Alkali Metal Species in the Reversible Activation of H₂

Abstract

MP(tBu) 2 (M = Li, Na, K), KH and KN(SiMe 3 ) 2 are shown to activate HD reversibly.Inthe case of MP(tBu) 2 this leads to isotopic scrambling and the formation of H 2 ,D 2 , H(D)P(tBu) 2 and MH(D) in C 6 D 6 .Intoluene,KP(tBu) 2 reacts with H 2 but also leads to isotopic scrambling into the methyl groups of the solvent toluene.D FT calculations reveal that these systems effect H 2 activation via cooperative interactions with the Lewis acidic alkali metal and the basic phosphorus, carbanion, or hydride centres,mimicking frustrated Lewis pair (FLP) behaviour.Heterogeneous transition-metal-mediated hydrogenation was discovered by Sabatier in 1899. [1] Ther eduction of multiple bonds has since become au biquitous process throughout academic chemistry and across chemical industry. [2] Moreover,t he field of catalytic hydrogenation has evolved considerably.I nt he 1960-1990s,t he advent of homogeneous precious transition-metal catalysts provided access to specifically designed catalysts allowing for enhanced substrate selectivity and the added sophistication of enantioselectivity. [3] These findings and the numerous successful applications of transition-metal systems established the dogma dictating the need for transition-metal catalysts for hydrogenation.In the early 1960s,W alling and Bollyky [4] reported metalfree hydrogenation of benzophenone in the presence of KOtBu, albeit at temperatures ca. 200 8 8Ca nd H 2 pressure of > 100 bar. This system was subsequently studied in detail by Berkessel et al. [5] Similarly,soluble LiAlH 4 [6] or suspensions of NaH, KH, and MgH 2 [7] were reported to effect catalytic hydrogenation of several olefinic and acetylenic substrates, although again rather extreme temperatures (150-225 8 8C) and H 2 pressures (60-100 bar) were required. Under similar forcing conditions (280 8 8C, 150 bar H 2 pressure), Haenel et al. reported the reductions of polyaromatics in coal via ac atalytic hydroboration/hydrogenolysis process. [8] In 2006, the discovery of frustrated Lewis pairs (FLPs) broadened the scope of hydrogenation catalysts to main-group compounds. [9] Combinations of Lewis acids and bases were found to act in concert to activate dihydrogen in aheterolytic fashion under comparatively mild conditions and have been applied to reduce aw ide range of organic substrates including imines, enamines,olefins,alkynes,polyaromatic species,ketones and aldehydes (Figure 1). [9c, 10] Moreover,c hiral FLP catalysts have yielded highly enantioselective reductions. [11] In 2008, Harder and coworkers [12] reported the use of NacNac dippcalcium and strontium complexes (dipp = 2,6-diisopropylphenyl;N acNac dipp = CH{(CMe)(2,6-iPr 2 C 6 H 3 N)} 2 )t oe ffect the hydrogenation of olefins,a nd showed that related alkalimetal systems require high H 2 pressures (100 bar). Subsequently,t hese Group 2s ystems were expanded to include Okudasw ork on trimetallic calcium [13] and strontium [14] hydride cations,[ (Me 3 TACD) 3 M 3 (m 3 -H) 2 ] + (Me 3 TACD = 1,4,7-trimethyl-1,4,7,10-tetraazacyclododecane;M= Ca, Sr)...

Synthesis of Sterically Demanding Secondary Phosphides and Diphosphanes and Their Utilization in Small-Molecule Activation

“…Therefore, 4a was reacted in the first step with CS 2 to form the bulky potassium phosphanyl dithioformate 10 . The reaction behavior of 4a toward CS 2 is in accordance with previously reported reactions of bulky group 1 phosphides with small molecules. − …”

Cesium Amide‐Catalyzed Selective Deuteration of Benzylic C‐H Bonds with D2 and Application for Tritiation of Pharmaceuticals

“…(4)], did take place in presence of CsHMDS, suggesting the existence of the carbanion or hydride intermediates. On the basis of these observations and former reports, [16, 18] we proposed a plausible process as shown in Scheme 2B. The cesium amide could undergo the kinetic deprotonative process to produce benzyl anion as a thermodynamically uphill intermediate ( 1 ) at very low concentrations.…”

“…The poor activity of base catalysts towards T 2 (H 2 ) remains a significant challenge for high‐specific‐activity tritiation. In 2018, Stephan and co‐workers revealed that alkali metal species can mimic frustrated Lewis pair (FLP) behavior for reversible activation of H 2 , and specifically KH was shown to deuterate benzylic C−H bonds of toluene in presence of D 2 gas, albeit with only 15 % deuterium incorporation (Scheme 1C1) [16] . Furthermore, Harder and co‐workers later found the heavier alkaline earth metal amides to be active catalysts for HIE of arenes with D 2 gas, but showing moderate level of isotope incorporation at benzylic C−H bonds (Scheme 1C2) [17] .…”