Protic ligands

One of the most intensively studied protic chiral ligands, which moreover allows enantioselectivities of up to 95% ee, is the proline-derived (25 )-l- [(25,)-l-methyl-2-pyrrolidinyl]methyl -2-pyrrolidine methanol (6)19-21. 

Likewise, triphenyltin hydride reacts with ethylzinc chloride, or triphenyltin chloride with metallic zinc, to give the compound PhaSnZnCl, which is stable in the presence of a strongly coordinating ligand, but, in its absence, apparently undergoes an intermetallic shift of the organic group, so that protic acids react to liberate benzene (272). 

These ligands were used in protic and biphasic media by modifying their structure using hydroxyalkyl groups [28,29]. The solubihty of the corresponding Ru(II) complexes was significantly increased in protic solvents. Hence, by performing the reaction in mixtures of toluene and water or al-... 

Ligand screening experiments were conducted on the alkenes 1-pentene and pent-4-en-l-ol, because such substrates were inert to 3a-3c (15). Pentene lacks any polar or protic group and pentenol contains the alkene and OH separated by 3 carbons. The preliminary studies involved phosphines with both imidazol-2-yl and pyrid-2-yl substituents on P as well as t-Bu, i-Pr, Ph, and Me groups (16). From the screening, complex 1 derived from the phosphine ligand 4 (17) was identified as the most capable (in terms of both reaction rate and final yield) of promoting isomerization of both 1-pentene and pent-4-en-l-ol. 

The ligand group can be introduced either on the meso or on the /5-pyrrole position of the porphyrin ring, but the synthesis of the meso-functionalized derivatives is easier and has been more widely exploited. Balch (50-53) reported that the insertion of trivalent ions such as Fe(III) (32) and Mn(III) (33) into octaethyl porphyrins functionalized at one meso position with a hydroxy group (oxophlorins) leads to the formation of a dimeric head-to-tail complex in solution (Fig. 11a) (50,51). An X-ray crystal structure was obtained for the analogous In(III) complex (34), and this confirmed the head-to-tail geometry that the authors inferred for the other dimers in solution (53) (Fig. lib). The dimers are stable in chloroform but open on addition of protic acids or pyridine (52). The Fe(III) octaethyloxophlorin dimer (52) is easily oxidized by silver salts. The one-electron oxidation is more favorable than for the corresponding monomer or p-oxo dimer, presumably because of the close interaction of the 7r-systems in the self-assembled dimer. 

Dehydrative condensation of transition metal hydroxo complexes is a versatile synthetic method, when the conjugated acid of the incoming ligand (H-A) is a protic compound. There are reports of the synthesis of two types of square-planar hydroxopalladium complexes, mononuclear [TplPr2 x(L)Pd-OH] (X = H, Br) and dinuclear complexes [TplPr2Pd(H20)( -0H)2 (H20)PdTplPr2] (TplPr2 = hydrotris(3,5-diisopropylpyrazolyl)borate), and some dehydrative condensations of them.278... 

Dimeric zinc complexes of tertiary phosphines, [Zn(PR3)I2]2, are also formed from zinc powder and R3PI2, (R = Me, Et, n-Pr, ra-Bu). The crystal structure of the ethyl derivative demonstrates the dimeric nature of the complexes.295 Metallation of diphenylphosphine with ZnEt2 results in a trimeric species with a Zn3P3 core and bridging diphenyl phosphide ligands. Two protic (HPPh2)... 

Recently, a rhodium-catalyzed tandem cyclization has been reported with an arylboronic ester bearing a pendant Michael-type acceptor olefin and acetylenic65 or olefinic66 derivatives. This transformation proceeds in a water-containing medium as solvent and proton source. This catalyst system is optimized with electron-rich and bulky ligands to stabilize the rhodium intermediate and decrease the protonolysis of boron derivatives in a protic solvent. 

For example, the substituted aniline Ar-NH2 (Ar = />-CH3OC6H4) reacts with the ruthenium nitrosyl complex Ru(bpy)2(Cl)(NO)2+ (bpy = 2,2 -bipyridine) to give a complex of the diazo ligand, namely Ru(bpy)2(Cl)(NNAr)2+ (57). Upon employing the 15N labeled nitrosyl complex Ru(bpy)2Cl(15NO)2+ this reaction resulted in the 15N coordinated product, Ru(bpy)2Cl(15NNAr)2+, demonstrating that the reaction occurs within the metal complex coordination sphere. When the reactions were conducted in non-protic solvents, these nucleophile-nitrosyl adducts could be isolated. 

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