Hydrozirconation regioselectivity

A variety of other addition reactions occurring regioselectively are also known. These include hydrocyanation, hydroalumination, hydrosilylation, and hydrozirconation. 

Terminal heterosubstituted alkynes X-C=CH such as RO-C=CH [16, 17, 151-153], RSe-teCH (R= alkyl, aryl) [154-157], MejSi-teCH [158] and R P-feCH [159] were regioselectively converted into the ( )-2-ethenylzirconium complexes via hydrozirconation using 1. In marked contrast, with terminal BuTe-C=CH [154]... 

It should be pointed out that, compared to a-bromoboronic esters, p-bromoboronic esters are much less stable. For instance, dibutyl (2-bromoethyl)boronate readily undergoes P-elimination, even under solvolytic conditions [87]. Therefore, the reaction of 18 with NBS also reaffirms the regioselectivity of the hydrozirconation step. The reaction is highly general and works equally well for the preparation of a-chloro- and a-iodoboronic... 

The amination of styrene, however, led to two products (1-phenyl-1-ethylamine and 2-phenyl-l-ethylamine) in a 1 3 ratio [113], indicating that the hydrozirconation was not completely regioselective [114,115]. Since it is well known that hydrozirconation of trisubsti-tuted alkenes places zirconium at the least hindered carbon of the chain by a process involving zirconium migration, this class of alkenes was not investigated [5,116], On the other hand, hydrozirconation/amination of 3-methyl-l,2-butadiene gave an allylic amine. Reaction of the latter could either occur at the terminal carbon or proceed with... 

The alkyne hydrozirconation protocol was also applied to acetylenic tellurides furnishing the zirconated vinyl tellnrides in cis fashion and high regioselectivity. Subsequent treatment with tellurenyl halides affords telluro ketene acetals with total retention of configuration. ... 

The kinetic product distribution can be further shifted in excess reagent to get organozirconium derivatives with high regioselectivity. Even subtle steric differences such as in 2-pentyne ensure high selectivity [Eq. (6.77), a]. As Eq. (6.77) demonstrates, hydrozirconation of alkynes is an exclusive syn process. 

Unusual regioselectivity has been observed180 during the reductive coupling of alkynes and allenes by hydrozirconation and zinca-Claisen rearrangement. 

Hydrozirconation of terminal triple bonds is an essential method to obtain alkenes with defined stereochemistry. In the case of internal alkynes the zirconocene moiety adds to the sterically less hindered position of the triple bond. C/s-selectivity is high, but the regioselectivity is sometimes moderate depending on the nature of the substrate.8... 

Schwartz s reagent, ZrHClCp2 n (Figure 2.29b), readily (and often regioselectively) hydrozirconates alkynes to provide o-vinyl complexes from which the vinyl ligand may be cleaved by a wide range of electrophiles, in a manner reminiscent of hydroboration chemistry (Figure 4.15). 

The addition of Cp2Zr(H)Cl, known as the Schwartz reagent [30], to different alkenes and alkynes is known to be a facile process [31]. Therefore, the hydrozirconation of a variety of readily available enynes 12 is among the first methods developed for the stereoselective preparation of dienyl zirconium reagents 13. This process is both completely chemo- and regioselective with a syn addition of the zirconium hydride across the alkyne [32] (Scheme 5). From the same intermediate, the Zr atom can be isomerized in its internal position such as in 15 via a zirconacyclopropene intermediate 14. Moreover, the addition of trimethylstannyl chloride to 14 led to the stannylated dienyl zirconocene 16 [33] (Scheme 5). 

Allylic zirconocenes, generated by hydrozirconation (see Hydrozirconation) of allenic systems, react with terminal alkynes when activated with MAO to regioselectively afford 1,4-dienes. The same MAO catalyzed process, applied to haloalkynes, leads to excellent yields of the 1,4-enyne (100) (Scheme 23). ... 

Hydroboration of acetylenic selenides with 9-BBN led to the regio- and stereoselective formation of a-selanylalkenyl boranes which were then converted into Z-a, -disubstituted vinyl selenides by cross-coupling reaction with aryl bromides [80] (Scheme 58). With unsubstituted acetylenic selenides, an inversion of regioselectivity during the hydrozirconation was observed [81,82]. 

Generation of organozirconium species 18 follows the hydrozirconation of the triple bond using zirconocene hydrochloride (Schwartz reagent) 16. This 16-electron, d° Zr(IV) complex is coordinatively unsaturated, so alkyne 15 coordinates to the electrophilic Zr center followed by insertion of the triple bond into the Zr-H bond. The resulting a-vinyl-Zr(lV)-species 18 is formed with a high cA-selectivity and regioselectivity, such that the bulky zirconocene moiety always adds end-on to the terminal multiple bond. 

The hydrozirconation of alkynes by Cp2ZrHCl has been studied in detail . The addition of Zr-H is uniquely cis, and in 1-alkynes the zirconium attaches to the terminal carbon atom with high regioselectivity ( 98%). The direction of cis-fi addition of Zr-H to internal alkynes is sensitive to the steric bulk of the two substituents of the alkyne and, in the absence of excess Zr-H, is subject to kinetic control. The presence of excess Zr-H results in rapid equilibration of the initial mixture. The results of some hydrozirconations of internal alkynes are shown in Table H. 

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