Stannanes palladium-catalyzed reactions with acid

Radhakrishnan and coworkers reported desymmetrization of bicyclic hydrazines, derived from the [4+2] cycloaddition of fulvene 27 with diethy-lazodicarboxylate, followed by a palladium/Lewis acid-catalyzed reaction with allyl and vinyl stannane to provide the alkylidene cyclopentenes 106 and 107, respectively (Scheme 7.25) [26]. 

Li and Yue also reported the intermolecular palladium catalyzed cross coupling reactions of bromo quinoxalines 214 and 216 with aryl boronic acids and heterocyclic stannanes, respectively <99TL4507>. The Suzuki couplings (i.e., 214 215) required the use of a... 

Pyrrolyl- and indolyl-stannanes and -boronic acids, which can be prepared from the corresponding organolithium derivatives, have received increasing use in palladium-catalyzed coupling reactions with aryl halides (Scheme 82) (91S613,92JOC1653). 

Despite the lack of examples of Friedel-Crafts acylations catalyzed by Lewis acids, - reactions of stannanes with acyl halides catalyzed by palladium species have found considerable use for the preparation of ketones. Since alkyl groups are only transferred slowly from tin, more rapid transfer to the acyl chloride is observed for alkynyl, alkenyl and allyl, as well as aryl and benzyl, groups. This leads to a versatile synthesis of ketones.Acylations of alkenylstannanes are both regio- and stereo-specific. 

Boronic acids derived from 3- and 5-substituted indoles are useful intermediates for palladium-catalyzed vinylation, arylation, and heteroarylation (Equation (47)) <92H(34)1395, 93TL2235>. As with the stannanes, a reverse process can be utilized. 6- and 7-Bromoindole can be coupled directly with arylboronic acids to give the 6- and 7-arylindoles, respectively a feature of the reaction is that protection of the indole nitrogen is unnecessary (Equation (48)) <94SL93>. A new route to 2-substituted indoles has been developed from trialkyl-(l-methylindol-2-yl)borates. The borate (153),... 

The most important palladium-catalyzed processes include Heck vinylation of halides and sulfonates and various cross-coupling reactions in which a nucleophilic intermediate (stannane, organo-zinc halide or boronic acid) is coupled with an electrophile (halide or sulfonate). These coupling reactions are usually restricted to arylation and vinylation because of the tendency of alkylpalladium species to undergo elimination. The pyrrole and indole rings can participate in cross-coupling as either the nucleophilic or electrophilic component. 

Palladium-catalyzed cross coupling of resin bound iodopyridine with a boronic acid (Suzuki reaction) (12) or with an alkenyl-, alkynyl-, or arylstannane (Stille reaction) (13) was effected by treatment of resin 5 with 4 equivalents of the boronic acid or stannane, 8 equivalents of K2CO3 (Suzuki reaction only), and catalytic Pd(PPh3)4 in DMF at 50 C for 20 hours. (14) Best results were achieved by running each reaction twice, with an intermediate wash of the resin, in order to drive the reaction to conq>letion. The corresponding Pd-catalyzed amination (Buchwald reaction) (15) worked well in protocol development but the reaction failed with the 3 amines attenpted during actual library preparation. 

Recently, the Blum group showed that the synthesis of a variety of tri- and tetra-substituted olefins by gold/palladium-catalyzed addition of sp - and sp-hybridized stannanes across monoester and diester alkynes can be accomplished with complete regioselectivity and high stereoselectivity. TTie reaction is proposed to proceed via a bimetallic mechanism where a Lewis acidic gold(I) activates an alkyne toward oxidative addition across palladium(O). 

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