Manganese‐Catalyzed Insertion of Aldehydes into a CH Bond

Abstract

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(I)‐Catalyzed C−H Activation: The Key Role of a 7‐Membered Manganacycle in H‐Transfer and Reductive Elimination

“…Manganacycle 4gsits at the selectivity junction to reductive elimination or H-transfer steps.Depending on the reaction conditions, 5g or 6g products form that correspond to reductive elimination and protonation pathways,respectively.Double alkyne insertion to give 10 has also been revealed in these studies.O ur observations provide the first clear cut evidence that manganacycles such as 4 are key intermediates in Mn I -mediated CÀHb ond activation processes involving substrates containing directing groups. [3,4,7] More generally,s uch intermediates may be considered as leading to side reactions,b ut here we have shown that it presents an opportunity to control product selectivity.Serendipitously we have uncovered arare example of aD AR of ap yridine derivative,w here the intermediate fragments to form products such as 8 and 9.T aken together, our findings provide aunique insight into Mn I -mediated CÀH bond activation processes,e specially how relatively minor changes in substrate structure influence product selection; Mn I -based metallocycles clearly offer rich chemistry, [3] much potential, and warrant further study more generally in organic and organometallic chemistry.…”

“…When compared with the formation of IV Pyr and IV Ph ,w hich is the next step in forming H-transfer products 5g and 5a,r espectively,i ti se vident that the reductive elimination to form 5g is competitive,b ut in the case of 5a the much larger energetic span to reductive elimination allows for productive catalysis via alkyne coordination to give IV Ph . [4] While no double alkyne insertion products were detected in reactions of 2g with phenylacetylene 3,t he reaction of related derivative 2h with 3 resulted in exclusive formation of double alkyne insertion product 10 (Scheme 6; the structure of 6h is shown as an expected alkenylated product). This remarkable result shows the impact that as ubtle change to the pyridyl directing group has on the barriers to these steps.…”

“…Interestingly,c atalytic reactions of 1g with 3,u nder the reaction conditions reported by Wang et al [4] for2 -phenylpyridine 1a (conditions:B rMn(CO) 5 ,C y 2 NH, Et 2 O, 100 8 8C for 6-24 h), do not lead to formation of alkenylated products (for example, 6g). This indicates that the rate of reductive elimination from 4g to give 5g is faster than the rate for alkyne H-transfer to give 6g(see above).…”

“…[3] Fore xample, 1 gives cyclomanganated complex 2,w ith subsequent reaction with alkyne 3 forming aproposed 7-membered ring intermediate 4 (Scheme 1). [4] Formation of either 5, 6,o r7 results from reductive elimination, H-transfer,o rd ehydrogenative annulation, respectively.…”

“…Them echanistic features of the remarkable synthetic work of Ackermann and Wang, [3,4] where intermediates 4a-c have been proposed, prompted us to examine whether they could be detected and characterized and then subsequently be shown to deliver organic products such as 5-7.C omplexes 4d-f,f ormed by insertion of internal alkynes are known, [6,7] but their competence in terms of af ully connected reaction system, affording organic products,h as not been examined. As 18-electron species containing four CO ligands,p ossessing high thermodynamic stability,t hey are unlikely to be directly involved in the catalytic cycle.…”