Arylcyclopropanes, from

The preparation of arylcyclopropanes from arylhalomethanes and aryidihalomethanes is usually accompanied by formation of various byproducts. Stilbene formation is generally observed and compounds resulting from C-H insertion are often formed. In some cases a formal byproduct may become predominant and even be formed in good yield. This is the case when benzal chloride is treated with methyllithium. Thus, treatment of a mixture of 2,3-dimethylbut-... 

Table 1. Arylcyclopropanes from Alkenes and Carbenes, Generated from Arylhalomethane or Aryldihalomethane with Organolithium Compounds... 

Arylcyclopropanes from Benzal Bromides, Alkenes and Alkyllithium General Procedure ... 

Table 3. Arylcyclopropanes from Direct and Benzophenone-Sensitized Irradiation of Aryldiazomethanes in 1,1-Dimethylallene...

Arylcyclopropanes from Aryldiazomethanes and Zinc Halides General Procedure 62... 

Arylcyclopropanes from Phenyldiazomethane and Rhodium(II) Acetate General Procedure 64 6... 

Table 5. Arylcyclopropanes from the Reaction of Alkenes with Arylcarbenes, Generated by Metal-Catalyzed Decomposition of Aryldiazomethane...

A study of the photo-sensitized ring-opening reactions of the radical cations (76) of arylcyclopropanes (75) with methanol, water, and cyanide nucleophiles suggests a three-electron 5k2 mechanism (Scheme 11).185 The isolated products are methyl propyl ethers, derived from nucleophilic attack of methanol on the radical cation (76). They were detected by UV-VIS spectroscopy and shown to react with nucleophiles by transient kinetic methods. The benzyl radical (77) reacts with the DCB radical anion to afford monoaromatic ether (78) by oxidation and protonation or the disubstituted ether (79) by addition of DCB. Regio- and stereo-selectivity of the substitution were complete regiochemistry and rate constant were profoundly effected by the electronic nature of the aryl substituents.186 Elsewhere, a combined ab initio and CIDNP study... 

Arylcyclopropanes and their heterocyclic analogues are liable to electron transfer induced fragmentation of a carbon-carbon bond that in some cases leads to synthetically useful products. Thus, 1,2-diarylcyclopropanes [240-243] as well as 2,3-diaryloxirans [244-246] and -aziridines (in the last case, also 2-monophenyl derivatives) [247,248] are cleaved upon photoinduced electron transfer sensitization. The final result, after back electron transfer, is trans-cis isomerization of the ring. In the presence of a suitable trap, however, a cycloaddition reaction takes place, involving either the radical cation or the ylide. Thus, dioxoles, ozonides or azodioxoles, respectively, are formed in the presence of oxygen and oxazolidines have been obtained from cyclopropanes in the presence of nitrogen oxide (Sch. 23). 

When ct-selanylalkyllithium compound 123 is reacted with double bonds of type 124, a methaneselenolate displacement generates functionalized arylcyclopropane derivatives 125.210 Depending on the substituents R1 and R2, the ratio of 125a 125b can vary from 97 3 to 0 100 as shown in Table 4 (Scheme 31). 

From these results it appears that the substitution regiochemistry is not determined by steric effects but rather by the ability of the alkyl group at Cp to stabilize a positive charge in the substitution transition state, indicating that three-electron Sn2 reactions of arylcyclopropane radical cations are dominated by electronic rather than steric effects (see later). 

Hydroxyalkyl selenides derived from a-selenobenzyllithiums and epoxides have proved to be valuable precursors of arylcyclopropanes. The reaction proceeds through the corresponding 7-tosyloxy-benzyllithiums. Although the synthesis of the 7-hydroxyalkyl selenides is not stereoselective, the subsequent steps occur with high stereoselectivity, and have led to the conclusion that the benzyllithiums possess the opposite stereochemistry to that of the selenides and that this stereochemistry is retained in the next step (Scheme 18). ... 

