Markovnikov adducts

NMR monitoring of the reaction of the palladium complex with 1-octyne suggested that the alkyne inserts into the Pd-H bond. Further heating produced a mixture of the two regioisomeric alkenylphosphine oxides, the anti-Markovnikov adduct being the favored product (54 46, 65% yield). 

Although the path (a), which is the initiation stage of the catalytic reaction, was actually confirmed by a stoichiometric reaction, no direct evidence has been provided about paths (b)-(d). When the reaction was carried out by using PdCl2(PhCN)2 as a catalyst precursor, the Markovnikov adducts formed in situ isomerized into internal vinyl sulfide 14 (Eq. 7.10) [22]. 

In the phosphonium salt synthesis, the addition reaction of tertiary phosphines to activated alkenes has been reported (Scheme 3). PPh3 is added to electron-deficient alkenes such as enones or enals at the p-position in the presence of acids.4 The reaction of styrenes with phosphine has recently been reported by Okuma, which gave Markovnikov adducts.5 Although no catalyzed reactions of... 

Adducts of type 13, arising from the rearrangement of the allylic intermediate, have never been observed. The product distribution in methanol depends, however, on the reaction conditions. When the addition of XeF2 is carried out in the presence of boron trifluoride as a catalyst, the formation of the complex b has been suggested. This complex would react with 2,3-dimethylbutadiene as a positive oxygen electrophile to give, besides 1,2- and 1,4-difluoro derivatives, 1,4- and 1,2-fluoromethoxy products with a predominance of the anti-Markovnikov adduct (equation 26). 

The addition of iodine electrophiles, fert-butyl hypoiodite (t-BuOI) in the presence of BF3, acetyl hypoiodite (AcOI), iodine monochloride (IC1) and iodine monobromide (IBr), to 1,3-butadiene gives always, under ionic conditions, mixtures of 1,2- and 1,4-Markovnikov adducts (equation 57). These mixtures are the kinetic products, since rearrangement to the thermodynamically stable products occurs under the appropriate conditions86. 

Usually, 5- and 6-membered Markovni-kov-type products are formed in other cases the process results in various open-chain products. The formation of an anti-Markovnikov adduct from 2-cyclohex-1-enyl-ethanol was explained by the cyclization of an episulfonium intermediate... 

Irradiation of 1-phenylcycloalkenes (160) with cyano-aromatics electron-accepting sensitizers in MeCN and benzene containing 1 m methanol gave trans- 6 ) and di-isomers (162) of anti-Markovnikov adducts. The (161)7(162) isomer ratio was found to depend on the ring size of 1-phenylcycloalkene but not on the sensitizer used. The mechanism of the reactions was studied by semiempirical MO calculations. 

The stereoselectivity of anti-Markovnikov adducts (161) and (162) produced through photo-induced electron-transfer reaction of (160) with MeOH in MeCN depends on the optimum structures and stabilities of the corresponding radical and carbanion intermediates (163) and (164). In PhH, steric hindrance in an exciplex, comprising an excited singlet sensitizer and (160), forced cis addition of MeOH to (160) to give trans-isomer (161) as the major addition product. 

Cklorohydrins. Chloramine-T reacts with olefins in an acidic medium to form chlorohydrins in 40 75% isolated yield. The reaction involves a /ran.v-addition Terminal olefins give a mixture of Markovnikov and anti-Markovnikov adducts in a ratio of 4 1. ... 

The ratio of the three products depends on the reacting silane and alkyne, the catalyst, and the reaction conditions. Platinum catalysts afford the anti-Markovnikov adduct as the main product formed via syn addition.442- 146 Rhodium usually is a nonselective catalyst404 and generally forms products of anti addition.447 151 Minor amounts of the Markovnikov adduct may be detected. Complete reversal of stereoselectivity has been observed.452 [Rh(COD)Cl]2-catalyzed hydrosilylation with Et3SiH of 1-hexyne is highly selective for the formation of the Z-vinylsilane in EtOH or DMF (94-97%). In contrast, the E-vinylsilane is formed with similar selectivity in the presence of [Rh(COD)Cl]2-PPh3 in nitrile solvents. 

While the reaction of alkenes with h at temperatures of 125-130 C reportedly produces regioisomeric mixtures of iodoalkanes,187 in the presence of dehydrated alumina in refluxing petroleum ether, this reaction gives low to reasonable yields of Markovnikov adducts.188,189 This approach does not appear very general, however, and rearrangement has been observed. 

The overall process is generally highly regiospecific. Mono- or di-substituted terminal alkenes and tri-substituted alkenes afford >99% of the Markovnikov adduct (equation 206).312-314... 

