Bicyclobutane reaction

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. 

A special case is the reaction of the tosylhydrazone 7 of cyclopropane carbalde-hyde. It conveniently gives access to bicyclobutane" 8 ... 

In the case of an intramolecular Wurtz reaction less side-reactions are observed this variant is especially useful for the construction of strained carbon skeletons. For example bicyclobutane 5 has been prepared from l-bromo-3-chlorocyclobutane 4 in a yield of > 90% ... 

In the reaction with epoxides, y-hydroxysulfones are obtained278-280. For example, Kondo and coworkers279 synthesized various (5-lactols 226 by treating sulfone acetals 225 with terminal epoxides as shown below. Dilithiated phenylsulfonylmethylene reacted with haloepoxide and afforded 3-(phenylsulfonyl)cycloalkanols281. Treatment of y, 5-epoxysulfones 227 and 229 with n-butyllithium resulted in cyclization to form cyclopropane derivatives 228 and bicyclobutane 230, respectively282. 

The disappearance of the spectra of the biradicals was attributed to ring closure to form the corresponding bicyclobutanes. H-abstraction from the surrounding matrices was excluded because (a) rates of decay were much too fast below 65 K compared with known radical H-abstraction reactions, (b) no radical signals were observed to grow in the EPR spectra, and (c) no rate differences were observed in deuterated compared with protio matrices. NMR and GC analyses of the EPR samples showed... 

The highly hindered disilene 2 did not react with white phosphorus, even under forcing conditions. With disilene 3, which is more hindered than 1 but less so than 2, the reaction with P4 was more complicated. It proceeded slowly, producing small amounts of both stereoisomers of the bicyclobutane compounds 70 and 70. The major product, however, was a more complex compound containing four phosphorus and four silicon atoms, also obtained as a mixture of two stereoisomers. Two-dimensional 31P NMR spectroscopy established the probable structures to be 71.98... 

Compounds 71 are precursors to the bicyclobutanes, as shown in Scheme 18 heating of the reaction mixture led to their disappearance, with formation of 70 and 70. At the same time rearrangement of 70 to the exo-exo isomer 70 took place. This appears to be the only reaction of disilenes known to occur with inversion of configuration at silicon. 

The reaction wtih As4, generated by vaporization of metallic arsenic into cold toluene, has so far been reported only for l.101 Two principal products (79%) were formed in 1 3 ratio the bicyclobutane 72 and the remarkable compound 73 (Scheme 19). The structure of the latter was established by X-ray crystallography. It appears to be an intermediate on the reaction pathway, since long heating at 95°C converted it completely to 72. 

Transition-metal catalyzed decomposition of alkyl diazoacetates in the presence of acetylenes offers direct access to cyclopropene carboxylates 224 in some cases, the bicyclobutane derivatives 225 were isolated as minor by-products. It seems justified to state that the traditional copper catalysts have been superseded meanwhile by Rh2(OAc)4, because of higher yields and milder reaction conditions217,218) (Table 17). [(n3-C3H5)PdCl]2 has been shown to promote cyclopropenation of 2-butyne with ethyl diazoacetate under very mild conditions, too 2l9), but obviously, this variant did not achieve general usage. Moreover, Rh2(OAc)4 proved to be the much more efficient catalyst in this special case (see Table 17). 

The recent syntheses of this compound have made it available for thermal studies (Wiberg and Lampman, 1963 Frey and Stevens, 1964 Srinivasan, 1963). Above 190° C, bicyclobutane isomerizes to butadiene. The reaction is predominantly homogeneous but there is some evidence for a small heterogeneous component of the reaction. The results of Srinivasan et al. (1965) yield the Arrhenius equation... 

Many reducing agents are capable of severing a carbon-halogen bond. Cathodic cleavage provides perhaps the most versatile method, and has been put to excellent use. The electrochemical variation of the Wurtz reaction constitutes a powerful method for the construction of a variety of rings, particularly strained systems. Dramatic examples are provided by the assembly of bicyclobutane (308) [89], bicyclohexene (310) [90-92], [2.2.2]propellane (312) [93], spiropentane (316) [94], j -lactams 318 [95], and a variety of small-ring heterocycles (320) [96,97]. 

