Acetylenes hydrozirconation

Another interesting approach is that described by Panek, who employed an acetylene hydrozirconation process followed by a Negishi coupling (Figure 9). 

The regiochemistry of the hydrozirconation of disubstituted stannyl- [24, 167-170] and silyl- [171] acetylenes and boron- [118, 172-175] and zinc- [34, 126] alkynyl derivatives result in the formation of 1,1-dimetallo compounds. Hydrozirconation of alkynyliodonium salts affords alkenylchlorozirconocenes with the Zr-C bond geminal to the iodonium moiety [176]. These zirconocene complexes allowed the preparation of ( )-trisubstituted olefins (Scheme 8-20). 

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]. 

Cp(PMe3)2RuC=CH and the corresponding vinyl compound react with 1 to give the expected hydrozirconated complexes [225, 226]. Hydrozirconation reactions were also observed with the vinyl and acetylenic ferrocenyl complexes [227]. In marked contrast, the iron complex Cp (dppe)Fe-C=CH reacts anomalously with Schwartz s reagent to form acetylide Cj-bridged heterodinuclear complexes (Scheme 8-31) [228]. 

Acetylenic ethers 7 can be hydrozirconated, and subsequent iododezirconation leads to (fc)-iodo enol ethers 8 (Scheme 4.4) [18], These species undergo efficient Sonogashira couplings to give (E)-enynes, which are ultimately converted to stereodefined dienol ethers. These dienes have proven useful in studies of diastereoselective cycloaddition reactions with singlet oxygen, where R in 8 is a nonracemic auxiliary (e. g., menthyl) (Procedure 3, p. 140). 

As an alternative to hydrozirconation of acetylenic tellurides or selenides, Dabdoub and co-workers have more recently described the first additions of the Schwartz reagent (one equivalent) to acetylenic selenide salts 51 (Scheme 4.30) [52]. Subsequent alkylation at selenium produces 1,1-dimetallo intermediates 52, which are cleanly converted in a one-pot process to stereodefined products 53. It is noteworthy that ketene derivatives 52 are of ( )-geometry, the opposite regiochemistry to that which results from hydrozirconation of acetylenic tellurides (vide supra). This new route also avoids the mixtures of regio-isomers observed when seleno ethers are used as educts. The explanation for the stoichiometric use of Cp2Zr(H)Cl in these reactions, in contrast to the requirement for two equivalents with seleno ethers, may be based on cyclic intermediates 54, in which Li—Cl coordination provides an additional driving force. Curiously, attempted hydrozirconation of the corresponding telluride salt 55 under similar conditions was unsuccessful (Scheme 4.31) (Procedure 12, p. 143). 

Hydrozirconations of both vinyl and acetylenic boranes by Srebnik et al. led to 1,1-dime-tallo reagents, which offer the benefits as coupling partners of alkyl- and vinylboranes, respectively [59—62], Initial trials were conducted with B-alkenylborabicyclo[3.3.1]nonanes, but these led to unstable dimetallics. Replacement of the 9-BBN fragment with the pina-colborane-derived analogue produced stable dioxaborolanes 61 (Scheme 4.34). 

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. 

During the last ten years, the chemistry of 1,1-boriozirconocene complexes has been studied. Both hydrozirconation and hydroboration reactions are well established, and are widely applicable to a wide variety of vinyl and acetylene derivatives [1], Alkenylboranes and alkenylzirconium compounds can also be readily prepared. Therefore, hydrometalla-tion of the corresponding alkenyl metals should offer a convenient method for preparing gem-boriozirconocenes [24]. 

The preparation of (Z)-vinylstannanes by hydrozirconation of stannylacetylenes has been reported [21]. Earlier, a simple, efficient procedure for generating (Z)-vinylstannanes from acetylenic precursors 90 had also been developed (Scheme 7.27) [156—158]. 

The above-described procedure is advantageous towards the precedent method since (a) the starting lithium alkynyl selenolate is prepared in situ, avoiding the laborious preparation of the acetylenic selenides and (b) the hydrozirconation step is regio- and stereoselective, in contrast with the previously discussed hydrozirconation of acetylenic selenides resulting in a mixtnre of the regioisomers, and requires only 1 equiv of the Schwartz reagent instead of 2 eqniv of the precedent procednre. 

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. ... 

Following the precedent methodologies, telluro(stannyl) ketene acetals are achieved by the hydrozirconation of stannyl acetylenes and successive reactions with butyl tellurenyl... 

The E configuration of the products, assessed by NOE experiments, is a consequence of the known 100% regio- and ii-stereoselectivity of the hydrozirconation of acetylenic stan-nanes, and the retained configuration in the Zr/Te exchange. 

Similarly, hydrozirconation of stannyl acetylenes 108 followed by treatment of the intermediate vinylzirconium species 109 with -butyltellurium bromide and then with sodium borohydride gives tintelluroketene acetals 110 (Scheme 65).182... 

The regiochemistry of the hydrozirconation of acetylenic selenides with the Schwartz reagent [Cp2Zr(H)Cl] is dependent on the nature of the substituents. For simple acetylenic selenides (R = Ar, n-Bu), ( j-j9-zirconated vinyl selenides were formed exclusively. The reaction with electrophilic reagents has allowed the stereoselective synthesis of ( j-2-halovinylaryl selenides [75], (Ej-2-aryl-vinyl aryl selenides [76], ( , j-l-arylselanylbutadienes [77] and ( j-2-butyltel-luranylvinyl selenides [78] (Scheme 55). 

Dabdoud et al. have shown that the hydrozirconation of alkyn-l-ylbutyl selenides was complete with two equivalents of Cp2Zr(H)Cl. The nature of the products, formed after reaction with BuTeBr, is dependent on the substituent R [78] (Scheme 56, reaction 1). With R=Ph, the ketene butylselanyl(butyl-telluranyl)acetal was obtained exclusively. In the other cases (R = alkyl, MeOCH2), small amounts of (Rj-(2-butyltelluranyl)vinyl butyl selenides were also produced. The hydrozirconation of substituted acetylenic butyl selenides could be considered as a convenient approach for the synthesis of... 

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]. 

Schwartz and Carr found that hydrozirconation of olefins or acetylenes using Cp2Zr(H)Cl and subsequent transmetalation by AICI3 resulted in clean formation of the corresponding alkylaluminum dichlorides these were readily acylated to give ketones in excellent yields as depicted in Sch. 3 [15]. It should be emphasized that because hydrozirconation of internal (or terminal) olefins gives the terminal zirco-... 

Reductive deformylation of 71 then gave a 50% yield of pentanoate 72, which still required an additional carbon atom to obtain the required fragment. This homologation was then accomplished via reduction of ester 72 to the aldehyde, chain extension to the terminal dibromide, and conversion to the acetylene 73 (55%). Hydrozirconation followed by quenching with iodine afforded the desired fragment 74 in 67% yield. 

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