C–H Activation at a Ruthenium(II) Complex – The Key Step for a Base‐Free Catalytic Transfer Hydrogenation?

Abstract

Keywords: Transfer hydrogenation / Ruthenium / C-H activation / Mass spectrometry / Density functional calculationsRuthenium(II) complexes [(η 6 -cymene)RuCl(apypm)]BPh 4 with bidentate 2-amino-4-(2-pyridinyl)pyrimidine (apypm) ligands catalyze the transfer hydrogenation of acetophenone. Their activities are strongly dependent on the substituent pattern of the pyrimidine ring. Complexes bearing a primary amino group in the 2-position of the pyrimidine ring do not perform the catalysis in terms of a "bifunctional mechanism", although they possess protic hydrogen atoms at the amino moiety in close proximity to the metal site. Systems contain-

Mechanistic Studies on Ruthenium(II)‐Catalyzed Base‐Free Transfer Hydrogenation Triggered by Roll‐Over Cyclometalation

“…For ad erivativec ontaining as econdary amino group (ArNH(nPr)) that is comparable to 3bBF 4 ,aRu1ÀN2 distance of 2.108 and aN 1-C5-C6-N2 dihedral angle of 3.48 were found. [9] The RuÀNd istances for compound 3gPF 6 ,w hich crystallizes with two crystallographicallyi ndependent molecules in the unit cell, were reported to be in the range of 2.087-2.101 ; [15] this is in agreement with the Ru1ÀN1 distances found in our study.T he dihedrala ngle N1-C5-C6-N2 of 3gPF 6 is closet o0 8.T ogether with previously published solid-state structures, [9] the structurald ata of the new ruthenium(II) complexes clearly prove the pronounced influence of the substituent attached in the 2-position of the 2-(pyrimidin-4-yl)pyridinetype ligand on the Ru1ÀN2 distance and, in particular, on the dihedrala ngle N1-C5-C6-N2. While the distances between the ruthenium(II) site Ru1 and the pyridine nitrogena tom N1 differ only slightly in all ruthenium complexes, there is clearly ab road variation in the Ru1ÀN2 distances.…”

“…The functionalized pyrimidine moieties are subsequently generatedb yc ondensation of 1 with formamidinium acetate, which resultsi nt he formation of 2-(pyrimidin-4-yl)pyridine (2a). [13] Alternatively, an appropriate guanidinium salt leads to 2-(2-aminopyrimidin-4-yl)pyridines 2b-d. [9] Condensation with urea gives 2-(2-hydroxypyrimidin-4-yl)pyridine (2e), which can then be convertedi nto the correspondingchloro derivative 2f.…”

“…Although the cyclization of 1 with formamidinium and guanidiniums alts, which leads to ligands 2a-d,requires basic conditions, [9] the condensation of 1 with urea, giving compound 2e,h as to be performedi nt he presenceo fa na cid. The yield for this transformationi sh igh, as it is for the subsequent introductionofachloro substituent by treating 2e with POCl 3 .…”

“…In 2013, we reported af irst but limited study on the two [(h 6cymene)(N,N'-ligand)RuCl] + cations 3a + + (tertiary amino group) and 3d + + (primary amino group), as wella so na na nalogous system containing at ertiary (2-pyrrolidinylpyrimidinyl) moiety. [9] It could clearly be shown that all three complexes underwent fragmentation of HClu nder CID EIS-MS conditions and the protono riginated from the ortho-CH moiety of the pyrimidine ring. The latter fact was proved by selective deuteration of this site.…”

“…In 2013, we reported that ruthenium(II) complexes with N,N'chelating 2-(2-aminopyrimidin-4-yl)pyridinel igands could undergo as elf-activation process:c leavageo ft he ortho-CH bond at the pyrimidine ringt hrough ar oll-over cyclometalation mechanism [8] resulted in the in situ formation of ar utheniumcoordinated carbanion. This was the key step to develop novel moisture-and air-stable catalysts that allowed the transfer hydrogenation of ketones, [9] and quite recently the reductiveamination of aromatic aldehydes, to be performed under base-free conditions. [10] However,t his special feature has, to date, been restricted to 2-(2-aminopyrimidin-4-yl)pyridine ligands with tertiary amino groups attached to the pyrimidine ring.…”