Derivatives of Zirconocene

Schwartz and Gell have reported that a benzene solution of Cp2Zr(PMePh2)2 containing 2 equivalents of PMePPh2 absorbed 1.4 equivalents of CO (based on Zr) over a 4-hour period to give at 

Rausch and Sikora have recently described the photochemically induced reaction of Cp2Zr(CO)2 (2) and triphenylphosphine from which Cp2Zr(CO)(PPh3) could be isolated as highly air-sensitive, maroon microcrystals in 28% yield (58). Similarly, when hexane solutions of 2 and PMe3 or PF3 were irradiated, the corresponding complexes, 

Cp2Zr(CO)(PR3) (R = Me, F), were isolated as oily, air-sensitive solids. The competing photodegradation of Cp2Zr(CO)2 (2) to oligomeric zirco-nocene prevented the formation of these monocarbonyl-phosphine complexes in higher yields (51). 

While Cp2Zr(CO)(PPh3) was found to be more reactive toward acetylenes than Cp2Zr(CO)2 (2), no monocarbonyl-Tj2-acetylene complexes of zirconocene were observed in contrast to the reaction of acetylenes with Cp2Ti(CO)(PPh3) (42) (50). Instead the reaction of Cp2Zr(CO)(PPh3) with RC=CR (R = Et, Ph) led directly to the respective zirconacy-clopentadienes (58). 

The IR and H-NMR spectral data for the various zirconocene monocarbonyl-phosphine complexes are compiled in Table IV. 

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R= Bu or Pr ), have been obtained from [TiO<-CH2)(M-Cl)(AlMe2)( -C5H5)2] and RCH=CH2 in the presence of 4-vinylpyridine/styrene copolymer which functions as a Lewis base. Some reactions of the t-butyl compound are shown in Scheme 2. A metallocyclic derivative of zirconocene has been obtained as shown in equation (4). The chloro-derivative rearranges thermally to [Zr-(CHPMe3)Cl( 7-C5H5)2]. ... 

The addition of any one of several dialkyl chlorophosphates to an arylalkyne-derived vinyl zirconocene in the presence of catalytic amounts of CuBr in THF leads to the corresponding vinyl phosphonate in high yields (78—92% see, for example, Scheme 4.38) [25]. Here, alkyl-substituted acetylenic starting materials do not react beyond the initial hydrozirconation stage. Vinyl phosphonates may be readily converted to acyloins by oxidation to the diol followed by base-induced cleavage. 

Acylzirconocene chloride derivatives are easily accessible in a one-pot procedure through the hydrozirconation of alkene or alkyne derivatives with zirconocene chloride hydride (Schwartz reagent) [Cp2Zr(H)Cl, Cp = cyclopentadienyl] and subsequent insertion of carbon monoxide (CO) into the alkyl— or alkenyl—zirconium bond under atmospheric pressure (Scheme 5.1) [2],... 

The aforementioned observations have significant mechanistic implications. As illustrated in Eqs. 6.2—6.4, in the chemistry of zirconocene—alkene complexes derived from longer chain alkylmagnesium halides, several additional selectivity issues present themselves. (1) The derived transition metal—alkene complex can exist in two diastereomeric forms, exemplified in Eqs. 6.2 and 6.3 by (R)-8 anti and syn reaction through these stereoisomeric complexes can lead to the formation of different product diastereomers (compare Eqs. 6.2 and 6.3, or Eqs. 6.3 and 6.4). The data in Table 6.2 indicate that the mode of addition shown in Eq. 6.2 is preferred. (2) As illustrated in Eqs. 6.3 and 6.4, the carbomagnesation process can afford either the n-alkyl or the branched product. Alkene substrate insertion from the more substituted front of the zirconocene—alkene system affords the branched isomer (Eq. 6.3), whereas reaction from the less substituted end of the (ebthi)Zr—alkene system leads to the formation of the straight-chain product (Eq. 6.4). The results shown in Table 6.2 indicate that, depending on the reaction conditions, products derived from the two isomeric metallacyclopentane formations can be formed competitively. 

An approach to derivatives of polyhydroxylated cyclooctane from D-ribose was proposed by Paquette.47 The crucial step involved a ring scission induced with zirconocene (Fig. 32). 

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