Cyclization-anion capture reactions

Grigg and coworkers have used 2-tributylstannylfuran to terminate a variety of interesting tandem cyclization-anion capture reactions. In one example, aryl iodide 105 reacts via a 5-exo-trig process to provide dihydroindolone 106 [105-107]. 

Grigg and coworkers also pioneered the work on cyclization-anion capture reactions using hydride-, [45] carbonyl- [46] or cyanide- [47] capture (Scheme 6.10). The terminating anion capture is, however, not limited to these three examples in fact, the organopalladium(n) intermediate can be captured by a wide variety of anionic, neutral and organometallic agents [47]. 

Heck cyclization/anion capture process 576 Heck reaction 566 ff. 

Nuss and co-workers have demonstrated that a tandem Heck cyclization-anion capture process can access substructures related to vitamin D [58] and the neocarzinostatin chromophore (162) (Scheme 6-28) [59]. In a one-pot procedure, diiodide 159 was converted into dienediyne 160 in low yield. In this reaction, the alkyne undergoes Heck... 

The use of allene as a relay switch introduces the benefits/challenges of regio- and stereo-selectivity. Our initial studies of cyclization-anion capture with allene as a relay switch generated allylic amines in excellent yield. Reactions were conducted in toluene (90 °C, 20 h) in a Schlenk tube, and employed 0.5-1 atm of allene and various amines, using alkenes/alkynes as relay species (Scheme 5.6.23)." The use of organostannanes as anion-capture agents in this cascade has yet to be explored. 

A one-pot sequential and cascade sequence involving the formation of allylic azides, from aryl/heteroaryl/vinyl halides, allene and sodium azide, by palladium catalyzed anion capture, and cyclization-anion capture, followed by 1,3-dipolar cycloaddition provided a variety of 1,2,3-triazoles in good yields <01T7729>. Reaction of a,P-acetylenic aldehydes 107 with sodium azide in dimethylsulfoxide followed by hydrolysis afforded 5-substituted-4-carbaldehyde-1,2,3-triazole derivatives 108 <01TL9117>. 

The first example of an enantioselective intramolecular cascade Mizoroki-Heck-cyanation sequence was recently reported which included the reaction of amide 104 (Scheme 12.24) [33], The cyanide source employed was potassium ferro(II)cyanide, which has been utilized for the palladium-catalysed cyanation of aryl halides. The proposed reaction pathway for the Mizoroki-Heck-cyanation involves capture of a a-alkylpalladium intermediate. Previous examples of enantioselective Mizoroki-Heck cyclization-anion capture most often involve trapping of the 7r-allylpalladium complexes in group-selective reactions. Reaction conditions were surveyed for the Mizoroki-Heck cyanation sequence. It was found that Pd(dba)2 afforded better enantioselectivities than Pd(OAc)2 with Ag3P04 as the additive. Using PMP under neutral conditions led to racemic product. To improve the enantioselectivity, several bidentate ligands were screened, and the ligand DIFLUORPHOS 54a was found to give the best enantioselectivity. 

CycUzation-Anion-Capture Process. Grigg et were interested in devising ring-forming processes with concomitant introduction of functionality by replacing the /3-hydride elimination step of the Heck reaction with a group or atom transfer. This led to the development of cascade cyclization-anion-capture processes. 

Z-tamoxifen 403 tandem cyclization 290, 295 tandem Heck reaction-anion capture 253-4 tandem Heck reaction-phenoxide capture 253 tandem Heck reactions 251, 252-4 tandem intramolecular Heck-intermolecular Stille cross-coupling 255 taxol 140, 143,243,245 ( )-tazettine 146,234 telomerization 352 telomerization products 343, 345 template effect 140 teraconic anhydride 468 terminal acetylenes, synthesis of 216-20 terminal alkynes 6, 213 terminal 2,2-diorgano-l-aIkcnylboronates 51 terminal diynes 207 ternary complex 444 ternary coupling 177... 

Some examples of termination by anion capture are as follows. Reactions are terminated by carbonylation. Domino carbopalladation and carbonylation of the iododiene 163 under 1 atm of CO via a neopentylpalladium intermediate gave the cyclized ester 164 with high yield and diastereoselectivity [82], The iodide 165 underwent 5-exo cyclization to generate the neopentylpalladium 166 and its carbonylation gave the acylpalladium 167. Intramolecular attack by malonate anion affords 168 [83]. 

The Balme group studied the cyclization-couphng reaction of alkene-containing car-bonucleophiles with organic halides using an anion capture approach [100] in the preparation of bi- and tri-cyclic compounds. A short entry to triquinanes, such as 183, via a transient palladium(II) complex formed by a Mizoroki-Heck reaction of the vinyl bromide 181, which reacts with the carbanion of a malonate moiety present in the substrate, is an illustrative example (Scheme 8.47). However, depending on the halide used, the products... 

For previous reports of tandem Heck reaction-anionic capture sequences, see (a) Grigg, R., Sridharan, V. and Xu, L.-H. (1995) Palladium-catalyzed cyclization-amination of allenes - effect of base on regioselectivity of formation of allylic amines. J. Chem. Soc., Chem. Commun., 1903 and references cited therein (b) Ma, S. and Negishi, E. (1995) Palladium-catalyzed cyclization of general route to common, medium, and large ring compounds via cyclic carbopaUadation. J. Am. Chem. Soc., 117, 6345-7 and references cited therein. 

Acyliminium ions are also derived from rearrangement of Bischler-Napieralski cyclization products, which are obtained from amide product of reaction of arylethylamines and 2-methoxycarbonyl benzoyl chloride. These can be captured, for example, with methoxide anion (Scheme 43) (94TL(35)2751). 

Asymmetric carbopalladation can also be combined with other reactions to give domino asynunetric processes. A group selective example from a synthesis of ( capnellene 216 features the cyclization of the alkenyl triflate 210 (Scheme 32). The resultant TT-allylpalladium intermediate has effectively been captured with a variety of nucleophiles including acetate anion for the synthesis. A domino Suzuki coupling/in-tramolecular Heck reaction converts ditriflate 212 into tricycle 213 in modest yield with 85% The transformation accomplises an annnelation, two carbon-carbon bond... 

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