Polymers hypervalent iodine

In 2003, Togo and co-workers described a radical cyclization and ionic cyclization onto the aromatic rings of 2-(aryl)ethanesulfonamides 21 to produce 3,4-dihydro-2,l-benzothiazine 2,2-dioxides with polymer-supported hypervalent iodine reagents in good yields <03ARK11>. 

Scheme 2.4 Polymer-supported oxidations using a hypervalent iodine reagent. 

Following a similar strategy, an ingenious mixed resin bed quench and purification strategy was devised for the Dess-Martin periodinane mediated conversion of alcohols to carbonyls. This hypervalent iodine oxidant was viewed as containing an inherent masked carboxylic acid functionality that was revealed at the end of the reaction (Species (11) Scheme 2.30). Therefore purification was easily achieved by treatment of the reaction mixture with a mixed-resin bed containing both a thiosulfate resin and a polymeric base. The thiosulfate polymer was used to reduce excess hypervalent iodine lodine(V) and (III) oxidation states species to 2-iodoben-zoic acid (11), which was in turn scavenged by the polymeric base [51]. 

Polymer-supported hypervalent iodine compounds in general are readily prepared and they have gained recently considerable popularity as reagents for clean oxidations. However, they are not newcomers since they have been known since 1961. A detailed procedure for the iodination of polystyrene and its conversion to poly[(diacetoxyiodo)styrene] appeared in 1972 [85]. However, this and other related methods were time consuming and despite encouraging results did not gain popularity. 

Keywords. Hypervalent iodine, Oxidation, Polymer-supported reagents, Rearrangement... 

The products 58 of this reaction are of high interest as they can be used in a variety of subsequent reactions, Scheme 26. They can be used in next condensation reactions [120,121], which sometimes can be carried out as one-pot procedures [122]. Some examples are shown in Scheme 27. The combination of a-tosylation and reaction with amidines generates different substituted lH-imi-dazoles 60 [123]. Also polymer bound sulfonic acids can be used to trap the hypervalent iodine intermediates and to transfer subsequent steps to solid-phase synthesis [124]. Products of type 61 can be reacted with a variety of different bisnucleophiles to generate polycyclic compounds 62. 

Ley and co-workers applied the polymer-supported hypervalent iodine reagents, poly(diacetoxyiodo)styrene (PDAIS) or polybis(trifluoroacetoxyio-do)styrene (PBTIS) to the above-mentioned spiroannulation reaction. They succeeded in the concise syntheses of ( )-oxomaritidine (40) and ( )-epimari-tidine (41) using this methodology as the key steps [116] (Scheme 26). 

Scheme 38. Polymer-supported hypervalent iodine-mediated oxidative cyclization of a maritidine precursor.

Polymer-supported hypervalent iodine heterocyclic reagents 02SL1966. 

For other polymer-supported hypervalent iodine reagents, see Togo, H. and Sakuratani, K. (2002) Synlett, 1966-75. 

The use of sources of positive halogen to bring about oxidations Is well known. While the molecular halogen Itself Is often sufficient to bring about the desired reaction, a less nucleophilic counterion Is often quite useful. If this counterion becomes bound to a polymer, then reaction workup Is substantially simplified. Below are discussed the major polymer-bound halogenatlng agents. Those involving hypervalent iodine are discussed later. 

A similar oxidative protocol has been used for the oxidation of (fluoroalkyl)alkanols, Rf(CH2) CH20H, to the respective aldehydes [146], in the one-pot selective oxidation/olefination of primary alcohols using the PhI(OAc)2-TEMPO system and stabilized phosphorus ylides [147] and in the chemo-enzymatic oxidation-hydrocyanation of 7,8-unsaturated alcohols [148]. Other [bis(acyloxy)iodo]arenes can be used instead of PhI(OAc)2 in the TEMPO-catalyzed oxidations, in particular the recyclable monomeric and the polymer-supported hypervalent iodine reagents (Chapter 5). Further modifications of this method include the use of polymer-supported TEMPO [151], fluorous-tagged TEMPO [152,153], ion-supported TEMPO [154] and TEMPO immobilized on silica [148],... 

The first polymer-supported hypervalent iodine reagent, poly[(dichloroiodo)styrene], was prepared by chlorination of iodinated polystyrene in the early 1980s [8]. This method, however, involves the initial preparation of iodinated polystyrene under harsh conditions (160 h, 110 °C), requires the use of hazardous chlorine gas and affords poly[(dichloroiodo)styrene] with a relatively low loading of active chlorine. An optimized one-pot preparation of polystyrene-supported (dichloroiodo)benzene 2 (loading of -ICI2 up to 1.35 mmol g" ) from polystyrene 1, iodine and aqueous sodium hypochlorite (bleach) was reported in 2011 (Scheme 5.1) [10]. 

Oxidative iodination of aromatic compounds by the combination of a hypervalent iodine reagent with iodine is a synthetically important reaction (Section 3.1.4) [34]. Polymer-supported diacetate 4 is a particularly convenient reagent for oxidative iodination since it can be regenerated and reused many times. Reagent 4 gives the best results for the iodination of electron-rich arenes 13, with predominant formation of the para-substituted products 14 (Scheme 5.8) [12,21]. 

Tsarevsky has found that hypervalent iodine compounds can be used for the direct azidation of polystyrene and consecutive click-type functionalization [49]. In particular, polystyrene can be directly azidated in 1,2-dichloroethane or chlorobenzene using a combination of trimethylsilyl azide and (diacetoxyiodo)benzene. 2D NMR HMBC spectra indicate that the azido groups are attached to the polymer backbone and also possibly to the aryl pendant groups. Approximately one in every 11 styrene units can be modified by using a ratio of PhI(OAc)2 to trimethylsilyl azide to styrene units of 1 2.1 1 at 0 °C for 4 h followed by heating to 50 °C for 2 h in chlorobenzene. The azidated polymers have been further used as backbone precursors in the synthesis of polymeric brushes with hydrophilic side chains via a copper-catalyzed click reaction with poly(ethylene oxide) monomethyl ether 4-pentynoate [49],... 

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