Alkenes transfers

If the initial organopalladium complex is formed in the presence of an alkene, transfer of the organyl group to the alkene can occur (Heck reaction, Scheme 8.7) [5, 78], Electron-deficient and electron-rich alkenes and even some arenes [79] can be used. In the last step of this process a Pd-hydrido complex is formed by /3-hydride elimination, leading to the formation of the C-C double bond. In the presence of bases the hydrido complex eliminates HX, regenerating the catalytically active Pd(0) complex (Scheme 8.7). 

ALLYLIC ALCOHOLS BY ALKENE TRANSFER FROM ZIRCONIUM TO ZINC 1-[(tert-BUTYL-DIPHENYLSILYL)OXY]-... 

Another cyclopropanation procedure that is quite general involves the use of Rh-carbene complexes, which can act catalytically to effect ring formation. Scheme 10.7 shows some of the details of this method. Ccaibene is derived from corresponding diazo compounds, which were traditionally used directly as sources of free carbenes. The scheme includes a catalytic cycle for conversion of the diazo compound to the Rh-carbene complex, which then delivers Ccarbene to the alkene. Transfer of Ccaibene regenerates an active catalyst that can react with another mole of diazo compound. The detailed mechanism of step c in the cycle resembles path b from Scheme 10.6. 

TMG (1) assisted alkene transfers from alkenyl bismuth reagents to reactive electrophiles have been reported [83]. Treatment of p-keto esters, p-diketones and phenols with alkenyltriarylbismuthonium salts in the presence of TMG (1) smoothly affords a-alkeny-lated products [83b] (Table 4.13). 

Treatment of the intermediate alkenylborane, such as 52 or 53, with iodine in the presence of a base (such as sodium hydroxide or methoxide) forms, stereose-lectively, a Z-1,2-disubstituted or trisubstituted alkene. Transfer of one alkyl group from boron to the adjacent carbon atom occurs stereospecifically, resulting, after anti elimination of boron and iodine, in a new alkene in which the two substituents of the original alkyne become trans to each other (2.53, 2.54). ... 

Wipf, P. Xu, W. Allylic Alcohols by Alkene Transfer from Zirconium to Zinc l-[(rert-Butyldiphenylsilyl)oxy]-dec-3-en-5-ol OS, (1996), 74,205. 

Alkenes in (alkene)dicarbonyl(T -cyclopentadienyl)iron(l+) cations react with carbon nucleophiles to form new C —C bonds (M. Rosenblum, 1974 A.J. Pearson, 1987). Tricarbon-yi(ri -cycIohexadienyI)iron(l-h) cations, prepared from the T] -l,3-cyclohexadiene complexes by hydride abstraction with tritylium cations, react similarly to give 5-substituted 1,3-cyclo-hexadienes, and neutral tricarbonyl(n -l,3-cyciohexadiene)iron complexes can be coupled with olefins by hydrogen transfer at > 140°C. These reactions proceed regio- and stereospecifically in the successive cyanide addition and spirocyclization at an optically pure N-allyl-N-phenyl-1,3-cyclohexadiene-l-carboxamide iron complex (A.J. Pearson, 1989). 

A cr-aryl-Pd bond is formed by the transfer of an aryl group even from arylphosphines to Pd and alkene insertion takes placefl 17-119], This reaction is slow and it is not a serious problem when triarylphosphine is used as a ligand. The cinnamate 149 is obtained by the reaction of PhsP with acrylate in the presence of Pd(OAc)2 in AcOH. 

The alkynyl iodide 150 undergoes the oxidative addition to form an alky-nylpalladium iodide, and subsequent insertion of an alkene gives the conjugated enyne 151 under phase-transfer conditions[120]. 

In these cydizations, the reaction can be terminated in other ways than elimination of /3-hydrogen. Typically the reaction ends by an anion capture process[154]. The following anion transfer agents are known H, OAc , CN, S02Ph, CH(C02R)2, NHRj, CO/ROH, and RM [M = Sn(IV), B(lll), Zn(II)]. Trapping with an amine after alkene insertion to give 189 and 190 is an example. A-Acetyl protection is important in this reaction[155]. 

The reaction of perfluoroalkyl iodides with alkenes affords the perfluoro-alkylated alkyl iodides 931. Q.a-Difluoro-functionalized phosphonates are prepared by the addition of the iododifluoromethylphosphonate (932) at room temperature[778], A one-electron transfer-initiated radical mechanism has been proposed for the addition reaction. Addition to alkynes affords 1-perfluoro-alkyl-2-iodoalkenes (933)[779-781]. The fluorine-containing oxirane 934 is obtained by the reaction of allyl aicohol[782]. Under a CO atmosphere, the carbocarbonylation of the alkenol 935 and the alkynol 937 takes place with perfluoroalkyl iodides to give the fluorine-containing lactones 936 and 938[783]. 

