Lateral Stereocontrol Using π-Allylmolybdenum Systems:  The Use of Methyl Substitution To Effect Conformational Control Leading to High Levels of Stereoselectivity

Abstract

Stereocontrol exerted by a pi-allyl-Mo(CO)(2)Tp system (Tp = hydrotris(1-pyrazolyl)borato), during addition of Grignard reagents to neighboring aldehyde functionality and during dihydroxylation of alkenes, is found to be dependent on conformational preferences of the substituent relative to the organometallic moiety. Incorporation of a methyl group at C(2) of the pi-allyl ligand leads to excellent conformational control when the lateral substituent is a vinyl group, and this results in a diastereomer ratio of 25:1 for the diol that is obtained from osmylation. Poorer conformational control is observed for an aldehyde, and nucleophile additions show correspondingly lower stereoselectivity. Introduction of a methyl substituent at C(1) of the pi-allyl ligand does not effect conformational control, as expected, and very poor lateral stereocontrol is observed during both alkene dihydroxylation and nucleophile additions to aldehyde.

Molybdenum and Tungsten Tricarbonyl Complexes with the Tripodal Ligands [nBuSn(2-pyridyl)3] and [RSn(methylthiomethyl)3]

“…These ligands are conveniently prepared by reaction of pyrazoles with the tetrahydroborate anion. Complexes with tris(pyrazolyl)borates are now known with almost every metallic element, and their applications include stoichiometric organic transformations, − catalysis, and modeling of enzymatic metal sites …”

Stereo- and Regiocontrolled Construction of Trisubstituted Piperidines Using a TpMo(CO)2(Dihydropyridine) Scaffold (Tp = Hydridotrispyrazolylborate)

“…The sequential, multistep, stereo- and regiocontrolled functionalization−demetalation of stoichiometric transition metal π-complexes has received considerable attention as a means of producing highly substituted organic molecules. − Stoichiometric π-complexes of molybdenum bound to polyene ligands are especially well suited to this task. ,, In this regard, hydridotris(1-pyrazolyl)borate (Tp) − used in place of the more commonly studied cyclopentadienyl (Cp) auxiliary ligand has significantly extended the applicability of the molybdenum-based methodology. ,,− In particular, the TpMo(CO) 2 ligand set has rendered useful the functionalization of a variety oxygen-containing heterocyclic systems. ,, Since the core units of a vast number of biologically active synthetic and natural products are nitrogen-containing heterocycles, the TpMo(CO) 2 -based synthetic methodology could prove of use in the stereocontrolled construction of these systems, also. To that effect, we report herein the synthesis, functionalization, and demetalation of a chiral, racemic TpMo(CO) 2 (dihydropyridine) complex and its use in the synthesis of 2,3,6-trisubstituted tetrahydropyridines and piperidines.…”

Diastereoselective Conjugate Additions to π-Allylmolybdenum Complexes: A Stereocontrolled Route to 3,4,5-Trisubstituted γ-Butyrolactones

“…Conjugate addition of Grignard reagents to 6b and 8b occurred with very good yields and complete diastereoselectivity (Table ). , The relative stereochemistry of products 9 − 14 was assigned on the basis of the known mode of attack of the reagent, anti to the [Mo(CO) 2 Tp] moiety, with the substrate in a s-trans conformation with respect to the C(3)−C(4) single bond . As expected, with substrates in anti configuration, diastereoselectivity was no longer dependent on the Me group at position 2, as opposed to the reactions previously performed on syn substrates, where this Me group was essential to shift the conformational equilibrium associated with rotation about the C(3)−C(4) bond toward a s-trans conformer. ,, Thus, addition of PhMgBr to 8b was also completely diastereoselective (Table , entry 6).…”