Propargyl leaving groups

Organocuprates react rapidly witli adylic balides for acetates), propargyl balides for acetates), and vinyloxiranes, frequently witli u2 regioselectivity. Tbe reaction ordinarily takes place witli oiui fwitli respect to tlie leaving group) steteoclieniistry. 

Cuprates react rapidly witli allylic halides for acetates) [17, 23], propargyl halides for acetates) [ 106-108], and vinyloxiraties, often witli S 2 regioselectivity iSclieme 10.9) [17]. Hie reaction takes place witli emit stereodieniistry iwitli respect to tlie leaving group), while syn substitution occurs when an allylic carbamate is employed as tlie substrate [109]. 

Propargylic substitution reaction is one of the most important routes to allenic compounds [1, 2], As shown in Scheme 3.1, replacement of a leaving group at the propargylic position with an incoming nucleophile via an SN2 pathway rearranges the C=C-C skeleton into a C=C=C moiety to give a propadienyl species. With certain... 

Changing to a trisubstituted aromatic precursor such as 1,3,5-tribromobenzene (153) yields the trisallene 155 when l-bromo-2-butyne (154) is used as the coupling partner. When leaving groups of different reactivity are present in the aromatic substrate, such as in l-bromo-4-iodobenzene, different propargyl halides can be connected to the aromatic core, resulting in the formation of arylallenes with different allene substituents. 

Table 9.21 Effect of propargylic leaving group on the efficiency of allenyltitanium additions...

Organocuprates react rapidly with allylic halides (or acetates), propargyl halides (or acetates), and vinyloxiranes, frequently with 5 2 regioselectivity. The reaction ordinarily takes place with anti (with respect to the leaving group) stereochemistry. 

SCHEME 23. Pd-catalyzed allylation and propargylation with various leaving groups... 

For alkynes bearing a leaving group at the propargylic position, bis-addition also took place and subsequent 1,3-elimination afforded disubstituted cyclopropanes (equation 153)176. 

SN2-type Reactions Other leaving groups are also prone to undergo direct Sn reactions in the presence of Fe salts. Zhan and Liu described the direct Fe-catalyzed etherification of propargylic acetates by O-nucleophiles to give rise to a variety of functionalized propargylic ethers (Scheme 7.10) [14]. 

Benzotriazole is an excellent leaving group, and the readily available imidazolidin-2-ones react with aryl-, alkenyl-, or alkylzinc derivates via an elimination-addition mechanism providing the /raor-products with >99% diastereo-selectivity (Equation (65)).125 Propargylic mesylates such as fluorine-substituted derivatives react with PhZnCl in the presence of Pd(PPh3)4 (5mol%) to provide the anti-S Z -product in excellent yield and complete transfer of the stereochemistry leading to the allene species (Equation (66)).126... 

The introduction of a double bond between the triple bond and the leaving group of a propargyl electrophile leads to enyne electrophiles (e.g., 198) which would give access to vinylallenes 199 if the attack of the nucleophile takes place at the triple bond in an Sn2" (l,5)-substitution reaction (Scheme 48). Besides the regioselectivity, two types of stereoselectivity have to be considered in this transformation, that is, the configuration of the olefinic double bond of the vinylallene and the (relative or absolute) configuration of the allenic chirality axis. In the event, the reaction of enyne acetates 198 with various lithium cuprates proceeds... 

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