Optically Active and C3-Symmetric Tris(pyrazolyl)hydroborate and Tris(pyrazolyl)phosphine Oxide Ligands: Synthesis and Structural Characterization

A New Synthesis of Charge‐Neutral Tris‐Pyrazolyl and ‐Methimazolyl Borate Ligands

“…[11] A number of bases were therefore selected (Table 1) and employed in the reaction with BA C H T U N G T R E N N U N G (NMe 2 ) 3 and methimazole (1:1:3 stoichiometry) heated to reflux in toluene, which provided the products (1) as colorless precipitates upon completion of the reaction (Scheme 4). Attempts to observe the reactive intermediates 3 in mixtures of BA C H T U N G T R E N N U N G (NMe 2 ) 3 and the bases by NMR spectroscopy at ambient temperatures provided spectra consistent only with mixtures of the two components, even for the strongest base examined 4.0]undec-7-ene), and we therefore concluded that the equilibrium concentration of the adducts [(Base)BA C H T U N G T R E N N U N G (NMe 2 ) 3 ] is insufficient to be observed spectroscopically, and thus that equilibrium constant K 1 is small. It was noted, however, that increased basicity resulted in increased rates of reaction, as measured by the time taken for cessation of the evolution of HNMe 2 from the reaction mixture (Table 1).…”

“…Ligands synthesized from a wide range of substituted pyrazoles are accessible by means of this route. [2] Analogous neutral tripods such as the tris(pyrazolyl)methanes (RC(pz) 3 ) [3] and phosporyl-centered ligands (O=P(pz) 3 ) [4] have also been developed. More recently, a new family of sulfur-donor ligands with the methimazolyl group (1-methylimidazolyl-2-thione) and its derivatives as the donor heterocycles, [HB(methimazolyl) 3 ] À (Tm), has been developed based upon a similar synthetic methodology (Scheme 1).…”

“…The previously postulated mechanisms to account for the formation of different products upon the reaction of BA C H T U N G T R E N N U N G (NMe 2 ) 3 with azoles of differing basicity: a) azoles with basic pK a < 3.5; b) azoles with basic pK a > 3.5 as exemplified by methimazole and imidazole, respectively. www.chemeurj.org temperature melt reaction of alkali metal tetrahydroborate salts with pyrazole provides the tetrakis(pyrazolyl)borate system [B(pz) 4 ] À , [1] the substitution of the remaining B À H group in [HBA C H T U N G T R E N N U N G (azolyl) 3 ] À systems has not been otherwise achieved in a controlled fashion. The preparation of such substituted ligands requires the use of [RBH 3 ] À salts or RB(OR) 2 systems.…”

The structures of chiral and racemate campho[2,3-c]pyrazole: A combined crystallographic, solid-state NMR and computational study

“…Campho [2,3-c]pyrazole (1) has been the subject of many studies: synthetic [4][5][6], structural [7], reactivity [3,[8][9][10][11][12], and as ligands in coordination chemistry [13][14][15][16][17][18][19]. The tautomerism has been studied proving that the proton is at the 2-position (2H-indazole) [7,20], a fact that has been related to the Mills-Nixon effect [21].…”

One‐Pot Synthesis of an Enantiopure N,N,O Scorpionate Ligand Derived from (+)‐Camphor

“…[13] Reaction of 1 with phosgene to afford 2,2Ј-carbonylbis(camphorpyrazole) (2b) was previously described in a footnote by Rebek and coworkers. [14] According to a very recent report by Artaud, phosgene can be replaced in such carbonylbis(pyrazole) syntheses by triphosgene.…”