Addition reactions zirconocene dichloride

Addition of zirconocene to unsymmetric acetylenes is often regiospe-cific. Thus the reaction of 1-trimethylsilyl-l-propyne (50) with zirconocene dichloride and magnesium amalgam gives the zirconocycle 51, where the large trimethylsilyl groups are adjacent to zirconium.21 Iodination of 51 affords diiodide 52, which has been converted to the corresponding di-stibaferrocene 55 and dibismaferrocene 56. 

The stereospecific construction of the trisubstituted double bond of the side chain at C-1 of carbazomadurins A (253) and B (254) was achieved using Negishi s zirconium-catalyzed carboalumination of alkynes 758 and 763, respectively. Reaction of 5-methyl-l-hexyne (758) with trimethylalane in the presence of zirconocene dichloride, followed by the addition of iodine, afforded the vinyl iodide 759 with the desired E-configuration of the double bond. Halogen-metal exchange with ferf-butyllithium, and reaction of the intermediate vinyllithium compound with tributyltin chloride, provided the vinylstannane 751a (603) (Scheme 5.79). 

TrisubsHtuted alkenes. Negishi et al have extended the reaction of alkynes with trimethylalane and zirconocene dichloride (this volume) to a corresponding addition reaction with trimethylalane-titanocene dichloride. This latter reagent is somewhat more reactive. It reacts with diphenylacetylene to form (Z)-a-methyl-stilbene (2) in high yield. The yield of 2 is <30% when 1 is replaced by the corresponding zirconocene reagent. 

Carbotitanation of alkyuylsilanes, The reaction of an alkynylsilane with 1 equiv. each of titanocene dichloride and dimethylaluminum chloride followed by hydrolysis with aqueous sodium bicarbonate or sodium hydroxide results mainly in c -carbometalation (equation I). Addition of triethylamine prior to quenching results in loss of stereochemistry. Use of trimethylaluminum results in similar CM-addition, but this reaction is slower than that with (CH3)2A1C1. Surprisingly, alkynylsilanes do not react with zirconocene dichloride and dimethylaluminum chloride. 

Heterobimetallic /x-oxo complexes 854 are formed via halide displacement reactions between trioxo anions such as [Cp M(0)3] (M = Mo, W) and Cp2ZrCl2655 (Scheme 214). The corresponding heterotrinuclear complexes 855 are obtained by addition of 2 equiv. of trioxoanion complexes to zirconocene dichloride. Other heterobimetallic complexes such as //-773-C02-bridged ruthenium-zirconium and rhenium-zirconium complexes have also been prepared.656... 

Addition of water to the carboalumination reaction mixture led to a considerable increase in the reaction rate (Scheme 6.118) [150] and even at -70°C methylalumination of 1-hexyne was essentially complete in 10 min in the presence of 1.5 equiv. H2O. Zirconocene dichloride is necessary as a co-catalyst, and the rate-... 

Branch at C-4 - The syntheses of some C-4 branched derivatives of NeuNAc have been achieved from a protected 4-keto analogue, methyl (methyl 5-acetamido-3,5-dideoxy-8,9-6>-isopropylidene-p-D-manno-2,4-nonudiulopyrano-sid)onate. For example conversion of the latter into spiro-epoxide 12 by reaction with zirconocene dichloride-Zn-diiodomethane, then peracid, provided a reactive centre for the addition of carbon centred nucleophiles giving products such as 13. The synthesis of furo[2,3-c]pyran-P-D-thymidine, a potential antiviral agent, is mentioned in Chapter 20. 

Electrophilic attack on olefin ligands coordinated to electron-rich, strongly backbonding metals is illustrated by the reactions of (P group 4 olefin and alkyne complexes, as well as some electron-rich olefin complexes. Zirconocene- and and hafnocene-olefin complexes generated by reaction of zirconocene dichloride with two equivalents of alkyl lithium and isolated upon addition of a phosphine ligand react with carbonyl compounds and weak protic acids to form insertion products and alkyl complexes. Several examples of the reactions of these complexes with electrophiles are shown in Equations 12.65-12.66. Zirconocene-alkyne complexes prepared by thermolysis of vinyl alkyl complexes and titanium-alkyne complexes generated by the reduction of Ti(OPr ) also react with electrophiles, such as aldehydes and acid, to form products from insertion into the M-C bond and protonation of the M-C bond respectively. 

The synthesis of vinylphosphines has been accomplished using a complimentary process (Scheme 4.314) [416]. The first part of the chemistry consisted of metallocycle formation by treatment of zirconocene dichloride with two equivlents of BuLi followed by the addition of two equivalents of an internal alkyne such as diphenylacety-lene. Treatment of this intermediate with a chlorophosphine should have generated vinylphosphines. However, the authors found that this reaction was low yielding. After some experimentation, they discovered that the conversion of the zirconium metallocycle into a vinylcopper species through the addition of copper chloride followed by addition of the chlorophosphine was an effective route to the formation of the vinylphosphines. 

Coupling Reactions.—Acetylenes react with organoalanes, e.g. MesAl, and zirconocene dichloride to form the alkenyl metal complexes (38) in high yield. The exact structure of these complexes is not known but both aluminium and zirconium are essential. The stereoselectivity of the reaction is shown to be 98% c/s-addition, and once formed the complexes (38) undergo a variety of useful... 

---