Carbonyl complexes electron donor-acceptor

IV. CARBONYL GROUP. ELECTRON DONOR-ACCEPTOR COMPLEXES... 

The scope of the Patemo-Buchi cycloaddition has been widely expanded for the oxetane synthesis from enone and quinone acceptors with a variety of olefins, stilbenes, acetylenes, etc. For example, an intense dark-red solution is obtained from an equimolar solution of tetrachlorobenzoquinone (CA) and stilbene owing to the spontaneous formation of 1 1 electron donor/acceptor complexes.55 A selective photoirradiation of either the charge-transfer absorption band of the [D, A] complex or the specific irradiation of the carbonyl acceptor (i.e., CA) leads to the formation of the same oxetane regioisomers in identical molar ratios56 (equation 27). 

The Patterno-Buchi coupling of various stilbenes (S) with chloroanil (Q) to yield fran -oxetanes is achieved by the specific charge-transfer photo-activation of the electron donor-acceptor complexes (SQ). Time-resolved spectroscopy revealed the (singlet) ion-radical pair[S+% Q" ] to be the primary reaction intermediate and established the electron-transfer pathway for this Patterno-Buchi transformation. Carbonyl quinone activation leads to the same oxetane products with identical isomer ratios. Thus, an analogous mechanism is applied which includes an initial transfer quenching of the photo-activated (triplet) quinone acceptor by the stilbene donors resulting in triplet ion-radical pairs. ... 

They acknowledge that the exact mechanism of the adsorption of heavy metal chelates is quite complex but do not hesitate to propo.se the formation of an electron donor-acceptor complex of the chelate and the active sites (e.g., carbonyl groups) and possible beneficial effect of hydrogen bonding between the... 

Contrary to the effects of surface carboxyl and hydroxyl groups, the surface quinone (or carbonyl) groups acmally increased the adsorption of aromatics. Epstein et al. (1971) observed these effects with the adsorption of p-nitrophenol. Their explanation was that the carbonyl groups aid the adsorption of aromatics by involving the formation of an electron donor-acceptor complex of the aromatic ring with the surface carbonyl groups, as proposed earlier by Mattson et al. (1969). 

Latham, W. and Morokuma, K. Molecular Orbital Studies of Electron Donor-Acceptor Complexes I. Carbonyl Cyanide- ROR and Tetracyanoethylene Complexes , J. Am. Chem. in press. 

Kochi and co-workers reported the photochemical addition of various stilbenes to chloroanil 53, which is controlled by the charge-transfer (CT) activation of the precursor electron-donor/acceptor (EDA) complex. The [2-1-2]-cycloaddition products 54 were established by an x-ray structure of the trans-oxetane formed selectively in high yields. Time-resolved (fs/ps) spectroscopy revealed that the (singlet) ion-radical pair is the primary reaction intermediate and established the electron-transfer pathway for this Patern6-BUchi transformation. The alternative pathway via direct electronic activation of the carbonyl component led to the same oxetane regioisomers in identical ratios. Thus, a common electron-transfer mechanism applies that involves quenching of the excited quinone acceptor by the stilbene donor to afford a triplet ion-radical intermediate, which appears on a nanosecond/microsecond time scale. The spin multiplicities of the critical ion-pair intermediates in the two photoactivation paths determine the time scale of the reaction sequences and also the efficiency of the relatively slow ion-pair collapse k = 10 s ) to the 1,4-biradical that ultimately leads to the oxetane product 54. 

Figure 2.10. Example of a donor-acceptor fluoroionophore in which the electron-withdrawing character of the acceptor (carbonyl group of the coumarine) is cation-controlled. Absorption and fluorescence spectra of ClS3-crown(Oj) and its complexes with perchlorate salts in acetonitrile. (Adapted from Ref. SO.)...

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

Basicity in the gas phase is measured by the proton affinity (PA) of the electron donor and in solution by the pAj,. A solution basicity scale for aldehydes and ketones based on hydrogen bond acceptor ability has also been established [186]. Nucleophilicity could be measured in a similar manner, in the gas phase by the affinity for a particular Lewis acid (e.g., BF3) and in solution by the equilibrium constant for the complexation reaction. In Table 8.1 are collected the available data for a number of oxygen systems. It is clear from the data in Table 8.1 that the basicities of ethers and carbonyl compounds, as measured by PA and p , are similar. However, the nucleophilicity, as measured by the BF3 affinity, of ethers is greater than that of carbonyl compounds, the latter values being depressed by steric interactions. 

The carbon dioxide molecule exhibits several functionalities through which it may interact with transition metal complexes and/or substrates. The dominant characteristic of C02 is the Lewis acidity of the central carbon atom, and the principle mode of reaction of C02 in its main group chemistry is as an electrophile at the carbon center. Consequently, metal complex formation may be anticipated with basic, electron-rich, low-valent metal centers. An analogous interaction is found in the reaction of the Lewis acid BF3 with the low-valent metal complex IrCl(CO)(PPh3)2 (114). These species form a 1 1 adduct in solution evidence for an Ir-BF3 donor-acceptor bond includes a change in the carbonyl stretching frequency from 1968 to 2067 cm-1. 

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