Electron donating ligands

It is apparent from the results with the cyclization of 61 to 62 versus 63 (Eq. 4) that more electron-withdrawing ligands on the rhodium make the derived carbene more reactive. These results suggest that more electron-donating phosphines would be the optimum ligands for the preparation of analogs of (M)-60 and (M)-56. Unfortunately, all attempts to prepare such derivatives have failed at the ortho-metallation stage. 

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Phosphino-oxazoline)copper complex 28 was found by Helmchen et al. to be an excellent Diels-Alder catalyst [37] (Scheme 1.47, Table 1.20). The nitrogen atom acts as an electron-donating ligand, whereas phosphorus is a cr-donor-Tt-acceptor ligand. The copper complex of this phosphino-oxazoline ligand is therefore expected to have... 

Using more basic (electron-donating) ligands generally speeds up the reactions while bulky ligands slow it down. The use of tris(o-tolyl)phosphine is an extreme example of the latter as IrCl(CO)[P(o-tolyl)3]2 fails to add 02, H2 or S02 and only adds HCI slowly the reason is that methyls in an ortho-position tend to block the axial positions (Figure 2.77) [126b]. 

As part of ongoing research into the behavior of (vinylcarbene)iron complexes,119120 Mitsudo and Watanabe found that the trifluoromethyl-substituted vinylcarbene 174 exhibited a reactivity different from that of both 166 and 169.107 Upon treatment of the complex 174 with triphenylphos-phine the vinylketene complex 175 is formed, a reaction identical to that seen in the series of vinylcarbene complexes 166 (R = H). However, when the vinylcarbene 174 is exposed to a high pressure of carbon monoxide, it is converted cleanly to the ferracyclopentenone 176. Remember that when the vinylcarbene complex 166 (R = H) was treated in the same manner, conversion stopped at the vinylketene complex 167 Even when exposed to a pressure of 80 atmospheres of CO(g), no further reaction was seen to occur. An electron donating ligand (L = PR3) is required for conversion to cyclopentenone structure 168. Conversely, when the more electron-rich vinylcarbene 169 is exposed to carbon monoxide in the same manner, the pyrone complex 172 is formed. 

In cobalt-germanium complexes, substitution of one carbonyl by electron-donating ligands should increase the nucleophilicity of the transition metal moiety . ... 

A basic hydride ligand should be present. This is usually realized for electropositive early transition metals or late transition metals in low oxidation states supported by electron-donating ligands. The metals are preferably from the second and third transition series to ensure strong covalent bonding. 

Although a large proportion of stable carbenes behaves as strongly 2-electron donating ligands, several carbenes are known to show a different coordination mode, coming from their unique electronic and geometric properties. This is illustrated by the case of acyclic strucmres which allow the free rotation of the substituents in contrast to the cyclic ones. In this section we will present several particular coordination modes of non-NHCs stable carbenes. 

KOBu-f in hexane gives a precipitate of butylpotassium" " , the reactivity of a slurry of BuLi—KOBu-f does not match that of BuK. Simplistically, superbases can be considered to be organolithiums solvated by very electron-donating ligands (much more so than THF or TMEDA). As synthetic tools they provide a useful top end to the armoury of bases for regioselective functionalization by deprotonation. 

Coupling reactions of aldehydes or ketones to 1,2-diols proceed with low-valent metals such as magnesium, zinc, and aluminum.Because it is not easy to control the stereoselectivity (diastereoselectivity and/or enantioselectivity) of the reactions with such main group metals, low-valent species of early transition metals are frequently employed with electron-donating ligands. The representative reagents are low-valent titanium and samarium species. 

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