Transient Four-membered Rings

Transient Four membered Rings.—Evidence has been presented for the intermediacy of Dewar-benzenes in the thermal rearrangement of bis-cyclopropenyl to benzene. When (572) was heated in acetonitrile, in the presence of 9,10-dibromoanthracene, fluorescence of the dibromoanthracene was observed. The nature of the excited state produced was identified as triplet xylene, which was shown to be produced from (572) via an intermediate. The activation energy for thermolysis of the intermediate, ca. 26kcalmol S was less than that for the disappearance of (572). Of the three possible benzene isomers, only Dewar-benzene produces a significant amount of the indirect chemiluminescence, and the activation energies for the decomposition of the Dewar-benzenes (573) and (574) are close to that found for the intermediate in the thermolysis of (573). Comparison of the chemiluminescence yields from (572), (573), and (574) allowed the conclusion that the rearrangement of (572) could proceed almost totally via a dimethyl-Dewar-benzene. 

The perfluoroxylenes are interconverted on irradiation in the vapour phase. The reaction probably involves the para-bonded and prismane valence isomers. The perfluor0-1,2-, -1,3-, and -2,5-dimethylbicyclo[2,2,0]hexadienes have been characterized. 

Free cyclobutadiene has been detected in solution using an insoluble polymer-bound precursor, with trapping of the cyclobutadiene by a second polymer-bound reagent. As only a negligible fraction of the active polymer sites are at the surface of the solid, any reaction between the two solid phases requires the existence of an unbound intermediate. The cyclobutadiene precursor, a polymer-bound -phenan-throline derivative of cyclobutadieneiron carbonyl (580) was oxidized in presence of a polymer-bound maleimide (581). The imide was then released from the polymer by treatment with methylamine. The reaction proceeds with 96% transfer of cyclobutadiene between the two polymers. 

The reaction of acetylene in a glow discharge in a specially constructed reaction tube yields vinylacetylene, diacetylene, benzene, phenylacetylene, styrene, and indene as the major volatile products. The formation of these products can be most economically rationalized by assuming that the dimerization of acetylene to cyclobutadiene is the first reaction step. 

Russell has reported the preparation of bicyclo[3,2,0]hept-2-en-6,7-semidione (582). Reaction of the bicyclo[2,2,l]heptene acyloin (583) with potassium t-butoxide in DMSO gives (582). The rearrangement may occur via the enediol dianion, as the bicyclo[2,2,l]heptene semidione itself, generated by an alternative route, is stable. The methylated bicyclo[2,2,l]heptene acyloins (584a) and (584b) both yielded mixtures of the possible bicydo[3,2,0]heptene semidiones (585) and (586) on treatment with butoxide in DMSO. The rearrangement was shown to be reversible, since the same mixture of semidiones was formed from l-methylbicyclo[3,2,0]hept-2-en-6,7-acyloin (587). 

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The mechanism for the transformation of 5 to 4 was not addressed. However, it seems plausible that samarium diiodide accomplishes a reduction of the carbon-chlorine bond to give a transient, resonance-stabilized carbon radical which then adds to a Smni-activated ketone carbonyl or combines with a ketyl radical. Although some intramolecular samarium(n)-promoted Barbier reactions do appear to proceed through the intermediacy of an organo-samarium intermediate (i.e. a Smm carbanion),10 ibis probable that a -elimination pathway would lead to a rapid destruction of intermediate 5 if such a species were formed in this reaction. Nevertheless, the facile transformation of intermediate 5 to 4, attended by the formation of the strained four-membered ring of paeoniflorigenin, constitutes a very elegant example of an intramolecular samarium-mediated Barbier reaction. 

Thermolysis at 180°C of dithiaphosphadigerminane 1929 leads to transient 2-thia-l,3-digermetane 20, which gives a [4] —> [2 H- 2] decomposition with formation of both dimethylgermene 15 and dimethylgermanethione 21 [Eq. (4)]. Note that a silene and a silanethione were also simultaneously obtained by West et al in a rather similar decomposition by photolysis of a 3-thia-1,2-disiletane,30 a four-membered ring with a Si-Si-S-C linkage. 