Irreversible acetoxyselenenylation of terminal and disubstituted olefins has been achieved on addition of PhSeBr in an acetate-buffered solution. Styrenes afford only Markovnikov adducts, while simple terminal olefins and olefins containing an allylic oxygen substituent (RCO2 or ArO group) furnish 50-80% of the anti-Markovnikov isomer. The product mixture can be isomerized to contain 90-97% of the Markovnikov product by a catalytic amount 6-41%) of BF3.Et20 in CHCI3259. 

Cyanosulfenylation is generally conducted by mixing the alkene with 1 in CH3CN followed by addition of finely powdered NaCN at 25°. Addition of dimethyl sulfide sometimes improves the yield. The adducts are all assumed to be trans, but regioselectivity is a more critical question. With monosubstituted alkenes, only anti-Markovnikov addition is observed. Markovnikov adducts are favored by use of the less nucleophilic (CH3)3SiCN. 

In the presence of BiCU, the hydroarylation of styrenes Ar,C(R)=CH2 with electron-rich arenes ArH afforded Markovnikov adducts Ar C(R)(Ar)Me selectively in good to high yields as a result of the C-H activation of ArH. Under arene-free conditions, the intermolecular hydroarylation of a-substituted styrenes and subsequent intramolecular hydroarylation produced the cyclic dimers of a-substituted styrenes in good yields.122,123... 

The treatment of terminal and internal alkynes with 1-tosyloxybenziodoxolone (11) in the presence of molecular iodine results in trans iodo-oxytosylation of the C,C-triple bond, and in the case of unsymmetrical alkynes, leads to Markovnikov adducts (Scheme 18) [46]. Co-iodinations of alkynes with the... 

Reaction with SC12) although more complex, also leads to exclusive formation of anti-Markovnikov adducts, along with an expected chlorination product. Addition of S2Cl2 to CF2=CHCF3 is another example[135,136] ... 

The results obtained57 were explained by competition of ionic and radical mechanisms, which lead to Markovnikov and anti-Markovnikov adducts, respectively. At this competition the nature of the unsaturated compound play an important role in determining the preferred mechanism. Thus, the major formation of Markovnikov adducts and therefore the preference of the ionic mechanism in the series of olefins styrene > 1-methylcyclohexene > 2,3-dimethyl-1-butene > isobutene > 1-heptene correlates with the ability of substituents to stabilize the intermediate carbenium ion. 

Formation of the anti-Markovnikov adduct points to the intermediacy of GeCl3 radicals. Formation of the radicals is explained by one electron transfer in different ionic pairs and separation of the formed radical pairs without recombination. This one-electron transfer at the stage of trichlorogermane ionization is expected because of the low potential of GeCl3 oxidation ( 1/2 = 0.48 V) and was confirmed by examination of CIDNP in the reactions (see equation 19 below). 

Numerous attempts to obtain the Markovnikov adduct by varying the reaction conditions, including its realization in concentrated HC1, had failed. Moreover, in a competitive reaction of a mixture of 1-heptene and styrene only the anti-Markovnikov adducts were formed for both olefins and, surprisingly, 1-heptene was found to be more reactive than styrene. This is also in agreement with the concept of two mechanisms. Here, 1-heptene assists in the formation of GeCl3 radicals and styrene acts as a radical trap, forming selectively only the anti-Markovnikov adduct. 

Formation of polarized anti-Markovnikov adducts here results from GeCl3 attack on the polarized olefin. 

Palladium chloride, propylene and carbon monoxide react in benzene solution to form 3-chlorobutyryl chloride in 27% yield 19>. The Markovnikov adduct apparently reacts with carbon monoxide and then the addition product undergoes a reductive elimination of Pd(O). 

Ailylic chlorides and "arylpalladium chlorides apparently react to form anti-Markovnikov adducts which then decompose by eliminating palladium chloride rather than hydride, producing allylaromatic compounds 81). 

The addition to allyl nitrile also shows some preference for the anri-Markovnikov adduct (equation III). However, both products undergo a Ritter-type hydrolysis of the nitrile group to provide the same lactone. 

Addition of CC14 to chloroethene and 1,1-dichloroethene occurred selectively under the influence of a catalytic amount of complex 1 and afforded the 1 1 anti-Markovnikov adducts in 86% and 89% yield, respectively. No reaction was observed with chlorotrifluoroethene [24]. 

Similar reactions with O-ethyl carbamate and terminal olefins yielded mixtures of 2-(ethoxycarbonylamino)-l-alkyl (Markovnikov adduct) and l-(ethoxycarbonylamino)-2-alkyl (anti-Markovnikov adduct) phenyl telluroxides that were again isolated as the corresponding diorgano tellurium compounds3. 

The Markovnikov adducts are the predominent species in the reaction mixture. With phenylethene, only the Markovnikov adduct was observed1. Benzenetellurinyl trif-luoroacetate did not react with norbornene or 1-methyl-2-phenylethene. 

---