1,4 Hydroboration JACS 09 - Iron-Catalyzed...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

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: ritter@chemistry.harvard.edu 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-...
View Full Document

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).

Page1 / 3

1,4 Hydroboration JACS 09 - Iron-Catalyzed...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online