2-Chloro-l-alkenes

The above-postulated overall mechanism considers two alternative pathways depending on the nature of the acetylene derivative. Region A outlines a proposal in which the formation of the a-complex intermediate is supported by the fact that the treatment of aliphatic terminal acetylenes with FeCl3 led to 2-chloro-l-alkenes or methyl ketones (Scheme 12). The catalytic cycle outlined in region B invoked the formation of the oxetene. Any attempt to control the final balance of the obtained... 

Chloro-l-alkenesS A regioselective route to these chloroalkenes involves thermodynamically controlled addition of C6H5SeCl to a 1-alkene followed by chlorination to provide a (2-chloroalkyl)phenylselenium dichloride (2). These products undergo elimination when treated with NaHC03 in a two-phase system to provide 2-chloro-l-alkenes (3). 

Addition of Ph SeCl to terminal olefins, followed by chlorination with sulfuryl chloride and hydrolysis, leads to 2-chloro-l-alkenes (equation 143)952. 

Homologation. 2-Chloro-l-alken-3 with 1,1,1-trichloroethane in the presenc alcohols. Conversion of aldehydes to complished by the CrCl2-catalyzed reactic... 

Homologation. 2-Chloro-l-alken-3-ols are obtained from aldehydes on treatment with 1,1,1-trichloroethane in the presence of CrCl2. They are precursors of propargyl alcohols. Conversion of aldehydes to alkenylsilanes with one more carbon is accomplished by the CrCl2-catalyzed reaction with Mel, Mn, and Me SiCl in THF. ... 

When acetic anhydride is used in the CF3CCI3 and zinc reaction with aldehydes, the initial addition product undergoes an elimination reaction to give 2-chloro-l,l,l-trifluoro-2-alkenes exclusively [60, 63] (equation 51)... 

Table 3. Preparation of 2-Chloro-l,l,l trifIuorO 2-alkenes from Aldehydes, Zinc, and Acetic Anhydride [63]...

Many polyhalogenated alkenes are potent nephrotoxins. 2-Chloro-l, 1,2-trifluoroethylene (CTFE), trichloroethylene (TCE) and tetrafluoroethylene (TFE) are notable examples. The initial step in bioactivation of such compounds is the GSH-trans-ferase-catalyzed addition or addition-elimination of GSH. For example, in the liver, the action of GSH- transferase converts CTFE to S-(2-chloro- l,l,2-trifluoroethyl)glu-tathione (CTFG), TCE to S-(l,2-dichlorovinylglutathione (DCVG) and TFE to S-... 

The currently available methods for the synthesis of the title compounds are confined to the preparation of homo-1,1-dihalo-1-alkenes 180 while only a few reports are available for mixed 1,1-dihalo-1-alkenes of defined stereochemistry 18u. As the hy-droboration reaction proceeds in a stereospecific manner, the hydroboration-oxi-dation-bromination-debromoboration sequence of 1-chloro-l-alkynes produces selectively (Z)-l-bromo-l -chloro-l-alkenes (Eq. 116),82>. The oxidation with anhydrous trimethylamine oxide of the alkenylborane prior to the addition of bromine is necessary to avoid the competing transfer of one of 1,2-dimethylpropyl group from boron to the adjacent carbon atom. Similar reaction sequence provides 1,1-dibromo-l-alkenes (Eq. 117)182). 

Hydroalumination of 1-chloro-l-alkynes. Lithium aluminum hydride adds to 1-chloro-l-alkynes (1) regio- and stereoselectively to form the a-chlorovinyl alanates 2, which are moderately stable at 0°C. On methanolysis they are converted into (E)-l-chloro-l-alkenes (3). They can also be converted into (Z)-l-bromo-l-chloro-l-alkenes (5) and into (Z)-l-chloro-l-iodo-l-alkenes (6). 

Bromo-l-chloro-l-alkenes, 276, 277 a-Bromocyclohexanone, 73 Bromodimethylsulfonium bromide, 66-67 3/3-Biomo-8-epicaparrapi oxide, 295 Bromo-2-ethoxycyclopiopyllithium, 67-68 5-Bromo-2-furylcaibinols, 398 Bromohydrins, 70-71... 

To a freshly prepared solution of 0.221 g (1 mmol) of SeCI4 in 25 mL of anhyd CH2C12 are added dropwise 2 mmol of the appropriate alkene. The solution turns a clear yellow within a few seconds. Evaporation of the solvent gives an yellow-orange oil. Crystallization of the oil from pentane or cyclohexane generally gives the bis(2-chloro-l-methylpropyl)selenium dichlorides as colorless, air- and moisture-sensitive solids. 

The zinc procedure has been extended to CF3CCI3 to form 2 chloro-l,l,l,-trifluoro-2-alkenes (84 equation 19). and a-fluoro-a,p-unsaturated carboxylic acid methyl esters (87) with methyl dichlorofluo-roacetate (8jS equation 20). Both of these transformations are carried out with zinc and acetic anhydride. Several variations of this reaction have appeared in the literature." The mechanism appears to dramatically different from the Oshima methylenation. In this case, the reaction proceeds through the alcohol intermediate, which is converted to the acetate and reductively cleaved. 

Tetrafluoroethylsilanes decompose by thermally induced a-elimination to give diflu oromethyl(fluoro)carbenes and fluorosilanes. Following this strategy, l-(chlorodifluoro-methyl)-l-fluorocyclopropanes 6 and 7 were obtained by thermolysis of (2-chloro-l,1,2,2-tetrafluoroethyl)trifluorosilane in the presence of excess alkene in a sealed Pyrex tube. ... 

