Ylides donor properties

The k C bonding mode includes most of the reported work on ylides. However, modifications of the structure of the yhde could be advantageous, in particular the introduction of additional donor atoms to form chelate hgands. The combination of the pure a-donor properties of the ylide with those of the auxiliary donor atom could be used for tuning the steric and electronic properties of yhde complexes. There are reports of useful C,P- and C,C-chelates, which will be detailed here. 

Recent research on aminocarbenes has led to the development of a very fruitful field. The synthesis of relevant complexes (Scheme 19) such as aminobis(yhde) carbene species (69) [147], cyclic C-amino P-ylides (70) (easily transformed into carbenes) [148] and their corresponding complexes (71) [149], and special ylides (72), which also transform very easily into carbenes by loss of pyridinium group, has been reported. Emphasis has been made on the transformation between ylides and carbenes and on the donor properties of the ylides. From the results obtained the ylides have shown a stronger a-donor behavior compared with the carbenes. 

These derivatives have not only provided new synthetic pathways but have shown improved thermal stability (as in the case of arsenic ylides) and a modified pattern of chemical reactivity. The donor properties of ylides 55, 24), and most of their synthetic applications 103), have been covered in other reviews and articles 3, 26) and are not duplicated here. The general organometallic chemistry of arsenic, antimony, and bismuth is the subject of the invaluable monograph by Doak and Freedman 11). The broad scope of phosphorus ylide and pentaorganophosphorane chemistry was covered in the leading multivolume series on organophosphorus chemistry edited by Kosolapoff and Maier 3, 21). Finally, the recent... 

Kawashima and co-workers attempted to prepare the phosphorus ylide-sta-bilized AYC ligand 129 which could, however, not be isolated because of an intramolecular rearrangement giving 130. The intermediate formation of 129 was confirmed by its reaction with sulfur to give 131 and by the formation of the rhodium complexes 132 and 133 (Scheme 1.17). Infrared spectroscopy of the carbonyl complex 133 confirmed the superb donor properties of AYC 129. A palladium complex bearing the phosphorus ylide-stabilized NHC was also prepared. Fiirstner et al. synthesized the analogous... 

Diaminocarbene complexes were reported as early as 1968 [152], Preparation and applications of such complexes have been reviewed [153], Because of 7t-electron donation by both nitrogen atoms, diaminocarbenes are very weak tt-acceptors and have binding properties towards low-valent transition metals similar to those of phosphines or pyridines [18,153]. For this reason diaminocarbenes form complexes with a broad range of different metals, including those of the titanium group. Titanium does not usually form stable donor-substituted carbene complexes, but rather ylide-like, nucleophilic carbene complexes with non-heteroatom-substituted carbenes (Chapter 3). 

Fischer-type complexes such as 1 were first prepared in 1964 and their chemical properties studied [1], Schrock-type nucleophilic complexes such as 2 were prepared later [2], They are formed by coordination of strong donor ligands such as alkyl or cyclopentadienyl with no 7i-acccptcr ligand to metals of high oxidation states. The nucleophilic carbene complexes show Wittig s ylide-type reactivity and the structures may be considered as ylides (eq. 8.1)... 

Ferrocene is composed of a pair of 6-7r-electron carbon arrays and a 6-d-electron iron(II) atom. Ferrocene-fullerene donor-acceptor dyads carry all the requisites for electron-transfer phenomena. However, data for the formation of ferrocene-fullerene hybrids are not abundant. Some such dyads have already been synthesized following the methodology of 1,3-dipolar cycloaddition of the appropriate azome-thine ylides to C60, with either variable-spacing building blocks or a rigid-bridge all-cj-bonded framework, in order to tune the redox properties of the system [40,234, 248-251]. Another novel dyad that contained two covalently bound ferrocene units was recently synthesized via cyclopropanation of the fullerene core [252]. 

A different approach to the modulation of steric and electronic properties has been reported using NHC moieties as ancillary ligands. A very rich chemistry has been developed around this topic in the last few years [145-148]. Complexes (67) have been prepared by deprotonation of the corresponding phosphonium-imidazo-lonium salt. The analysis of several Rh derivatives (67) shows that the ylide behave as a very strong n-donor, even more than the NHC ligand, and that Pd"" complexes... 

Obviously, ylide complexes such as 1, in which the ylide ligand is coordinated via the ylidic anion center to a suitable transition metal, are of interest concerning their properties as polymerization catalysts in comparison to commonly used ligands such as phosphanes, which differ significantly in the energetic availability of the ligand lone pair electrons as well as in the ligand donor/acceptor ratio. This chapter reviews research results in this area, obtained in the Central Research Laboratories of Bayer in Leverkusen. 

A new ylide, tris(4-methylphenyl-)benzoylmethylenephosphorane, has been synthesized and its properties and reactivity studied.The metallation of the donor-functionalized ylide ligand (2-methoxyphenyl)methylene-triphenylphosphor-ane with conujlexes of yttrium and lutetium gave unprecedented phosphoranylidene complexes.The reaction of triphenyl-methylenephosphorane with Mo(NAr)-(CHCMe3)[OCMe(CF3)2l2> (Ar = 2,6- Pr2QH3), produced an anionic alkylidyne complex (Scheme 103). ... 

The linear and nonlinear optical properties of C60-triphenylamine (TPhA) hybrids are reported. The synthesized materials were prepared following the 1,3-dipolar cycloaddition of azomethine ylides onto the skeleton of Cgo forming the TPhA-based monoadduct, equatorial bis-adduct and dumbell Ceo- It was proved that in all considered cases, Ceo serves as an acceptor while triphenylamine unit acts as a donor. It was found that the total second-order hyperpolarizability of C60-TphA-C60 system is several times larger than that of TPhA-C60. The results of experimental measurements are supported by quantum-chemical calculations. 

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