Allylic ethers hydroboration

Allyl formate, physical properties, 6 292t Allyl glycidyl ester (AGE), physical properties of, 2 244, 246 Allyl glycidyl ether, 2 267-268 Allylic derivatives, hydroboration of, 73 641... 

Very recently, three alternatives to the /-selective hydrosilylation-oxidation of 6 and its analogs have been reported. Two of them involve catalyzed hydroboration occurring with moderate to good selectivity11,12, and the other involves conversion to a vinyl allyl ether which then undergoes a double (in ter-intramolecular) hydroboration in which the intramolecular step is highly stereoselective13. These methods are discussed in Section D.4.2.2. 

Hydroborations. Addition of Catecholborane to alkenes is accelerated by Wilkinson s catalyst, and other sources of rhodium-(I) complexes. Unfortunately, the reaction of Wilkinson s catalyst with catecholborane is complex hence if the conditions for these reactions are not carefully controlled, competing processes result. In the hydroboration of styrene, for instance, the secondary alcohol is formed almost exclusively (after oxidation of the intermediate boronate ester, eq 37) however, the primary alcohol also is formed if the catalyst is partially oxidized and this can be the major product in extreme cases. Conversely, hydroboration of the allylic ether (12) catalyzed by pure Wilkinson s catalyst gives the expected alcohol (13), hydrogenation product (14), and aldehyde (15), but alcohol (13) is the exclusive (>95%) product if the RhCl(PPh3)3 is briefly exposed to air before use. The 5yn-alcohol is generally the favored diastereomer in these and related reactions (eq 38), and the catalyzed reaction is therefore stereocomplementary to uncatalyzed hydroborations of allylic ether derivatives. ... 

Silicon.—Alkenyl, Vinyl-, and Allyl-silanes. Hydroboration of alkynylsilanes proceeds stereospecifically, subsequent carbodeboronation leading to stereochemically defined vinylsilanes (Scheme 26). A similar route using hydroalumination can also be employed, but the stereochemistry is dependent upon the conditions of the hydroalumination step (product of cw-hydroalumination in ether-hexane, transin hexane alone). ... 

Another example for block copolymer synthesis is the preparation of poly (methylene-fc-styrene), which was achieved by using a hydroxyl-terminated living polystyrene obtained by TEMPO-mediated living radical polymerization [75] (Scheme 55). The chain end hydroxyl group was transformed into an allyl ether moiety, which was subjected to hydroboration with BH3 to afford polystyrene macroinitiator for the polymerization of 11. After chain elongation with 11 oxi-dation of the C-B chain ends furnished poly(methylene- -styrene) bearing TEMPO and hydroxyl group at chain ends. 

As summarized in Table 1, this hydroboration polymerization can be applied to various combinations of dienes such as 1,9-decadiene, p-divinylbenzene, p-diallylbenzene, bis(allyl ether)s of ethylene glycol, tetramethylene glycol, triethylene glycol, hydroquinone and bisphenol-A with thexylborane to produce the corresponding organoboron polymers. 

As expected, allylic alcohols and ethers afford v/c-diol derivatives upon hydroboration. However, in the presence of Wilkinson s catalyst, a reverse regiochemistry for the hydroboration is observed [108]. There is the possibility that boron, rather than hydrogen, is transferred from the metal to the olefin. In other words, the boron acts as a donor. 

Hydroboration of alkenes1 (15, 91). The Rh(I)-catalyzed hydroboration provides a highly diastereoselective reaction in a synthesis of a poly ether antibiotic. Thus the derivative (1) of an acyclic allylic alcohol is converted to the primary alcohol 2 by hydroboration with catecholborane (CB) catalyzed by ClRh[P(C6H5)3]3 with 94 6 selectivity. Note that hydroboration of 1 with disiamylborane (12, 484) proceeds with the opposite selectivity at Cio (8 92). 

Intramolecular hydroboration of allyl vinyl ethers.1 The hydroboration and subsequent oxidation of allyl vinyl ethers 1 with ThxBH2 (2 equiv.) leads to 1,3-diols with almost exclusive syn selectivity. High syn selectivity obtains regardless of the bulk of R1, but is lowered when R is phenyl. Apparently, electronic effects of R1 are important for stereoselectivity. 

The palladium-catalyzed hydroboration of allyl phenyl ether 456 is followed by cyclization of the corresponding triflate to afford the chroman core of the tocopheryls 457 (Equation 186) <1998JA9074>. The intramolecular hydroarylation of l-(but-3-enyloxy)-3,5-dimethylbenzene to afford 4,5,7-trimethylchroman can be accomplished using a RuCh/AgOTf catalytic system (Equation 187) <20040L581>. 2,2-Dimethylchromans 458 are formed by a Mo(CO)6 catalyzed intramolecular cyclization of aryl prenyl ethers 459 (Equation 188) <1998S256>. 

Straightforward is the stereoselective introduction of two oxygen functionalities onto carbon-carbon double bonds, as represented by the Sharpless epoxidation4 or osmium tetroxide oxidation of allyl alcohols.5 An alternative route may be envisioned by anti-Markovnikov hydration of enol ether derivatives, as in the present method, but such an approach has so far been rarely studied. The only other reported method is hydroboration which affords syn/anti ratios in the range of 83/17 to 3/97 in essentially the same systems as those examined in this procedure. The method described here is a highly stereoselective route to 2,3-syn isomers of 1,2,3-triol skeletons from a-hydroxy enol ethers. 

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