An important mechanistic question is the structure of the C3H5N product generated in the C2H4 elimination reaction. One possibility would be to simply lose C2H4, which would retain the aza-bicyclobutane skeletal structure. This is rather unlikely, since this simple aza-bicyclobutane molecule is substantially less stable [25] than the ethyl derivative studied. 

Dicarboxylic acids give very poor yields of the cyclopropane under Kolbe reaction conditions. ITie cyclobutanedicarboxylic acid 25 affords a bicyclobutane [132], 2,2-DiraethyIglutaric acid gives low yields of a hydrocarbon, which is not... 

Attempts to prepare a bicyclobutane by reductive cydization of cyclobutane-1,3-diones have been unsuccessful [114]. a,o)-Dibenzoylalkanes are reduced at a mercury cathode to cycloalkanediols only when a ring size of 5 or 6 results. These reactions have been achieved using either aqueous ethanolic sodium hydroxide [115] or acetonitrile containing tetramethylammonium tetrafluoroborate [113] as electrolyte. Attempts to extend the process to formation of 7-membered rings lead only to oligomeric materials. 

Reactions of cycloproparenes with (ri -allyl)(Tv" -cyclopentadienyl)palla-dium(III) in the presence of trimethylphosphane affords a bicyclobutane (387) with... 

Although the entire gaseous product is caught in the first trap, this trap tends to plug during the reaction. Therefore, the second trap is used as a safety measure to collect the bicyclobutane as the first trap is thawed to open the system. 

Such a reaction, in which two bonds are broken in a single step, has precedent in the two-bond pluck mechanism proposed for the reactions of bicyclobutanes (and quadricyclanes) with carbenes (Scheme 7.15). " In this reaction the transition state... 

Occasionally the cyclopropylidene to allene isomerization cannot take place for structural reasons. If, for example, the expected allene would be very highly strained, as is the case for certain cyclic allenes, then the reaction is forced to follow an alternative path. A case in point is provided by 1-alkyl-7,7-dibromonorcaranes which undergo a carbene insertion reaction when treated with methyl lithium to provide bicyclobutanes rather than allene derivatives. 

As can be seen, both routes lead to the same product. Nevertheless, in compounds where the conformational flexibility of the bicyclobutane frameworks is restricted, 1,3-addition is found to be favored via the diequatorial mode.12,13 This aspect is illustrated in a reaction in which iodine reacted with tricyclo[4.1.0.02,7]heptane in carbon tetrachloride to give 6,7-diiodobicy-clo[3.1. l]heptane (9) in 55% yield.13 In support of this stereochemistry, the majority of results obtained from the dculeration of tricyclo[4.1.0.02,7]heptane suggested that the attack is from the equatorial position.14 Theoretical studies also support the notion that the equatorial approach of an electrophile to the bridgehead of bicyclo[l.1.0Jbutane is thermodynamically favored.14,15... 

Bicyclobutanes as a structural subunit of complex hydrocarbon frameworks also undergo radical addition reactions with benzenethiol and bromotrichloromethane. In compounds such as 23, where the azo functionality can also be involved in a radical reaction, a half-cage compound 24 was produced in 64% yield.38 The generation of other half-cage compounds from 23 and its analogs are shown in Table 10.38... 

Bicyclobutanes also undergo thermal and photochemical cycloaddition reactions with cyclic azo compounds to form 2,3-diazabicyclo[2.1.1]hexanes 31.45,46 Some typical examples are shown. 

Table 2 summarizes the preparation of bicyclobutanes by elimination reactions.25-29-30... 

Dehalogenation and related reactions have also been employed in the synthesis of many bicyclobutanes. Sodium induced dechlorination of l,3-dichloro-2,2,4,4-letramethylcyclobu-tane (69) gave 2,2,4,4-tetramethylbicyclo[1.1.0]butane (70) in 47% yield,29 while magnesium also converted l-chloro-3-ethoxy-l-phenyl-2,2,4,4-tetramethylcyclobutane (71) into 1-phenyl-2,2,4,4-tetramethylbicyclo[1.1.0]butane (72) in 40% yield.31 Other typical preparations are summarized in Table 3 25-29-31 35... 

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