Palladium chiral conformation

The palladium-catalyzed addition of silylstannanes to 1,6-diynes invokes intramolecular cyclization, giving rise to 1,2-dialkylidene cyclopentanes (Equation (110)).269 The resulting Z,Z-l-silyl-4-stannyl-l,3-diene moiety fixed in an i -m-conformation makes the molecule axially chiral. Rapid equilibrium between the two helical forms is observed by NMR spectroscopy.270,161... 

Distinction between enantiodiscrimination by complexation and by alkylation of equilibrating intermediates is less clear in a number of related cases. It is likely that more than one type of chiral discrimination may be involved. For example, when a conformational ly flexible four-membered ring substrate is used for the same reaction, the enantioselectivity was only 56% ee (Eq. 8E.15) [175]. In this case, it has been proposed that equilibration via a tertiary e-palladium species may be possible, switching the origin of enantio-discrimination to the alkylation step. A more contrasting example involves the formation of an asymmetric diene via selective P-elimination of similar diastereomeric Jt-allyl intermediates (Eq. 8E.16). Evidence suggests that the enantio-determining elimination process occurs after the equilibration of the 7t-allyl intermediates [176]. 

A new type of iodonium salts constitute the conformationally rigid, tetranu-clear macrocyclic ring systems dubbed molecular boxes. The relatively simpler tetraaryltetraiodonium salts were obtained from 4,4 -bis(diacetoxyiodo)bi-phenyl and 4,4 -bis(trimethylsilyl)biphenyl [119]. The iodonium salt derived from 4-(4 -lithiophenyl)pyridine was made using the method of /J-(dichloroio-do)chloroethylene and it was used for the construction of hybrid iodonium-platinum (or palladium) cationic tetranuclear macrocyclic squares including some in which the ligand of the metal was a chiral biphosphine [120,121]. 

The final neutral palladium(II) complex with the amide functionalised carbene hgand features two six membered metallacycles, just as the cationic not yet deprotonated one does. The amine nitrogen atom has four substituents (H, Pd and two C). Since the two metallacycles can adopt two distinct conformations that slowly flip into each other for steric reasons, the nitrogen atom appears chiral due to atropisomerism despite its overall C -symmetry. The deprotonated complex lacks this feature, clearly visible in the respective NMR spectra. 

Only the geometry of the rt-allyl ligand in the intermediate metal complex determines the enantioselectivity. Racemization cannot occur via anti attack of a free palladium(O) species onto the xc-allyl complex. A stereoscrambling T-a-n rearrangement is rather unlikely, since it would involve the unfavorable formation of a cr-bond from palladium to a tertiary carbon atom. Ionization of the allylic ester is under stereoelectronic control. In particular, the C —X bond has to be orientated orthogonal to the plane of the double bond. Two enantiomeric conformers of the allylic substrate which are in rapid equilibrium meet this requirement. The chiral palladiuni(O) complex discriminates between these enantiomeric conformers, whose conversion to the corresponding jt-allyl complexes occurs at markedly different rates. 

Several polyarenes obtained by palladium-catalyzed cocycloaddition of arynes and DMAD are conformationally stable chiral helicenes. For example, cocycloaddition of aryne 141 and DMAD affords a mixture of polyarenes from which helicene 142 can be isolated in yields up to 30%. Furthermore, this reaction proceeds with good enantioselectivity when performed in the presence of chiral bidentate ligands such as BINAP, which leads to an ee of 67% when the solvent is THF (Table 5, entry 2) [73]. 

The authors propose that, for the case of rapidly interconverting racemic o-iodoanilides, oxidative addition of a chiral palladium catalyst to the Ar—I bond constimtes a dynamic kinetic resolution (Scheme 12.11). Theoretically, the oxidative addition using a chiral palladium catalyst would preferentially occur with one enantiomeric conformer of the racemic o-iodoanilide 39 to generate the enantioenriched intermediate 47. The subsequent insertion step (47 48 49) is expected to be more rapid than N—Ar bond rotation, and the axial chirality of the oxidative addition is relayed to the final product, oxindole 40. 

The plausible mechanism of asymmetric induction is proposed (Scheme 2). The palladium catalyst reacts from the sterically less crowded downward direchon of the lone pair side of the chiral sulfinyl group in the conformationally most stable form of (5s)-l with syn-coplanality between the sulfinyl-oxygen bond and the carbon-carbon double bond of the chiral vinyl sulfoxide, forming 3. Based on the stereochemistry of the product, the... 

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