Preparation of Dicarbonyl[hydrotris(1-pyrazolyl)borato](η³-allyl)molybdenum Complexes Bearing Electron-Donating Substituents (1-((tert-Butyldimethylsilyl)oxy), 1-Alkoxy, and 1-Acetoxy) via the Nucleophilic Addition of Mo(CO)₃(DMF)₃ to Enals and Enones

Abstract

Cyclic and acyclic dicarbonyl[hydrotris(1-pyrazolyl)borato][η 3 -1-((tert-butyldimethylsilyl)oxy)allyl]molybdenum complexes and a variety of their 1-acetoxy and 1-alkoxy (RO ) MeO, i-PrO) analogues were prepared and characterized by IR and 1 H and 13 C NMR spectroscopy and, in the case of [TpMo(CO) 2 [η-(1,2,3)-(()-(1R,2S,3S)-1-methoxy-2-cyclohexen-1-yl], by X-ray crystallography. These complexes were prepared in moderate to excellent yields by the tert-butyldimethylsilyl chloride promoted oxidative addition of R,β-unsaturated aldehydes and acyclic and cyclic ketones to (DMF) 3 Mo(CO) 3 followed by ligand metathesis with potassium hydrotris(1-pyrazolyl)borate. The 1-tert-butyldimethylsiloxy-and the 1-methoxyand 1-isopropoxy-substituted acyclic complexes were formed solely as the syn isomer; however, the 1-acetoxy analogue underwent isomerization to a thermodynamic mixture of the syn and anti isomers in which the anti isomer predominated (3.7 : 1). The 1-((tertbutyldimethylsilyl)oxy)-3-alkyl-or 1-alkoxy-3-alkyl-disubstituted acyclic complexes were formed with syn-silyloxy/anti-alkyl or syn-alkoxy/anti-alkyl stereochemistry, while the disubstituted allyls bearing a 1-acetoxy substituent existed as mixtures of both possible syn/ anti isomers and the syn/syn and anti/anti isomers. The conformation and configuration of the isomers was confirmed through nOe studies on several complexes and by X-ray crystallography in the case of [TpMo(CO) 2 [η-(1,2,3)-(()-(1R,2S,3S)-1-methoxy-2-cyclohexen-1-yl].

Studies on Lateral Stereocontrol Using the (π-Allyl)molybdenum System

“…Complex 1 could be reduced uneventfully to the alcohol 2 , which was oxidized to aldehyde 3 using the standard Swern protocol . The syn stereochemistry for 1 − 3 (L = Tp) was assigned by comparison with 1 H NMR coupling constant data reported by Liebeskind et al13a While 3 could be converted to the olefin 4 (Chart ) by standard Wittig methylenation, we decided to investigate a more direct route to 4 that would furnish significant quantities for further study, as described in the next section.

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Regio- and Stereospecific Uncatalyzed Reactions of Electron-Rich Arenes and Olefins at Organomolybdenum Enantiomeric Scaffolds

“…Readily available enantiopure, air- and moisture-stable TpMo­(CO) 2 (5-oxo-η 3 -6 H -pyranyl)­and TpMo­(CO) 2 (5-oxo-η 3 -6 H -pyridinyl) complexes, 1 and 2 , and their derivatives are synthetically useful scaffolds for the enantiocontrolled construction of substituted heterocycles (Scheme ; Tp = hydridotris­(pyrazolyl)­borato). − The preparation of multigram quantities of high-enantiopurity complexes is relatively straightforward. , For carbon–carbon and carbon–heteroatom bond constructions, both TpMo­(CO) 2 -stabilized cation ,,,,,− , and anion − , pathways have been observed and applied in the enantiocontrolled synthesis of natural products. The TpMo­(CO) 2 -stabilized carbocations react regio- and stereospecifically with a variety of sp 3 -, sp 2 -, and sp-hybridized Grignard and lithium reagents. ,,,,− Complementing the stabilized cation pathway, a TpMo­(CO) 2 -mediated nucleophilic functionalization pathway was also described.…”

Practical, Scalable, High-Throughput Approaches to η3-Pyranyl and η3-Pyridinyl Organometallic Enantiomeric Scaffolds Using the Achmatowicz Reaction

“…High enantiopurity TpMo(CO) 2 (η 3 -pyranyl) and TpMo(CO) 2 (η 3 -pyridinyl) complexes have proven to be versatile organometallic enantiomeric scaffolds. − Taking advantage of novel trends in reactivity and selectivity, single enantiomers of these air- and moisture-stable pyranyl and pyridinyl organometallic scaffolds have been used in the enantiocontrolled construction of a diverse set of heterocyclic organic systems exemplified in Figure . Of strategic interest in organic synthesis, the principles of organometallic enantiomeric scaffolding allow the development of parallel reaction profiles applicable to both pyran- and piperidine-derived systems, an option not available with traditional organic enantiomeric scaffolds.…”