Rhodium anti-Markovnikov

Hydroamination of olefins under most catalytic conditions proceed with Markovnikov addition of the N-H bond across the olefin. Shown below is a rhodium-catalyzed intramolecular, anti-Markovnikov, hydroamination developed for the synthesis of 3-arylpiperidines 167 <06JA6042>. Further evaluation of this reaction as a synthesis of multisubstituted piperidines revealed that substrates with substituents a or y to the amino group did not produce the expected piperidine, however, substrates with a substituent (1 to the amino group produce piperidines in high yield. 

Although extremely rare, a recent report documents the intramolecular S -type displacements of (TPP)Rh-alkyl complexes (TPP = tetraphenylporphyrin) by alcohols and phenols to form THFs (Scheme 19). The intermediate rhodium alkyl complexes are themselves prepared by anti-Markovnikov hydrorhodation, and although the (TPP)Rh-H... 

Rhodium(I) and ruthenium(II) complexes containing NHCs have been applied in hydrosilylation reactions with alkenes, alkynes, and ketones. Rhodium(I) complexes with imidazolidin-2-ylidene ligands such as [RhCl( j -cod)(NHC)], [RhCl(PPh3)2(NHC)], and [RhCl(CO)(PPh3)(NHC)] have been reported to lead to highly selective anti-Markovnikov addition of silanes to terminal olefins [Eq. 

Although ruthenium and Group 6 metal catalysts are commonly employed for anti-Markovnikov alkyne hydrofunctionalization (Chapter 10), some interesting rhodium- and iridium-catalyzed methods have also been reported. These can be divided into three groups based on the nature of the incoming functional group ... 

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. 

Of the isomeric aldehydes indicated in Eq. (7.1), the linear aldehyde corresponding to anti-Markovnikov addition is always the main product. The isomeric branched aldehyde may arise from an alternative alkene insertion step to produce the [RCH(Me)Co(CO)3] or [RCH(Me)Rh(CO)(PPh3)2] complexes, which are isomeric to 2 and 8, respectively. Alternatively, hydroformylation of isomerized internal alkenes also give branched aldehydes. The ratio of the linear and branched aldehydes, called linearity, may be affected by reaction conditions, and it strongly depends on the catalyst used. Unmodified cobalt and rhodium carbonyls yield about 3-5 1 mixtures of the normal and iso products. 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

The intramolecular anti-Markovnikov hydroamination of l-(3-aminopropyl)vinyl-arenes (71 R = H, Me, CH2OMe, CH2OTBS) in the presence of a rhodium catalyst to form 3-arylpiperidines (72) has been reported. In contrast to intermolecular hydroamination of vinylarenes, which occurs in high yields in the presence of rhodium catalysts... 

Catalytic hydroboration of perfluoroalkenes 68 with catecholborane provides either terminal 69 or internal alcohols 70 regioselectively <19990L1399>. The regioselectivity is controlled by a judicious choice of catalyst. The anti-Markovnikov alcohol can be obtained with very high selectivity by using cationic rhodium catalysts such as Rh(COD)(DPPB)+BF4, while neutral Rh catalysts such as Wilkinson s catalyst provide the Markovnikov product (COD = cyclooctadiene Equation 3) <19990L1399>. 

The introduction of rhodium has allowed the development of processes which operate under much milder conditions and lower pressures, are highly selective, and avoid loss of alkene by hydrogenation. Although the catalyst is active at moderate temperature, plants are usually operated at 120°C to give a high n/iso (linear/ branched) ratio. The key to selectivity is the use of triphenylphosphine in large excess which leads to >95% straight chain anti-Markovnikov product. The process is used for the hydroformylation of propene to n-butyraldehyde, allyl alcohol to butanediol, and maleic anhydride to 1,4-butanediol, tetrahydrofuran, and y-butyrolactone. 

