7t donor

Eor the selective pre-concentration of deactivated phenols a new silica-based material with the grafted 2,3,5-triphenyltetrazole was proposed. This method is based on the formation of molecular chai ge-transfer comlexes of 2,3,5-triphenyltetrazole (7t-acceptor) with picric acid (7t-donor) in the phase of the sorbent. Proposed SPE is suitable for HPEC analysis of nitrophenols after their desorption by acetonitrile. Test-system for visual monitoring of polynitrophenols under their maximum concentration limits was developed using the proposed adsorbent. 

The high reactivity of the acyl chlorides also reflects the polar electron-withdrawing effect of the chlorine, which more than outweighs the small 7t-donor effect. 

First examine the geometry of methyl radical. Is it planar or puckered Examine the geometries of 2-methy 1-2-propyl radical, trifluoromethyl radical, trichloromethyl radical and tricyanomethyl radical. Classify each of the substituents (methyl, fluoro, chloro and cyano) as a n-electron donor or as a Tt-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. 

The structure of the complex of (S)-tryptophan-derived oxazaborolidine 4 and methacrolein has been investigated in detail by use of H, B and NMR [6b. The proximity of the coordinated aldehyde and indole subunit in the complex is suggested by the appearance of a bright orange color at 210 K, caused by formation of a charge-transfer complex between the 7t-donor indole ring and the acceptor aldehyde. The intermediate is thought to be as shown in Fig. 1.2, in which the s-cis conformer is the reactive one. 

The photoindueed 1,7-cycloaddition of carbon monoxide across the divinyl-cyclopropane derivative 32 yields the two cyclic dienyl ketones 34, via the ferracyclononadiene intermediate 33 [18]. (Scheme 11) cyclopentene rearrangement. The dienylcyclopropane 35 is capable of forming the complex 36, followed by ring enlargement to 37 [19]. 1,1-Dicyclopropylethylene 29 is also converted to the 1-cyclopropyl-1-cyclopentene 38. The additional functionality of vinylcyclopropanes is necessary to serve as a 7t-donor... 

T-donors such as tryptophan, tyrosin, etc. a-addition reactions of suitably fixed neighboring groups R—XH may occur (cf. Scheme 3) ... 

Using a similar strategy, Dias and coworkers were able to prepare classical AgI-CO and AgJ-C2H4 complexes supported by the mesityl-substituted tris(pyrazolyl)borate ligand L9 (Fig. 11) (102). The isolation of such complexes is a challenging task because silver(I) is a poor u-acceptor, a poor 7t-donor, and in... 

Figure 5.45 The optimized structure and leading intermolecular 7t->-7t donor-acceptor interaction of the C2H4- -NO+ complex.

Further, if we follow these ideas through, we may find at least one reason why the monomers containing hetero-atoms appear to polymerise so much more rapidly than the hydrocarbons under the most common conditions. The reason may well be that because for these n-donor monomers the KM is much greater than for the 7t-donors, the fraction of the Pn+M chain-carriers is much greater than that of the much more slowly growing Pn+A. In other words, it is a question of the composition of the ionic population rather than of rate-constants, although these may, of course, contribute to the effect. This idea is open to test by measurements of KM and kp+, kp+M, and kp. ... 

The Kus can be estimated as follows an extrapolation of the Kus for the CT complex formed by any one donog such as mesitylene or hexamethylbenzene, with 1,3,5-trinitrobenzene and 1,4-dinitrobenzene to PhN02, and an extrapolation from solvent CC14 to one of DC > ca. 10 (Foster, 1969) shows that for our system Kus is very unlikely to be greater than 0.01 hmol"1. Therefore, with m = 1 mold"1, and [Sv] = 10 mold"1, [MSv] < 0.1 mold 1. This means that for styrene and other 7t-donors effectively all the monomer is free. For n-donor monomers such as the VE, however, the fraction of uncomplexed monomer may be somewhat smaller. Therefore it appears that the formation of CT complexes probably did not affect significantly at least the results for the three hydrocarbon monomers. 

In these equations, the substituents are changed from weak (F) to strong (OMe) 7C electron donor. Likewise the ability of the metal fragment to back donate into the empty p orbital of the carbene is varied through ligand (n acid CO vs. 7t donor Cl with associated charge adjustment) or through metal (the 5d orbital of Os are more efficient in back donation than the 4d of Ru). 

On the other hand, pyridine stabilizes the dioxygen molecule less well than the 1-alkyl imidazoles, because it is itself a weak 7r-acceptor and not a 7t-donor. The displacement reaction (7) occuring in excess pyridine may illustrate this argument (102). 

The small spectral discrimination between CO, 02, and NO in the Fe(P) system as compared with the Ru(P) or Os(P) systems may also be due to the less pronounced 7T-donor ability of the Fe11 ion relative to the Ru11 or Os11 ion in a porphyrin system. The spectral differences between the nitrosyls and the dioxygen complexes are especially small. The a- or (3-bands indicated for these complexes (Series a, c, d, Table 13) fall in the range where the a- or 0-bands of the corresponding Zn11 porphyrins are observed (Xa for Zn(Proto-IX) 586 nm (43) for Zn (TPP) 548 nm, see Table 3). As... 

As a general guideline it can be stated that a-deprotonation of transition metal alkyl complexes will be easy when the metal is in a high oxidation state, the complex is positively charged, and the additional ligands are poor 7t-donors. 

Abstract In this chapter, aspects related to the synthesis, structure, and reactivity of metallacumulenes [M]=C(=C) =CR R ( = 1, 3, 5) in which one or both substituents R /R are heteroatomic 7t-donor groups (NR2, OR, SR, SeR) are discussed. The coordination chemistry of related heteroatom-cumulated ligands, i.e., phosphonioacetylides and tricarbon monoxide, is also presented. 

Effects on benzene geometry stemming from substituent electronegativity are dominated by changes in internal angles. Nonetheless, the idea that the electronegative character of an annelation can induce bond fixation has been forwarded. In order to resolve the situation, a direct test of 7t-donor character (i.e. 

Scheme 28 Comparison of Bi(OTf)3-catalyzed alkylations with o-donors and 7t-donors...

It has been demonstrated that Bi(OTf)3 is a versatile catalyst for the activation of both ct- and 7t-donors. Thus, in order to evaluate their potential in terms of a ji-donor/71-acceptor interaction, Bi-catalyzed hydroalkynylations were investigated. As an example, Takaki and coworkers developed a Bi(OTf)3-catalyzed intramolecular carbo-oxycarbonylation of alkynyl esters (Scheme 35) [89]. 

The discovery of metallic conductivity in organic solids stands as a major scientific discovery of the twentieth century <1995S215>. From a total of several organic 7t-donors described in the literature, five (438a-c, 439, and 440) are four-heteroatom bicyclic 5 6 systems with two heteroatoms in each moiety. 

The CH3, CF3, acetyl, fluoro and chloro substituents are not sufficiently strong 7t donors or acceptors to significantly influence the size of kisc (Table 2). 

For example, in AOM terms, the octahedral splitting A t is given by (5) and illustrated in Fig. 5 for the particular case of a 7T-donor ligand for which en is positive. 

Because the C(1)—P(1) bond in 64 is shortened, the (Me2N)PC(SiMe3) group is a strong 7t-donor in this compound and the mesomeric structures 64b and 64c give a good description of the heteroallyl anion113. 

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