Cyclopropa naphthalenes synthesis

Reductive 1,2-eIimination of chlorine and bromine from adducts of l-bromo-2-chlorocyclo-propene (see Section 5.2.2.1.2.5.) with oxygen and sulfur hetarenes has served in the synthesis of a number of cycloproparenes. This transformation is effected by low-valent titanium together with lithium aluminum hydride or an organolithium compound. Thus, reaction of the adduct 3 of l-bromo-2-chlorocyclopropene and 1,3-diphenylisobenzofuran with tita-nium(III) chloride and lithium aluminum hydride overnight in tetrahydrofuran led to elimination of both halogens together with extrusion of the oxygen and formation of 2,7-diphenyl-l/f-cyclopropa[ ]naphthalene (4) in 72% yield. 

Aromatization of dihalocarbene adducts to 1,4-cyclohexadiene or synthetic equivalents is the method of choice for the synthesis of the parent benzocyclo-propene (1). ° The mechanism of the aromatization step of the intermediate 7,7-dihalogenobicyclo[4.1.0]hept-2-ene (51) has been shown by labeling experiments with 51 depleted of C at Cl, to proceed via a series of elimination and double bond migration steps via cyclopropene- and alkylidenecyclopropane intermediates 52 to 54 with preservation of the original carbon skeleton. The synthesis of the benzannelated homologue, l//-cyclopropa[b]naphthalene (42), by the same route confirms these findings. Some skeletal rearrangement has, however, been observed in an isolated case. ... 

Alder-Rickert cleavage has not been widely used for cycloproparene synthesis, since the preparation of the precursors is often tedious, except for the simple cases like 7,7-difluorobenzocyclopropene (21). The approach offers, however, decisive advantages in special situations. If the Alder-Rickert cleavage is carried out under flash-vacuum pyrolysis conditions, the products may be isolated under neutral conditions and at low temperature. Thus the synthesis of the highly reactive li/-cyclopropa[a]naphthalene (56) by pyrolysis of 68 has been achieved by this approach. Several other approaches to 56 failed. 

The linear cyclobuta[e]cyclopropa[b]naphthalene (114) was synthesized very early by the carbene addition approach, while the synthesis of the angular isomer failed. The most strained of all isolable cycloproparenes, 1,4-dihydrocyclo-propa[b,g]naphthalene (115), was obtained originally by the same approach from the tetrabromide 116. The synthesis was recently improved. An alternative access to 115 via 117 as precursors is also available. The strain energy of 116 is extremely high, and it explodes upon melting (132 °C). ... 

Synthesis and subsequent thermal decomposition of aryldiazomethanes under standard Bam-ford-Stevens conditions has also been utilized to prepare aryl-substituted cyclopropanes. When benzaldehyde tosylhydrazone is mixed with solid sodium methoxide in naphthalene, phenan-threne, and anthracene and heated above 130°C, phenylcarbene is generated. All the three aromatic hydrocarbons are attacked at the site of highest double-bond character to give, albeit in low yield, norcaradienes. The best results were obtained with naphthalene, which gave 1-phenyl-la,7b-dihydro-l//-cyclopropa[a]naphthalene (7) in 9% yield. ... 

The same technique has been used more recently to prepare higher cycloproparenes whose instability makes their synthesis by other methods dilEcult (see Sections 5.2.2.1.2.5. and 5.2.2.4.). Thus, l//-cyclopropa[fl]naphthalene (7) was made in almost quantitative yield by the pyrolysis of the adducts 6 of butynedinitrile or dimethyl butynedioate with 2,3-benzo-l, 6-methanocyclo-decapentaene. The product could be crystallized from pentane at — 78 C but underwent partial polymerization or decomposition at its melting point of 19-20°C. 

Several extensions of this approach have been attempted, but none was successful. Only a faint odor results from the attempted synthesis of cyclopropa[Z)]naphthalene and none of the expected cyclopropa[a]naphthalene or cyclopropa[/]phenanthrene were generated in the corresponding reactions.- The same difficulties have been experienced with other benzyl derivatives. The reaction of 2-bromobenzyl bromide, 2-bromobenzyl chloride and 2-iodobenzyl bromide with butyllithium gave (partially) reduced bibenzyls or 9,10-dihydroanthracene, but no benzocyclopropene. Similarly, lithiation of 2-bromobcnzylsulfonates afforded only trace amounts (< 5%) of bcnzocyclopropene, while only methoxymethylbenzene was isolated on treatment of l-methoxymethyl-2-trimethylsiIylbenzene with fluoride ions. ... 

The dihalocyclopropane route is the method of choice for the synthesis of benzocyclopropenes and linearly fused cyclopropanaphthalenes, but is unsuccessful for angular cyclopropa[u]naph-thalene. Treatment of l,l-dichloro-la,2,3,7b-tetrahydro-l/f-cyclopropa[a]naphthalene(5) with potassium /ert-butoxide gave a mixture of 1-chloromethylnaphthalene, 1-terr-butoxymethyl-naphthalene and 2-terf-butoxy-l-chloromethylene-l,2,3,4-tetrahydronaphthalene, but none of the expected cyclopropa[n]naphthalene. The failure of the reaction has been attributed to difficulties in the aromatization. Isomerization of the initially produced cyclopropenyl double bond into the cyclohexane ring would require disruption of the aromatic character of the adjacent benzene nngT t-r>> 62 Similarly, attempted aromatization of l-chloro-l,2,3,7b-tetra-hydro-l//-cyclopropa[a]naphthalene (6) with 2,3-dichloro-5,6-dicyanobenzoquinone or via N-bromosuccinimide bromination followed by reaction with base afforded 4,5-benzotropone instead of cyclopropa[fl]naphthalenc. ... 

The synthesis of alkylidcnecyclopropa[()]naphthalencs 11 used disilylated cyclopropanaph-thalenc 9 as the starting material, as silylation of 17/-cyclopropa[(b]naphthalene cannot be stopped at the monosilylation stage. The required l-trimethylsilyl-l//-cyclopropa[/)]naph-thalenyl anion 10 was generated from the bis-silylated cycloproparcne with potassium tert-butoxide and trapped with the appropriate aldehyde or ketone. Yields varied from 10-70%, and were dependent on the thermal stability of the products. 

In the related synthesis of cyclopropa[h]naphthalene (167X product distribution is related to base concentration and the solvent employed. In THF at high base concentrations, (167) predominates, whereas at low base concentrations (168) is the major product. In DMSO (167) and (168) are both formed in low yields, (169) being the major product. Possibly pathways such as A and B above may be competing in this case. 

Full details of the synthesis of cyclopropabenzene from 7,7-dichlorobicyclo[4,l,0]-hept-3-ene and further details of the mechanisms by which cyclopropabenzenes are produced from diaza[4,4] spirenes have appeared. The retrodiene route which was developed by Vogel to provide the first synthesis of cyclopropabenzene has been extended by him to provide cyelopropa[a]naphthalene (161) in high yield. Unlike the cyclopropa[h]-analogue, (161) is thermally labile and suffers decomposition at temperatures above — 20°C the speetroscopic data recorded for the compound... 

---