Benzyl, and Allyl Bromides

The study of these systems provided important information for both mechanistic and synthetic purposes. The reactivity of bromobenzene was evaluated against 1-bromooctane by following the same procedure used for alkyl bromides (THE, 0°C, GEC analysis). It reacts with active zinc ca. 20 times slower than this primary alkyl bromide and, subsequently, 27 times slower 

Benzyl and allyl bromides could not be studied using the normal experimental protocol. The reaction was too fast to adequately monitor by simply sampling at different time intervals. On the other hand, GLC response calibration of benzyl bromide gave inconsistent results [177]. We used a variation of the technique, in which several reactions—one for each point to be plotted— were set up. They were loaded with a previously prepared mixture of bromides/ internal standard. A substoichiometric amount of active zinc, different for each reaction, was then injected. The reactions went to completion within minutes. Direct analysis of the unreacted bromides by NMR provided, if not 

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A copper(O) complex, electro-generated from Cu(acac)2, is able to undergo an oxidative addition with benzyl and allyl bromides. Further reduction leads to the coupling products bibenzyl and 1,5-hexadienes Methyl-3-hexene-l,6-dicarb-oxylate can be prepared from butadiene and CO by electroreduction if di-Fe dicyclopentadienyl tetracarbonyl is used as redox catalyst Electro-generated low-valent tungsten species are able to reductively dimerize benzaldehyde to stilbene according to Eq. 83. The reduction potential was controlled at the third wave of the WClg catalyst (V = -1900 mV/SCE)... 

KOH/RX n-BcuNBr or Aliquat 336 (RX= 1°, 2° alkyl bromides benzyl and allyl bromide Mel 1° alkyl sulfates)... 

The at complex from DIB AH and butyllithium is a selective reducing agent.16 It is used tor the 1,2-reduction of acyclic and cyclic enones. Esters and lactones are reduced at room temperature to alcohols, and at -78 C to alcohols and aldehydes. Acid chlorides are rapidly reduced with excess reagent at -78 C to alcohols, but a mixture of alcohols, aldehydes, and acid chlorides results from use of an equimolar amount of reagent at -78 C. Acid anhydrides are reduced at -78 C to alcohols and carboxylic acids. Carboxylic acids and both primary and secondary amides are inert at room temperature, whereas tertiary amides (as in the present case) are reduced between 0 C and room temperature to aldehydes. The at complex rapidly reduces primary alkyl, benzylic, and allylic bromides, while tertiary alkyl and aryl halides are inert. Epoxides are reduced exclusively to the more highly substituted alcohols. Disulfides lead to thiols, but both sulfoxides and sulfones are inert. Moreover, this at complex from DIBAH and butyllithium is able to reduce ketones selectively in the presence of esters. 

R. J. K. Efficient and selective Stille cross-coupling of benzylic and allylic bromides using bromobis (triphenylphosphine) (N-sucdnimi-de)Pd(II). Tetrahedron Lett. 2004, 45, 461—465. 

Benzylic and allylic bromides may also be converted into aldehydes as shown in equation 43368. 

Problems can arise at the last stage due to difficulties in die isolation of die aldehyde and/or preferential vinyl sulfide formation. Nonetheless, the method has some potential Sulfoxides are prone to thermal elimination, and this has been used by Trost in his mediod, which can also be used for the oxidation of primary amines (Scheme 14). The procedure is limited to benzylic and allylic bromides. 

Direct Reaction of Zn with Alkyl Halides. The direct insertion see Insertion) reaction of Zn metal into alkyl halides - alkyl iodides being the ideal snbstrates - is a nseful reaction to prepare simple or polyfunctional organozinc halide compounds (equation 1). With primary alkyl iodides, the reaction requires an excess of Zn dnst (ca. 3 eqniv), previonsly treated with few mol % of 1,2-dibromoethane and TMSCl, and a temperature of 40 °C in THF. In these conditions, secondary alkyl iodides react at room temperatnre and benzylic and allylic bromides at 0 °C. The insertion see Insertion) into less activated C-X bonds may reqnire more reactive forms of zinc (Riecke zinc), higher temperatures, or the use of polar see Polar Compounds) solvent or cosolvent. 

The BiBrs-Sm binary reagent promotes reductive C-S and C-Se bond formation between benzyl and allyl bromides and diorganyl disulfides and diselenides in aqueous media, affording the corresponding sulfides and selenides, respectively (Scheme 14.105) [217, 218]. Intramolecular reductive C-S bond formation by use of a BiCl3-M (M = Sn, Zn) redox system is used in the synthesis of 3-hydroxyceph-ems and 2-exo-methylenepenams (Scheme 14.106) [219]. Alkyl and arylsulfonyl chlorides couple with allylic halides in the presence of Bi to afford the corresponding allylic sulfones [220]. 

Many aspects of the regioselective manipulation of polyols through dialkylstannylene acetals have been studied and some interesting modifications have improved this procedure [53, 55], For example, the regioselective formation of monobenzyl, monoallyl and monomethyl ethers, which normally proceeds at very slow speed, is markedly enhanced when the reaction of benzyl and allyl bromides or methyl iodide on dialkylstannylene derivatives of polyhydroxy compounds is carried out in the presence of stoichiometric amounts of quaternary ammonium halides [37,56,57]. Several examples of this modified procedure, such as the regioselective mono-O-alkylation of disaccharide glycosides (Scheme 2), have been reported [58]. 

Tokuda and coworkers also reported the electrochemical alkylation of phenylacetylene with alkyl halides (Mel, EtBr, EtI, n-Bul) in HMPA/BU4NI, in good yields. Secondary halides and benzyl and allyl bromides did not give the expected products. It was suggested... 

Several N-protected indol-2-yltributylstannanes were examined in Pd-catalyzed cross-coupling with aryl halides and triflates, acyl chlorides and benzylic and allylic bromides. <94J0C4250> The 1-methyl and l-(2-trimethylsilylethoxymethyl) (SEM) derivatives reacted readily whereas the 1-t-butoxycarbonyl derivative was somewhat less reactive. The SEM group is removable with BU4N F , providing acces to the deprotected 2-substituted indoles. 

KHMDS has been used to effect a-deprotonation of O-sUyl protected cyanohydrins derived from 2-/7-tolylsulfinyl henzalde-hyde followed by trapping of the C-nucleophile with diverse C-electrophiles, providing a powerful alternative approach to cyanohydrins of ketones. The remote 1,4-asymmetric induction was equally effective for either epimer (diastereomer) of the 0-TIPS protected cyanohydrin, and an equimolar mixture of the two epimers was employed. Both KHMDS and LHMDS bases provided the substituted cyanohydrins from reactions with highly reactive electrophiles (ClCOOMe and ClCOMe) in excellent yields and diastereoselectivities (dr > 98 2) (eq 65). The deprotonation induced by KHMDS led to more reactive nucleophiles, shortening the reaction times. Notably, in alkylations of Eschen-moser s salt, and benzyl and allyl bromides, the application of LHMDS instead of KHMDS improved the diastereoselectivity. The stereoselectivity of the alkylations mediated by KHMDS could be increased by the inclusion of the 18-crown-6 ether... 

Indeed, remarkable enantioselectivity was observed with the enolates of cyclohexanone, a-tetralone, lactams, and lactones upon treatment with benzylic and allylic bromides or iodides but also methyl iodide. As the presence of lithium bromide was found to have a beneficial effect on the stereoselectivity, it was concluded that a complex between the lithium enolate, the amine 2, and lithium bromide was crucial for high enantioselectivity some illustrative examples are shown in Scheme 5.2 [2]. 

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