Cyclization-anion capture cascade

Scheme 10 A cyclization-anion capture cascade with ensuing [4 + 2] cycloaddition [52]...

Propargylic carbonates are valuable substrates in Pd(0)-catalyzed processes. Their chemistry differs mechanistically from that of simple alkynes in that they form allenyl palladium species. We and others have shown these versatile intermediates can serve a double function as both starter and terminating species in bis-cyclization-anion capture cascades A typical example is the process 45 46, which generates azabicyclo [3.1.0] hexanes (Scheme 5.6.12) in good yield. ... 

Palladium-Catalyzed Cascade Cyclization-Anion Capture Processes Employing Pre- and In Sitn-Formed Organostannanes... 

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. 

A novel, three-component, palladium-catalyzed, cascade cyclization-anion-capture process which involves in situ generation of a zipper molecule has been reported. Thus, 2-iodobenzoyl chloride, an acetophenone imine, and carbon monoxide react in the presence of Pd(PPh3)2Cl2 to give isoindolin-l-one in moderate yield (eq 86). 

Sequential and cascade palladium catalyzed cyclization-anion capture-olefin metathesis. Tetrahedron Letters, 40,... 

Grigg and co-workers [203] have very recently described an ingenious cascade process. Tliis is initiated by the addition of Bu3SnH to alkyne cyclization and anion capture follow and afford polycyclic products (Scheme 4-52). A palladium-catalyzed hydrogenation of the double bond in ajS-unsaturated A -acyloxy azolidinones was described by Wu et al. [204] the diastereoselectivity was low. 

Table 5.6.4 Cascade hydrostannylation-cyclization-carbonylation-anion-capture processes ...

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