It was reported that the pyrazolyl-borate complex of rhodium (Tp Rh(PPh3)2, Tp =hydrotris(3,5-dimethylpyrazolyl)borate) is active not only in the hydrothiolation with ArS H, but also with AlkS H in good to high yields of Markovnikov isomer at room temperature (Scheme 3.91) [162,163]. However the reaction of 1-octyne gave a mixture of isomers in 70% yield. In addition to the double bond isomerization, a Markovnikov/anti-Markovnikov ratio of 12 1 was found in the case of 1-octyne. 

Alkyne hydroamination has been extensively reviewed [3, 4, 10] and important contributions using late transition metals have been realized to give the Markovnikov-type products most typically. Interestingly, in 2007, Fukumoto reported a tris(pyrazolyl borate)rhodium(l) complex for the anti-Markovnikov hydroamination of terminal aUcynes with both primary and secondary amine substrates, although yields with primary amines are always reduced compared to those with secondary amines (Scheme 15.26). Desirable functional group tolerance is also noteworthy for this regioselective hydroamination catalyst [187]. 

Nakao, Hiyama, et al. [141] could also demonstrate that a Ni/P(Cyp)3 catalyst was highly effective for the direct alkylation of polyfluoroarenes with vinylarenes or 1,3-dienes, again occurring with the Markovnikov regioselectivity (Scheme 19.98). This method nicely complements the anti-Markovnikov rhodium-catalyzed process (Scheme 19.90) [132]. 

When 1.88 was treated with n-butylamine and a rhodium catalyst, 1 l-(N-butyl-amino)undecanoic acid (1.267) was obtained. 51 This rhodium catalysis method allows an amine to add to an alkene in an anti-Markovnikov manner, as shown. The formation of 1.260 from the previous section also involves a rhodium species. 

Cationic rhodium complexes catalyse the oxidative anti-Markovnikov amination of aromatic alkenes to enamines, a process that is accompanied by a simultaneous formation of 1 equiv. of ethylbenzene. Kinetic and mechanistic studies reveal that the yield and the rate of the reaction increase on increasing the styrene amine ratio. Furthermore, the type of phosphine ligand greatly influences the reaction. The formation of cationic rhodium-alkene-amine complexes has been proposed to be the first step towards the active catalytic species. ... 

SCHEME 2.96 Rhodium-catalyzed anti-Markovnikov addition of carboxylic acids to alkynes [141]. 

While the majority of metal catalysts generated the Markovnikov addition product, a few catalyst systems were able to generate the anti-Markovnikov vinyl esters with Z-stereochemistry [136]. In related work, rhodium catalysts were also able to promote the addition reaction to regioselectively generate the anti-Markovnikov vinyl esters with Z-stereochemistry (Scheme 2.96) [141]. A wide range of carboxylic acids and a number of alkylalkynes were used in this chemistry. Curiously, phenylacetylene was unable to be converted into the vinyl ester following this approach. Additionally internal alkynes were also unresponsive under the reaction conditions. 

Both rnthenium- and rhodium-catalyzed routes are known for to generate the anti-Markovnikov addition products [136, 141]... 

SCHEME 3.127 Rhodium-catalyzed anti-Markovnikov hydroamination of bulky anilines [139]. 

The formal addition of HF to alkenes would be an attractive approach to the synthesis of alkyl fluorides. To circumvent the issues surrounding the use of hazardous HF for this reaction, a two-step approach has been developed (Example 7.7) [27]. The first step in the process consisted of a rhodium-catalyzed anti-Markovnikov hydroboration of an alkene in order to generate an intermediate alkylboronate. The second step entailed the use of a silver salt to promote a fluorodeboronation of the intermediate. The result of this two-step process was an overall hydrofluorination of an unactivated alkene. In related work, the regi-oselectivity of the hydrofluorination reaction was reversed using a cobalt-catalyzed process (Markovnikov selective) (Scheme 7.14) [28]. 

