Joyce Y. Corey

THE X-RAY CRYSTAL STRUCTURES OF THE ANSA-METALLOCENES, ME2C(C5H4)2MCL2 (M=TI, ZR AND HF)

Abstract Group 4 metallocenes and ansa-metallocenes are known for their role in the Ziegler-Natta catalysis of olefins and in the catalytic dehydrocoupling of hydrosilanes to form polysilanes. This report describes the crystal structures of three carbon-bridged ansa-metallocene dichlorides, Me2C((C5H4)2)MCl2 ( M = Ti ( 1 ), Zr ( 2 ) and Hf ( 3 ) ). Comparisons of the structures of the molecules with previously characterized systems is made; in particular, the Cpcentroid-M-Cpcentroid angle, the position of the metal relative to the cyclopentadienyl rings and the uniformity of the CC bond lengths in the Cp rings are discussed. Comparisons between structure and reactivity in the dehydrocoupling of hydrosilanes are described.

Conversion of hydrosilanes to alkoxysilanes catalyzed by Cp2TiCl2/nBuLi

Abstract The combination of Cp 2 TiCl 2 and n BuLi provides an effective catalyst for alcoholysis of the model silanes n-HexSiH 3 , PhMeSiH 2 , Ph 2 SiH 2 and PhMe 2 SiH by ethanol, isopropanol, t-butyl alcohol and phenol. Increasing the steric bulk of the substituents on either the alcohol or the silane generally requires longer reaction periods and/or increasing temperature. All SiH bonds are converted to SiOEt groups by ethanol and a single SiH bond in secondary silanes and two SiH bonds in tertiary silanes are replaced by t-butyl alcohol. Diols including pinacol, 2,4-pentanediol and 2,5-hexanediol react with PhRSiH 2 (R  Me, Ph) to give 1,3-dioxa-2-silacyclopentanes, -hexanes and -heptanes, respectively. Attempts to form caged structures by condensation of primary silanes and triols was unsuccessful. Hydrolysis of PhRSiH 2 is promoted by Cp 2 TiCl 2 / n BuLi and the siloxane is produced in quantitative yield when R  Ph and a mixture of linear disiloxanes and trisiloxanes in addition to cyclopolysilanes are produced when R  Me. Other protic reagents including acids, mercaptans, amines and enolizable ketones did not react. The effects of reaction parameters such as temperature, silane to catalyst ratio, solvent, transition metal and replacements for n BuLi were also determined.

Condensation of primary silanes in the presence of Cp2MCl2/nBuLi (M = Ti, Zr, Hf)

The condensation of RSiH3 (R  Ph, nBu, nHex) to silicon oligomers in the presence of the catalyst precursor combination Cp2MCl2/2nBuLi (M  Ti, Zr, Hf) has been monitored by gas chromatography and the intermediate oligomers characterized by GC-MS. The average molecular weights of the isolated products from PhSiH3 depend on the catalyst and increase through the sequence, Hf < Ti < Zr, to a maximum of about 1800 for reactions conducted at room temperature. In the condensation of alkylsilanes linear oligomers are produced in the initial stages of the reaction and as the reaction continues cyclic polysilanes are formed with increasing numbers of isomers as the number of silicon atoms increases. A large rate difference is observed in the formation of the diastereomers of the tetrasilane produced from PhSiH3 for all three catalysts. For reactions conducted at 50°C both diastereomers of H(nBuSiH)4H are produced in equal amounts throughout the reaction period for M  Ti but at different rates for M  Zr and only one diastereomer is observed for M  Hf. The observations are discussed in terms of chain growth through sigma-bond metathesis processes.

Synthesis and characterization of 10,11-dihydro-5H-dibenzo[b,f]metallepins of group IV☆

Direct reaction of 2,2′-dilithiobibenzyl with (CH3)2MCl2 or (C6H5)2MCl2 where M  Si, Ge, Sn, or Pb results in the formation of 10,11-dihydro-5H-dibenzo-[b,f]metallepins. The germanium derivaives were successfully converted to 5,5-dimethyl- and 5,5-diphenyl-5H-dibenzo[b,f]germepin by bromination followed by dehydrobromination with 1,5-diazabicyclo[4.3.0]non-5-ene. Reactions of dihydrodibenzo[b,f]stannepins uniformly gave ring cleavage products. The abundances of the metal fragments produced in the mass spectrum of the dihydrodibenzometallepins are recorded and a fragmentation pattern common to the system is given.

