Cyclobutenes 3,4-dichlorocyclobutene

A number of examples involving nitrile oxide cycloadditions to cyclic cis-disubstituted olefinic dipolarophiles was presented in the first edition of this treatise, notably to cyclobutene, cyclopentene, and to 2,5-dihydrofuran derivatives (15). The more recent examples discussed here also show, that the selectivity of the cycloaddition to 1,2-cis-disubstituted cyclobutenes depends on the type of substituent group present (Table 6.8 Scheme 6.41). The differences found can be explained in terms of the nonplanarity (i. e., pyramidalization) of the double bond in the transition state (15) and steric effects. In the cycloaddition to cis-3,4-diacetyl-(197) and cis-3,4-dichlorocyclobutene (198), the syn-pyramidalization of the carbon atoms of the double bond and the more facile anti deformability of the olefinic hydrogens have been invoked to rationalize the anti selectivity observed. 

Simple alkenes which have been observed to undergo meta cycloaddition to benzene to give three-membered rings from ring contractions, sometimes in high chemical and quantum yields, are (Z)- and ( )-but-2-ene, cyclobutene, 2,3-dimethylcyclobutene. methylenecy-clobutane, 2,3-dichlorocyclobutene, bicyclo[3.2.0] hepta-2,6-diene, 8,9,10-trinorborn-2-ene (but not 8,9,10-trinorborna-2,5-diene"), 5,6-dichloro-8,9,10-trinorborn-2-ene, cyclo-... 

The reduction of 3,4-dichlorocyclobutene (222) in the presence of metal carbonyls has been utilized to prepare the parent complex [223, MLn = Cr(CO)4, Mo(CO)3, W(CO)3, Fe(CO)3, Ru(CO)3 orCo2(CO)6] (equation 32) .Morerecently, reaction ofNi(CO)4 with 3,4-dihalocyclobutenes (X = Br or I) or with 222 in the presence of AICI3 produced the corresponding (cyclobutadiene)nickel dihalides . Methodology for the preparation of 1,2- or 1,3-disubstituted (cyclobutadiene)Fe(CO)3 complexes from 1,2- or 1,3-disubsli-tuted-3,4-dibromocyclobutenes has been presented - In turn, the substituted dibromo-cyclobutenes are prepared from squaric esters. The reaction of cz5-3,4-carbonyldioxycy-clobutene and substituted variants with l c2(CO)9 orNa2Fe(CO)4 also produces (cyclobu-tadiene)Fe(CO)3 complexes . Photolysis of a-pyrone generates 3-oxo-2-oxabicyclo [2.2.0]hex-5-ene (224) which undergoes photolysis with a variety of metal carbonyls to afford the parent cyclobutadiene complex 223 [MLn = CpV(CO)2, Fe(CO)3, CoCp. or RhCp] (equation 33) 2 0. 

The tu c-dichloride (1), the adduct of benzyne and cis-3,4-dichlorocyclobutene, was successfully dechlorinated by disodium phenanthrene to give (2) in 27% yield.8 Other reagents give mixtures of tetralin and 1,4-dihydronaphthalene. The product (2) is correctly named as benzobicyclo 2.2.0]hexa-2,5-diene, but can be called hemi Dewar naphthalene. Similar dehalogenations of n c-dihalides to give substituted cyclobutene derivatives have been carried out using naphthalene-sodium. ... 

Disubstituted cyclobutene-1,2-diones." These products can be prepared by reaction of 3,4-dichlorocyclobutene-l,2-dioncs with functionalized organocopper/zinc reagents. 

It is difficult to choose a reference compound against which to judge the stability of the dication. That it can be formed at all, however, is suggestive of special stabilization associated with the two TT-electron system. The dianion formed by adding two electrons to the TT system of cyclobutadiene also meets the 4 - -2 criterion. In this case, however, four of the six electrons would occupy HMO nonbonding orbitals, so high reactivity could be expected. There is some evidence that this species may have a finite existence. " Reaction of 3,4-dichlorocyclobutene with sodium naphthalenide, followed a few minutes later by methanol-0-<7 gives a low yield of 3,4-di-deutero-cyclobutene. The inference is that the dianion [ 4114 ] is present, but there has not yet been direct experimental observation of this species. Cyclooctatetraene is reduced by alkali metals to a dianion. 

The steric course of the reaction of cyclic nitrones (476) and (477) with cis-3,4-disubstituted cyclobutenes depends on the nature of the substituents on the cyclobutene ring. With 3,4-dichlorocyclobutene the syn-isomers (479) or (481) are the major products. The syn anti ratio is highest in solvents of high polarity. Reaction of (477) with cis-3,4-dimethoxycarbonylcyclobutene or with the anhydride gives the anti addition products (480b) and (480c). Simple steric and dipole-dipole interactions... 

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