Carbonyl compounds donor power

Optical detection of intermediates produced in the reactions of triplet carbonyl compounds with electron donors has some obvious limitations. However, the technique of CIDNP is proving particularly effective at elucidating the reaction pathways in these systems. The outstanding work of Hendriks et al. (1979) illustrates the power of the technique. Not only was the role of radical ions in the reactions of alkyl aryl ketones with aromatic amines defined but the rate constants for many of the processes determined. The technique has been used to show that trifluoracetyl benzene reacts with electron donors such as 1,4-diazabicyclo[2.2.2]octane and 1,4-dimethoxy-benzene by an electron-transfer process (Thomas et al., 1977 Schilling et al., 1977). Chemically induced dynamic electron polarisation (CIDEP) has been... 

Aldehyde 20 is reduced by NaBH4 to the alcohol in presence of the amide. Sodium borohydride is one of the weakest hydride donors available. The fact that it can be used in water is evidence of this, as more powerful hydride donors such as LiAlH4 react violently with water. Sodium borohydride does not usually react with carbonyl compounds other than aldehydes and ketones. 

However, if the reaction is not under thermodynamic control, the regioselectivity will be determined by the coefficients and charges at the a- and y-carbon atoms of the allyl cation. We can treat an X-substituted allyl cation as resembling an a,/3-unsaturatcd carbonyl compound. The orbitals of acrolein show us that a powerful donor substituent like an oxyanion conjugated to an allyl cation (on the left in Fig. 4.17) leads the /3-carbon atom (y in the allyl system) to have the higher coefficient. However, the... 

The transition metal carbonyl groups which act as ligands to tin clearly also form very covalent bonds, and function as good donors (McAuliffe, Niven Parish, 1977a, b Dickinson et ai, 1975). The donor power of carbonylate anions parallels their nucleophilicities, and increases as carbonyl groups are replaced by phosphines. Detailed examination of data for compounds of the type SnM2X2 (M=carbonylate anion, X=halide) show considerable discrepancies between calculated and observed values of quadrupole splitting (Dickinson et al., 1975). The calculated values are between 10 and 30% lower than those observed. The trends in the... 

Compounds with a low HOMO and LUMO (Figure 5.5b) tend to be stable to selfreaction but are chemically reactive as Lewis acids and electrophiles. The lower the LUMO, the more reactive. Carbocations, with LUMO near a, are the most powerful acids and electrophiles, followed by boranes and some metal cations. Where the LUMO is the a of an H—X bond, the compound will be a Lowry-Bronsted acid (proton donor). A Lowry-Bronsted acid is a special case of a Lewis acid. Where the LUMO is the cr of a C—X bond, the compound will tend to be subject to nucleophilic substitution. Alkyl halides and other carbon compounds with good leaving groups are examples of this group. Where the LUMO is the n of a C=X bond, the compound will tend to be subject to nucleophilic addition. Carbonyls, imines, and nitriles exemplify this group. 

With chromium, molybdenum, and tungsten carbonyls, 1 and 2 displace two CO molecules and form the trans complexes, (l)2M(CO)4 and (2)2M(CO)4 (M = Mo, Cr and W). As an example, the structure of (l>2Cr(CO)4 is shown in Fig. 6. To provide a measure of the strength of these ligands as electron donors, the C=0 stretching frequencies for these complexes were determined by IR spectroscopy [9]. The frequencies for the molybdenum complexes, and for a number of isostmctural Mo compounds, are shown in Table 1. The data indicate that toward Mo(CO)4 as a reference acid, 1 is about equal to triphenylphosphine in donor ability, whUe 2 is slightly weaker, more resembling a trialkoxyphosphine. The stable carbene isostmctural with 1 is an extremely powerful Lewis base toward molybdenum, however, surpassing even trialkylphosphines. 

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