Push-pull dipoles

Scheme 6.276 An intramolecular [3+2] cycloaddition reaction of a push-pull dipole across a heteroaromatic JT-system.

Mejla-Oneto and Padwa have explored intramolecular [3+2] cycloaddition reactions of push-pull dipoles across heteroaromatic jr-systems induced by microwave irradiation [465]. The push-pull dipoles were generated from the rhodium(II)-cata-lyzed reaction of a diazo imide precursor containing a tethered heteroaromatic ring. In the example shown in Scheme 6.276, microwave heating of a solution of the diazo imide precursor in dry benzene in the presence of a catalytic amount of rhodium I) pivalate and 4 A molecular sieves for 2 h at 70 °C produced a transient cyclic carbonyl ylide dipole, which spontaneously underwent cydoaddition across the tethered benzofuran Jt-system to form a pentacyclic structure related to alkaloids of the vindoline type. 

A recent example of this intramolecular tandem transformation is the Rh(ii)-catalyzed reaction of diazo keto ester 71. Depending on the structure of the diazo compound, a push-pull dipole intermediate derived from 71 can be trapped either by a tethered vinyl group (when n = 0) or by an indole 7r-bond (when n=l) (Equation (11)). This result clearly demonstrates a critical role of the conformation of the cycloaddition transition state. 

Padwa A, Price AT (1995) Tandem cyclization-cycloaddition reaction of rhodium carbenoids as an approach to the aspidosperma alkaloids. J Org Chem 60 6258-6259 59. Padwa A, Price AT (1998) Synthesis of the pentacyclic skeleton of the aspidosperma alkaloids using rhodium carbenoids as reactive intermediates. J Org Chem 63 556-565 Mejia-Oneto JM, Padwa A (2004) Intramolecular [3+2]-cycloaddition reaction of push-pull dipoles across heteroaromatic p-systems. Org Lett 6 3241-3244 61. Padwa A, Lynch SM et al (2005) Cycloaddition chemistry of 2-vinyl-substituted indoles and related heteroaromatic systems. J Org Chem 70 2206—2218... 

As a further extension of push-pull dipole cycloaddition chemistry, the Rh (I I)-catalyzed cycHzation/cycloaddition cascade was applied toward the hexacyclic framework of the kopsifoline alkaloids. The kopsifolines 14 are structurally intriguing compounds, related to and possibly derived from an aspidosperma-type alkaloid precursor 12. A possible biogenetic pathway to the kopsifolines from 12 could involve an intramolecular epoxide-ring opening followed by loss of H2O as shown in Scheme 4. The interesting biological activity of these compounds, combined with their... 

The total synthesis of several members of the vinca and tacaman class of indole alkaloids has also been accompHshed using push-pull dipoles in the critical cycloaddition step (2007OL3249, 2008JOC2792). The central step in the synthesis consists of an intramolecular [3-F2]-cycloaddition reaction of an a-diazo indoloamide (i.e., 19), which delivers the pentacyclic skeleton of the natural product in excellent yield (Scheme 6). The acid lability of... 

Compare the geometry of para-nitroaniline to those of both aniline and nitrobenzene. Is there any evidence for push-pull resonance contributors Is there shortening of bonds to the amino and nitro groups Are the bonds in the ring localized Is the dipole moment for para-nitroaniline smaller, larger or about the same as the sum of the dipole moments for aniline and nitrobenzene What does your result say about the importance of push-pull resonance contributors ... 

Red emission chomophores having a long wavelength emission band are usually polar, such as the above DCM series. The push-pull red emitters are normally prone to aggregation in the solid state owing to dipole dipole interactions or through intermolecular tt-tt stacking, especially when the molecules are flat as is the case for DCM. As a consequence, the push-pull red... 

The separation of formal charges in a polar limiting structure like 2b creates a dipole moment of ca. 20 D. Therefore, if such structures were of great importance, quite high dipole moments should be expected for push-pull ethylenes. Data for a reasonable number of mostly symmetrical and rather rigid compounds are known (Table 20). Several high dipole moments are observed, though not in the vicinity of those required for a complete transfer of the double-bond it... 

Summing up, we find that most push-pull ethylenes are comparatively strongly polarized, but that even the largest it polarizations found by combined use of dipole moments and CNDO/2 calculations do not allow the molecules to be pictured with a dominant weight for the dipolar limiting structures. 

The second chapter, by Jan Sandstrom, deals with stereochemical features of push-pull ethylenes. The focus is on rotational barriers, which span a large range of values. The ease of twisting is partly a matter of electron delocalization and partly a matter of steric and solvent effects. Electronic structure and such related items as dipole moments and photoelectron spectra for these systems are discussed. The chapter also deals with the structure and chiroptical properties of twisted ethylenes that do not have push-pull effects, such as frans-cyclooctene. 

Apart from the class of push-pull molecules many other polyconjugated systems exhibit peculiar electronic properties which make them very attractive for technological applications. Most of these systems do not posses a permanent dipole moment and their mutual interactions are due only to London dispersion forces. Also in this case, however,... 

Prompted by our earlier work dealing with the internal dipolar cycloaddition reaction of mesoionic oxazolium ylides of type 74, we subsequently studied the rhodium(II) catalyzed reactions of the related a-diazo ketoamide system 154 <97JOC2001 04OL3241 05JOC2206>. Attack of the amido oxygen at the rhodium carbenoid produces a push-pull carbonyl ylide dipole (i.e., 155) that is isomeric with the isomiinchnone class of mesoionic betaines. 

Similarly, (heterocyclic) jS-enamino esters substituted with an electron donor as well as with an electron acceptor group ( push-pull ) show a decreased cycloaddition tendency towards 1,3-dipoles such as azides see (71c3B351o) and Section 4.11.4.7. 

---