Cyclobutanes Cyclobutenes

The synthesis of 2-chloro-2,3,3-trifluorocyclobutyl acetate illustrates a general method of preparing cyclobutanes by heating chlorotrifluoroethylene, tetrafluoroethylene, and other highly fluorinated ethylenes with alkenes. The reaction has recently been reviewed.11 Chlorotrifluoroethylene has been shown to form cyclobutanes in this way with acrylonitrile,6 vinylidene chloride,3 phenylacetylene,7 and methyl propiolate.3 A far greater number of cyclobutanes have been prepared from tetrafluoroethylene and alkenes 4,11 when tetrafluoroethylene is used, care must be exercised because of the danger of explosion. The fluorinated cyclobutanes can be converted to a variety of cyclobutanes, cyclobutenes, and butadienes. 

New Cyclobutane, Cyclobutene and Cyclobutanone Derivatives derived from the Adduct of Phenylacetylene with l.l-Difluoro-2.2-dichloroethyle-ne. J. Amer. chem. Soc. 75, 4765 (1953)-... 

To allow fast orientation. Table 1 summarizes the most important methods for the synthesis and rearrangement of the cyclopropylmethyl compounds available today. The first column indicates the basic structure(s) to be rearranged, the second details the substituent(s), and the third gives information on the most general methods for the synthesis of the substrates. The fourth column specifies whether cyclobutanes, cyclobutenes and/or cyclobutanones may be obtained, and in the last column the section is given where the corresponding transformation is discussed. Asymmetric versions are indicated with an asterisk ( ). 

The first example of Lewis acid-promoted [2 + 2]-cycloaddition of allylsilanes was introduced by Snider et al. in 1979.185 In the last decade, similar cycloadditions to various electron-deficient unsaturated bonds have been developed for efficient syntheses of cyclobutanes, cyclobutenes, oxetanes, and azetidines as described below. 

The chiral titanium reagent (6) also catalyzes the [2 + 2] cycloaddition reaction of 1,3-oxazolidin-2-one derivatives of a,(3-unsaturated carboxylic acids and ketene dithioacetals in the presence of MS 4A to give cyclobutanone dithioacetal derivatives with high optical purity (eq 10). Vinyl sulfides, alkynyl sulfides, and 1,2-propadienyl sulfides can also be employed in this reaction to give the corresponding cyclobutanes, cyclobutenes and methylenecyclobutane derivatives with high optical purity (eqs 11 and 12). ... 

Photoinduced [2 + 2] cycloaddition (Section 4.9) of alkenes (alkynes) to form cyclobutane (cyclobutene) derivatives is one of the best studied reactions in photochemistry.680 682 According to the Woodward Hoffmann orbital symmetry rules,336 the cycloaddition of one singlet excited (Si) and one ground-state alkene is allowed by a suprafacial suprafacial concerted stereospecific pathway (Scheme 6.45) 695 699 700 Rare concerted [4 + 2] and [4 + 4] photocycloadditions of conjugated singlet excited dienes must occur in a suprafacial antarafacial and suprafacial suprafacial manner, respectively.690 Since the suprafacial antarafacial reactant approach is geometrically difficult to achieve, [4 + 2] reactions usually proceed stepwise (involving biradical intermediates). [2 + 2] or [4 + 4] photocycloadditions can occur in either a concerted or stepwise fashion. 

The e.s.r. spectra of the radical ions of benzo[h]biphenylene have been reported. Bauld and Cessac have discussed the consequences of 1,3-carbon-carbon interactions in the ion radicals of cyclobutane, cyclobutene, and benzocyclobutene. 

The classic example is the butadiene system, which can rearrange photochemi-cally to either cyclobutene or bicyclobutane. The spin pairing diagrams are shown in Figure 13. The stereochemical properties of this reaction were discussed in Section III (see Fig. 8). A related reaction is the addition of two ethylene derivatives to form cyclobutanes. In this system, there are also three possible spin pairing options. 

Simple cyclobutanes do not readily undergo such reactions, but cyclobutenes do. Ben-zocyclobutene derivatives tend to open to give extremely reactive dienes, namely ortho-c]uin(xlimethanes (examples of syntheses see on p. 280, 281, and 297). Benzocyclobutenes and related compounds are obtained by high-temperature elimination reactions of bicyclic benzene derivatives such as 3-isochromanone (C.W. Spangler, 1973, 1976, 1977), or more conveniently in the laboratory, by Diels-Alder reactions (R.P. Thummel, 1974) or by cycliza-tions of silylated acetylenes with 1,5-hexadiynes in the presence of (cyclopentadienyl)dicarbo-nylcobalt (W.G, Aalbersberg, 1975 R.P. Thummel, 1980). 

Cycloaddition of norbornadiene with allene takes place to yield the cyclobutene derivative 10[5], Cyclodimerization of 1,2-cyclononadiene (11) affords a mixture of stereoisomers of the cyclobutane derivatives 12[6,7],... 

