Double hydrozirconation

The bis-zirconocene complex CpjClZrCHjCHjZrCpjCl has been isolated upon double hydrozirconation of acetylene with 1 [102]. Recently, the preparation of a heterogeneous bis-zirconocene catalyst was succesfully achieved from zirconocene dichloride complexes containing alkenyl or alkynyl substituents [224]. 

Selective monoalkylation of Cp2ZrCl2 to form Cp2Zr(R)Cl in which R is alkyl or alkenyl is best accomplished by hydrozirconation with [Cp2Zr(H)Cl]n (1). Most syn-thehc applications involve the use of terminal organozirconocene complexes. The hydrozirconahon reachon on carbon-carbon double or triple bonds with 1 is generally clean and gives high yields of essentially 100% cis addition products. 

Hydrozirconation across Carbon-Carbon Double Bonds... 

Formation of the least-hindered alkylzirconium complex during the course of the hydrozirconation-isomerization process of alkenes can be altered by the presence of aromatic rings, additional conjugative double bonds, chelating heteroatoms [81, 94] or groups that can be lost by elimination [95, 96]. 

Hydrozirconation of the sterically less hindered double bond in conjugated dienes is generally observed [103]. However, the presence, as the minor regioisomer, of the allyhc product has been reported [5, 86]. Depending on the reaction conditions and the reagents used in the subsequent transformation, the allylic isomer did not form (Scheme 8-12) [2]. 

Scheme 8-24 Hydrozirconation of phosphorus-X double bond derivatives...

Some hydrometalation reactions have been shown to be catalyzed by zirconocene. For instance, CpiZrCf-catalyzed hydroaluminations of alkenes [238] and alkynes [239] with BU3AI have been observed (Scheme 8-34). With alkyl-substituted internal alkynes the process is complicated by double bond migration, and with terminal alkynes double hydrometalation is observed. The reaction with "PrjAl and Cp2ZrCl2 gives simultaneously hydrometalation and C-H activation. Cp2ZrCl2/ BuIi-cat-alyzed hydrosilation of acyclic alkenes [64, 240] was also reported to involve hydrogen transfer via hydrozirconation. 

Besides hydrozirconation of terminal triple bonds Cp2Zr(H)Cl (16) also reacts with double bonds.8 The mechanism is similar to that described for alkynes. After coordination of alkene 5 to the Zr center giving -complex 23 the terminal double bond inserts into the Zr-H bond to form the stable (T-alkyl complex 24. The bulky zirconocene moiety again adds to the end-position of the terminal double bond. 

The steric demand of the zirconocene moiety leads to rearrangements in the case of hydrozirconation of internal double bonds. Regardless of position or configuration of the double bond in the substrate, zirconium migrates to the terminal position of the alkyl chain via insertion and /TH-elimination steps. Such isomerization does not occur upon hydrozirconization of internal alkynes. 

Alkynes are universally hydrozirconated as well the only failure in the literature is that of a perfluori-nated compound, C7Fi5(2sCH. Alkynes appear to be more reactive than alkenes, both by qualitative comparison and from the results on enynes (Table 6 also see Section 3.9.3.3.1), Furthermore, hydrozirconation of alkynes can compete with reduction of unsaturated functional groups such as nitriles and esters (Table 5), which is generally not true for alkenes. Dienes can be cleanly monohydrozirconated if one of the double bonds is terminal other cases are considered in Section 3.9.3.3.I. 

Definitive evidence for the role of heteroatom coordination may be seen in the hydrozirconation of al-kenoxide (29 equation 33) where the five-membered ring favors (30) (confirmed by the X-ray crystal structure). A related structure was obtained in equation (34) the 0-coordinated product (31) does not isomerize to the terminal alkylzirconium product (16) below 140 "C. (At 140 C it slowly decomposes to 17, presumably via 16 cf. equation 22 above.) In contrast, hydrozirconation of dienols shows preferred reactivity of the double bond remote from the —OH group an example is shown in Table 3. Probably the first equivalent of (6) reacts with the —OH group both to increase steric bulk and prevent... 

Hydrozirconation of vitamin D3 (39) gives a mixture of four diastereomers resulting not only from addition to either face of the reacting exocyclic double bond, but also isomerization of the nonreacting 5,6-double bond (equation 41), even though no isomerization of unreacted (39) was detected. No explanation was offered. Partial isomerization of a double bond during hydrozirconation of a triple bond has also been observed. ... 

Halogenative cleavage likewise appears completely general for both alkyl- and alkenyl-zirconium complexes (Scheme 10), and proceeds with complete retention of configuration at carbon. For the hydrozirconation products of 3-methyl-1,3-dienes, such as (40), cyclization competes (equation 43). Analogs without the 3-methyl subsitutent, or with more remote double bonds, give only the uncyclized products in good yields. " ... 

In one example alkyne 96 is activated by hydrozirconation. The zirconium is in the terminal position 99. The two Cp rings are quite big and so it is usual for the zirconium to add preferentially to the less hindered end of the triple (or double) bond. This is the regioselective aspect of the reaction and not what concerns us here. Transmetallation to the organolithium and again with Cu(I) gives the cuprate and this cuprate does a conjugate addition to the double bond of an enone that looks suspiciously Robinson annelation-derived.13 Before we move on, notice that the double bond has a trans geometry - we shall come back to this. 

---