Donor stability

TT-Conjugating groups tend to favor attack at C, but the ratio of Ca. C attack depends strongly on a balance of steric and electronic factors arising from both substituent and nucleophile (Table 4). The results can be rationalized, to a first approximation, by assuming that with good vr-donors stabilization of the incipient carbocation in (50) offsets steric hindrance. 

Electron-donor substituents are known to accelerate the rate of electrophilic substitution on benzene, while electron-withdrawing groups are known to retard the reaction. One explanation is that electron donors stabilize the positive charge in the benzenium ion intermediate while electron-withdrawing substituents destabilize the positive charge. 

Very recently, even examples of donor-stabilized bissilylene complexes have been introduced. For instance, the two cyclic systems 13 and 14 are prepared by an aminolysis reaction of the respective bis(chloro)silyl complexes. The x-ray structure analysis of 13 has been performed [39]. 

A phosphine-donor stabilized adduct 29 of the methylenephosphenium ion, 7, was synthesized [70] in a way similar to the iminophosphenium cation 6 from the corresponding chloroderivative 28 and PPh3 with addition of AICI3. Interestingly the base-free methylenephosphenium cation has recently been observed in the gas phase [71] (Scheme 16). 

Fig. 17. Plot of change in complex stability, Alog K, that occurs for the pairs of ligands THEEN and EN (O), and PDTA-amide and EDDA ( ), as a function of metal ion radius (20). The diagram shows how netural oxygen donors stabilize the complexes of large metal ions relative to small metal ions. Alog Ki for THEEN and EN, for example, is log Kx for the THEEN complex of the particular metal ions, minus log Kx for the EN complex. Data from Ref. (11).

An EPR study of the monomeric 02 adducts of the Schiff base complexes of Co(bzacen)(py) (71a) and the thiobenzoyl analog Co(Sbzacen)(py) (71b) characterized the five-coordinate mono (pyridine) precursors and the six-coordinate 02 adducts.327 Increased covalency in the Co—S bonds was seen in the EPR parameters, indicative of 7r-backbonding. Substituent effects on the aromatic rings had no effect on the EPR spectra, but these were reflected in the observed redox potentials. Furthermore, the S-donors stabilize the Co ion in lower oxidation states, which was consistent with destabilization of the 02 adducts. 

The cyclopentadienes la-e can be deprotonated by potassium in toluene to give the potassium cyclopentadienides 2a-e [2]. These compounds are very soluble in THF and also in hot benzene or toluene, which is indicative of the presence of essentially monomeric units in solution this is due to an intramolecular donor stabilization by the tentacle function [6]. 

Donor-stabilized compounds with simple, non-functional alkyl substituents R are known mainly for gold in its standard oxidation states +1 and +3, that is, (L)AuR and (L)AuR3, respectively. For the latter, R may be replaced by halide, pseudohalide, or other anionic or neutral ligands leading to compounds of the general formula (L)AuR X3 x. 

Figure 2.1 Preferred characteristics of glycosyl donors stability of C—X bond and mildness of promoter.

Donor-stabilized silylene complexes of manganese include 45 and 46, which have been prepared by the reaction of the corresponding R2SiH2 or RSiH3 compound with (77S-CsH4R)Mn(CO)3 (R = H, Me).71 128... 

The donor-stabilized lead(n) dialkyl 54 is prepared in 65% yield by the reaction of the lithium compound 53 with Pb[N(SiMe3)2]2- The orange-red crystals of 54 are stable in the solid state in solution, they slowly decompose to deposit metallic lead at room temperature (Equation (20)). 

The reaction of the lithium compound 55, accessible from the iminophosphorane CH2(SiMe3)PPh2=NSiMe3 and BuLi, with PbCl2 gives colorless crystals of the lead complex 56 in 50% yield (Equation (21)).77 Similarly, treatment of PbCl2 with the lithium compound 57 affords yellow crystals of the donor-stabilized lead(ll) diaryl 58 in 70% yield (Equation (22)).78... 

Substituents perturb the relative energies of the singlet and triplet state. In general, alkyl groups resemble hydrogen as a substituent, and dialkylcarbenes have triplet ground-states. Substituents that act as electron-pair donors stabilize the singlet state more than the triplet state by delocalization of an electron pair into the empty p orbital.55,56... 

The silicon atom in Wiberg s silenes is a Lewis acidic center and can be coordinated with donors. These donors may be halide ions [like, e.g., in 104 LiX(T2-C-4)831, ethers or nitrogen bases84. Coordination with the donor stabilizes the silenes and, when the basicity suffices (LbOcBr, THF, NMe3, F ), the adducts serve as stock compounds from which the silenes may be liberated (equation 32). 92 decomposes at —100 °C, but its trimethylamine adduct is stable at room temperature85. Noticeable amounts of 97 are available from 97-LiBr already at — 78 °C, whereas from 97 LiF the temperature has to be raised to +30 °C. Structures of such silene adducts will be discussed in Section I.C.l. The silene adduct 124-TIIF has been discussed in Section I.A.2. 

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