Adducts with Other Metal Fragments

Although the chemistry of bi- and poly-nuclear vinylidene complexes is beyond the scope of this chapter, several examples of the interaction of metal-vinylidene fragments with a second metal-ligand fragment have been described. This type of reaction usually produces complexes containing bridging vinylidene ligands (Equation 1.21)  

A range of homo- and hetero-nuclear complexes has been obtained from Rh (=C=CHR)(L)Cp addition of RhCl(PPr 3)2 2 gives Cp(L)Rh(p-C=CHR)RhCl 

Complexation of ]Fe =C=CH(nap) (dppe)Cp ] with the areneophile ]RuCp ]+ gives derivatives in which the RuCp fragment is attached to the naphthyl group, the isomer in which it is attached to the unsubstituted C(, ring being the major product (92%) ]337]. 

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Thermodynamic control. Note that it is also possible for the aldolate adduct to revert to aldehyde and enolate, and equilibration to the thermodynamic product may afford a different diastereomer (the anti aldolate is often the more stable). The tendency for aldolates to undergo the retro aldol addition increases with the acidity of the enolate amides < esters < ketones (the more stable enolates are more likely to fragment), and with the steric bulk of the substituents (bulky substituents tend to destabilize the aldolate and promote fragmentation). On the other hand, a highly chelating metal stabilizes the aldolate and retards fragmentation. The slowest equilibration is with boron aldolates, and increases in the series lithium < sodium < potassium, and (with alkali metal enolates) also increases in the presence of crown ethers. ... 

A typical example of this type of doping is a fragmentation stutfy of the flavo-noid glycoside ratin, adducted with different alkah metal ions (LL, Na% or K+), by post-soirrce decay MALDI time-of-flight (TOF) MS [14]. Differences in fragment ions and especially the relative abimdance of fragment ions were observed (see Sect. 7.5.2). Nitmerous other examples of this type of studies are available. 

Mobile-phase additives can also influence the relative abundance of the various adduct ions. Karlsson [105] performed post-column addition of alkali cations to enhance ESI-MS of carbohydrates and other compormds without nitrogen atoms. For most analytes, the adduct formation increased with the size of the cation. Optimum concentration of the cation in the solution was ca. 5x10 mol/1. Alkali-metal affituties and alkali-metal influence on fragmentation in MS-MS have been studied by others as well [106-107]. 

Dotz and coworkers prepared several interesting and novel glycosylidene carbenes (259-261) by reaction of lithiated glycal 258 with the appropriate metal carbonyl derivative. The synthetic utility of the formed carbenes was demonstrated by reaction with 3-hexyne to give a mixture of complexed and uncomplexed adducts 262 and 263, respectively. The anomeric chromium carhene 264 was converted to 265 hy exposure to ethoxy ethyne (Scheme 48) [70]. The same workers have carried out a similar chromium-mediated henzannulation (266 —> 268), this time with the chromium on the aromatic fragment [71]. Other sugar-hased carhenes have also been prepared [72]. 

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