Cyclohexene, reaction with electrophiles

In general the Stork reaction gives moderate yields with simple alkyl halides better yields of alkylated product are obtained with more electrophilic reactants such like allylic, benzylic or propargylic halides or an a-halo ether, a-halo ester or a-halo ketone. An example is the reaction of 1-pyrrolidino-l-cyclohexene 6 with allyl bromide, followed by aqueous acidic workup, to yield 2-allylcyclohexanone ... 

As for many other nucleophiles, the nitrite anion undergoes addition to the iodonium ion generated by the reaction of alkenes and 1,3-alkadienes with electrophilic iodine reagents. Two procedures have been described bis(pyridine)iodine(I) tetrafluoroborate136,137 [prepared from mcrcury(II) oxide and tctrafluoroboric acid supported on silica gel and pyridine on dichloromethane] and copper(II) tetrafluoroborate [prepared from copper(II) oxide and te-trafluoroboric acid] and iodine138 139. trans Addition would be expected for all products from mechanistic considerations, however, only the cyclohexene adduct 1 has been shown to have trans configuration ( H-NMR spectroscopy)139. 

Nitration of 2-methyl-l-(trimethylsiloxy)cyclohexene (31) with nitronium tetrafluoroborate leads to 2-methyl-2-nitrocyclohexanone (32 equation 12), while 6-methyl-l-(trimethylsiloxy)cyclohexene (33) gives under similar conditions a 30 70 mixture of cis- and fran5-6-methyl-2-nitrocyclohexanone (equation 13). The desilylative nitration of allylsilanes with nitronium tetrafluoroborate proceeds readily. The reaction is considered to pass through initial electrophilic attack of the nitronium ion on the allyl system followed by desilylative elimination. So, on treatment of l-(trimethylsilyl)but-2-ene (34) with nitronium tetrafluoroborate the product is 3-nitrobut-l-ene (35) and not l-nitrobut-2-ene (equation 14). ... 

BICYCLOPROPYLIDENE possesses unique reactivity toward a wide range of electrophiles and nucleophilic carbenes. (E)-I-DIMETH YLAMINO-3-tert-BUTYLDIMETHYL-SILOXY-1,3-BUTADIENE is a highly reactive diene for Diels-Alder reactions, as described in an accompanying procedure for the synthesis of 4-HYDROXYMETHYL-2-CYCLOHEXEN-1-ONE via the Diels-Alder reaction with methyl acrylate. Finally, this section concludes with the preparation of DIETHYL [(PHENYLSULFONYL)METHYL]PHOSPHONATE, a reagent that is very useful for synthesis of [Pg.285]

As in the Negishi reaction, various alkylboron reagents have also been successfully coupled with electrophilic partners [32], For example, Suzuki et al. coupled 1-bromo-l-phenylthioethene with 9-[2-(3-cyclohexenyl)ethyl]-9-BBN (34), prepared by a simple addition of 9-borabicyclo[3.3.1]nonane (9-BBN) to 4-vinyl-1-cyclohexene (33), to furnish 4-(3-cyclohexenyl)-2-phenylthio-l-butene (35) in good yield [33],... 

Zeifirov s group has reported on the activation of the electrophilic reagents ethyl nitrite and phenylsulphenyl chloride by insertion of S03 into the molecules and has subsequently described their reactions with olefins. The reaction of activated ethyl nitrite (146) with cyclohexene yields ethyl 2-oxocyclohexylsulphate (147) (equation 15)193 while the reaction of activated phenylsulphenyl chloride in acetonitrile with cyclohexene gives the trans amide 148 in 62% yield and 28% yield of the chlorosulphide 149 (equation 16)194. 

The electrophilic addition reactions of A -unsaturated steroids and other rigid cyclohexenes are controlled mainly by the conformational preference for diaxia) addition HOBr, for example, gives mainly a 5a-bromo-6)5-alcohol. A study of similar reactions with B-nor-A -unsaturated steroids suggests that the reaction of a cyclopentene is under electronic rather than conformational control.A variety of reagents (HOBr, BrF, Brj, BrOMe, and BrOAc) gave mainly 6a-bromo-5)S-substituted derivatives (155), indicating that the initial product, a 5a.6a-bromonium ion (154). reacts further according to Markovnikoff, with attack of the anion at the tertiary 5 -position. 

The chiral anisole derivative 119 has been used in the synthesis of several asymmetric functionalized cyclohexenes (Fig. 27) [55]. In a reaction sequence similar to that employed with racemic anisole complexes (see above), 119 adds an electrophile and a nucleophile across C4 and C3, respectively, to form the cy-clohexadiene complex 120. The vinyl ether group of 120 can then be reduced by the tandem addition of a proton and hydride to C2 and Cl respectively, affording the olefin complex 121. Direct oxidation of 121 liberates the cyclohexenes of the type 122 having the initial asymmetric auxiliary still intact. Alternatively, the auxiliary may be cleaved under acidic conditions to afford q -allyl complexes, which can undergo reactions with nucleophiles regioselectively at Cl. Oxidative decomplexation liberates the cyclohexenes 123-127. 

The intermediate in both reactions is a cation but the first (from cyclohexene) adds an anion while the second (from benzene) loses a proton so that the aromatic system can be restored. Notice also that neutral bromine reacts with the alkene but the cationic AICI3 complex is needed to get reaction with benzene. Bromine itself is a very reactive electrophile. It is indeed a dangerous compound and should be handled only with special precautions. Even so it does not react with benzene. It is difficult to get benzene to react with anything. 

ABSTRACT. Dicarbonyl(t 5-cyclopentadienyl)carbyne complexes of molybdenum and tungsten prove to be a valuable synthetic tool Reaction with phosphines provides substituted carbyne complexes and leads via an intramolecular CC-coupling to t 1- or Tj -ketenyl complexes respectively. Electrophiles attack the metal carbyne triple bond forming hetero- and acyclic carbene complexes, r 2-acyl compounds, T -ketene complexes and metalla-dithia-bicyclobutane cations. Dithio-carboxylates are formed in reaction of these dicarbonyl(Ti5 cyclo-pentadienyl)carbyne complexes with sulfur or cyclohexene sulfide. 

Aromatic compounds are not fundamentally different from cyclohexene. They can also react with electrophiles. However, because of resonance in the ring, the electrons of the tt system are generally less available for addition reactions because an addition would mean the loss of the stabilization that resonance provides. In practice, this means that aromatic compounds react only with powerfully electrophilic reagents, usually at somewhat elevated temperatures. 

The intermediate first formed is somewhat stabilized by resonance and does not rapidly undergo reaction with a nucleophile in this behavior, it is different from fhe unsfabilized carbocation formed from cyclohexene plus an electrophile. In fact, aromaticity can be restored to the ring if elimination occurs instead. (Recall that elimination is often a reaction of carbocations.) Removal of a proton, probably by HS04, from the sp -ring carbon restores the aromatic system and yields a net substitution wherein a hydrogen has been replaced by a nitro group. Many similar reactions are known, and they are called electrophilic aromatic substitution reactions. 

Yadav et al. have reported using cyclopropanes as the electrophilic component in the Danheiser annulation leading to the formation of cyclohexene products.Treatment of cyclopropane 102 with Et2AlCl at 25 °C resulted in ring opening of the cyclopropane, which then underwent reaction with silylyallene 34 to afford cyclohexene 103, formally a [3 + 3] annulation product. 

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