Zirconocene Chemistry

Since the initial reports of Schwartz s reagent ([Cp2Zr(H)Cl]n, 1) over 30 years ago there has been explosive growth of zirconocene chemistry. Hydrozirconation is an efficient method of hydrometalation because of (i) the mild nature of the conditions involved, (ii) the excellent regio- and stereo-chemical control of hydrozirconation, (iii) the one-pot nature of the procedure, and (iv) the price of Zr, which is one of the... 

For recent symposiums on zirconocene chemistry, see E. Negishi, Recent Advances in the Chemistry of Zirconocene and Related Compounds, Tetrahedron Symposia-in-print No. 57, Tetrahedron 1995, 51 (special issue). R. F. Jordan Metallocene and Single Site Olefin Catalysis, f. Md. Catal. 1998, 128 (special issue) and references cited therein. R. F. Jordan, A. S. Guram, in Comprehensive Organometallic Chemistry II, E. W. Abel, F. G. A. Stone, G. Wilkinson, M. F. Lappeet (eds.), Pergamon Press, Oxford, 1995, Vol. 4, p. 589. 

However, the development of cyclopropane synthesis through zirconocene chemistry is still in its infancy. The reactions presented in this chapter have only recently been reported for the most part, and not systematically studied. Further investigations appear to be desirable. Practical procedures involving optimized reaction conditions and simpler reagents would be welcomed. Advances should focus on the development of catalytic and asymmetric cyclopropanation reactions. 

Phosphoies and other reiated heterocycies are an important class of main group compounds. The chemistry of phosphoies and their preparation has been reviewed extensiveiy by Mathey.3 We provide details here for a simple, one-pot procedure for the preparation of 1-phenyl-2,3,4,5-tetramethylphosphole applying zirconocene chemistry.4 The procedure involves reduction of (ri-C5H5)2ZrCl2 with butyllithium in the presence of 2-butyne which (as reported initially by Negishi, et al. ) forms a zirconium metallacycle. Addition of dichiorophenylphosphine to this reaction mixture produces the phosphole. One other procedure for the preparation of 1-phenyl-... 

TRIFLUORO-2-BUTYNOATE. C-Acylation of an enolate using methyl cyanoformate provides a convenient source of the a-carbometh-oxyoctalone, METHYL (la,4Ap,8Aa)-2-OXO-DECAHYDRO-l-NAPH-THOATE and represents a good example of generating jS-keto esters under mild conditions. The nitrone functionality is featured in a procedure which makes it in a single step from secondary amines and 6-METHYL-2,3,4,5-TETRAHYDROPYRIDINE N-OXIDE is the example described. Finally, the synthesis of phospholes l-PHENYL-2,3,4,5-TETRAMETHYLPHOS-PHOLE is described as an example of the versatility of zirconocene chemistry. 

Stelck, D. S. Advancements in boryl-bridged araa-zirconocene chemistry. Ph.D. Thesis, University of Idaho, 2001. 

Majoral J-P, Igau A, Cadierno V, ZablockaM (2002) Benzyne-Zirconocene Reagents as Tools in Phosphorus Chemistry. 220 53 -77... 

Majoral J-P, Igau A, Cadierno V, Zablocka M (2002) Benzyne-Zirconocene Reagents as Tools in Phosphorus Chemistry. 220 53-77 Manners I (2002), see McWMams AR (2002) 220 141-167 March NH (1999) Localization via Density Functionals. 203 201-230 Marchivie M,see Guionneau P (2004) 234 97-128... 

Negishi E, Tan Z (2005) Diastereoselective, Enantioselective, and Regioselective Carbo-alumination Reactions Catalyzed by Zirconocene Derivatives. 8 139-176 Netherton M, Fu GC (2005)Pa]ladium-catalyzed Cross-Coupling Reactions of Unactivated Alkyl Electrophiles with Organometallic Compounds. 14 85-108 Nicolaou KC, King NP, He Y (1998) Ring-Closing Metathesis in the Synthesis of EpothUones and Polyether Natmal Products. 1 73-104 Nishiyama H (2004) Cyclopropanation with Ruthenium Catalysts. 11 81-92 Noels A, Demonceau A, Delaude L (2004) Ruthenium Promoted Catalysed Radical Processes toward Fine Chemistry. 11 155-171... 

In deciding on the material to be covered in this chapter, limitations had to be set. The first section will present the synthesis of various zirconocene hydrides. The focus of the subsequent sections is to present a general synopsis of the different aspects of the hydrozirconation reactions using 1 not only on carbon-carbon multiple bonds but also on heteropolar multiple bonds. Those aspects of hydrozirconation that were covered in previous reviews [1-5, 27] and in the excellent chapter by Labinger in Comprehensive Organic Synthesis [28] are summarized or briefly mentioned here. For other aspects of organozirconium chemistry not covered by the above-mentioned reviews, the reader is referred to a number of monographs and reviews [29, 30]. 

In a somewhat different area of organozirconium chemistry, Erker s group has recently reported the formation of Cp2Zr(CO)2 (2) in very good yield via the carbonylation of the zirconocene diene complexes 26a and 26b (57). In cases where the diene was structurally less complex (e.g.,... 

The gas-phase reaction of cationic zirconocene species, ZrMeCp2, with alkenes and alkynes was reported to involve two major reaction sequences, which are the migratory insertion of these unsaturated hydrocarbons into the Zr-Me bond (Eq. 3) and the activation of the C-H bond via er-bonds metathesis rather than /J-hydrogen shift/alkene elimination (Eq. 4) [130,131]. The insertion in the gas-phase closely parallels the solution chemistry of Zr(R)Cp2 and other isoelec-tronic complexes. Thus, the results derived from calculations based on this gas-phase reactivity should be correlated directly to the solution reactivity (vide infra). 

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. 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

Organometallic Chemistry of Titanocene and Zirconocene Complexes with Bis(trimethylsilyl)acetylene as the Basis for Applications in Organic Synthesis... 

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