Dimethoxy-3,6-dihydropyrazine (109), prepared by methylation of 2,5-piperazinedione with trimethyloxonium tetrafluoroborate, is susceptible to lithiation because the protons at C-3 and C-6 are activated by adjacent imine moieties. The lithium salt of this bislactim ether reacts with the 2-chloro-l-phenylsulfonyl alkene (110) to give the 3-substituted pyrazine (111) (Scheme 25) <89JCS(P1)453>. The bislactim ether from piperazinedione cyclo(L-Val—Gly) is lithiated with butyl-lithium and then treated with ketones, alkyl halides, or others to form, nearly stereospecifically, ran5-3-isopropyl-6-substituted piperazinediones due to the steric influence of the isopropyl group <828866, 838673). Similar stereoselective syntheses have been achieved in reactions starting from cyclo(L-Val—D,L-Ala) <828864, 918939). Acid hydrolysis of these products affords chiral a-amino acids. 

Chloromethylenetriphenylphosphorane (2). Treatment of 1 with potassium f-butoxide forms chloromethylenetriphenylphosphorane (2). Chloromelhylation of aldehydes and ketones with reagent prepared in this way results in 1-chloro-l-alkenes [(E)- and (Z)-isomers] in 60-95%, yield. If excess base is used, aromatic aldehydes give arylacetylenes, ArCHO. - ArCs=CH (40-90% yield). 

Dichloroiodo)benzene can be convenientiy generated in situ from other hypervalent iodine reagents and used for subsequent chlorination of organic substrates. In a specific example, an efficient chlorination of p-keto esters, 1,3-diketones and alkenes has been performed using iodosylbenzene with concentrated HCl, selectively giving a-chloro-p-keto esters, 2-chloro-l,3-diketones and 1,2-dichloroalkanes, respectively [63]. A stereoselective anti-addition was observed in the chlorination of indene under these conditions. 

The mixed halogens ICl, IBr, and BrCl also add to alkenes. White and Robertson found third-order kinetics for the reaction and determined the relative reactivities to be BrCl > ICl > Br2 > IBr > l2- The l ge reactivity of BrCl means that addition of BrCl can occur when alkenes react with mixtures of Br2 and Cl2. The regioselectivity of addition of mixed halogens was reported by Ingold and Smith. Addition of ICl to propene gave 69% of 2-chloro-l-iodo-propane (30) and 31% of l-chloro-2-iodopropane (31), and addition to styrene gave more than 95% of l-chloro-2-iodo-l-phenylethane. ... 

This regiochemistry can be rationalized by imagining that the X-Y molecule first adds to the alkene when Y is the more electronegative of the atoms X and Y. Furthermore, attachment of X+ may result in a halonium ion if such an intermediate is formed in the addition of X2 to the same alkene. For example, ds-stilbene reacted in a mixture of Br2 and CI2 to produce 68% of t/(reo-l-bromo-2-chloro-l,2-diphenylethane and only about 5% of the erythro diastereomer. ... 

Free radical polymerization, another example of a chain reaction (this chapter), is quite common and, for many alkenes and dienes, is the preferred method of polymer formation. Typically, as shown in Table 6.3 and Scheme 6.45, the initiator of the free radical process is a peroxide (such as di-fm-butylperoxide [(CH3)3C-0-0-C(CH3)3]). In Scheme 6.45a, the radical polymerization of ethylene (ethene, CH2=CH2) normally carried out at high pressure (>1(F atm) is shown, while as shown in Scheme 6.45b, the radical polymerization of a diene, 2-chloro-l, 3-butadiene [chloroprene, CH2=C(C1)CH=CH2], produces the all tram or (Z)-polymer called neoprene. ... 

Dehydrohalogenation of the diastereomeric forms of l-chloro-l,2-diphenylpropane is stereo-specific. One diastereomer yields ( )-l,2-diphenylpropene, and the other yields the Z isomer. Which diastereomer yields which alkene Why ... 

The parent five-membered nitronate having no substituent at the 3-position was too unstable to be isolated. However, 3-substituted derivatives were highly stabilized. Especially, the 3-ethyl derivatives having a terminal electron-withdrawing substituent are readily available by the dehydrochlorination of 3-chloro-l-nitropropane in the presence of electron-deficient alkenes. It was our delight that the reaction of 3-al-kyl-substituted five-membered nitronates was also successfully catalyzed by R,R-DBFOX/Ph-Ni(SbFg)2 complex to at room temperature. This reaction was highly endo-selective (cisjtrans= 91 9) and enantioselective for the endo cycloadduct (92% ee). 

Problem 13,23 How could you use lH NMR to determine the regiochemistry of electrophilic addition to alkenes For example, does addition of HC) to 1-methylcvclohexene yield 1-chloro-l-methylcyclohexane or l-chloro-2-methylcyclohexane ... 

One of the most striking differences between conjugated dienes and typical alkenes is in their electrophilic addition reactions. To review briefly, the addition of an electrophile to a carbon-carbon double bond is a general reaction of alkenes (Section 6.7). Markovnikov regiochemistry is found because the more stable carbo-cation is formed as an intermediate. Thus, addition of HC1 to 2-methylpropene yields 2-chloro-2-methylpropane rather than l-chloro-2-methylpropane, and addition of 2 mol equiv of HC1 to the nonconjugated diene 1,4-pentadiene yields 2,4-dichloropentane. 

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