Iron Borohydride Pincer Complexes for the Efficient Hydrogenation of Ketones under Mild, Base‐Free Conditions: Synthesis and Mechanistic Insight

Abstract

The new, structurally characterized hydrido carbonyl tetrahydridoborate iron pincer complex [(iPr-PNP)Fe(H)(CO)(η(1)-BH(4))] (1) catalyzes the base-free hydrogenation of ketones to their corresponding alcohols employing only 4.1 atm hydrogen pressure. Turnover numbers up to 1980 at complete conversion of ketone were reached with this system. Treatment of 1 with aniline (as a BH(3) scavenger) resulted in a mixture of trans-[(iPr-PNP)Fe(H)(2)(CO)] (4a) and cis-[(iPr-PNP)Fe(H)(2)(CO)] (4b). The dihydrido complexes 4a and 4b do not react with acetophenone or benzaldehyde, indicating that these complexes are not intermediates in the catalytic reduction of ketones. NMR studies indicate that the tetrahydridoborate ligand in 1 dissociates prior to ketone reduction. DFT calculations show that the mechanism of the iron-catalyzed hydrogenation of ketones involves alcohol-assisted aromatization of the dearomatized complex [(iPr-PNP*)Fe(H)(CO)] (7) to initially give the Fe(0) complex [(iPr-PNP)Fe(CO)] (21) and subsequently [(iPr-PNP)Fe(CO)(EtOH)] (38). Concerted coordination of acetophenone and dual hydrogen-atom transfer from the PNP arm and the coordinated ethanol to, respectively, the carbonyl carbon and oxygen atoms, leads to the dearomatized complex [(iPr-PNP*)Fe(CO)(EtO)(MeCH(OH)Ph)] (32). The catalyst is regenerated by release of 1-phenylethanol, followed by dihydrogen coordination and proton transfer to the coordinated ethoxide ligand.

Iron/Brønsted Acid Catalyzed Asymmetric Hydrogenation: Mechanism and Selectivity‐Determining Interactions

“…[8] Milstein has reported pincerbased iron hydrogenation catalysts for symmetric reduction of ketones. [10,11 ] Morris and co-workers have applied chiral ironbased hydrogenation catalysts with tridentate [Fe(P-N-P)] and tetradentate [Fe(P−N−N−P) ligands in the asymmetric hydrogenation of ketones and imines. [ 12 , 13 ] Casey and Guan reported symmetric hydrogenation of aldehydes, ketones, and imines with Knölker's iron complex, which possesses a cyclopentadienone ligand.…”

Hydrogenation of Polar Bonds Catalysed by Ruthenium-Pincer Complexes

“…in direct, efficient hydrogenation of CO 2 to methanol). The development of efficient catalysts based on iron pincer complexes is an important challenge; promising results in the hydrogenation of ketones and CO 2 were already obtained [23][24][25]. …”

“…While there are many examples of Ru-catalysed hydrogenations of ketones and aldehydes [7,11], and recently Fe-catalysed hydrogenation of ketones was also reported [35][36][37][38][39], including our system based on pincer-iron complexes [24,25], catalytic hydrogenation of esters, particularly non-activated ones, under mild conditions is a challenging task [6,7,9,10,40,41]. In pioneering work by Elsevier et al various aromatic and aliphatic esters were hydrogenated in fluorinated solvent using in situ prepared ruthenium complexes bearing P,P,P ligands at high pressure of dihydrogen under basic conditions [42].…”

“…In this chapter we describe straightforward hydrogenation of various carbonyl compounds, in particular, carboxylic and carbonic acid derivatives catalysed by Ru(II)-pincer complexes. We have also reported Fe(II)-pincer catalysed hydrogenation of ketones and CO 2 under very mild conditions [23][24][25].…”

Binuclear Copper(I) Borohydride Complex Containing Bridging Bis(diphenylphosphino) Methane Ligands: Polymorphic Structures of [(µ2-dppm)2Cu2(η2-BH4)2] Dichloromethane Solvate

“…Copper(I) tetrahydroborates with phosphine ligands featuring relative stability to air oxygen and moisture are used as selective reducing agents [36][37][38][39][40], catalysts of photosensitized isomerization of dienes [41][42][43] and hydrolytic dehydrogenation of ammonia borane [44]. Since metal tetrahydroborates have great potential in hydrogen storage technology [45][46][47][48][49][50], as catalysts [51][52][53][54][55] and selective reducing agents [56][57][58][59][60][61] their structural and dynamic properties have been actively investigated [52,[62][63][64]. These studies revealed different modes of BH 4 − coordination to the metal atom, which can behave as mono-, bi-, or tridentate ligand [64].…”