In 1985, Fluck et al. reported that treatment of the P-fluoromethylene-phosphorane 3f with 2 equiv of butyllithium at -95°C gave the 1,1,3,3-tetrakis(dimethylamino)-lA5,3A5-diphosphete 5f in 34% yield.30 The formation of this four-membered ring could result from a [2 + 2] head-to-tail dimerization of the transient phosphinocarbene 2f. However, an alternative... 

Although phosphorus-containing four-membered rings have been found in some reactions as intermediate, they are only a transient species in most cases. 

Chelidonine, a representative benzo[c]phenanthridine alkaloid (1,2), was synthesized as the first application of this reaction in natural product synthesis (145). Initial thermal opening of the four-membered ring in 299 led to the formation of a transient -o-quinodimethane (301), which has a dienamide structure in the diene part and is then trapped by the suitably positioned multiple bond in the same molecule. They applied this intramolecular reaction to the acetylenic cyclobutene 300 for the synthesis of ( )-chelidonine (Scheme 110). 

Several stereochemical facets show up on Diels-Alder reactions of 7,8-disubstituted o-quinodimeth-anes. A seminal study of addition reactions of 7,S-diphenylbenzocyclobutenes provided strong evidence for a conrotatory opening of the four-membered ring, followed by a supra-supra-facial endo-selective Diels-Alder reaction with TV-phenylmaleimide, as exemplified by the transformation (607) - (609) (Scheme 137). Based on this result, it is now generally agreed that the trans relation of substituents at C-7 and C-8 in benzocyclobutenes translates, via the transient ( , )-o-quinodimethanes, e.g. (608), into the cis disposition in the corresponding cycloadducts. 

Transient Three-and Four-membered Ring compoonds.—Protonated Cyclopropanes. The C3H7 energy surface has been re-examined by MO theory with an improved basis set. The conclusion remains that the 2-propyl cation is the most stable, but the second most stable structure is now found to be a corner-protonated cyclopropane. Furthermore, the energy gap between the edge- and comer-protonated species is now calculated to be only 6 kcal mol ... 

The four-membered ring-system 250 is formed transiently in the next sequence (Reaction scheme 174), which looks like a technical process to produce the tetramethyl cyclopropane carboxylic 247 acid for fenpropathrin [496]. 

Vacuum thermolysis (160°C) of the hemiaminal 11 generates the azaphos-phetane 65 in 85% yield.32 This product clearly results from the intramolecular insertion of the transient (amino) (phosphino)carbene 2h into the C-H bond of a diisopropylamino group bonded to phosphorus. Note that the four-membered heterocycle 65 is formed exclusively in spite of the ratio of six methyl-CH bonds to one methine-CH bond, and that only one of the two possible diastereomers is detected. The same regio- and diastereo-selectivity have already been observed with the di(phosphino)carbene 2g,74 but is in marked contrast to the exclusive formation of five-membered rings... 

Starting from (silyl)(phosphino)diazomethane derivatives, stable pseudo-diazoalkenes and carbenes have been obtained. The diazomethylenephosphoranes are very reactive dipolarophiles but are also synthetic equivalents of naked carbon C . The stable carbenes react in a similar manner to their transient analogues, but also exhibit new reactions the synthesis of unsaturated three-membered heterocycles is particularly noteworthy. Ring expansion reactions involving the latter are of special interest since they afford novel non-antiaromatic four-7t-electron four-membered heterocycles. 

It is well documented that carbenium ions undergo nucleophilic attack by the carbon atom of isocyanides [46]. Significantly, diphosphirenium 2 Pa reacts at -50°C with ferf-butyl isocyanide in a similar manner to give the transient ni-trilium salt 21, which undergoes a ring expansion reaction affording 22 [45] (Scheme 16). Note that 22 is one of the very rare four-membered heterocycles featuring a phosphaalkene moieties [47]. 

Crystal structure analyses of fourteen aluminum(I) compounds have uncovered three structural motifs (1) four-membered AI4 rings for solvated aluminum(I) halides (AlX Do)4, (2) AI4 tetrahedra for aluminum(I) compounds with organic substituents (RA1)4, and (3) a monomer for the nacnac derivative. In addition, a tetramer-monomer equilibrium for (Cp Al)4 was established through VT NMR studies, and the gas-phase structure of Cp Al was determined by electron diffraction [20]. A dimeric structure similar to that of (Dipp Ga)2 [33, 34] was suggested for the transient Dipp Al species, a proposal based on the structure of the isolated cycloaddition product [27]. 

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