Heterocycles three-membered

TRANSFORMATION OF EXISTING HETEROCYCLES 3.03.S. 1 Three-membered Heterocycles... 

Table 1 Structures of Known Three-membered Heterocyclic Compounds...

For the antiaromatic three-membered heterocycles, experimental data are available only for thiirenes (and there is some doubt about the true antiaromaticity of thiirenes). Bond lengths have been calculated, however, for these antiaromatic 47r-systems (80PAC1623). In comparison with the corresponding saturated heterocycles, the C—X bond lengths are increased by 0.05 to 0.17 A and the C—C bond length is decreased by 0.2 A. 

Table 5 Ranges of NMR Data of Three-membered Heterocyclic Systems...

Table 6 Ranges of NMR Data for Three-membered Heterocyclic Systems with Exocyclic Unsaturation...

Radicals of most small and many large heterocycles are known, but their chemistry has not always been explored in depth. The ESR spectra of small ring radicals have been measured and generally found to be in good agreement with theoretical predictions. Table 1 gives some data for three-membered heterocyclics. Noteworthy is the close similarity of the ESR spectra of 1-aziridinyl, 1-azetidinyl and dimethylaminyl (71TL2247). The radicals in the table are all tt-radicals. 

The proton affinities (gas phase) of thiirane and other three-membered heterocycles have been determined azirane (902.5), thiirane (819.2), phosphirane (815.0), oxirane (793.3 kJ moF ) (80JA5151). Increasing s character in the lone electron pairs decreases proton affinities. Data derived from NMR chemical shifts in chloroform indicate the order of decreasing basicity is azirane > oxirane > thiirane (73CR(B)(276)335). The base strengths of four-, five- and six-membered cyclic sulfides are greater than that of thiirane. 

Thiurets — see 1,2,4-Dithiazolidines, diimino-Thonzylamine antihistamine, 3, 153 Thorpe reaction benzothiophenes from, 4, 876 Thorpe-Ziegler cyclization, 2, 74 Three-membered heterocyclic compounds basicity, 7, 23... 

Three-membered heterocyclic compounds are available when one atom fragments like carbenes [108 or phosphorus compounds [109] add to C=0 ot C=N bonds (equation 26)... 

When lithiated, the ring strain of the three-membered heterocycle remains important, and this strain, combined with a weakening of the a-C-O bond, due to its greater polarization, make metalated epoxides highly electrophilic species [2], They react with strong nucleophiles (often the base that was used to perform the a-deprotonation) to give olefins following the elimination of M2O (Scheme 5.2, Path B), a process often referred to as reductive alkylation . 

These polymers are based on the three-membered heterocyclic system either as the epoxy or oxirane ring (1.3). 

Presumably, 9 is actually formed from carbene 8 in the pyrolysis zone by a P/C phenyl shift, but then apparently succumbs to fast transformation into the thermodynamically stable final products. Formation of the methane derivative 13 should be preceded by a 1,2-phenyl shift to give the shortlived 10, the production of fluorene (14) by the occurrence of diphenylcarbene (II), and the formation of benzophenone (15) by isomerization to the angle-strained three-membered heterocycle 12, which is followed by elimination of phenylphospbinidene. No direct evidence is available for the intermediacy of 10-12. 

Available experimental data suggest that the decomposition of betaines I occurs via direction (iii) with R3E15 elimination giving three-membered heterocycles or via retro-Wittig type (ii) to eliminate R3E15=CR1R2 leading to the compounds with an E14=X bond (Scheme 19). 

Figure 1-3. Solution stable three-membered heterocyclic ring systems.

The first isolable alkenetitanium complex, the bis(pentamethylcyclopentadienyl)-titanium—ethylene complex 5, was prepared by Bercaw et al. by reduction of bis(penta-methylcyclopentadienyl)titanium dichloride in toluene with sodium amalgam under an atmosphere of ethylene (ca. 700 Torr) or from ( (n-C5Mc5)2Ti 2(fJ-N2)2 by treatment with ethylene [42], X-ray crystal structure analyses of 5 and of the ethylenebis(aryloxy)trimethyl-phosphanyltitanium complex 6 [53] revealed that the coordination of ethylene causes a substantial increase in the carbon—carbon double bond length from 1.337(2) A in free ethylene to 1.438(5) A and 1.425(3) A, respectively. Considerable bending of the hydrogen atoms out of the plane of the ethylene molecule is also observed. By comparison with structural data for other ethylene complexes and three-membered heterocyclic compounds, the structures of 5 and 6 would appear to be intermediate along the continuum between a Ti(11)-ethylene (4A) and a Ti(IV)-metallacyclopropane (4B) (Scheme 11.1) as... 

Known and Potential Monocyclic Three-Membered Heterocyclic Rings... 

Similarly, the (phosphino)(silyl)carbene 2a reacts at -30°C with a slight excess of the tert-butylphosphaalkyne cleanly affording the 2-phosphino-2//-phosphirene 34.53 The reaction leading to 34 is strictly analogous to that observed on reacting the transient dichlorocarbene with the tert-butyl-phosphaalkyne, in which the 2//-phosphirene 36 was obtained.54 The three-membered heterocycle 34 appeared to be rather unstable and rearranged, quantitatively, to afford the lA5,2A3-diphosphete 35 after 3 h at room temperature.55 Once again, these results as a whole indicate that a concerted [1 + 2]-cycloaddition process is involved in the formation of the 2//-phosph-irene 34. 

There are only a few syntheses of three-membered heterocycles (Fig. 3.4) reported in solid-phase chemistry. Filigheddu et al. [305] described the synthesis of aziri-dines (223) (Scheme 3.33), which are important heterocycles in organic and medicinal chemistry. 

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