Silver, ylide complexes

At the same time, the synthesis of mononuclear silver ylide complexes were achieved, Schmidbaur and his colleagues discovered that the ylide anions produced dinuclear species, Ag2[(CH2)2PMe2]. The remarkably stable methyl compound melts at 153-155°C and can be sublimed at 150°C and 0.1 atm. Mass spectral and H and P H NMR studies demonstrated the ring structure of the compound, although no structural analysis could be performed due to the photosensitivity of the compound. The synthesis of the dinuclear silver(I) ylide complex (6) is described in Inorganic Syntheses, and although colorless crystals are obtained, no X-ray stracture was possible due to sample decomposition in the X-ray beam. 

The first structural report of a silver-ylide complex appeared in 1988. This compound, C6F5-Ag-CH2PPh3, shows that the C-Ag-C geometry is linear with a Ag-CH2 distance of about 2.2 A. Additional structural reports have appeared. ... 

The phosphorus ylide complex [(Ph3PCH2)2Au]2Ag2(C104)4 445 also contains Au-Ag bonds [(2.783(2)/2.760(2) A] unsupported by any covalent bridge within a Au2Ag2 ring, but each silver atom is further bonded to two oxygen atoms from two... 

Computational studies yielded the models shown in Figure 2.2. These models illustrate how the existence or absence of hydrogen bonding between the silver/ligand complex and the dipolarphile may lead to selection of opposite faces in the reaction with the silver-bound azomethine ylide derived from 111. 

In addition to the phosphorus ylide complexes, arsenic and snlfur yUde complexes of silver have been obtained, althongh no structural reports exist. While the arsenic derivatives match the phosphorus ylide complexes for stability of the Ag products, the sulfoxonium ylide complexes are somewhat less stable. 

Four types of ylide complexes of silver have been reported so far (a) mononuclear neutral [Ag(R)(ylide)], (b) mononuclear cationic [Ag(ylide)2]X, (c) dinuclear neutral [Ag2 (CH2)2ERR, 2] (E = P, As) and (d) dinuclear dicationic [Ag2 CH(PPh3) 2CO 2]. 

The dinuclear dicationic complex 7 with the di-yUde [(Ph3PCH)2CO] as a bridging ligand between two silver atoms has been obtained from a transyUdation reaction between the diphosphonium salt [(Ph3PCH2)2C0]C104 and the bis(ylide) complex [Ag (Ph3P)CHC(0)CH3 2]C104 described above (equation 7)19. 

Uson, R., Laguna, A., Laguna, M., Uson, A. and Gimeno, M.C. (1988) Reactions of pentafluorophenyl(ylide)silver(I) or -gold (I) complexes with chlorobis... 

Scheme 6.7 shows some other examples of enantioselective catalysts. Entry 1 illustrates the use of a Co(III) complex, with the chirality derived from the diamine ligand. Entry 2 is a silver-catalyzed cycloaddition involving generation of an azome-thine ylide. The ferrocenylphosphine groups provide a chiral environment by coordination of the catalytic Ag+ ion. Entries 3 and 4 show typical Lewis acid catalysts in reactions in which nitrones are the electrophilic component. 

The mechanism proposed involves desilyation of the silver complexed imidate and cycloaddition by the azomethine ylide 35 to give 37 followed by elimination (22). 

Another approach employing chiral acyclic azomethine ylides was published in two recent papers by Alcaide et al. (85,86). The azomethine ylide-silver complex (51) was formed in situ by reaction of the formyl-substituted chiral azetidinone (50) with glycine (or alanine) in the presence of AgOTf and a base (Scheme 12.18). Azomethine ylides formed in this manner were subjected to reaction with various electron-deficient alkenes. One example of this is the reaction with nitrostyrene, as illustrated in Scheme 12.18 (86). The reaction is proposed to proceed via a two step tandem Michael-Henry process in which the products 52a and 52b are isolated in a... 

During the reaction silver(0) deposits on the sides of the reaction vessel. The sequence of events leading to the azomethine ylide is unclear. However, evidently single-electron transfer (SET) from the amine to silver takes place either prior or subsequent to fluoride-enabled silyl cleavage. This process is repeated with a second equivalent of silver fluoride resulting in the formation of 38 either in free form or more likely as its silver complex. The scope of the method was expanded to the synthesis of bicylic systems exemplified here by tropinone 42 (Scheme 2.11).19 Pandey has also extended the protocol to the synthesis of tricycloalkanes20 and applied it to a total synthesis of the poisonous frog alkaloid epibatidine.21... 

Zhou also reported a series of related P,S-ferrocenyl ligands and their use in the [3+2] cycloaddition of aryl-substituted azomethine ylides with A-phenylmalei-mide.52 While these silver complexes were able to efficiently catalyze the reaction, the enantioselectivity was lower than in the protocol described above. 

While virtually all of the research described above has focused on the inter-molecular cycloaddition of azomethine ylides, the intramolecular process holds considerable promise for the synthesis of polycyclic natural products. The Pfaltz group reported an intramolecular catalytic asymmetric cyclization of aryl iminoesters 112 using a complex of silver acetate with PHOX type ligand 100 (Scheme 2.29,... 

In addition to participating in [2 + l]-cycloaddition reactions, divalent reactive intermediates can form ylides in the presence of carbonyl or other Lewis basic functionalities.108 These ylides participate in cycloaddition or other pericyclic reactions to furnish products with dramatically increased complexity. While carbenes (or metal carbenoids) are well known to participate in these pericyclic reactions, silylenes, in contrast, were reported to react with aldehydes or ketones to form cyclic siloxanes109,110 or enoxysilanes.111,112 Reaction of silylene with an a,p-unsaturated ester was known to produce an oxasilacyclopentene.109,113,114 By forming a silver silylenoid reactive intermediate, Woerpel and coworkers enabled involvement of divalent silylenes in pericyclic reactions involving silacarbonyl ylides115 to afford synthetically useful products.82,116,117... 

Similar to the abovementioned silver nhc coordination compounds, carbene chemistry has also been dominant in the field of gold organometallic chemistry. Noteworthy examples include a Au(PPh3)-compound derived from tetraaminoallene, that can be rationalised in terms of a dicarbene with ylide character and which, owing to the electron-rich character of the central carbon atom, offers the potential for dimetallation products.108 Non-activated allenes and alkynes have been found by Lavallo to be readily aminated by cationic carbene gold complexes.109 For this purpose, a 2,6-diisopropylphenyl functionalized cyclic alkylaminocarbene gold(I) complex... 

The stabilized versions of (99)—(101) have easily been obtained from the silver derivative (103), shown in Scheme 31 [212], Treatment of (103) with ClAu(tht) gives (104), which can be oxidized to give the dinuclear complexes (105) and (106) [213], Interestingly, treatment of (103) with Pd precursors gives complexes in which the bis-ylide is always behaving as a chelate [214]. 

Bimetallic gold-silver complexes may be prepared from gold bis(ylide) species such as [Au(CH2PPh3)2]ClC>4 and AgC104 (leading to neutral clusters, equation 54) or... 

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