Enantioenriched Molybdenum Dearomatization: Dissociative Substitution with Configurational Stability

Abstract

The preparation and properties of the complex (R Mo ,R)-MoTp(NO)(DMAP)(η 2 -α-pinene) are described (∼10 g scale; DMAP = 4-(dimethylamino)pyridine; Tp = hydridotris(pyrazolyl)borate). This complex undergoes exchange of the pinene with a wide range of other π ligands including acetone, ethyl acetate, N,N-dimethylformamide, acetonitrile, and naphthalene. Treatment of the α-pinene complex with iodine results in the complex (S)-MoTp(NO)(DMAP)(I), which is recovered in enantioenriched form (er = 99:1; yield >90%; scale 4.6 g). Reduction of this molybdenum(I) precursor results in enantioenriched molybdenum(0) complexes, including (R)-MoTp-(NO)(DMAP)(η 2 -trifluorotoluene). Sequential treatment of this arene complex with acid, a masked enolate, and iodine regenerates MoTp(NO)(DMAP)(I) along with an alkylated 1-(trifluoromethyl)cyclohexa-1,3-diene with an er value as high as 99:1. This process demonstrates the efficient transfer of asymmetry from α-pinene to the diene product. Accompanying studies with (1R)-myrtenal reveal a redox-catalyzed pinene/myrtenal ligand exchange occurring through Mo(I) intermediates.

Highly Functionalized Cyclohexenes Derived from Benzene: Sequential Tandem Addition Reactions Promoted by Tungsten

“…To effect this 3,4-η 2 to 1,2-η 2 diene isomerization, our strategy was to utilize a redox catalyst similar to that used for ligand substitution reactions with the {TpMo(NO)(DMAP)} system. 19 Gratifyingly, we found that stirring the 3,4-η 2 -diene 6A with 0.1 equiv of the mild oxidant ([Fe(Cp*) 2 ]Tf 2 N) resulted in almost complete conversion to the 1,2-η 2 isomer 29 within 15 min (by 1 H NMR), with an isolated yield of 87% (Scheme 4). The initial identification of 29 as the 1,2-η 2 form of 6A was accomplished using 2D NMR analysis.…”

“…If successful, this could give rise to η 2 -cyclohexene products with complementary substitution patterns to those reported above. To effect this 3,4-η 2 to 1,2-η 2 diene isomerization, our strategy was to utilize a redox catalyst similar to that used for ligand substitution reactions with the {TpMo­(NO)­(DMAP)} system . Gratifyingly, we found that stirring the 3,4-η 2 -diene 6A with 0.1 equiv of the mild oxidant ([Fe­(Cp*) 2 ]­Tf 2 N) resulted in almost complete conversion to the 1,2-η 2 isomer 29 within 15 min (by 1 H NMR), with an isolated yield of 87% (Scheme ).…”

“…Recently, our group has shown the use of an oxidant to catalyze ligand substitution reactions for {MoTp(NO)- (DMAP)} systems. 19 Similarly, in this work, we have demonstrated a redox-catalyzed intramolecular alkene isomerization from the 3,4-η 2 diene 6A to its 1,2-η 2 isomer 29. We theorize that upon oxidation, the diminished electron density of the W(I) system and coinciding reduced ability to backdonate weakens the tungsten alkene bond and enables an intramolecular linkage isomerization.…”

Asymmetric organic synthesis with stoichiometric transition metal complexes

“…Reasoning that oxidation of the Mo(0) center to Mo(I) would minimize the racemization at the metal stereocenter, an impressive solution to this problem was designed. 57 Thus, oxidative (I2, THF) replacement of the -pinene ligand in the Mo(0)-complex afforded a Mo(I)-iodo complex in an excellent yield and high enantiopurity. Next, the enantioselective dearomatization of trifluoromethyl benzene was explored.…”

“…Towards this goal, reduction of racemic Mo(I)-complex with Na in the presence of (R)--pinene afforded the Mo(0)-complex with the chiral ligand as a single diastereomer with (RMo,RLig) configuration (Scheme 30). 57 Due to the lower -basicity of the Mo-complex in comparison to Re or W analogs, the mismatched (SMo,RLig) diastereomer proved too unstable to be isolated. The matched (RMo,RLig) diastereomer participated in subsequent ligand exchange reactions quite easily at ambient temperature.…”