Conjugation spiro

The two orbitals that do interact, tp2 and n, which have the same symmetry, create the usual pair of new orbitals, one raised and one lowered. Since there are only two electrons to go into the new orbitals, the overall energy of the conjugated system is lowered. The effect, AES, is small, both because of the poor overlap, and because the two orbitals interacting are far apart in energy, which we shall see later is an important factor. Nevertheless, it is a general conclusion that if the total number of n electrons is a (4 2) number, the spiro system is stabilised, leading to the concept of spiroaromaticity. 

There is equally a phenomenon of spiroantiaromaticity when the total number of n electrons is a 4 number, as in spirononatetraene 1.36 (Fig. 1.52). Here the only orbitals with the right symmetry to interact productively are the i 2 orbitals on each side (ignoring the interaction of the unfilled tp4 orbitals with each other, which has no effect on the energy because there are no electrons in these 

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Regioselectivity of Nitration of Fluorene and Its Reversal by a Spiro Conjugation... 

Facial selectivities of spiro[cyclopentane-l,9 -fluorene]-2-ones 30a-30e were studied by Ohwada [96, 97]. The carbonyl tz orbital can interact with the aromatic % orbital of the fluorene in a similar manner to spiro conjugation [98-102]. The ketones 30 were reduced to alcohols by the action of sodium borohydride in methanol at -43 °C. The anti-alcohol, i.e., the syn addition product of the reducing reagent with respect to the substituent, is favored in all cases, irrespective of the substituent at C-2 or C-4 of the fluorene ring (2-nitro 30b syn anti = 68 32), 4-nitro... 

In the epoxidation of an olefin with a peracid, the occupied n orbital of the olefin group HOMO) interacts with the vacant orbital (LUMO) of the peracid [143, 144]. The higher-lying aromatic n orbital of the substituted fluorenes (69) can interact with the orbital in a similar manner to spiro conjugation (Fig. 12). 

The sterically unbiased dienes, 5,5-diarylcyclopentadienes 90, wherein one of the aryl groups is substituted with NO, Cl and NCCHj), were designed and synthesized by Halterman et al. [163] Diels-Alder cycloaddition with dimethyl acetylenedicarbo-xylate at reflux (81 °C) was studied syn addition (with respect to the substituted benzene) was favored in the case of the nitro group (90a, X = NO ) (syrr.anti = 68 32), whereas anti addition (with respect to the substituted benzene) is favored in the case of dimethylamino group (90b, X = N(CH3)2) (syn anti = 38 62). The facial preference is consistent with those observed in the hydride reduction of the relevant 2,2-diaryl-cyclopentanones 8 with sodium borohydride, and in dihydroxylation of 3,3-diarylcy-clopentenes 43 with osmium trioxide. In the present system, the interaction of the diene n orbital with the o bonds at the (3 positions (at the 5 position) is symmetry-forbidden. Thus, the major product results from approach of the dienophile from the face opposite the better n electron donor at the (3 positions, in a similar manner to spiro conjugation. Unsymmetrization of the diene % orbitals is inherent in 90, and this is consistent with the observed facial selectivities (91 for 90a 92 for 90b). 

In order to smdy the effect of perturbation arising from spiro-conjugation on the chemical reactivities, in particular the facial selectivities, sterically unbiased dienes (96 and 97) based on fluorenes in spiro geometry have been synthesized [165]. These dienes react as Diels-Alder dienes with several dienophiles (maleic anhydride (MA), A-phenylmaleimide (PMI), A-phenyl-l,3,5-triazoUne-2,4-dione (PTD) and iV-methyl-l,3,5-triazoline-2,4-dione (MTD)). 

Ohwada extends his theory, unsymmetrization of n orbitals, to Orbital Phase Environment including the secondary orbital interaction (Chapter Orbital Phase Environments and Stereoselectivities by Ohwada in this volume). The reactions between the cyclopentadienes bearing spiro conjugation with benzofluorene systems with maleic anhydride exemplified the importance of the phase environment. The reactions proceed avoiding the out-of-phase interaction between dienophile LUMO and the HOMO at the aromatic rings. The diene 34 with benzo[b]fluorene favored syn addition with respect to the naphtalene ring, whereas the diene 35 with benzo[c]fluorene showed the reverse anti preference (Scheme 22) [28]. 

The class of 2-amino-4H-pyrans has been known for several decades. The first representatives 1 and 2 are from the late 1950s to the early 1960s. Spiro-conjugated pyranopyrazole 1 was obtained in studies on pyrazolone dyestuffs (56ACS587), while pyranopyranone 2 served as a precursor for the blood anticoagulant warfarin (62JOC3086). 

Aminopyrans 244, spiro-conjugated with an polycyclic N,0,S-system, have been synthesized using N,S-acetal derivatives of actylacetone and acetoacetic ester 245 (00PS(160)105). Diacetyl derivative 245 (Z = COMe) undergoes deacetylation in the course of pyran synthesis (Scheme 93). 

A three-component reaction of thienopyridinone 211, substituted piperidin-4-ones 246, and malononitrile 27a leads to spiro-conjugated polycycles 251 (03RCB1380) (Scheme 96). 

A new type of spiro-conjugated aminopyrans, pyrans, containing a cyclophane moiety 254, were obtained from a carbonyl[2.2]paracyclo-phane derivative 255 in a two-step procedure with isolation of UN 256 (03T1739) (Scheme 98). 

Spiro-aromaticity (Section 1.5.5) is barely detectable. Explain why the spiro cation 1.54, might have more tt stabilisation by spiro conjugation than spiro-heptatriene 1.30. 

Tetrahedral structures have been studied occasionally (Lequan et al., 1994 Lambert et al., 1998 Wolff and Wortmann, 1998). Only one tensor element, /3ayj, is significant. The conjugation between the substituents through the central element seems to be unsatisfactory, despite the enhancement by through-space coupling in [122] and possibly also [121]. Spiro-conjugation in >2d molecular symmetry does improve interaction to some extent as shown on p. 173 (cf. [5] in Scheme 4). 

As we have just seen, it is possible to have some bonding even when the overlap is neither strictly head-on nor sideways-on. It is easily possible to retain much more of the bonding when the orbitals are rather better aligned than those in spiro-conjugated systems, as is the case in several strained molecules, epitomised by cyclopropane. 

An investigation into spiro-conjugation in dioxane systems has been carried out using UV spectroscopy. The UV spectrum of a spirocyclic orthocarbonate was compared with the spectra of two other nonspirocyclic dioxane-derived compounds <82BCJ1106>. 

A particular kind of through space interaction is known as spiro conjugation The He(I) and He(II) PE ectra of the spiro compounds 15 and 16 are shown in Fig. 17 The bands in the low energy r on are relatively more... 

The similarity of the absorption spectra and the strong solvent dependence of the emission spectra show that the two halves of the molecule demonstrate neghgible interaction in the ground state but strong interaction in the vibrationally relaxed exited state. These effects can be imderstood in terms of a photoinduced intramolecular electron transfer reaction. A possible explanation can be given by the effect of spiro conjugation (see next section) [142-145]. For compoimd 119, Chien et aL determined the free energy of electron transfer in different solvents to a value of AG > - 0.63 eV [ 141 ]. 

Spiro-conjugated polyamides [154], polyimides [21], and polyquinolines [155] have also been reported in the hterature. 

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