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Unformatted text preview: Iron-Catalyzed 1,4-Hydroboration of 1,3-Dienes Jessica Y. Wu, Benot Moreau, and Tobias Ritter* Department of Chemistry and Chemical Biology, Har V ard Uni V ersity, 12 Oxford Street, Cambridge, MA 02138 Received June 13, 2009; E-mail: firstname.lastname@example.org Iron can adopt formal oxidation states ranging from- II 1 to + VI. 2 Iron complexes are used as catalysts in synthetic chemistry for carbon- heteroatom 3 and carbon- carbon bond-forming reac- tions. 4 Low-valent iron complexes can catalyze cross-coupling, 4 cycloisomerization, 5 and cycloaddition reactions. 6,7 We are inter- ested in iron catalysis for the identification of useful, previously inaccessible reaction chemistry and report here C- B bond formation by hydroboration of 1,3-dienes. To our knowledge, there are no other examples of Fe-catalyzed hydroboration reactions of olefinic substrates. The allylborane products are formed regio- and stereo- selectively with ( E )-double bond geometry exclusively and are challenging to access selectively with conventional chemistry. We have previously reported a 1,4-addition reaction of R-olefins to dienes using an iminopyridine- ferrous chloride complex 8 with magnesium metal as an in situ reducing agent. 9 In this communica- tion, we describe the use of analogous, readily prepared iminopy- ridine-derived iron complexes as catalysts for the regioselective 1,4-addition of pinacolborane (HBPin) to substituted 1,3-dienes (eq 1). Allylboranes are versatile intermediates employed in oxidation to form allylic alcohols, 10 allylation to give homoallylic alcohols 11 and amines, 12 and Suzuki cross-coupling reactions. 13 Traditional methods for synthesizing allylboranes involve basic main-group organometallics, such as Grignard and organolithium reagents, that are incompatible with electrophilic functional groups. 11a While transition-metal-catalyzed hydroboration of olefins has been studied extensively with great success, 14 hydroboration of dienes to access allylboranes is less-established. Pd(0) catalyzes the 1,4-addition of catecholborane to unfunctionalized 1,3-dienes such as 1,3-penta- diene and isoprene to give ( Z )-branched allylboranes, 15 while Ni II- 16 and Rh I-catalyzed 17 reactions are selective for 1,2-addition. The synthesis of linear ( E )- -disubstituted allylboranes is at- tractive because, for example, they can afford trisubstituted allylic alcohols stereospecifically and add to electrophiles to generate quaternary stereocenters with control of diastereoselectivity. Chal- lenges in the synthesis of linear ( E )- -disubstituted allylboranes via hydroboration of 1,3-dienes include control of chemoselectivity to favor 1,4- over 1,2-addition, control of regioselectivity to favor C- B bond formation at a single diene terminus, and control of stereoselectivity to favor ( E )-olefin geometry. The hydroboration reaction presented herein controls all three types of selectivity. A general method for synthesizing linear ( E )- -disubstituted allylbo-...
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This note was uploaded on 02/11/2010 for the course BIOPL 10738 taught by Professor Davies,peter during the Spring '10 term at Cornell University (Engineering School).
- Spring '10