Fluorinated cyclobutanes and cyclobutenes are relatively easy to prepare because of the propensity of many gem-difluoroolefins to thermally cyclodimerize and cycloadd to alkenes and alkynes. Even with dienes, fluoroolefins commonly prefer to form cyclobutane rather than six-membered-ring Diels-Alder adducts. Tetrafluoroethylene, chlorotrifluoroethylene, and l,l-dichloro-2,2-difluoroethyl-ene are especially reactive in this context. Most evidence favors a stepwise diradical or, less often, a dipolar mechanism for [2+2] cycloadditions of fluoroalkenes [S5, (5], although arguments for a symmetry-allowed, concerted [2j-t-2J process persist [87], The scope, characteristic features, and mechanistic studies of fluoroolefin... 

The types of cycloadditions discovered for enamines range through a regular sequence starting with divalent addition to form a cyclopropane ring, followed by 1,2 addition (i) of an alkene or an alkyne to form a cyclo-cyclobutane or a cyclobutene, then 1,3-dipolar addition with the enamine the dipolarophile 4), and finally a Diels-Alder type of reaction (5) with the enamine the dienophile. 

The NMR spectra of cis and trans-diphenylbenzo-cyclobutene are nearly the same except for the chemical shifts of the hydrogens on the four-membered ring. The hydrogens in one compound produce a singlet at 6 4.44, while those in the other compound produce a singlet at 6 5.2. Examine the geometries of cis and trans diphenylbenzo-cyclobutane. Which compound is responsible for which NMR spectrum Explain your reasoning. 

Conjugated chains, 14, 46 Correlation diagrams, 44, 50 Cyclobutadiene, 171 Cyclobutane, 47, 222 orbital ordering, 26 through-space interactions, 26 Walsh orbitals, 27 Cyclobutene, 200 Cyclohexane, 278 Cyclohexene (half-boat), 274 Cyclopen tadiene, 225 Cvclopen tadienone, 269 Cyclopentadienyl anion, 237 Cyclopentane, 254 Cyclopen ten e, 241 Cyclopropane, 41, 47, 153 construction of orbitals, 19, 22 Walsh orbitals, 22, 36, 37 Cyclopropanone, 48, 197 bond lengths, 38 Cyclopropen e, 49, 132 reactivity, 40... 

Crotonaldehyde, hydrogenation of, 43-48 Cubane, isomerization of, 148 Cyclic dienes, metathesis of, 135 Cyclic polyenes, metathesis of, 135 Cycloalkenes, metathesis of, 134-136 kinetic model, 164 ring-opening polymerization, 143 stereoselectivity, 158-160 transalkylation, 142-144 transalkylidenation, 142-144 Cyclobutane configuration, 147 geometry of, 145, 146 Cyclobutene, metathesis of, 135 1,5,9-Cyclododecatriene, metathesis of, 135... 

Oxidative cleavage of l,2-bis(trimethylsilyloxy)cyclobutene with Br2, to give cyclobutane-l,2-dione. 

Carbocation-carbanion zwitterionic intermediates were proposed for the thermal cleavage of several cyclic compounds. In most of these reactions the ionically dissociating bond belongs to one of four strained ring systems, i.e. cyclopropane (13), cyclobutane (14), cyclobutene (15) or norbornadiene (16). The mechanism is distinguished from the formation of a diradical intermediate through homolysis in terms of solvent and substituent effects... 

Dispiro[2.2.2.0]octane 22 was obtained in good yield along with a fragmentation product 23 by treating 1,1,2,2-tetrakis-bromomethyl-cyclobutane 21 with zinc dust in aqueous ethanol4g). The cyclobutene analog of 22 has also been reported 4h). 

Maleic anhydride adds to ethylene or to acetylene to afford the corresponding cyclobutanes (or cyclobutenes) (4.70)482a,b). 

The chemistry of cyclobutanes was remarkably exploited by Oppolzer42, who reported that the aldol condensation of the acetal (122) with l,2-Ws(trimethylsiloxy)-cyclobutene was catalyzed by BF3 Et20 to afford the eyclobutanone (123), which was refluxed with pTsOH in benzene to give 1,3-cyclopentanedione (124). On treatment with acetylchloride in pyridine at 0 °C, the compound (124) was transformed to the enol acetate (125) Irradiation of (125) gave the intramolecular photoadduct (126). Then on successive treatments with MeMgl, KOH and MsCl in... 

There are ample evidences which suggest that a cyclic periodate ester is the intermediate 148). Cyclobutane-1,2-diols can also be cleaved oxidatively and this aspect has been used in organic synthesis. Thus, photocycloaddition of l,2-bis(tri-methylsiloxy)cyclobutene to (—)piperitone (381) gave the photoadduct (438). Desilyla-... 

Cyclobutane-1,2-dione, bromine used in formation of, 4 303 Cyclobutane derivatives, 75 489 Cyclobutene adducts, 75 489... 

The calculations above did not give satisfactory results concerning the structure of even the parent hydrocarbons (cyclobutane and bicydo[2.2.0] hexane), but highly strained cyclobutene, methylene-cyclobutene, Dewar benzene, and so on, were shown to be handled well by MM2 (83a). 

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