Carbon Carbonyls

Fig. 1. The rate-determining step in the neutral hydrolysis of paramethoxy-phenyl dichloroacetate. In the reactant state (a) a water molecule is in proximity of the carbonyl carbon after concerted proton transfer to a second water molecule and electron redistribution, a tetrahedral intermediate (b) is formed.

The first step is the interaction of the basic catalyst with the ester to produce the carbanion (I) the carbanion so formed then attacks the carbonyl carbon of a second molecule of ester to produce the anion (II), which is converted to ethyl acetoacetate (II) by the ejection of an ethoxide ion. Finally (III) reacts with ethoxide ion to produce acetoacetic ester anion (IV). This and other anions are mesomeric thus (IV) may be written ... 

The mechanism of the reaction probably involves the production, by into -action of the aldehyde with hydroxide ions, of two reducing anions, the first (I) more easily than the second (II). Either of these anions may transfer a hydride ion to a carbonyl carbon atom in another aldehyde molecule ... 

Reaction of the cnrbaiilon (acetone anion) with the carbonyl carbon of ethyl acetate, accompanied by the release of an ethoxide ion, to form acetyl-acetone ... 

The regioselectivity benefits from the increased polarisation of the alkene moiety, reflected in the increased difference in the orbital coefficients on carbon 1 and 2. The increase in endo-exo selectivity is a result of an increased secondary orbital interaction that can be attributed to the increased orbital coefficient on the carbonyl carbon ". Also increased dipolar interactions, as a result of an increased polarisation, will contribute. Interestingly, Yamamoto has demonstrated that by usirg a very bulky catalyst the endo-pathway can be blocked and an excess of exo product can be obtained The increased di as tereo facial selectivity has been attributed to a more compact transition state for the catalysed reaction as a result of more efficient primary and secondary orbital interactions as well as conformational changes in the complexed dienophile" . Calculations show that, with the polarisation of the dienophile, the extent of asynchronicity in the activated complex increases . Some authors even report a zwitteriorric character of the activated complex of the Lewis-acid catalysed reaction " . Currently, Lewis-acid catalysis of Diels-Alder reactions is everyday practice in synthetic organic chemistry. 

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

The ring nitrogen can be involved in intramolecular reactions on carbonyl carbon as exemplified by the preparation of 6,7-dimethoxy-4-methyl-10,ll-thiopega-2,9-diene (72) (Scheme 51) (228). 

Together these two products contain all eight carbons of the starting alkene The two carbonyl carbons correspond to those that were doubly bonded m the original alkene One of the doubly bonded carbons therefore bears two methyl substituents the other bears a hydrogen and a tert butyl group The alkene is identified as 2 4 4 trimethyl 2 pentene (CH3)2C=CHC(CH3)3 as shown m Figure 6 15... 

The chemical shift of the carbonyl carbon (8 209) is not included because it has no attached hydrogens... 

With ketones the e ending of an alkane is replaced by one in the longest con tinuous chain containing the carbonyl group The chain is numbered in the direction that provides the lower number for this group The carbonyl carbon of a cyclic ketone is C 1 and the number does not appear m the name... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

Methyl ketones give esters of acetic acid that is oxygen insertion occurs between the carbonyl carbon and the larger of the two groups attached to it... 

Section 17 2 The carbonyl carbon is sp hybridized and it and the atoms attached to It are coplanar Aldehydes and ketones are polar molecules Nucleophiles attack C=0 at carbon (positively polarized) and electrophiles especially protons attack oxygen (negatively polarized)... 

The step m which Ihe nucleophile attacks Ihe carbonyl carbon is rate determining m bolh base calalyzed and acid calalyzed nucleophilic addilion In Ihe base calalyzed mechanism Ihis is Ihe lirsl step... 

It IS convenient to use the Greek letters a p 7 and so forth to locate the carbons m a molecule m relation to the carbonyl group The carbon atom adjacent to the carbonyl carbon is the a carbon atom the next one down the chain is the p carbon and so on Butanal for example has an a carbon a p carbon and a y carbon... 

