, with respect to the allyl unit, the insertion takes place at the more substituted termin-us.248 249,251,252 Second, with regard to the diene it has been shown that 2-substituted-1,3-dienes insert at the more substituted alkene.248 249 These interesting features have been explained by the intermediacy of an n -allyl species which is bound to the palladium at the less substituted terminus and with the Pd com-... 

Scheme 20 Reaction mechanism of diene insertion with a catalyst system based on NdCl3 2 TBP and TIBA [131], reprinted with permission from Elsevier...

Stereochemical control is exerted by forces operating after diene insertion. 

Metallo-ene reactions involving the transfer of palladium and nickel have been described since the early 1960s, mostly in cormection with their crucid role in the Pd- and Ni-catalyzed polymerization of butadiene." Hence, insertions of 1,3-dienes into allylpalladium compounds were extensively studied. Thus preformed allylic complexes (49) underwent the metallo-ene reactions (49) —> (51) at 20 C (20 h) or 35 °C (<5 min) or 70 °C 0 h) the reaction rate depended on the substituents R and as well as on the ligand L and decreased in the order R = Cl > H > Me R = H > Me L = Feacac > acac > Cl. Further diene insertion into the resulting allylpalladium product (51) (polymerization) was generally slower than the initial step (49) —> (51) and again relies on the nature of the ligand L (Scheme 11). ... 

Substitution of 7r-allyl groups by other hydrocarbon ligands has been studied in less detail. Formally such reactions may resemble the recently reported interaction of 7r-allylpalladium acetylacetonate with butadiene 151) in which the diene inserts into the Pd—C3H5 bond. 

Alkyllithium compounds LiR react stoichiometrically with butadiene and isoprene in hydrocarbons to form the corresponding alkyl-substituted butenyllithium compounds. If the diene is applied in excess, the polymerization can be catalyzed by further diene insertion into the allyl-lithium bond. Both steps have been proved directly but the mechanism of the selectivity remains an open question [41, 42]. 

The regioselectivity of diene insertion into Pd-R bonds has been explored. ... 

AmUnotetruhydrofurans The complexes derived from Cp Zr and 1 -aza-1,3-dienes insert carbonyl compounds which on demetallation with pyridine Al-oxide lead to 2-anilinotetrahydrofurans, and thence to the y-lactols and other compounds. 

The attachment of a slightly polar carbon-metal bond to a symmetric or quasisymmetric diene is in correspondence with respect to symmetry (pseudosymmetry) and, according to the correspondence principle, should proceed relatively rapidly. It is interesting to note that as far back as 30 years ago Stearns and Forman [64] and Szwarc [65] drew an analogy between the elementary act of diene insertion on the carbon-metal bond and the Diels-Alder reaction. Moreover, taking into account the fact that dienes can take part in the diene synthesis only in the cisoid conformation [66] it should be assumed by analogy... 

Homoallylic alcohols and 1,3-dienes. Insertion of carbenoids to the C—Zr bond of alkenylzirconocene chlorides by gem-chloroalkyllithium generates reactive reagents that can be exploited in a carbon chain extension. Thus, consecutive reactions of the zirconocene species with Me SiCHjCljLi and aldehydes lead to homoallylic alcohols. Similar insertion with 1-chloro-l-lithioalkenes gives rise to conjugated dienes. This method is adaptable to the synthesis of more extended conjugated systems. 

Carbon-metal bond, diene insertion 165 Cationation 1, 4, 7-10, 30, 37, 43-46 Cationogen 5 Chain conformation 280... 

Dicumyl chloride 61 p-Dicumyl methyl ether 10 Dielectric constant 123 Diene insertion, carbon-metal bond 165... 

The diene insertion may take place into the o-allyl-metal bond or into rj -allyl-metal bond. In the latter case, after the first insertion of the diene into M-R bond to give a substituted r/ -allylic complex, there are two possible sites for the further insertion of butadiene, one into the substituted site to give the 1,2-insertion... 

The diene insertion reaction will take place intramolecularly if the diene system is part of the acyl group in the acylcobalt tetracarbonyl (13). Thus, /ra j-2,4-pentadienoyl(triphenylphosphine)cobalt tricarbonyl, prepared by the acid chloride method, cyclizes on heating in ether solution in a closed vessel at 75° C for an hour, to 7r-cyclopentenonyl(triphenylphosphine)cobalt dicarbonyl in 30% yield. 

Dienes and allenes i.e. 1,2-dienes) insert into palladium-allyl bonds to give, for example, (1) from allene itself and the allyl compound (2). The... 

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