59th Annual Report on Research 2014

Tethered Ligands

Building upon our successful synthesis of a Cp*quinolineCoI₂ last year, we attempted to reduce the Co(III) to Co(I) in the presence of ethene to form our pre-catalyst.  Using a wide variety of reductants including KC₈, Na(Hg), Zn(Hg), nBuLi, and Et₂Zn, we saw only reduction to paramagnetic species consistent with reduction of the ligand and formation of Co(II) species.  Since the Cp* species is very electron rich, we began the synthesis of MeCp-quinoline through an analogous route which we believe will be less electron rich and less likely to form the Co(II) species.  Additionally, we have synthesized Cp*CH₂CH₂SCH₃, which we predicted will be less likely to undergo reduction at the tethered ligand. 

Scheme 1.  New tethered ligand species less likely to undergo reduction at the tether.

Isocyanides.

We synthesized and purified six para-substituted phenylisocyanides that span the electronic range from -NO₂ to -OMe.    Addition of stoichiometric amounts of the isonitriles to Cp*Co(ethene)₂ gives moderate yields of crystalline products with electron rich substituents (OMe, Me)but intractable materials when electron withdrawing substitution (NO₂, CF₃).  We successfully crystallized Cp*Co(ethene)(4-MeOPhNC) and structurally characterized it.  The complex appears to have a small amount of pi donation from the metal to the isocyanide, consistent with the CNC_ipso bond angle of 173.7¡ and CN bond distance of 1.173 versus 180¡ and approximately 1.15 in the uncoordinated species.  Addition of two equivalents of the methoxyphenylisonitrile gave low yields of Cp*Co(CNPhOMe)₂.

Scheme 2.   X-ray structure of Cp*Co(ethene)(4-MeOPhNC)

Planar Ligands.

We made strides in our attempt to find a general synthesis of para substituted phosphabezenes.   Phenyl (trimethylsilyl) ethynyl ketone was synthesized from benzoyl chloride and trimethylsilyl acetylene under Sonogashira conditions.  3-Hydroxy-3-phenyl-1-(trimethylsilyl)-1,4-pentadiyne resulted from addition of ethynyl magnesium chloride to the TMS protected ketone.  Etherification of the alcohol with dimethylsulfate simultaneously cleaved the TMS protecting group and generated 3-methoxy-3-phenyl-1,4-pentadiyne.  Benzolyl peroxide promoted addition of Bu₂SnH₂ to the diyne gave1,1-dibutyl-4-methoxy-4-phenylstannacyclohexa-2,5-diene in good crude yields.  We are currently working to improve the isolated yield.