Arylcyclopropanes are obtained from coupling using cyclopropylzinc halides. Carbonylative coupttng. Symmetrical ketones are formed on treatment of organozinc reagents with (PhjPl Pd under CO. In the presence of aryl iodides the reaction with RZnI gives RCOAr. Methacrylic acid derivatives are formed by a Pd-catalyzed reaction of allene, carbon monoxide, and nucleophiles. ... 

With respect to the carbene, arylcyclopropane formation occurs with rather low stereospecificity. In most cases the ewt/o-isomer predominates, which is in accordance with the general observation made when arylmethylenes from other sources are added to alkenes. Thus, photolysis of phenyldiazomethane dissolved in cyclohexene gave 7-phenylbicyclo[4.1.0]heptane (2) in 34% yield with a endojexo ratio of 1.1. Preference for the e (/o-isomer is also observed when monoarylcarbenes are reacted with cyclohepta-l,3,5-triene, but when 1-naphthylcar-bene, 2-naphthylcarbene and 9-anthrylcarbene are reacted with cycloocta-l,3,5,7-tetraene the exo-isomer predominates. ... 

Arylcyclopropanes can be prepared from a variety of alkenes enamines react sluggishly, electron-rich alkenes are, in general, the most reactive. Thus, phenylcarbene, generated by dirhodium tetraacetate catalyzed decomposition of phenyldiazomethane, gave 2-butyl-1-phenyl-cyclopropane (2) in 6% yield from hex-l-ene, but l-butoxy-2-phenylcyclopropane (3) was obtained in 92% yield from butyl vinyl ether. ... 

The yield of the arylcyclopropanes is very sensitive to the catalyst employed. Very low yields (< 10%) were observed when lithium salts were employed. Magnesium, cobalt and copper salts gave better results, but the best results were achieved with zinc halides and rho-dium(II) acetate. This was apparent when 7-phenylbicyclo[4.1. OJheptane (4) was prepared from cyclohexene and phenyldiazomethane. ... 

Arylcarbenes are formed from benzaldehyde and some of its derivatives if the Clemmensen reduction is performed using boron trifluoride or another Lewis acid instead of hydrochloric acid. If the reaction is carried out in the presence of an alkene, which is often used as solvent, addition to the C-C double bond takes place and cyclopropane formation results in low to fair yields. The arylcyclopropanes are always formed as mixtures of stereoisomers and, in all cases but two, the endo-isomcr is significantly predominant. Thus, when a mixture of 4-methoxybenzaldehyde, boron trifluoride-diethyl ether complex and a considerable excess of cyclohexene was allowed to react with amalgamated zinc, 7-(4-methoxyphenyl)bicy-clo[4.1.0]heptane (1) was obtained in 60% yield. 

The conversion of an arylvinylmethane reactant 8 to an arylcyclopropane 9 proceeds from both the excited singlet and triplet states. It is convenient to use sensitizers in these reactions as the arylcyclopropanes are often photostable under such conditions. Examples are summarized in Tables 2 and 3. 

Various arylcyclopropanes have been synthesized by heating mixtures of cyclopropyllithium, obtained from bromocyclopropane and lithium, and a halobenzene or a nitrogen-containing aromatic heterocycle. The cleanest reactions and the best yields are afforded when heterocyclic reactants are employed. Thus, reflux of an ethereal solution of cyclopropyllithium and fluorobenzene gave mainly phenylcyclopropane (1) and 2-cyclopropylbiphenyl (2), which were isolated in 37 and 25% yield, respectively. If the aromatic compound was pyridine, the analogous substitution product, 2-cyclopropylpyridine (3), was obtained in 58% yield. ... 

