Palladium complexes traps

Bidentate ferrocene ligands containing a chiral oxazoline substituent possess both planar chiral and center chiral elements and have attracted much interest as asymmetric catalysts.However, until recently, preparation of such compounds had been limited to resolution. In 1995, four groups simultaneously communicated their results on the asymmetric synthesis of these structures using an oxazoline-directed diastereoselective lithiation (Scheme 8.141). " When a chiral oxazolinylferrocene 439 was metalated with butyllithium and the resulting aryllithium species trapped with an electrophile, diastereomer 442 was favored over 443. The structure of the major diastereomer 442 was confirmed, either by conversion to a compound of known stereochemistry or by X-ray crystallography of the product itself or of the corresponding palladium complex. ... 

The palladium catalysed formation of indole derivatives has been extended by Grigg, who used carbon monoxide and unsaturated amines to trap the palladium complex formed in the insertion step. Reaction 3.10. provides an example of such a transformation. The amides were converted to the cyclic derivatives using ring closing metathesis.13... 

The formation of compound 175 could be rationalized in terms of an unprecedented domino allene amidation/intramolecular Heck-type reaction. Compound 176 must be the nonisolable intermediate. A likely mechanism for 176 should involve a (ji-allyl)palladium intermediate. The allene-palladium complex 177 is formed initially and suffers a nucleophilic attack by the bromide to produce a cr-allylpalladium intermediate, which rapidly equilibrates to the corresponding (ji-allyl)palladium intermediate 178. Then, an intramolecular amidation reaction on the (ji-allyl)palladium complex must account for intermediate 176 formation. Compound 176 evolves to tricycle 175 via a Heck-type-coupling reaction. The alkenylpalladium intermediate 179, generated in the 7-exo-dig cyclization of bro-moenyne 176, was trapped by the bromide anion to yield the fused tricycle 175 (Scheme 62). Thus, the same catalytic system is able to promote two different, but sequential catalytic cycles. 

Allylic carbonates produce the required alkoxide by decarboxylation of the carbonate anion that is displaced in the formation of the 7E-allyl palladium intermediate. Deprotonation creates the active nucleophile, which rapidly traps the 7t-allyl palladium complex to give the allylated product and regenerates the palladium(O) catalyst. 

The chemistry of secondary phosphine oxides, R2P(H)0 and their phosphi-nous acid tautomers, R2POH, has continued to attract attention. The study of the phosphinous acid tautomers has been aided by the development of stereoselective procedures for direct conversion of secondary phosphine oxides to the phosphinous acid-boranes (83). Treatment of the secondary phosphine oxide with either a base-borane complex or boron trifluoride and sodium borohyd-ride provides the phosphinous acid-borane with predominant inversion of configuration at phosphorus. The phosphinous acid tautomers are usually trapped as ligands in metal complexes and further examples of this behaviour have been noted. Discrimination of enantiomeric forms of chiral phosphinous acids, Ph(R)OH, coordinated to a chiral rhodium complex, has been studied by NMR. °° Palladium complexes of di(t-butyl)phosphinous acid have found application as homogeneous catalysts.A lithium salt of the tellurophos-phinite Ph2PTeH has been prepared and structurally characterised. ... 

The chiral R-ethylmethylpropylarsine can be obtained in 60% optical purity by reductive methylation of ethylpropylarsinic acid and trapped by the palladium complex 1. Arsenic coordination is completely regioselective giving 2, in which the arsine occupies a position trans to the dimethylamino group. 

The intermediate palladium complex may be trapped by an external anion, instead of undergoing HPdX elimination. For example, the reaction of 11.9 in the presence of Pd(OAc)2, (S)-3.43 and H-BU4NOAC in DMSO at 50°C gives 11.10 with a good ee [1659] (Figure 11.4). A similar trapping can be performed with benzylamine. 

Cycloaddition. A new trapping partner for the trimethylene-palladium complex is carbon dioxide. 3-Methyl-2-buten-4-olide is formed. A,A -Di-r-butyl-l,2-diaziridinone behaves as a 1,3-dipolar species in the presence of (Ph3P)4Pd and such is trapped by alkenes. ... 

The palladium complex isomer resulting from aryl migration to the norbornyl site was clearly the less reactive for ring closure to hexahydromethanotriphenylene and it could be trapped by norbornene itself, according to Scheme 5 [44]. The formation of the two isomers was interpreted as resulting from a common intermediate which forms by oxidative addition of the aryl halide to the palladacycle, according to Eq. 38. 

Interception of the Tr-allyl palladium complex by soft nucleophiles, particularly malonates, has been described above. Alkenes, alkynes and carbon monoxide can also insert into the Tr-allyl palladium complex, generating a u-alkyl palladium species. When an internal alkene is involved, a useful cycbzation reaction takes place (sometimes called a palladium-ene reaction).Addition of palladium(O) to the allylic acetate 225 gave the cyclic product 226 (1.225). The reaction proceeds via the -ir-allyl palladium complex (formed with inversion of configuration), followed by insertion of the alkene cis- to the palladium and p-hydride elimination. In some cases it is possible to trap the a-alkyl palladium species with, for example, carbon monoxide. 

The mechanistic propositions in Scheme 23 are further supported by a number of experimental findings, namely, the isolation and structural characterization by X-ray methods of both organic and organometallic spirocyclic derivatives in a number of reactions, such as IV and 93, the trapping of an analog of D to yield 92 by an intramolecular cyclization reaction in a triphenylphosphine-induced depalladation process," and the structural characterization of some rare q -bound aryl palladium complexes, such as 94 found in Chart 1." ... 

A wide number of chiral palladium complexes have been used in the context of DKR. In 1999, Cook et al. reported the palladium-mediated synthesis of chiral vicinal diamines from chiral oxazolidinones. The process involved successive oxidative insertion, loss of CO2 and subsequent cyclisation at the amide oxygen atom. The intermediate re-allyl palladium complexes underwent a rapid equilibration. Moreover, the intermediate oxazoline was also ionised by the palladium catalyst and was in equilibrium with the 7t-allylpalladium complexes, giving rise to thermodynamically controlled product ratios. These dynamic intermediates could be trapped with phthalimide under kinetic control to afford enantio- and diastereoselectively the corresponding syn-ch x 1,2-diamines (Scheme 2.46). 

The Diels-Alder reaction is a [4 -h 2] cycloaddition between a diene and a dienophile (a substituted alkene or alkyne). It leads to the formation of six-membered rings. Trost has developed a related procedure for making five-membered rings. A trimethylenemethane, briefly trapped as its palladium complex, adds to an alkene, which is activated by an electron-withdrawing group. 

---