Siloles: Part 1: Synthesis, Characterization, and Applications

Abstract Advances made in methods of synthesis of siloles during the period 1996 to the end of 2009 are described. The earliest general method involved reductive dimerization of tolan with lithium metal followed by quenching with a dichlorosilane, trichlorosilane, or silicon tetrachloride. Versatile new methods involve the intramolecular cyclization of dialkynylsilanes in the presence of lithium naphthalenide (Tamao method) and 1,1-organoboration with a trialkylborane (Wrackmeyer method) each of which provides an entry into different types of substitution patterns in the silole product. Supplementing the reductive-coupling route to siloles has been both stoichiometric and catalyzed processes mediated by transition metal complexes. Various, less general synthetic methods are also described. Reactions of siloles at either the silicon center or the ring carbon centers that retain the silole skeleton are included. Characterization methods used to confirm the siloles are included in tabular form, and uses or applications of the silole are also summarized. Fluorescent data are highlighted for selected siloles as is the aggregation-induced emission phenomenon generally exhibited by this class of compounds. Various oligomers and polymers that contain siloles have been reported and include oligo- and poly-(1,1-siloles) as well as polymers where organic bridges connect to the silole in the 1-position or at a ring carbon position. Siloles have been used as core components for efficient electron-transporting materials and their incorporation into OLED devices and the characteristics of such devices is summarized.

Siloles: Part 2: Silaindenes (Benzosiloles) and Silafluorenes (Dibenzosiloles): Synthesis, Characterization, and Applications

Abstract The preparation of benzannulated siloles including silaindenes (benzosiloles) and silafluorenes (dibenzosiloles) reported during the period 1996 through 2010 is reviewed. As is the case for nonannulated siloles, tolan and related systems were the precursors of choice in the traditional route to silaindenes, through formation of a dilithio reagent. More recently, transition metal-catalyzed reactions of various silyl-substituted dienes and ene-ynes have been introduced as is also the case for condensation of 2-silylphenylboronic acid with alkynes. Formation of ladder oligomers and polymers with ladder segments related to silaindene is included. The traditional route to silafluorenes involved starting with o , o ′-dibromobiphenyl followed by formation of the dilithio reagent and then quenching with chlorosilanes. Several extensions of this approach have been made through development of new biphenyl precursors with multiple halogen substituents that then have produced substituents at the benzo-positions of the silafluorene that are useful for further chemical elaboration of the basic core. The traditional route through dilithio reagents has been supplemented by new transition metal-catalyzed processes involving a variety of precursors in addition to a Friedel–Crafts route to silafluorenes. Reactions at both the silicon center and involving C–X substituents on the carbon framework are summarized. Compounds related to silafluorenes such as silicon-bridged benzene-heteroarenes and silicon-bridged bithiophenes that exhibit a commonality in the synthetic methods to silafluorenes are discussed. Silafluorenes and dithienosiloles have played a role in the formation of oligomers as well as homopolymers and copolymers. Silafluorenes and dithienosiloles have been employed in electronic devices particularly in photovoltaic cells, white light-emitting diodes and bulk heterojunction solar cells and field effect transistors. The characterization methods reported for monomers and polymers are tabulated as well as applications in device construction when reported.

Pentacoordinate silicon compounds: Pseudorotation at silicon

Abstract The variation with temperature of the 19 F and 1 H NMR spectra of pentacoordinate fluorosilanes which contain an intramolecular SiN coordination has been examined. A clear distinction between pseudorotation and ring opening-closing processes has been obtained for the first time and the corresponding activation energies determined.

ansa-Metallocenes: Catalysts for the dehydrocoupling of hydrosilanes

Abstract The dehydrocoupling of PhMeSiH2 to form oligomers and of PhSiH3 to form polysilanes has been performed with the precatalyst combination of [Me2El(C5H4)2MCl2 (El = Si, C; M = Ti, Zr, Hf) and nBuLi. The results are compared to those produced from (η 5 - C 5 H 5 ) 2 MCl 2 BuLi .

Synthesis of dibenzo[b, f]silepins

Abstract The reaction of o,o′-dilithiobibenzyl with substituted silicon halides results in the formation of previously unreported 10,11-dihydro-5H-dibenzo[b,f]silepins. The bromination of the dihydrodibenzosilepins followed by dehydrobromination with potassium acetate and 1,5-diazabicylo[4.3.0]non-5-ene results in the formation of the symmetrical dibenzosilepin derivatives. The spectral data for these new compounds are discussed.

Silicon analogs of the thioxanthenes

Abstract The 9,10-dihydro-9-silaanthracene framework generated from ( o -ClMgC 6 H 4 ) 2 CH 2 and a dichlorosilane provides the starting point for formation of derivatives which contain a tricyclic skeleton and a three-atom side chain terminated by a nitrogen group bonded to the central ring of the framework. Reaction of the ketone, 10-oxo-9,9-dimethyl-9,10-dihydro-9-silaanthracene with either N,N dimethylaminopropylmagnesium chloride or N -methylpiperidinylmagnesium chloride provided the alcohols which could be dehydrated with thionyl chloride/ pyridine to give the silicon analogs of thioxanthene derivatives (SiMe 2 replaces S). Several other dehydration attempts are described as well as two other synthetic routes for introduction of an exocyclic double bond in the 10-position. Hydride addition of 9-methyl-9,10-dihydro-9-silaanthracene to N,N -dimethylallylamine in the presence of H 2 PtCl 6 and bromination of 9,9-dimethyl-9,10-dihydro-9-silaanthracene followed by reaction with 1-methylpiperazine provided two further examples of derivatives based on the silaanthracene framework.