Anion capture process

In these cydizations, the reaction can be terminated in other ways than elimination of /3-hydrogen. Typically the reaction ends by an anion capture process[154]. The following anion transfer agents are known H, OAc , CN, S02Ph, CH(C02R)2, NHRj, CO/ROH, and RM [M = Sn(IV), B(lll), Zn(II)]. Trapping with an amine after alkene insertion to give 189 and 190 is an example. A-Acetyl protection is important in this reaction[155]. 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

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

The variety of products can be increased by an anion-capturing process of tbe intermediate Pd-compounds, and tbis has been intensively explored by Grigg and coworkers [18]. 

Heck reactions can also be combined with anion capture processes, animations, metatheses, aldol and Michael reactions, and isomerizations. The anion capture process has also been widely used with other Pd-catalyzed transformations. Outstanding examples of many different combinations have been developed by Grigg and coworkers, though not all of them match the requirements of a domino process. All of these reactions will be detailed here, despite the fact the nature of these intermediate transformations would also have permitted their discussion in Chapter 2. 

Grigg developed several palladium-catalyzed tandem cyclization-anion capture processes, and these include organoboron anion transfer agents [112, 164, 206]. Two examples of this methodology are shown. 

Scheme 3-25 A synthesis of capnellene (86) empioying an enantioselective intramolecular Heck and anion-capture process [40a, 11 la].

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... 

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

Table 5.6.1 Potential combinations for (poly) cyclization-anion capture processes...

Mono-Cyclization-Anion Capture Processes Carbamyl and Oxycarbonyl Starter Species... 

Carbamyl 1,2 and oxycarbonyl 5 chlorides have proven effective starter species although their cyclization-anion capture processes have been only briefly explored (Scheme 5.6.1)." The terminating species in Scheme 5.6.1 are alkynyl (1,5) and alkenyl (3) with pre-formed Sn(IV) capmre agents. These processes, as expected, are regio-and stereoselective and their full potential remains to be explored, but reference to Table 5.6.1 indicates the substantial leverage available from alternative terminating and anion capture species. 

Suffert et reported the first examples of palladium-catalyzed 4-exo-dig cyclization-anion capture processes (Scheme 5.6.3). In an ingenious sequence, which incorporates two separate electrocyclic steps, the vinyl starter species 10, in the presence of Pd(0) and iranx-vinyl distannane 11 in benzene at 90 °C for 30 min, affords 12, which on further heating undergoes disrotatory electrocyclization to the strained tricyclic diol 13 (Scheme 5.6.3). The electrocyclization process establishes the expected anti-stereochemistry of the 6-H and tri-n-butylstannyl group. Intermediate 13 undergoes elimination-... 

We have transferred the in situ generation of anion capture agents/cyclization-anion capture processes to solid supports Thus 35, attached to Wang resin, was reacted with alkyne 36 in the presence of Pd(0) and Bus SnH in toluene initially at 0 °C for 1 h, then 110 °C for 16 h. The expected product 37 was cleaved from the resin by transesterification with MeOH in good yield (Scheme 5.6.9). 

Sequential decarboxylative azomethine ylide cycloaddition-palladium catalyzed hydrostannylation-cyclization-anion capture processes have also been achieved (Scheme 5.6.10). ... 

We have provided numerous examples that show that these bis-cyclization-anion capture processes are diastereoselective. For example, 47a, b in the presence of Pd(0) and 2-thienyl tri-n-butylstannane in THF at 60 °C for 2-A h afford 48 and 49 as single diastereomers (Scheme 5.6.13). ... 

Typical examples of bis-cyclization anion capture processes forming fused and spirocyclic rings, triggered by aryliodide species, are shown in Scheme 5.6.16. Thus, enamide 50 reacts with Af-SEM-indolyl stannane in the presence of Pd(0) to afford a 5 1 diastereomer mixture of 51 and 52 in 74% yield, whilst 53 reacts with 2-pyridyltri- -butyltin in the presence of Pd(0) to afford 54a as a single diastereoisomer. ... 

Cyclization-Anion Capture Processes Involving Relay Switches... 

Cyclization-carbonylation-anion capture processes involving in situ generated vinylstannanes as terminating agents (as in Section 5.6.2) afford a variety of complex heterocyclic a, (3-unsaturated ketones in good yield. Typical examples are shown in Table 5.6.4. °... 

Queguiner made great use of Suzuki methodology to prepare heterobiaryls for use in carboline synthesis [169]. This chemistry is discussed in Chapter 4. As explored in more detail in Section 3.5, Grigg developed several Pd-catalyzed tandem cyclization-anion capture processes, and these include organoboron anion transfer agents [109, 170, 171], Two examples of this methodology are shown. 

Asymmetric Heck reaction.3 Treatment of the vinyl triflatc I with tclrabulylam-monium acetate (1.7 equiv.) and a catalytie amount of Pd(OAc) and (S)-BINAP in DMSO at 20° results in the fused bicyclic pentanoid 2 in 80% ee, formed by a Heck reaction followed by an anion capture process. This product can be converted into 3, an... 

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