The formation of 1-and 2-aIkenes can be understood by the following mechanism. In the presence of formate anion, the 7r-allylpalladium complex 572 is converted into the 7r-allylpalladium formate 573. The most interesting feature is the attack of the hydride from formate to the more substituted side of the (T-allylic system by the cyclic mechanism shown by 574 to form the 1-alkene 575[367]. The decarboxylation and hydride transfer should be a concerted... 

Step 4 The second hydrogen atom is transferred forming the alkane The sites on the catalyst surface at which the reaction occurred are free to accept additional hydrogen and alkene molecules... 

In the mechanism for alkene hydrogenation shown m Figure 6 1 hydrogen atoms are transferred from the catalyst s surface to the alkene Although the two hydrogens are not transferred simultaneously they both add to the same face of the double bond... 

The stereoselectivity of this reaction depends on how the alkene approaches the catalyst surface As the molecular model m Figure 6 3 shows one of the methyl groups on the bridge carbon lies directly over the double bond and blocks that face from easy access to the catalyst The bottom face of the double bond is more exposed and both hydrogens are transferred from the catalyst surface to that face... 

FIGURE 6 6 Electron flow and orbital interactions in the transfer of a proton from a hydrogen halide to an alkene of the type H2C=CHR... 

We can consider the hydroboration step as though it involved borane (BH3) It sim phfies our mechanistic analysis and is at variance with reality only m matters of detail Borane is electrophilic it has a vacant 2p orbital and can accept a pair of electrons into that orbital The source of this electron pair is the rr bond of an alkene It is believed as shown m Figure 6 10 for the example of the hydroboration of 1 methylcyclopentene that the first step produces an unstable intermediate called a tt complex In this rr com plex boron and the two carbon atoms of the double bond are joined by a three center two electron bond by which we mean that three atoms share two electrons Three center two electron bonds are frequently encountered m boron chemistry The tt complex is formed by a transfer of electron density from the tt orbital of the alkene to the 2p orbital... 

Peroxy acid and alkene Transition state for oxygen transfer from the OH group of the peroxy acid to the alkene Acetic acid and epoxide ... 

Epoxidation (Section 6 18) Peroxy acids transfer oxygen to the double bond of alkenes to yield epoxides The reaction IS a stereospecific syn addition... 

Step 4 Proton transfer from ammonia converts the alkenyl anion to an alkene ... 

Group I metals—sodium is the one usually employed—in liquid ammonia as the solvent convert alkynes to trans alkenes The reaction proceeds by a four step sequence in which electron transfer and proton transfer steps alternate... 

Methylene transfer from lodomethylzmc iodide is stereospecific Substituents that were cis in the alkene remain cis m the cyclopropane... 

Methylene transfer from lodo methylzinc iodide converts alkenes to cyclopropanes The reaction is a stereo specific syn addition of a CH2 group to the double bond... 

When applied to the synthesis of ethers the reaction is effective only with primary alcohols Elimination to form alkenes predominates with secondary and tertiary alcohols Diethyl ether is prepared on an industrial scale by heating ethanol with sulfuric acid at 140°C At higher temperatures elimination predominates and ethylene is the major product A mechanism for the formation of diethyl ether is outlined m Figure 15 3 The individual steps of this mechanism are analogous to those seen earlier Nucleophilic attack on a protonated alcohol was encountered m the reaction of primary alcohols with hydrogen halides (Section 4 12) and the nucleophilic properties of alcohols were dis cussed m the context of solvolysis reactions (Section 8 7) Both the first and the last steps are proton transfer reactions between oxygens... 

Many naturally occurring substances are epoxides You have seen two examples of such compounds already m disparlure the sex attractant of the gypsy moth (Section 6 18) and m the carcinogenic epoxydiol formed from benzo[a]pyrene (Section 118) In most cases epoxides are biosynthesized by the enzyme catalyzed transfer of one of the oxy gen atoms of an O2 molecule to an alkene Because only one of the atoms of O2 is trans ferred to the substrate the enzymes that catalyze such transfers are classified as monooxy genases A biological reducing agent usually the coenzyme NADH (Section 15 11) is required as well... 

Quaternary ammonium salts as we have seen are useful m synthetic organic chem istry as phase transfer catalysts In another more direct application quaternary ammo mum hydroxides are used as substrates m an elimination reaction to form alkenes... 

The alkene is allowed to react at low temperatures with a mixture of aqueous hydrogen peroxide, base, and a co-solvent to give a low conversion of the alkene (29). These conditions permit reaction of the water-insoluble alkene and minimise the subsequent ionic reactions of the epoxide product. Phase-transfer techniques have been employed (30). A variation of this scheme using a peroxycarbimic acid has been reported (31). 

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