Thus, with RhHL more linear isomer would be formed, while for RhH(CO)4 the branched isomer would be favored. The steric bulk of a ligand such as PPhj is clearly more than that of CO, and its presence in the coordination sphere would therefore favor the anti-Markovnikov pathway. It is for this reason a large excess of PPhj, with a rhodium to phosphorous molar ratio ranging from 1 50 to 1 100, is used. Under these conditions, most of the precatalyst is present as RhH(CO)L3. 

Rhodium catalysts have also found application in oxidative aminations of styrenes. Beller and co-workers observed that numerous styrenes reacted with various kinds of secondary aliphatic amines in the presence of the cationic rhodium complexe [Rh(cod)2]BF4 and PPhs. Regioselectively the corresponding anti-Markovnikov products ( -enamines) were formed [49], While the Markovnikov product was never observed under such conditions, the target enamine was mostly obtained along with hydrogenated olefin, and in some cases even small amounts of hydroaminated products were detected [50],... 

Kondo M, Kochi T, Kakiuchi F (2011) Rhodium-catalyzed anti-markovnikov intermolecular hydroalkoxylation of terminal acetylenes. J Am Chem Soc 133 32-34... 

Another useful Rh-catalyzed reaction includes Ph2P(0)H addition to the triple bond of ethynyl steroids as air and water insensitive microwave-assisted hydrophosphinylation [91]. The reaction proceeded via the anti-Markovnikov addition, leading to the -/i-isomer. Rhodium complexes with hydrotris(3,5-dimethylpy-razolyl)borate ligand (Tp ) have shown catalytic activity in alkynes hydrophosphinylation with diphenylphosphine oxide [92]. Anti-Markovnikov products ( -)S-isomers) were formed in this reaction in moderate yields of 17-51%. Catalytic performance of Tp Rh(PPh3)2 and Tp Rh(cod) complexes was less efficient than [ClRh(PPh3)4] catalyst in the same conditions. 

Kawaguchi S-i, Kotani M, Ohe T, Nagata S, Nomoto A, Sonoda M, Ogawa A (2010) Rhodium-catalyzed anti-Markovnikov-type hydrophosphination of terminal alkynes with diphosphines and hydrosilanes in the presence of oxygen. Phosphorus, Sulfur, Silicon 185 1090-1097... 

The mechanism of olefin hydroformylation catalyzed by rhodium complexes has been extensively studied. For TPP as a ligand, it corresponds to Wilkinson s dissociative mechanism, which involves the four-coordinated active intermediate HRh(CO)L2 (L = TPP, Figure 14.2). Coordination of olefin with HRh(CO)L2 yields the 7t-complex 2. The insertion of coordinated olefin to the Rh-H bond leads to the formation of alkyl complexes 3a or 3b, respectively, via the anti-Markovnikov or the Markovnikov path. Subsequently, the alkyl migration to the CO affords the acyl complexes 4a or 4b, which leads to linear or branched aldehyde and HRh(CO)L2 via hydrogenolysis, eventually. The water-soluble catalyst HRh(CO)(TPPTS)3 is considered to react according to the dissociative mechanism [10]. However, the reaction occurs at the liquid phase or the gaseous-Hquid interface [11], and the activity and selectivity are remarkably different from those... 

Secondary amines can be added to certain nonactivated alkenes if palladium(II) complexes are used as catalysts The complexation lowers the electron density of the double bond, facilitating nucleophilic attack. Markovnikov orientation is observed and the addition is anti An intramolecular addition to an alkyne unit in the presence of a palladium compound, generated a tetrahydropyridine, and a related addition to an allene is known.Amines add to allenes in the presence of a catalytic amount of CuBr " or palladium compounds.Molybdenum complexes have also been used in the addition of aniline to alkenes. Reduction of nitro compounds in the presence of rhodium catalysts, in the presence of alkenes, CO and H2, leads to an amine unit adding to the alkene moiety. An intramolecular addition of an amine unit to an alkene to form a pyrrolidine was reported using a lanthanide reagent. 

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