Homoallylic halides

The last group of reactions uses ring opening of carbonyl or 1-hydroxyalkyl substituted cyclopropanes, which operate as a -synthons. d -Synthons, e.g. hydroxide or halides, yield 1,4-disubstituted products (E. Wenkert, 1970 A). (1-Hydroxyalkyl)- and (1-haloalkyl)-cyclopropanes are rearranged to homoallylic halides, e.g. in Julia s method of terpene synthesis (M. Julia, 1961, 1974 S.F. Brady, I968 J.P. McCormick, 1975). 

Most preparations of the intermediate racemic 3-allyl(dicarhonyl)cyclopentadieny molybdenum complexes7 2 start from sodium or potassium tricarbonyl(cyclopentadienyl)molybdate8 9 10 (4) for a simple preparation see ref 11, p 493. The allylation of 4 hy homoallylic halides, such as 4-hromo-l-butene, is accompanied by rearrangement and decarhonylationI2. (Z)-if-2-butenyl(dicarbonyl)cyclopentadienylmolybdenum (5), like other comparable complexes, exists as a mixture of endo/exo- conformers, which interconvert rapidly at room temperature12. 

Most remarkably, the homoallylic halides 214 not only yield the thermodynamically unfavored ris-cyclopropanes 215 preferentially (see Sect. 2.2.3), but also give rise to enantioselective formation of the (1/ ) configuration, in contrast to the cyclopropanation of 1,3-butadienes with the same catalysts (see Table 15). Only in the case of olefin 214 (X = CF3, Y = Cl), may the (1 S)-trans isomer be obtained enantioselecti-vely, depending on the catalyst (Table 16, entries 8-11). In these few cases, optical induction occurs at C(3) of the cyclopropane rather than at C(l). 

Ion-derived products are also formed in the irradiation of bicyclic benzyl chlorides150,151. Compound 23, which is both a benzylic and a homoallylic halide, shows a comparable direct versus sensitized irradiation dichotomy as simple, less activated benzylic halides do (equation 32)150. Also, in the direct isomerization, substantial epimerization occurs, which is not observed upon sensitization. This means that the R+CT ion pairs produced in the singlet and triplet reaction are not identical and recombine or are captured before relaxing to identical species. Alternatively, the different behaviour may well be a manifestation of the intermediacy of an exciplex as product-directing intermediate in the sensitized reaction (cf equation 31). 

This method is recommended for conversion of cyclopropylcarbinyl alcohols into the corresponding halides without formation of homoallylic halides via ring cleavage.-Example ... 

These reactions, coupl with the alkylations of sulfides and selenides, allow the homologation of primary alkyl halides - - - and the transformation of allyl halides to homoallyl halides. A related process, employing 2-methylthio- and 2-allylthio-thiazoline, allows the iodomeAylation (ICH2—) and the iodopropenylation (ICH2CH=CH—) of alkyl halides (Scheme 54).i3iii,i32.i7i... 

Silver oxide converts homoallylic halides into cyclopropane derivatives, for example ... 

A striking example for the preferred formation of the thermodynamically less stable cyclopropane is furnished by the homoallylic halides 37, which are cyclopropanated with high ci>selectivity in the presence of copper chelate 38 . The cyclopropane can easily be converted into cw-permethfic acid. In contrast, the direct synthesis of permethric esters by cyclopropanation of 1,1-dichloro-4-methy 1-1,3-pentadiene using the same catalyst produces the irazw-permethric ester (tra s-39) preferentially in a similar fashion, mainly trons-chrysanthemic ester (rra 5-40) was obtained when starting with 2,5-dimethyl-2,4-hexadiene... 

Synthesis of homoallyl halides (usually E) by acid catalyzed rearrangement of cyclopropyl carbinols (see 1st edition). 

The facts shown above indicate that /3,y-unsaturated halides and other electrophiles are some of the most reactive classes of compounds.t Although generally less reactive, a,/3-unsaturated derivatives are also generally reactive and practically useM. i On the other hand, simple alkyl halides lacking a proximal unsaturation are generally insuffi-ciendy reactive. Relatively little is known about the effects of more remote Tr-bonds and of proximal n-electron donors, but the results observed for ( )-2-methyl-l,4-diiodo-l-butene shown in Scheme 2 of Sect. 1.2 clearly indicate that homoallyl halides must be far less reactive than alkenyl hahdes. These results collectively point to the probable significance of prior 7r-complexation as a binding process and probable subsequent intramolecular interaction of Pd with C—bonds. 

Examples of three different oxidant-promoted iV-allylic hydrazone rearrangements developed by the Thomson group are illustrated in Scheme 14.19a. These transformations convert N-allylic hydrazones to homoallylic halides in the presence of either CUCI2 or NBS and to... 

During a total synthesis of marasmic acid, it has been found that the phosphon-ate (80) condenses with aldehydes to give the -substituted butenolides (81). This will probably be the subject of a full paper in the near future. The coupling of zinc alkyls derived from homoallylic halides with vinyl halides in the presence of palladium(O) catalysts has been used to prepare butenolides (82) from /8-bromobutenolide. This useful reaction can also be used with /8-bromopropenoates to give dienoates in which the stereochemical integrity of... 

Details of a further variant of the Julia method for conversion of cyclopropyl-carbinols to homoallylic halides have appearedthe carbinols are treated directly with magnesium (or beryllium) halides [equation (20)], thus avoiding the use of strong mineral acids and the need to prepare cyclopropylcarbinyl halides. This procedure is closely related to another reported last year (4,159). 

Vinylic halides, allylic halides, homoallylic halides and other olefinic halides. 

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