Pd^II‐ and Pt^II‐Mediated Polycyclization Reactions of 1,5‐ and 1,6‐Dienes: Evidence in Support of Carbocation Intermediates

Abstract

Biomimetic polyolefin cascade reactions [1,2] represent some of the most challenging problems in reaction design, and their products are ubiquitous in the natural world.[3] Since the Brønsted-Lewis acids (BLAs) of Yamamoto et al. are the only known synthetic catalysts for asymmetric catalytic initiation of cation-olefin cyclizations, [4,5] [7] and Toyota, Ihara et al. [8] demonstrated that nucleophilic enols lead to carbocyclic products by b-hydride elimination [9] or protonation. [10] Although substituent effects [11] and stereochemical studies [6c] support a cationic and cyclic intermediate, respectively, the exact nature of this intermediate is unclear, and direct evidence for this cation is lacking.[12] We therefore initiated a plan that first gathered evidence supporting the intermediacy of the carbocyclic cation, while also determining whether it could function as a point for initiating new metal-mediated reactivity. The key to our pursuit was a recent report by Vitagliano et al. of a dicationic Pt complex of a pyridyl bisphosphane pincer ligand [Pt(PNP)](BF 4 ) 2 (PNP = 2,6-bis(diphenylphosphanylmethyl)pyridine) that catalyzes the dimerization of ethylene and 2-methyl-2-butene. [13,14] Intermolecular nucleophilic addition of 2-methyl-2-butene to coordinated ethylene was proposed, with turnover by a sequence of 1,2-hydride shifts. Most importantly, b-hydride elimination did not occur, since no open cis coordination sites were available, and this suggests that this complex might be capable of initiating cation formation while preserving the stereochemistry of the MÀC bond.Our first approach to trapping the putative cation utilized the Pd analogue of the Vitagliano complex 1 and dienylphenol 3. Unlike direct Brønsted acid [4] and Hg II [15] activation/ polycyclization, Pd II prefers to coordinate and activate the least substituted alkene, [16] ensuring activation at the terminus. As shown in Scheme 1, the isolable C 6 F 5 CN adduct [17] efficiently (1 h, RT, CH 2 Cl 2 ) converted 3 to a new metalcontaining product devoid of alkene resonances ( 1 H NMR spectroscopy) as a 93:7 mixture of isomers according to 31 P NMR spectroscopy. This compound is stable up to 100 8C, though demetalation with NaBH 4 at room temperature rapidly [14b] yielded tricycle 4 in 90 % yield (two steps) and d.r. > 99:1. [18] Since a single diastereomer was observable in the crude demetalation product, we surmised that the isomer mixture for the Pd alkyl complex must result from epimers at the metal-containing stereocenter, a situation suggestive of competing chair-chair and boat-chair transition states, [19] each with a degree of concertedness in the CÀC/CÀO bondforming event, that is, a free carbocation is not likely formed [cf. Eq. (1)]. [1,20] Pincer complex [Pt(PPP)](BF 4 ) 2 (2, PPP = bis(2-diphenylphosphanylethyl)phenylphosphane), [21] derived from commercially available triphos, provided the intermediate alkyl complex with a slightly higher diastereoselectivity (96:4), and 4 in similarly high yield (87 %) and selectivity (> 99:1...

Neutral Tridentate PNP Ligands and Their Hybrid Analogues: Versatile Non‐Innocent Scaffolds for Homogeneous Catalysis

“…[29,30] One special feature of these 2,6-lutidinederived systems is their ease of deprotonation of the methylene spacer group, which effectively renders this ligand class "non-innocent". [31] The aim of this Minireview is to give a tantalizing overview of the interesting and useful chemistry available with this type of ligands, with a particular emphasis on the non-innocent (cooperative) ligand reactivity and (potential) applications in catalysis.…”

Catalytic Enantioselective Cyclization and C3-Fluorination of Polyenes

“…9,10,11 The fate of the organometallic intermediate of these cascades can be controlled through ligand choice, and when the supporting ligand is a diphosphine, this intermediate is susceptible to β-H elimination and leads to net dehydrogenated products. If this complex could instead be intercepted by a Pt-C fluorination reaction, a catalytic cyclization/fluorination protocol would result with concomitant access to C3-fluorinated compounds.…”

“…As a starting point for our catalytic cyclization/fluorination goal, we adapted conditions previously optimized for cyclization/β-H elimination reactions. 10b The modified conditions included use of AgBF 4 and NCC 6 F 5 to generate the “active” [( S )-(xylyl-phanephos)Pt(NCC 6 F 5 ) 2 ][(BF 4 ) 2 ] catalyst. Subsequent addition of a base (to facilitate cyclization), substrate and an electrophilic fluorine source generated the desired product ( 1 ) as a single (stereoretentive) isomer, along with variable quantities of β-H eliminated product ( 3 ) and the Brønsted product ( 5 ) (Table 1); several additional phosphines are included for comparison.…”

Electrophilic fluorination of cationic Pt-aryl complexes

“…12,13 Complex 5 was prepared by treating chloro(2-phenylpyridine)[2-(2-pyridyl)- phenyl-C,N]Pt 14 with triphos, while its dicationic isostere 6 was obtained by reacting [(triphos)Pt(NCC 6 F 5 )](BF 4 ) 2 15 with 2-phenylpyridine. 12 …”