Formation of arylcyclopropanes also occurs when l-chlorotricyclo[3.1.0.0 ]hexane and 1-chlorotetracyclo[4.1.0.0 . 0 jheptane were reacted with phenyllithium. Both reactions involve formation of a strained cyclopropene by phenyllithium-promoted elimination and subsequent addition of the same organometallic species to the strained alkene. When the intermediate alkene is symmetric, such as tricyclo[3.1.0.0 ]hex-l(6)-ene (7), generated from 1-chlorotri-cyclo[3.1.0.0 ]hexane, the nucleophilic addition of phenyllithium leads to one product only, l-phenyltricyclo[3.1.0.0 ]hexane (8). However, when 4-chlorotetracyclo[4.1.0.0 ". 0 ]hept-ane was reacted with phenyllithium, the intermediate formed, tetracyclo[4.1.0.0 . 0 ]hept-3-ene, was unsymmetric and gave a 1 1 mixture of cis- and ra s-3-phenyltetracyclo-[4.1.0.0 . 0 ]heptane (9) in 90% yield when nucleophilic addition of phenyllithium to the double bond occurred.The cM-product can be regarded as the formation of one C-H and one C-C bond (Section 5.2.1.3.5). 

Cation radicals of arylcyclopropanes, produced by photoinduced one-electron transfer to an electron acceptor, undergo ring-opening reactions accompanied by regioselective polar additions. 1,4-Dicyanobenzene (5) sensitized photolysis of l,l-bis(4-tolyl)cyclopropane (4) in methanol/acetonitrile resulted exclusively in anti-Markovnikov addition of methanol to the cyclopropane. It is assumed that excitation of 5 followed by electron transfer from 4 gives a dicyanobenzene radical anion 5(- ) - cyclopropane radical cation 4(4-) pair. Attack by methanol on the radical cation 4(4-) followed by proton loss affords benzylic radical 7 that is reduced by 5( —) to give benzylic anion 8 and quenched by methanol to afford product... 

The requirement of extensive allyl anion stabilization as a prerequisite for cyclopropyl anion opening has only been overcome in the treatment of arylcyclopropanes such as 16 and 20 with Lochmann s base (BuLi//-BuOK) in refluxing hexane. Under these conditions poly-metalated 17 and dimetalated species 21, respectively, were formed. The ring opening results from the need for the charge in the benzylic cyclopropyl position to be further removed from... 

PET is the key feature of many other photo-oxidations involving aromatic substrates. It has been shown, for example, that pyrene and anthracene which are covalently attached to silica, gold or indium-doped tin oxide (ITO), undergo a photo-oxidation forming dihydroxy/dione derivatives. The reaction involves 02, formed by ET between excited pyrene, or anthracene, and O2, and it is suggested that the implications of such a photodegradation need to be considered when polycyclic aromatic hydrocarbons (PAHs) are used as spectroscopic probes in surface adlayers. The redox photosensitized amination of 1,2-benzo-1,3-cycloalkadienes, arylcyclopropanes, and quadricyclane with ammonia and primary amines, using 1,2,4-triphenylbenzene (1,2,4-TPB) or 2,2 -methylenedioxy-1,1 -binaphthalene in the presence of m- or p-dicyano-benzene (DCB), has been described (Scheme 51). The process involves the formation of the radical cation of 1,2,4-TPB, for example, by PET to the DCB, followed by hole transfer from the radical cation to the substrate, the latter... 

Ammonia or amines were likewise used as nucleophiles in place of alcohols [52]. Here, PET between an arene S (e.g. triphenylbenzenes) and 1,4-dicyanoben-zene (DCB) led to the arene radical cation S and the radical anion DCB. Secondary ET from a donor (D) to S produced the radical cation of the former (D ) that added ammonia. After deprotonation, coupling of the resulting neutral radical with DCB and cyanide loss led to the corresponding NOCAS product Suitable donors were arylcyclopropanes, quadricyclane and dienes that gave 4-(l-aryl-... 

Table 7.3 y-Sultines from arylcyclopropanes and sulfur dioxide ... 

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