Transition metals of the fourth row are abundant and cheap compared to those of the fifth and sixth rows. Therefore, by introducing new reactivities with fourth-row metal complexes, it might be possible to replace fifth-and sixth-row metals in some fundamental and important reactions. Catalytic Grignard-type addition of nucleophiles to aldehydes is one such reaction. Grignard reagents are usually prepared from organic halides and magnesium metal, [1] but this procedure results in the unwanted formation of stoichiometric amounts of metal salts. One way to solve this problem is to generate the nucleophiles by CÀH bond activation; [2] however, it has been difficult to promote nucleophilic addition after the CÀH activation step. Although nucleophilic addition of species generated by CÀH activation has been reported using ruthenium, [3] rhodium, [4] palladium, [5] and rhenium [6] catalysts, it has been difficult to catalyze such reactions using fourthrow transition-metal complexes. [7] We report herein that 1) complexes of manganese, a fourth-row transition metal, can be employed for CÀH bond activation of aromatic compounds; 2) insertion of aldehydes into CÀH bonds occurs to give benzyl alcohols; and 3) catalytic transformation is achieved with the manganese complex by the addition of Et 3 SiH.We initially investigated stoichiometric CÀH bond activation and insertion of aldehydes with the manganese complex [MnBr(CO) 5 ]. A mixture of 1-methyl-2-phenyl-1H-imidazole (1 a) and [MnBr(CO) 5 ] in toluene was heated at 100 8C for 5 min, at which point a solution of benzaldehyde (2 a) was added. The mixture was heated at reflux for 10 h to give alcohol 3 in 52 % yield (Scheme 1). Although stoichiometric CÀH bond activation and insertion of the aldehyde occured with [MnBr(CO) 5 ], only a trace amount of 3 was produced with a catalytic amount of the manganese complex.To recycle the manganese complex, 2.0 equiv of triethylsilane (4) was added to the reaction mixture from the beginning. As a result, silyl ether 5 a was obtained in 93 % yield with 5 mol % [MnBr(CO) 5 ]. [8,9] We examined the catalytic activity of several metal complexes using the reaction between 1 a, 2 a, and 4 as a probe. A different manganese complex, [Mn 2 (CO) 10 ], showed similar catalytic activities (82 % yield of 5 a). However, the reaction did not proceed at all with the following metal complexes: [MnCl 2 ], [Mn(acac) 3 ] (acac = acetylacetonate), [{ReBr(CO) 3 (thf)} 2 ], [6]

Transition metals of the fourth row are abundant and cheap compared to those of the fifth and sixth rows. Therefore, by introducing new reactivities with fourth-row metal complexes, it might be possible to replace fifth-and sixth-row metals in some fundamental and important reactions. Catalytic Grignard-type addition of nucleophiles to aldehydes is one such reaction. Grignard reagents are usually prepared from organic halides and magnesium metal, [1] but this procedure results in the unwanted formation of stoichiometric amounts of metal salts. One way to solve this problem is to generate the nucleophiles by CÀH bond activation; [2] however, it has been difficult to promote nucleophilic addition after the CÀH activation step. Although nucleophilic addition of species generated by CÀH activation has been reported using ruthenium, [3] rhodium, [4] palladium, [5] and rhenium [6] catalysts, it has been difficult to catalyze such reactions using fourthrow transition-metal complexes. [7] We report herein that 1) complexes of manganese, a fourth-row transition metal, can be employed for CÀH bond activation of aromatic compounds; 2) insertion of aldehydes into CÀH bonds occurs to give benzyl alcohols; and 3) catalytic transformation is achieved with the manganese complex by the addition of Et 3 SiH.We initially investigated stoichiometric CÀH bond activation and insertion of aldehydes with the manganese complex [MnBr(CO) 5 ]. A mixture of 1-methyl-2-phenyl-1H-imidazole (1 a) and [MnBr(CO) 5 ] in toluene was heated at 100 8C for 5 min, at which point a solution of benzaldehyde (2 a) was added. The mixture was heated at reflux for 10 h to give alcohol 3 in 52 % yield (Scheme 1). Although stoichiometric CÀH bond activation and insertion of the aldehyde occured with [MnBr(CO) 5 ], only a trace amount of 3 was produced with a catalytic amount of the manganese complex.To recycle the manganese complex, 2.0 equiv of triethylsilane (4) was added to the reaction mixture from the beginning. As a result, silyl ether 5 a was obtained in 93 % yield with 5 mol % [MnBr(CO) 5 ]. [8,9] We examined the catalytic activity of several metal complexes using the reaction between 1 a, 2 a, and 4 as a probe. A different manganese complex, [Mn 2 (CO) 10 ], showed similar catalytic activities (82 % yield of 5 a). However, the reaction did not proceed at all with the following metal complexes: [MnCl 2 ], [Mn(acac) 3 ] (acac = acetylacetonate), [{ReBr(CO) 3 (thf)} 2 ], [6]

Manganese-Catalyzed Insertion of Aldehydes into a C-H Bond.-(KUNINOBU*, Y.; NISHINA, Y.; TAKEUCHI, T.; TAKAI, K.; Angew. Chem., Int. Ed. 46 (2007) 34, 6518-6520; Div. Chem. Biochem., Okayama Univ.,

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