As a 71 Donor

One of the first 71-donors applied in intermolecular photochemical substitution reactions is acrylonitrile 26 b, which can function as a 71-donor (C=C) and as a M-donor (—C=NI) 205> as demonstrated with complexes (2)-(4). 

From such a study, it appears that the sulfur atom acts as a 71-donor. The spectrum of benzo [b]thiophene in a nematic solvent is consistent with a planar molecule without significant distortion in the geometry of the benzene and thiophene rings.71... 

It is interesting to note that orthometallation occurs preferentially at the aryl ring in the thiolactone 69 despite the availability of oxygen as a potential donor ligand oxidation of the dinuclear complex 70 by ceric ion gives the condensed isobenzothiophene derivative 71 in unspecified yield152 (Scheme 85). 

Spectroscopic investigations of the lithium derivatives of cyclohexanone TV-phcnyli-mine indicate that it exists as a dimer in toluene and that as a better donor solvent, THF, is added, equilibrium with a monomeric structure is established. The monomer is favored at high THF concentrations.71 A crystal structure determination has been done on the... 

Conseqnently, the magnesinm chelate 71 can also react as a nucleophilic donor in aldol reactions. In the chemistry involving magnesium chelates, these two aspects model their mode of action as nucleophilic partners in aldol condensations. This is exemplified in aldol condensations of y-diketones . Thus, sodium hydroxyde catalyzed cyclization of diketone 73 to give a mixtnre of 3,5,5-trimethyl-cyclopent-2-enone 74 and 3,4,4-trimethyl-cyclopent-2-enone 75 in a 2.2/1 isomeric ratio (equation 100). When treated with magnesinm methanolate, the insertion of a a-methoxy carbonyl group as control element, as in 76, allows the formation of a chelated magnesium enolate 77, and the major prodnct is now mainly the aldol 78. This latter treated with aqueous NaOH provides the trimethylcyclopent-2-enones 74 and 75 in a 1/49 ratio. 

First examine the geometry of methyl radical. Is it planar or puckered Examine the geometries of 2 methyl-2-propyl radical, trifluoromethyl radical, triehloromethyl radical and tricyanomethyl radical. Classify each of the substituents (methyl, fluoro, chloro and cyano) as a 71-electron donor or as a 7t-electron acceptor (relative to hydrogen). Does replacement of the hydrogens by 7t-donor groups make the radical center more or less puckered Does replacement by Jt-acceptor groups make the radical center more or less puckered Justify your observations. 

Mukaiyama et al. examined 6-nitrothiazolyl mannoside 44 as a glycosyl donor [71,72] (O Scheme 23). They reported that in the presence of a catalytic amount of HB(C6F5)4,... 

Recently, Weiss and Kraut reported an unusual NHC in which the two substituents at the nitrogen atoms are oxygen [101]. The carbene center can be viewed as the terminus of two, mutually independent two-step 71-redox systems. This electronic configuration is expected to interact with high flexibility with a metal center coordinated to the carbenic center that is predicted to act as either 71-donor (Schrock-type carbene) or 71-acceptor (Fischer-type carbene). The multiplicity at the carbonic center depends on the nature of the metallic fragment. The carbene could not be isolated but the authors were able to trap it in the presence of palladium salts (Scheme 35). 

It has become apparent from the observation of vCO above 2200 cm l and from stretching force constants above 20 x 10 Nm"l for [Au(CO)2] " and [Pt(CO)4]2" ", diat these binary carbonyl cations differ drastically from classical transition-metal carbonyls with terminal CO groups, where vCO is foxmd in the general range of 2125 to 1850 cm (79, 80) and force constants usually between 15 and 1710 Nm l. It is obvious that Tc-backdonation, which is essential in transition-metal carbonyls (81), must be absent or substantially reduced and that carbon monoxide does not function as a 71-acceptor, but rather as a a-donor in noble-metal carbonyl cations and their cationic derivatives. It is not surprising that other distinguishing features, descriptive or experimental in nature, differ as well for both groups. 

The unsubstituted benzyne (8.26) from 2-diazoniobenzenecarboxylate can be trapped in the absence of a Bronsted acid as biphenylene, i.e., dimeric benzyne (8.30 Ford, 1971), or by cycloaddition to a 4-71 donor such as furan to give 8.31 (Stiles et al., 1963). 

In addition to the variation in electronic configuration, the geometric details of the coordination sphere and the properties of iron-ligand bonds (different a- or 71-donor strength) also influence the isomer shift as observed for a series of compounds ... 

At the same time Markham and Laidler (70) and also Veselovsky and Shub (71, 72) have shown that the photocatalytic activity of zinc oxide diminishes as a result of the calcination of specimens at high temperatures (around 1000°C) in the reduced atmosphere (such pretreatment results in an increase of the concentration of superstoichiometric zinc in the specimen). In other words, a donor impurity (zinc in excess of stoichiometry) retarded the reaction. 

Overall, the 2a - 4a equilibrium is seen to be predominantly determined by electronic factors, with steric interactions having a less pronounced influence. The predicted position of the 2a 4a equilibrium is in excellent agreement with experiment (cf. Section 2).14 For the strong a-donor PPr 3 as well as for the moderate a-donor PMe3 2a is favored relative to 4a by 3.3 and 0.5kcalmol-1 (AG), thus the [Nin(octadienediyl)L] complex is predicted to occur predominantly in the q3,q (C1) form 2a. On the other hand, the bis(r 3) form 4a is preferred by 0.4 and 1.1 kcalmol-1 (AG) in the case of the weak a-donor PPh3 as well as the 71-acceptor P(OPh)3, and is therefore predicted to have the highest thermodynamic population. 

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