Complexity via Gold-Catalyzed Molecular Gymnastics

Abstract

Reactions of 1,6-enynes catalyzed by gold(I) complexes usually proceed stereospecifically through highly distorted cyclopropyl gold carbenes. Substrates with an alkoxy substituent at the propargylic position undergo stereoselective transformations through intermediates in which the OR group and the gold carbene are anti-oriented. Intramolecular attack of carbonyl groups to the cyclopropyl gold carbene is faster than the 1,5-migration of the OR groups, which itself is faster than the intramolecular cyclopropanation by a pendant alkenyl group. The intramolecular attack of carbonyl groups is the key transformation in the [2?2?2] gold-catalyzed cycloaddition, which has been applied in the total synthesis of (?)-orientalol F.Electrophilic metals catalyze the cyclization of 1,n-enynes to give a variety of cyclic structures (recent reviews in [1-4]) [5][6][7][8][9][10][11][12][13][14][15][16]. In the absence of nucleophiles, simple 1,6-enynes 1 undergo skeletal rearrangement to give products 2 (single exo-cleavage) and 3 (double exo-cleavage) ( Fig. 1) . In a third type skeletal rearrangement, dienes 4 (single endo-cleavage) were found using cationic Au(I) catalysts [5,38]. Products of type 4 were later found in reactions catalyzed by InCl 3 [27, 28], Fe(III) [6], or Ru(II) [39]. Cyclobutenes resulting from a [2?2] cycloaddition process have also been obtained from 1,6-[40], 1,7-[25, 29-31, 34, 41], and 1,8-enynes [42, 43]. Other type of cyclobutenes have been obtained in the palladium-[17, 18], platinum-[44], and gold-catalyzed cyclization of enynes [15,16,[45][46][47].Although dienes 2 obtained by the single exo-cleavage are identical to those formed by the metal-catalyzed metathesis of enynes [48,49], the mechanism of the skeletal rearrangement is very different [50,51]. For Au(I), the rearrangement was proposed to proceed via intermediates 5 (Fig. 2) [52]. On the other hand, the double-cleavage skeletal rearrangement usually leads to dienes 3 with predominant [1-4, 17-21, 38], or exclusive Z [53, 54] configuration. For Au(I), products of type 3 are obtained via rearranged carbene 6, followed by 1,2-H shift and protodemetalation [52]. Experimental support for the involvement of 6 has been obtained in reactions in which these intermediates have been trapped with alkenes [55], indoles, allyl silanes [56][57][58], and carbonyl compounds [59,60]. Proton loss from 6 can form 1,4-dienes in InCl 3 -catalyzed reactions of substrate in which R 0 is an alkyl group [27].The dual character of intermediates 5 as metal carbenes and carbocationic species has been recently discussed [1][2][3][4][61][62][63]. Conventionally, these complexes are often depicted as cyclopropyl gold carbenes 5, although DFT calculations show that these species have highly distorted structures, intermediate between cyclopropyl gold carbenes and gold-stabilized homoallylic carbocations [38,52]. A gold carbene has been formed in the gas phase, see [64,65]. Intermediates of type 5 are involved in other processes such as nucleophilic additions of heteronucleo...

Synthesis and Study of Cationic, Two‐Coordinate Triphenylphosphine– Gold–π Complexes

“…However, as was noted in the introduction, broadening in the 31 P NMR spectrum (À20 8C) suggested the presence of a mixture of complexes, presumably including the free or solvated cationic species A·BF 4 , which may be more prevalent than was A·SbF 6 in the formation of 3·SbF 6 owing both to the weaker ligating ability of 4-methylstyrene to gold(I) relative to 2-methyl-2-butene and the stronger coordination of BF 4 À relative to SbF 6 À . [29] Indeed, the 31 P NMR spectrum of a 1:1:1.15 mixture of [(PPh 3 )AuCl], AgBF 4 , and 4-methylstyrene in CD 2 Cl 2 at À20 8C displayed a single broad resonance at d = 33.9 ppm (n 1/2 = 112 Hz).…”

“…The reported spectroscopy for 1·BF 4 , [23] in particular the significant perturbation of the vinylic proton resonances of bound 4-methylstyrene relative to free 4-methylstyrene, leave little doubt that complex 1·BF 4 is generated in solution from mixtures of [(PPh 3 )AuCl], AgBF 4 , and 4-methystyrene. However, as was noted in the introduction, broadening in the 31 P NMR spectrum (À20 8C) suggested the presence of a mixture of complexes, presumably including the free or solvated cationic species A·BF 4 , which may be more prevalent than was A·SbF 6 in the formation of 3·SbF 6 owing both to the weaker ligating ability of 4-methylstyrene to gold(I) relative to 2-methyl-2-butene and the stronger coordination of BF 4 À relative to SbF 6 À .…”

“…Unable to resolve the 31 P NMR spectrum of the solution generated from reaction of [(PPh 3 )AuCl], AgBF 4 , and 4-methylstyrene into its constituent resonances, a similar experiment employing the less coordinating SbF 6 À counterion was performed. To this end, an equimolar mixture of [(PPh 3 )AuCl], AgSbF 6 , and 4-methylstyrene in CD 2 Cl 2 was mixed at À78 8C for 5 min.…”

“…[1][2][3][4] In particular, cationic gold(I) complexes have been applied extensively as catalysts for the electrophilic activation of CÀC p bonds. Included in this family of transformations are the gold(I)-catalyzed hydrofunctionalization of C À C multiple bonds with carbon and heteroatom nucleophiles and the gold(I)-catalyzed cycloaddition of enynes and related substrates.…”

“…Included in this family of transformations are the gold(I)-catalyzed hydrofunctionalization of C À C multiple bonds with carbon and heteroatom nucleophiles and the gold(I)-catalyzed cycloaddition of enynes and related substrates. [3,4] Although a number of sterically hindered, electron-rich supporting ligands, such as Nheterocyclic carbenes (NHCs) and dialkyl o-biphenylphosphines, have been employed to good effect in gold(I) catalysis, the simplest triarylphosphine, triphenylphosphine, remains the workhorse ligand in gold(I) catalysis. In addition, axially chiral bis(triarylphosphines) remain the most common ligands utilized in enantioselective gold p-activation catalysis.…”