Acetylenic selenides

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). 

General procedure for the synthesis of telluroselenoethenes from acetylenic selenides... 

Telluro(seleno)ketene acetals have been synthesized by the Al/Te exchange reaction, as shown in the following scheme, starting from hydroalumination of acetylenic selenides. ... 

The following scheme illustrates the different regio- and stereochemical outcomes of the sequential addition of the Schwartz reagent (Cp2Zr(H)Cl) and butyl tellurenyl bromide to acetylenic selenides. ... 

With R=H or Ph unique products are formed with opposite regiochemistry, whereas aikylse-lenoalkynes (R=aikyl) give rise to a mixture of regioisomers. All the acetylenic selenides require 2 equiv of the Zr reagent to ensure the total conversion of the starting material. 

In a further line of experiments lithium alkynyl selenolate, instead of acetylenic selenides, has been used to generate the zirconated vinylselenides, precursors of (E)-telluro(seleno)ketene acetals. ... 

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. 

In the last section, the access to a-selanyl vinylmetals is considered. Besides the classical methods involving vinylic selenides and ketene selenoacetals, the hydrometallation (M = Sn, Zr, B) of acetylenic selenides constitutes an efficient method. These species can be considered as vinyl dianion equivalents. Some examples of cross-coupling reactions are presented. 

In this concluding section, we will recall that a-selanylvinylmetal derivatives can be produced by a-deprotonation of vinylic aryl selenides 11, cleavage of ketene selenoacetals 12 or bromine/lithium exchange of a-bromovinyl selenides 13 (Scheme 49). These strategies involve vinylic selenides whose syntheses are not simple. More recent works have used organocuprate additions, hydrometala-tions (M = Sn, Zr) or hydroborations of readily available acetylenic selenides 14 (Scheme 49). The process can be regio- and stereocontrolled and produce the a-selanylvinylmetal derivatives under mild conditions. The latter can be considered as vinyl dianion equivalents and are very useful intermediates for cross-coupling reactions. 

The reaction of various electrophiles with vinylcuprates, formed by addition of lithium n-butylcyanocuprate to acetylenic selenides afforded, a-substituted vinylic selenides in reasonable to good yields (50-76%) [661 (Scheme 53, reaction 1). Starting from phenylselanylacetylene, the two groups introduced have a cis configuration [67,681. A loss of stereoselectivity was observed after hydroly-... 

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). 

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]. 

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