Alkyl-diene complex

Figure 3.82 (a) The effect of or/Ao-substituents on substitution reactions of cis-Pt(PR3)2ArCl complexes (b) the effect of alkyl substituents on substitution reactions of dien complexes. 

Electron-deficient 1,3-dienes are known to react when heated with metho-xy(aryl)- or methoxy(alkyl)carbene complexes to afford vinylcyclopropane derivatives with high regioselectivity and diastereoselectivity [8a, 24]. Cyclo-propanation of the double bond not bearing the acceptor functional group and... 

As shown in Scheme 94, the fj -styrene zirconium complex Cp Zr (f/ -PhCHCH2)[MeC(NPr )2] also served as starting material in the synthesis of alkyl-substituted 1,3-diene complexes of (pentamethylcyclopentadienyl)zirco-nium amidinates. NMR spectroscopy as well as single-crystal X-ray analyses of these complexes revealed that they are best described by the Zr(lV) a, n-metallacyclopent-3-ene limiting resonance form rather than as Zr(ll) f/ -diene complexes. ... 

All of the reactions described above use anionic alkyl metal complexes as stoichiometric reductants. Cationic zirconium catalyst 58 was shown to re-ductively cyclize a variety of 1,5-dienes to give both mono- and bicyclic silane products when H3SiPh was employed as the stoichiometric reductant (Scheme 10) [32]. Poor yields due to competing polymerization processes were observed when less substituted dienes were employed. It is likely that... 

As mentioned above nonconjugated dienes give stable complexes where the two double bonds can form a chelate complex. A common pathway in palladium-catalyzed oxidation of nonconjugated dienes is that, after a first nucleophilic addition to one of the double bonds, the second double bond inserts into the palladium-carbon bond. The new (cr-alkyl)palladium complex produced can then undergo a /(-elimination or an oxidative cleavage reaction (Scheme 2). An early example of this type of reaction, although not catalytic, was reported by Tsuji and Takahashi (equation 2)12. 

For a decade or so [CoH(CN)5] was another acclaimed catalyst for the selective hydrogenation of dienes to monoenes [2] and due to the exclusive solubility of this cobalt complex in water the studies were made either in biphasic systems or in homogeneous aqueous solutions using water soluble substrates, such as salts of sorbic add (2,4-hexadienoic acid). In the late nineteen-sixties olefin-metal and alkyl-metal complexes were observed in hydrogenation and hydration reactions of olefins and acetylenes with simple Rii(III)- and Ru(II)-chloride salts in aqueous hydrochloric acid [3,4]. No significance, however, was attributed to the water-solubility of these catalysts, and a new impetus had to come to trigger research specifically into water soluble organometallic catalysts. 

As mentioned earlier, steric effects can be important in determining the outcome of the hydride abstraction reaction. This is particularly vexing in cases where an alkyl substituent is present at the sp carbon of the cyclohexadiene complex. For example, complexes such as (47 equation 19) are untouched by trityl cation, provided traces of acid are not present (these are formed by hydrolysis of the trityl tetra-fluoroborate due to atmospheric moisture, and will cause rearrangement of the diene complex). This is due to the fact that only the hydride trans to the Fe(CO)3 group can be removed, and the methyl substituent prevents close approach to this hydrogen. 

In an elegant synthetic application of iron-diene complexes, Knox30 has reported acylation of a series of 1-alkyl-substituted diene complexes which after cleavage, reduction and esterification give a series of moth pheromones (Scheme 11). 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

Addition of H[BF4] to 4-amino-l-metalla-l,3-dienes ( )-96 in ether leads to precipitation of a stable dia kyliminio(alkyl)carbene complex 106, which has been characterized by X-ray analysis. Compound 106 is more reactive toward insertion at the M=C bond than its precursor ( )-96. Thus, a marked enhancement of reactivity is achieved by protonation of 4-amino-1-metalla-l,3-dienes. Addition of LiBr to compound 106 and thermolysis... 

