Ylide coordination modes

The reaction of the stabilized yUde 46 (a-vinyl substituted) with the cycloocta-dienyl Pd(II) allows the synthesis of a novel complex, the (rj -allyl)palladium 47, in which the olefmic double bond participates in the coordination (Scheme 20) [83]. The coordination of the bis-yUde 48 with the same starting Cl2Pd(COD) leads to the formation of another new palladium complex 49 via COD exchange reactions. A C-coordination mode takes place between the carbanionic centers of the bis-ylide and the soft palladium and two stereogenic centers of the same configuration are thus created [83]. In contrast to the example described in Scheme 19, the Cl2M(COD) (M=Pd or Pt), in presence of a slightly different car-... 

Although, in principle, the chemistry here reported should be centered on the late transition metals, sometimes we will jump the frontier between late and middle or early transition metal since this line could be more or less diffuse and could change as a function of the history. At least seven different coordination modes have been identified (I-VII, Scheme 3) as the main bonding modes. In modes I and II the ylide behaves as neutral and monodentate, bonded exclusively through the Ca atom (kC mode) this is the case for simple ylides and carbodipho-sphoranes. Mode 111 covers the variants of a metallated ylide, that is, a situation in which the metal replaces a substituent of the ylide and transforms it into an anionic ligand. 

A rather rare coordination mode of pyridines is the if(C=C) mode, which is exclusively stabilized by the electron-rich [Os(NH3)s] fragment, for example, 2,6-lutidine. The olefin-like complex easily rearranges to the t7 (N) mode by a one-electron oxidation. Interestingly, the t7 (G=C) mode of lutidine rearranges with time to give an Os(ll) lutidinium ylide. In contrast, the if C,C) mode is maintained when [Os(NH3)s(77 -lutidine)] is protonated to give a lutidinium derivative. ... 

A careful inspection of several examples [51-54] leads to the conclusion that the O-coordination is produced trans to a soft (C or P) atom, while C-coordi-nation mainly occurs trans to a harder (N) atom. Complex (12) is the paradigm of this selectivity on bonding modes and sites [53] the C-bonded yhde is found trans to the N atom while the N-bonded ylide is trans to the paUadated C atom. In spite of this puzzling appearance (Scheme 5), the consideration of the nature of the donor atoms and the antisymbiotic behavior of the Pd center [55] provides a sensible explanation of the observed reactivity [21]. 

COjEt R"" = COPh, = COPh R = Bu", R = H, R = COPh Z = As, R = Ph, R2 = H, R3 = COPh or COjMe MejSCHCOPh CsHjNCHCOPh), in contrast to the dinuclear complexes [Hg2(Ylide)2Cl4] formed with HgCl2. It was found that v(CO) for the complexes exhibits a blue shift relative to the free ylide, the values approaching those of completely protonated onium salts, indicative of coordination via the methine C atom. Similar thiocyanato-complexes showed both N and S bonding modes with N co-ordination favoured by complexes of the least basic ylides. 

For alkynyl transfer, no direct evidence has been presented to determine whether organometallic nucleophiles interact with the electrophilic iodine centre or the jS-carbon although, in the case of transfer to the nitrogen nucleophile of a coordinated cyano ligand, the S-interaction mechanism is clearly implicated in the formation of 68 (Scheme 38, related to the mechanism in Scheme 14). This example indicates, for the synthesis of alkynylmetal species, the attractiveness of RC=C(Ph)I reagents where the R group cannot participate in such reactivity, e.g. R = SiMcs, Ph. Alternatively, this type of reaction, where the metal centre may act as a nucleophile at the -carbon, is worthy of exploration as a mode of organometallic synthesis as also is electrophilic attack at the K-carbon of ylides (illustrated by the mechanism in Scheme 14). 

---