Alkenyl Stannanes

The Stille reaction can be used with alkenyl stannanes, alkenyl halides, and triflates,196 and the reactions occur with retention of configuration at both the halide and stannane. 

Allylation of perfluoroalkyl halides with allylsilanes is catalyzed by iron or ruthenium carbonyl complexes [77S] (equation 119) Alkenyl-, allyl-, and alkynyl-stannanes react with perfluoroalkyl iodides 111 the presence ot a palladium complex to give alkenes and alkynes bearing perfluoroalkyl groups [139] (equation 120)... 

Alkenyl(tributyl)stannane (Method ) General Procedure27 ... 

A second consequence of the cyclic transition state of these reactions is that (Z)-alkenyl-stannanes are less reactive than their (E)-isomers and can in some cases be recovered unchanged from reactions in which mixtures of isomers are used109. a-Alkoxy-2-butenylstannanes fail to react with 0,/J-dialkylated a./J-unsaturated aldehydes, on heating, and give low syn/anli stereoselectivity with ,/i-acetylenic aldehydes, perhaps because of reduced steric effects110. 

Table 7. 4-Hydroxy-l-alkcnyl Diisopropylcarbamates from 2-Alkenyl Diisopropylcarbamates by Ho-moaldol Addition of Carbonyl Compounds to Trichlorotitanates (via Stannanes)...

The versatility of Pd-catalyzed coupling of stannanes has been extended by the demonstration that alkenyl triflates are also reactive.199... 

Alkenyl silanes and stannanes have the potential for nucleophilic delivery of vinyl groups to a variety of electrophiles. Demetallation also occurs in these reactions, so the net effect is substitution for the silyl or the stannyl group. 

There are several procedures for synthesis of terminal alkenyl stannanes that involve addition to aldehydes. A well-established three-step sequence culminates in a radical addition to a terminal alkyne.150... 

As with the silanes, the most useful synthetic procedures involve electrophilic attack on alkenyl and allylic stannanes. The stannanes are considerably more reactive than the corresponding silanes because there is more anionic character on carbon in the C-Sn bond and it is a weaker bond.156 The most useful reactions in terms of syntheses involve the Lewis acid-catalyzed addition of allylic stannanes to aldehydes.157 The reaction occurs with allylic transposition. 

Enantioselective Addition Reactions of Allylic Stannanes. There have been several studies of the enantiomers of a-oxygenated alkenyl stannanes. The chirality of the a-carbon exerts powerful control on enantioselectivity with the preference for the stannyl group to be anti to the forming bond. This is presumably related to the stereoelectronic effect that facilitates the transfer of electron density from the tin to the forming double bond.182... 

There are, however, serious problems that must be overcome in the application of this reaction to synthesis. The product is a new carbocation that can react further. Repetitive addition to alkene molecules leads to polymerization. Indeed, this is the mechanism of acid-catalyzed polymerization of alkenes. There is also the possibility of rearrangement. A key requirement for adapting the reaction of carbocations with alkenes to the synthesis of small molecules is control of the reactivity of the newly formed carbocation intermediate. Synthetically useful carbocation-alkene reactions require a suitable termination step. We have already encountered one successful strategy in the reaction of alkenyl and allylic silanes and stannanes with electrophilic carbon (see Chapter 9). In those reactions, the silyl or stannyl substituent is eliminated and a stable alkene is formed. The increased reactivity of the silyl- and stannyl-substituted alkenes is also favorable to the synthetic utility of carbocation-alkene reactions because the reactants are more nucleophilic than the product alkenes. 

Intramolecular nucleophilic assistance is another distinct possibility for providing activation, and can be realized by coordination of the metal center by a heteroatom (O, N, etc.) within the ligand which transiently forms a chelate-like attachment to the metal. Such a mode of activation was supposed to account for the ease of reaction of vinyl triflates with (zp-isomers of alkenyl-stannanes bearing OH groups in the proximity of the reaction center (71).2 5... 

Like most aryl halides, furyl halides and furyl triflates have been coupled with a variety of organostannanes including alkenyl, aryl, and heteroaryl stannanes in the presence of catalytic palladium. Carbamoylstannane 66 was prepared by treating lithiated piperidine with carbon monoxide and tributyltin chloride sequentially. The Stille reaction of 66 and 3-bromofuran then gave rise to amide 67 [61]. In another example, lithiation of 4,4-dimethyl-2-oxazoline followed by quenching with MesSnCl resulted in 2-(tributylstannyl)-4,4-dimethyl-2-oxazoline (68) in 70-80% yield [62], Subsequent Stille reaction of 68 with 3-bromofuran afforded 2-(3 -furyl)-4,4-dimethyl-2-oxazoline (69). 

Little use has been made of the reaction between acylstannanes and phosphorus ylides as a route to alkenyl-stannanes, though the feasibility of these reactions is well established, for example, Equation (69).217... 

As indicated in Section ni.B, deprotonation of a carbamate affords a dipole-stabilized a-amino-organolithium that can be transmetalated with copper salts to form cuprates, thereby expanding the versatility of the organolithium. Suitable electrophiles include enones, alkenyl, alkynyl, allenyl and dienyl carboxylic acid derivatives, nitriles and sulfoxides. Dieter and coworkers have shown that the same process can be accomplished via transmetalation of a stannane (Scheme 36). The procedure is particularly... 

In addition to aryl-aryl coupling, the Stille reaction can be used with alkenylstan-nanes and alkenyl halides and triflates.125 The reactions occur with retention of configuration at both the halide and the stannane. These reactions have become very useful in stereospecific construction of dienes and polyenes, as illustrated by some of the examples in Scheme 8.10. 

The fact that the cyclization is directed to an acetylenic group and leads to formation of an alkenyl radical is significant. Formation of a saturated iodide would be expected to lead to a more complex product mixture because the cyclized product could undergo iodine abstraction and proceed to add to a second unsaturated center. Vinyl iodides are much less reactive, and the reaction product is stable to iodine-atom abstraction. Because of the potential for competition from reduction by the stannane, other reaction conditions have been developed to promote cyclization. Hexabutylditin is frequently used.238... 

Die Michael-Addition an (E)- -Nitro-1 -alkene gelingt auch mit [(Z)-l-Alkenyl]-tributyl-stannan in Gegenwart von Lewis-Sauren ... 

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