Anion capture

Asymmetric Heck reaction of the conjugated diene 184 and subsequent acetate anion capture of the rr-allylpalladium intermediate afforded 185 in 80% ee. which was converted into the key intermediate 186 for the capnelle-... 

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]. 

Terminal alkynes undergo the above-mentioned substitution reaction with aryl and alkenyl groups to form arylalkynes and enynes in the presence of Cul as described in Section 1.1.2.1. In addition, the insertion of terminal alkynes also takes place in the absence of Cul, and the alkenylpalladium complex 362 is formed as an intermediate, which cannot terminate by itself and must undergo further reactions such as alkene insertion or anion capture. These reactions of terminal alkynes are also treated in this section. 

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. 

Grigg, R. Sridharan, V. (1999) Palladium Catalyzed Cascade Cyclisation - anion Capture, Relay Switches and Molecular Queues. Journal of Organometallic Chemistry, 576, 65-87. 

The yield of the photocathodic dissolution of CdS in a solution containing 1 x 10" M SOi" is only 0.005 molecules dissolved per photon absorbed. In the presence of 5 X 10 M excess Cd " ions it amounts to 0.05. Sulfate and dithionate are formed in the ratio 2.2 to 1. The oxidation of SO3" is effected by the positive holes produced upon illumination, two holes being necessary to convert one SO ion into SO " or 1/2 SjOg . If the SOj anion captured the two holes from the same colloidal particle ( two-hole mechanism ), only sulfate would appear as the oxidation product. However, if SO3" captured only one hole to form the radical SOj", the final products would be formed bj reactions of two such radicals, and these two radicals could even originate from different colloidal particles ( one-hole mechanism )... 

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 and coworkers developed bimetallic domino reactions such as the electro-chemically driven Pd/Cr Nozaki-Hiyama-Kishi reaction [69], the Pd/In Barbier-type allylation [70], Heck/Tsuji-Trost reaction/1,3 dipolar cycloaddition [71], the Heck reaction/metathesis [72], and several other processes [73-75]. A first example for an anion capture approach, which was performed on solid phase, is the reaction of 6/1-134 and 6/1-135 in the presence of CO and piperidine to give 6/1-136. Liberation from solid phase was achieved with HF, leading to 6/1-137 (Scheme 6/1.30) [76]. 

Scheme 6/1.36. Domino Heck/anion-capture reaction.

The Balme group [82] also used the anion capture approach to develop a short entry to triquinanes as 6/1-150 forming a transient Pdn-complex in a Heck reaction of the vinyl iodide 6/1-149 (Scheme 6/1.39). The latter reacts with the carbanion ofa malonate moiety in the substrate however, the products of the normal Heck reaction are also formed. 

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. 

Whereas Hegedus [335] and Danishefsky [336] were the first to discover a tandem Heck reaction from o-allyl-A -acryloylanilines leading to tricyclic pyrrolo[l,2-a]indoles or pyridino[l,2-a]indoles [336], it has been the fantastic work of Grigg to unleash the enormous potential of this chemistry. Grigg and his co-workers parlayed their Pd-catalyzed tandem polycyclization-anion capture sequence into a treasure trove of syntheses starting with IV-allyl-o-haloanilines [337-345], Diels-Alder and olefin metathesis reactions can be interwoven into the sequence or can serve as the culmination step, as can a wide variety of nucleophiles. An example of the transformation of 289 to 290 is shown below in which indole is the terminating nucleophile [340],... 

Elegant contributions to this fidd have also been made by Grigg et al espedally interesting are his palladium-anion capture domino processes. By this way a direct route to protected pseudoargiopinine III 127, a toxine from Argiope bbata, has been developed using a three-component process of allene 125... 

Scheme 25. Palladium anion capture domino process...

The attempted trifluoromethylation of pentamethylcyclopentadienyllithium by reaction with F3 Cl in the presence of A-phenylmaleimide gave rise to the unexpected Diels-Alder adduct (25).37 The authors proposed that the cyclopentadienyl anion captures the iodine of CF3I to give (26) and trifhjoromethyllithium which then produces difluorocarbene (F2C ) which inserts into the C—I bond of (26) to form the difluoroiodomethylated product. The observed regioselectivity of addition of thiazyl chloride (N=S—Cl) to 2,5-diarylftirans provided support for a mechanism involving carbene and nitrene intermediates (27).38... 

The alkyne insertion reaction is terminated by anion capture. As examples of the termination by the anion capture, the alkenylpalladium intermediate 189, formed by the intramolecular insertion of 188, is terminated by hydrogenolysis with formic acid to give the terminal alkene 192. Palladium formate 190 is formed, and decarboxylated to give the hydridopalladium 191, reductive elimination of which gives the alkene 192 [81]. Similarly the intramolecular insertion of 193 is terminated by transmetallation of 194 with the tin acetylide 195 (or alkynyl anion capture) to give the dienyne 196 [82], Various heterocyclic compounds are prepared by heteroannulation using aryl iodides 68 and 69, and internal alkynes. Although the mechanism is not clear, alkenylpalladiums, formed by insertion of alkynes, are trapped by nucleophiles... 

From a mechanistic viewpoint, the Pd(0)-eatalysed reactions of propargylic compounds so far discovered can be classified into four types I IV The allenyl complexes 5 undergo three types of transformations depending on reactants. Type I reactions proceed by insertion of unsaturated bonds to the n-bond between Pd and the sp2 carbon in 5. Type la is the insertion of alkenes to the palladium-carbon n-bond, and the 1,2,4-alkatrienes are formed by /f-elimination. Alkynes insert to form the alkenylpalladium 6, which undergoes various transformations such as insertion of unsaturated bonds and anion captures. 

Langer, P. Doring, M. Seyferth, D. Gorls, H. Synthesis of nitrile oligomers through multiple anion capture reactions of allene dianions. Eur.J. Org. Chem. 2003, 1948-1953. 

The same authors also reported on palladium-catalyzed domino cycli-zation-anion capture reactions employing allene (69) as a relay switch (Scheme 10) [52], Such a reaction of the alkynyl-tethered aryl iodide 68 starts with an oxidative addition to the catalytic metal species, and is followed by an intramolecular carbopalladation of the triple bond. An intermolecular car-... 

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

The same palladium-catalyzed domino cyclization-anion capture sequence involving carbopalladation of allene (69), which was employed by Grigg et al. for the synthesis of diene 70 (Scheme 10) can yield 1,3,5-hexatriene 194, when starting from 2-bromo-l-ene-6-yne 193 instead of the aryl iodide 68 (Scheme 32) [52], Under the conditions of its formation, 194 immediately undergoes thermal 67r-electrocyclization to give the bicyclic product 195. 

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