The d group 3 and lanthanide complexes Cp MR are isoelectronic with the [Cp2MR]+ catalysts discussed in the previous section. These nonionic complexes are soluble in most hydrocarbons and as one-component systems make ideal models for many of the fundamental processes in polymerization catalysis. For example, an alkyl-alkene complex can be observed by NMR when Cp 2 YH is allowed to react with an o , )-diene (equation 13). ... 

Highly enantioselective alkylations a to acyclic diene complexes have been developed. Deprotonation of (91) with LDA to form an ester enolate, followed by reaction with iodomethane, gives the alkylated prodnct (92) in excellent yield with 82% ee (Scheme 153). Stereospecific remote alkylation was used in a synthesis toward macrolactin A (Scheme 154). In the synthetic seqnence, the primary... 

Both 1,4- and 1,5-dienes form stable complexes with Pd. For most 1,3-dienes, such as 1,3-butadiene, reaction with Pd° compounds leads to 7r-allyl formation. These reactions are described in Section 7. The coordinated double bonds in palladium diene complexes are reactive toward attack by many nucleophiles, and the resulting chelating alkene palladium alkyls are easily isolated. Many useful reactions of dienes were discovered by Jiro Tsuji in the 1960s and 1970s. These have been recently reviewed in a historical memoir. ... 

Many nncleophiles add to one of the double bonds of chelating palladium(diene) complexes to give a chelating Pd alkyl(alkene) derivative, as exemphfied by the reaction of PdCl2(l,5-cod) with methoxide (equation 41). In most cases, the direction of attack is exo. If the nucleophile is in a form that can undergo transmetalation with the Pd l bond, such as Ph2Hg, the nucleophihc group can be delivered endo. In this case, prior formation of a Pd nucleophile bond accounts for the direction of attack (equation 42). 

Although butadiene reacts with Co2(CO)8 to yield the diene complexes (diene)C02(CO)o and (diene)2Co2(CO)4 (268), with alkyl- or acylcobalt tetracarbonyls it produces only the Tr-allylic species, 1-alkyl- or 1-acylmethyl-TT-allylcobalt tricarbonyls (281). These will react, in turn, with P(C3Hb)3 which displaces one CO ligand to form monotriphenyl-phosphine derivatives (281). 

These studies have indicated that (i) there is usually only one olefin molecule coordinated at each silver ion (621), (ii) alkyl substitution at the double bond decreases the stability of the complex (288, 416, 621), (in) with endo-cycloolefins, the stability constant increases with increasing ring strain (578), (iv) more stable complexes are formed with cis-than with diene complexes are formed by the 1,5-diene systems (416), and (vi) deuteration of an olefin increases the complex stability (154). 

Within a series of a,t j-dienes, the 1,5-diene forms the most stable AgNOs complex (416), perhaps because it possesses the optimum configuration for chelation. As with monoolefins, the presence of alkyl substituents decreases the stability of the diene complexes (416). 

Lithium alkynylcuprates react with haloallenes to give similar skipped diacetylenes (see below). The related skipped enynes can be prepared by treatment of (pentadienyl)iron(tricarbonyl) halide complexes with dilithium trialkynylcuprates, the compounds being isolated as the iron(tricarbonyl)(diene) complexes (Scheme 4). Further examples of alkylation reactions of copper alkynides are illustrated in Scheme 5. Reaction between a lithium cyanoaikynecuprate and an iodoallene leads to a skipped diacetylene. This useful reaction has been used by Corey in his synthesis of hybridalactone (Scheme 6). °... 

Cp"MCl3 compounds are very versatile precursors for a variety of mono-Cp" complexes with ligands of diverse hapticity. For example, mono-Cp" diene complexes Cp"Zr( /4-butadienc)CI 402 were obtained by the reaction of Cp"MCl3 with internally substituted 1,4-butadiene in the presence of Na/Hg.292 Subsequent alkylation of 402 with appropriate alkylating reagents leads to the corresponding 73-ally1 403292 and -benzyl 404 (M = Zr) or r)1 -benzyl... 

---