Cyclopropyl halide

Fonnation of allylic products is characteristic of solvolytic reactions of other cyclopropyl halides and sulfonates. Similarly, diazotization of cyclopropylamine in aqueous solution gives allyl alcohol. The ring opening of a cyclopropyl cation is an electrocyclic process of the 4 + 2 type, where n equals zero. It should therefore be a disrotatory process. There is another facet to the stereochemistry in substituted cyclopropyl systems. Note that for a cri-2,3-dimethylcyclopropyl cation, for example, two different disrotatory modes are possible, leading to conformationally distinct allyl cations ... 

It seems likely that aryl, vinyl, and cyclopropyl halides react by an alternative mechanism, since the corresponding radicals are less stable than alkyl radicals. It has been suggested that these halides may react through a dianion.13... 

The key cyclization in Step B-2 was followed by a sequence of steps that effected a ring expansion via a carbene addition and cyclopropyl halide solvolysis. The products of Steps E and F are interesting in that the tricyclic structures are largely converted to tetracyclic derivatives by intramolecular aldol reactions. The extraneous bond was broken in Step G. First a diol was formed by NaBH4 reduction and this was converted via the lithium alkoxide to a monomesylate. The resulting (3-hydroxy mesylate is capable of a concerted fragmentation, which occurred on treatment with potassium f-butoxide. 

Several studies of the electrochemical reduction of cyclopropyl halides have been reported in the literature 20,26,27,55) t it appears that reduction occurs without complete loss of stereochemical integrity. The predominent stereochemical pattern involves reduction with overall (partial) retention of configuration. The following examples are representative ... 

The partial loss of configuration often observed during reduction of cyclopropyl halides may actually occur via the corresponding cyclopropyl radicals, which lose configuration rapidly °>. In that event, their behavior would resemble that of vinyl halides, as exemplified by the 3-iodo-3-hexenes 16>. Occasional cases of partial inversion could be associated with shielding of the cyclopropyl carbanion by the electrode surface, with concomitant protonation on the other face of the carbanion S8>. 

As was indicated previously, several other parameters can affect the stereochemistry of reduction of cyclopropyl halides 20>. For example the optical purity of 59 produced from the electrochemical reduction of 58 in acetonitrile... 

Reduction of chiral cyclopropyl halides using Sml2 has been investigated... 

We have previously seen (p. 345) that this is the reason nucleophilic substitutions are not feasible at a cyclopropyl substrate. The reaction is often used to convert cyclopropyl halides and tosylates to allylic products, especially for the purpose of ring expansion, an example being1211... 

The cycloalkyl halides, except for cyclopropyl halides, have physical and chemical properties that are similar to those of the open-chain secondary halides and can be prepared by the same types of reactions (Table 14-5). All the cycloalkyl halides undergo SN2 reactions rather slowly and, with nucleophiles that are reasonably basic ( OH, OC2Hs, C=N, etc.), E2 reactions... 

The cyclopropyl halides are exceptional in that their behavior is much more like alkenyl halides than like secondary alkyl halides. Thus cyclopropyl chloride undergoes SN1 and SN2 reactions much less rapidly than isopropyl or cyclohexyl chlorides. A relationship between the reactivity of cyclopropyl chloride and chloroethene is not surprising in view of the general similarity between cyclopropane rings and double bonds (Section 12-5). This similarity extends to cyclopropylmethyl derivatives as well. Cyclopropylmethyl chloride is reactive in both SN-1 and SN2 reactions in much the same way as 3-chloropropene ... 

The equilibria between the allyl and the cyclopropyl cations and the corresponding equilibria of the anions, are not seen as such, but they are found in disguise. Thus the cationic reaction is seen when cyclopropyl halides are heated or treated with silver ions. The cyclopropyl cation itself is not an intermediate, because the ring opening (4.23, arrows) takes place at... 

The unsymmetrical halonium ions such as 165-Br are found in equilibrium with open-chain [3-bromocarbenium ion. Many of the above-mentioned halonium ions have been prepared by the protonation of the appropriate cyclopropyl halides in superacids.390... 

Even cyclopropyl halides are reducible by LiAlH4. Thus treatment of 7,7-dibromo-bicyclor4.1.01heptane (3) with LiAlH4 in ether for 18 hr. gives the anti- and syn-monobromo derivatives (4) and (5) together with the parent hydrocarbon (6) in the... 

A classical method using Na- or Li-liquid ammonia (Birch reduction conditions) is effective for reductive dehalogenations of aryl and vinylic halides, but it is not always successfully applied to alkyl halides, although cyclopropyl halides and bridgehead halogens are exceptions.Under such conditions, the reactions are often accompanied by side reactions, such as elimination, the Wurtz coupling reaction, cyclization and reduction of carbonyl compounds. An example, a synthesis of pentaprismane (1), is shown in Scheme 4. ... 

Mercury-photosensitised decompositions have been reported for fluorinated methanes , fluorinated ethanes , fluorinated ethylenes , perfluorocyclobu-tene and perfluorobuta-1,3-diene , chlorofluoromethane , dichloromethane (ref. 939), and octafluorobut-2-ene . Acetone has been used for sensitising the decomposition of 1,4-dichlorobutane , chlorobutenes , and allylic halides (the last to cyclopropyl halides) other studies have involved the use of hexafluoroacetone and benzene as sensitisers. 

B. Electron Transfer to a Bonds of Cyclopropyl Halides 1. Surface reactions. ... 

The reduction of tertiary cyclopropyl halides with alkali metal naphthalenides leads to a similar situation. After treatment of a 78 22 and a 25 75 mixture, respectively, of r-1-bromo-l-methyl-c-2-methyl-r-2-phenylcyclopropane (110) and its isomer (111) with LiN in THE at 20°C, protonation with methanol led to identical 45 55 mixtures of r-l-phenyl-l,c-2-dimethyl- and r-1-phenyl-l,r-2-dimethylcyclopropanes (112 and 113). 

Thus, the results in solution with the cyclopropyl halides mentioned lead to the following conclusions ... 

SCHEME 13. Alkali metal naphthalenide reductions of cyclopropyl halides... 

There is no experimental indication that either an ion paired cyclopropyl halide... 

Table 18 summarizes the net retentions of configuration of the cyclopropane 49 observed in the reactions of the optically active cyclopropyl halides 5(1-52 with alkali metal naphthalenides MN as the halide X, the gegenion M and the concentrations are varied. The effect of inverse versus normal addition is demonstrated in Table 19. The influence of the solvent is given in Table 20. Dicyclohexyl-18-crown-6 also affects the. amount of retention of cyclopropane 49 (Table 21).

What is the reason for the alternative pathways in the reactions of the cyclopropyl halides 106,107,110, 111, 114 Hal, 115-Haland 50-1,51-Br, 52-CI, 118-Hal with alkali metal naphthalenides ... 

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