The carbonyl group withdraws rr electron density from the double bond and both the carbonyl carbon and the p carbon are positively polarized Their greater degree of charge separation makes the dipole moments of a p unsaturated carbonyl compounds signifi cantly larger than those of comparable aldehydes and ketones... 

Electron release from the substituent X not only stabilizes the carbonyl group it decreases the positive character of the carbonyl carbon and makes the carbonyl group less electrophilic... 

Electron release from nitrogen stabilizes the carbonyl group of amides and decreases the rate at which nucleophiles attack the carbonyl carbon... 

Section 20 21 Acyl chlorides anhydrides esters and amides all show a strong band for C=0 stretching m the infrared The range extends from about 1820 cm (acyl chlorides) to 1690 cm (amides) Their NMR spectra are characterized by a peak near 8 180 for the carbonyl carbon H NMR spectroscopy is useful for distinguishing between the groups R and R m esters (RCO2R ) The protons on the carbon bonded to O m R appear at lower field (less shielded) than those on the carbon bonded to C=0... 

The systematic lUPAC name of ethyl acetoacetate is ethyl 3 oxobutanoate The presence of a ketone carbonyl group is indicated by the designation oxo along with the appro priate locant Thus there are four carbon atoms m the acyl group of ethyl 3 oxobutanoate C 3 being the carbonyl carbon of the ketone function... 

Like aldol condensations Claisen condensations always involve bond formation between the a carbon atom of one molecule and the carbonyl carbon of another... 

The anomenc carbon m a cyclic acetal is the one attached to two oxy gens It IS the carbon that corresponds to the carbonyl carbon m the open chain form The symbols a and (3 refer to the configuration at the anomenc carbon... 

When both building block units are m place on the acyl carrier protein carbon-carbon bond formation occurs between the a carbon atom of the malonyl group and the carbonyl carbon of the acetyl group This is shown m step 1 of Figure 26 3 Carbon-carbon bond formation is accompanied by decarboxylation and produces a four carbon acetoacetyl (3 oxobutanoyl) group bound to acyl earner protein... 

Step 2 On reaction with hydrogen chloride m an anhydrous solvent the thiocarbonyl sulfur of the PTC derivative attacks the carbonyl carbon of the N terminal ammo acid The N terminal ammo acid is cleaved as a thiazolone derivative from the remainder of the peptide... 

Hydrogen is less electronegative than carbon so the carbonyl carbon of —CH=0 is more shielded than the carbonyl carbon of a ketone... 

Carbonyl stretching frequency. Aldehyde proton, relative to TMS. Carbonyl carbon, relative to TMS. 

Monomer Reactivity. The poly(amic acid) groups are formed by nucleophilic substitution by an amino group at a carbonyl carbon of an anhydride group. Therefore, the electrophilicity of the dianhydride is expected to be one of the most important parameters used to determine the reaction rate. There is a close relationship between the reaction rates and the electron affinities, of dianhydrides (12). These were independendy deterrnined by polarography. Stmctures and electron affinities of various dianhydrides are shown in Table 1. 

Benzoates. The selective debenzoylation of sucrose octabenzoate [2425-84-5] using isopropylamine in the absence of solvents caused deacylation in the furanose ring to give 2,3,4,6,1/3/6 -hepta- and 2,3,4,6,1/6 -hexa-O-benzoyl-sucroses in 24.1 and 25.4% after 21 and 80 hours, respectively (54). The unambiguous assignment of partially benzoylated sucrose derivatives was accompHshed by specific isotopic labeling techniques (54). Identification of any benzoylated sucrose derivative can thus be achieved by comparison of its C-nmr carbonyl carbon resonances with those of the assigned octabenzoate derivative after benzoylation with 10 atom % benzoyl—carbonyl chloride in pyridine. 

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