Carbonyl dipoles

With (32), despite the intramolecular hydrogen-bonding possible in the enol form (326), the equilibrium lies essentially completely over in favour of the keto form (32a), because this can take up an anti-conformation in which the two electronegative oxygen atoms are as far from each other as possible, and in which the carbonyl dipoles are opposed. With (33), the C=0 groups are locked in the syn-con-formation in both keto (33a) and enol (336) forms, and the intramolecular hydrogen-bonding open to (336), but not to (33a), then decides the issue. 

In general, the Henry reaction proceeds in a non-selective way to give a mixture of anti (erythro) and syn (threo) isomers. Ab initio calculations on the Henry reaction suggest that free nitronate anions (not influenced by cations) react with aldehydes via transition states in which the nitro and carbonyl dipoles are antiperiplanar to each other. This kind of reaction yields anti-nitro alcohols. The Henry reaction between lithium nitronates and aldehydes is predicted to occur via cyclic transition states yielding syn-nitro alcohols as major products (Eq. 3.64).108... 

The reader was given a taste of the power of isomiinchnone dipolar cycloaddition chemistry in Section 10.2.1. As discussed by Potts (1) and Gingrich and Baum (10), the isomiinchnone ring system—a masked carbonyl dipole—is exceptionally reactive as a 1,3-dipole in 1,3-dipolar cycloaddition reactions. In the intervening years since these two excellent reviews the major research efforts in isomiinchnone chemistry have entailed synthetic applications to specific targets such as alkaloids and other natural and unnatural products. 

Fig. 5. Conjectured cross-sectional representation of complex for cucur-bituril plus C HjNHfCHjj NH-(CHjj NHj in acidic and in alkaline solution. Outlines are drawn as for Fig, 4. In all probability a slight buckling of the hexyl chain actually occurs in the upper situation, better filling the cavity and bringing the ammonium ions into improved alignment with the carbonyl dipoles of the receptor, from [8] with permission...

MibBtituent from axial to equatorial. On the other hand, this second effect iB not sufficient to offset the first with 8 - cetoxy-5j3,6j3 epoxy steroids. Finally, with respect to 3-oxo-G ,Qa.epoxidos, in wlikh mi furmational effect are negligible to a first approximation, the efeclron-withdrawing influence of the carbonyl dipole is die dominant directive factor, causing fluorohydrin formation exclusively. 

Presence of a phenyl 6ubetitu nt at C(jj, as in 2,3-epoxy-3-phemy) -cyclohexanone caused a preponderance of acyl migration (Eq. 47. j. presumably by stabilizing the carbonium ion generated by Cjar bond rupture. Formation of a oerbonium ton at Ct ) rather than is favored also on electrostatic grounds by the proximity of C( to tlie positive terminal of the carbonyl dipole.8 ... 

Azomethine derived ferroelectric liquid crystals As DOBAMBC, many ferroelectric LC s were prepared utilizing amyl alcohol as the chiral source. The reason for the small spontaneous polarization of DOBAMBC is the separation between the C=0 dipole moment and the chiral carbon. This effect can be explained by the intramolecular rotation or vibration of the carbonyl dipole moment relative to the chiral carbon, because they are not adjacent. There are some rules linking the molecular structure and the direction of the spontaneous polarization (minus or plus). In order to reduce the separation between the carbonyl dipole moment and the chiral carbon, ferroelectric LC s were synthesized utilizing a secondary alcohol as the chiral source. Ferroelectric LC s with large spontaneous polarizations have large dipole moments at the chiral centre. Ferroelectric LC s with halogen or nitrile units connected directly to the chiral carbon were synthesized from chiral lactic acids or amino acids. 

Figure 7.11 Irreversible stress-induced phase reorientation in 2,10-undecandione-urea. (a) Crystal before stress, (b) Schematic of crystal before stress arrows represent carbonyl dipoles for guests in one layer, (c) Crystal under uniform stress from bottom left of picture, (d) Schematic of stressed crystal. (Reproduced by permission from Macmillan Publishers Ltd).

SCF calculations have also been made on the dimers of HFU0 and formaldehyde.111 In the latter case two potential minima were found of comparable energy, one hydrogen-bonded and the other held together by the electrostatic interactions of the two carbonyl dipoles. The barrier between the two is, however, very low, and the two forms could almost certainly not be isolated. The situation in HCN is similar.112... 

It is not even necessary for the reagent to be charged. Ammonia also reacts with acetone and this time it is the lone pair of electrons —a pair of electrons not involved in bonding and concentrated on the nitrogen atom of the uncharged ammonia molecule—that is attracted to the positive end of the carbonyl dipole. 

For both terpene ketones, pulegone and camphor, the A ( C-ASIS) has been estimated by a chemical shift comparison method using t-butylcyclohexane as an additional reference compound [409], By comparing the A (ASIS) of both and nuclei it seems that the carbonyl plane rule is also valid for aromatic solvent-induced NMR chemical shifts of carbonyl compounds [409], Obviously, the particular geometrical arrangement of the aromatic solvent molecules around the carbonyl dipole inhu-ences both the and nuclei in the same way. 

These points prompt a few words of caution regarding the results discussed in this section and some comments on theoretical results in general. In particular, besides considerations of the size and effect of basis sets, one needs to examine the energy dependence on electron correlation. For donor-acceptor complexes, such as Lewis acid-carbonyls, electron correlation seems to be crucial. It is also instructive to consider the dipole moments of various structures in theoretical studies. Assumption of a gas phase environment may overestimate destabilization due to a large dipole, as dipoles can often be stabilized quite effectively in solution. At times the interpretation of theoretical results are the source of trouble and confusion. Thus two different groups propound apparently conflicting and opposite views on the effect of Lewis acids on the carbonyl dipole. 

FIGURE 19.18 Similar intercharge distances between the protonated nitrogen and die carbonyl dipole exist in bicuc-ulhne, GABA, acetylchohne, and carbachol. 

Mutual Orientation of Carbonyl Dipoles or Benzene Rings... 

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