Gem-dihalocyclopropanes

CHAIN ELONGATION OF ALKENES via gem-DIHALOCYCLOPROPANES l,l-DIPHENYL-2-BROMO-3-ACETOXY-l-PROPENE... 

Bicyclic gem-dihalocyclopropanes lead predominantly to the corresponding endo-monohalocyclopropanes, which are formed by reduction of the exo-halogen, whose approach to the cathode is less hindered [356, 357]. 

Cyclopropyl iodides like 70 and bromides are good substrates for the exchange reaction (equation 44)77,82,83 reaction is stereoselective and sufficiently fast at low temperatures, thus allowing the preparation of functionalized compounds. If a coordinating group like an ester is present in a. gem-dihalocyclopropane like 71, the cA-halogen substituent is exchanged selectively in ether (equation 45). ... 

The elimination reactions involving dehalogenation, dehydrohalogenation, and dehydration are often laborious compared to the more recent techniques involving dehalogenation of gem-dihalocyclopropanes [10a, b]. However, the availability of the starting materials is the deciding factor. 

Gem-Dihalocydopropanes belong to the most readily available cyclopropane derivatives known today. They have been shown to be extremely valuable starting materials for the preparation of cyclopropanes and cyclopropenes, they may be converted to bicyclobutane derivatives and spiropentanes, can lead to allenes and the higher cumulenes, cyclopentenes and cyclopentadienes, and many other classes of compounds, both hydrocarbon systems and derivatives with valuable functional groups. The article summarizes the preparative developments in the area of gem-dihalocyclopropane chemistry during the last decade. 

When the gem-dihalocyclopropanes are reduced by dissolving alkali metals in alcoholic solvents or in liquid ammonia the strained ring is retained. 

When subjected to strong bases, gem-dihalocyclopropanes undergo dehydro-halogenations, and cyclopropenes are formed. These are generally unstable under the reaction conditions and participate in further transformations. The most common of these processes is the isomerization of the newly formed double bond from the endo- to the exo-orientation, followed by a second dehydrohalogenation step. The methylenecyclopropenes thus generated are still not stable, and subsequently tend to rearrange to less strained systems. 

Phase transfer-catalyzed reactions have recently been employed to dehydro-halogenate gem-dihalocyclopropanes [156, 157]. Thus, 1-methylene-2-vinylcyclo-propane has been prepared from l,l-dichloro-2-ethyl-3-methylcyclopropane in 60 % yield. Under the reactipn conditions (solid KOH, DMSO in the presence of dibenzo-18-crown-6, 100-130 °C) further transformations may take place, however. For example, monoalkylated cyclopropanes have been converted to mixtures of acyclic enynes and conjugated trienes. And 7,7-dichloronorcarane is converted to toluene under these conditions. 

Although the gem-dihalocyclopropanes are fairly stable compounds, they can participate — as has been shown in the above sections — in quite a number of chemical transformations. Several reactions between dihalocarbenes and alkenes have been described in which no dihalocyclopropane formation could be observed that these intermediates might have been produced was only inferred from the type of products finally isolated. A typical process of this type is the e/ufo-addition of dihalocarbenes to norbomene and norbomadiene as discussed above. Comparable rearrangements have been observed, when dichlorocarbene additions either lead to aromatic products or when they cycloadd to rather inert aromatic systems. In the latter case a ring-enlargement takes place. A reaction related to the concerted opening of two cyclopropane rings in a bicyclopropyl system as discussed above takes place when dichlorocarbene is added to spiro[2.4]hepta-4,6-diene [227]. 

Although many recent improvements in the preparation of the Simmons Smith reagent might be helpful23 24, the authors of this chapter would recommend one to consider an alternative two-step cyclopropanation procedure, which includes cycloaddition of dichloro- or dibromocarbene to methylenecycloalkane25 followed by reductive dehalo-genation (equation l)26. The first reaction is usually carried under phase transfer conditions and presents a very simple and efficient procedure. Reduction of gem-dihalocyclopropanes with lithium in tert-butanol or with sodium in liquid ammonia usually proceeds without complications and with high yield. 

It has been reported that gem-dihalocyclopropanes, usually undergo electrochemical or metallic reduction with predominant retention of configuration as shown in Scheme 34. 124,126) The stereoselectivity of these reactions may result from the intermediacy of a cyclopropyl carbanion which is unusually stable to racemization compared to other alkyl carbanions. 125>... 

P Volume 1,1991,283 pp. 109.50/ 69.50 ISBN 1-55938-180-9 CONTENTS Introduction to the Series An Editor s Foreword, Albert Padwa. Preface, Brian Halton. Strain in Organic Chemistry A Perspective, Brian Halton. Gem-Dihalocyclopropanes in Chemical Synthesis, Martin G. Banwell and Monica E. Re-um. 1-Halo- and 1, 2-Dihalocyclopropenes Useful Synthetic Intermediates, Mark S. Baird. Cyciization and Cycloaddition Reactions of Cyclopropenes, Albert Padwa and Glen E. Fryx-ell. New Synthetic Pathways From Cyclobutanones, Edward Lee-Ruff. Cyclic Alkynes, Enynes and Dienynes A Synthetic Challenge, Herbert Meier. Index. 

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