Stereoselective Synthesis of Disubstituted Butadienes via Copper-Mediated Coupling of Alkenyl Silanes

Abstract

A strategy is described for the stereoselective synthesis of substituted (E)-, (Z)-, and α-disubstituted butadienes from terminal alkynes by the copper-mediated coupling of geometrically-defined alkenyl silanes. Proof-of-concept results that demonstrate the feasibility of this approach are presented.

Key words

copper - coupling - diversity-oriented synthesis - butadienes - hydrosilylation

References and Notes

• 1 Thirsk C. Whiting A. J. Chem. Soc., Perkin Trans. 1  2002,  999 
• 2See for example:
• 2 Robinson BH. Dalton LR. Harper AW. Ren A. Wang F. Zhang C. Todorova G. Lee M. Aniszfeld R. Garner S. Chen A. Steier WH. Houbrecht S. Persoons A. Ledoux I. Zyss J. Jen AKY. Chem. Phys.  1999,  245:  35 
• 3 Denmark SE. Tymonko SA. J. Am. Chem. Soc.  2005,  127:  8004 
• For recent reviews on DOS, see:
• 4a Galloway WRJD. Isidro-Llobet A. Spring DR. Nat. Commun.  2010,  1:  801 
• 4b Schreiber SL. Nature (London)  2009,  457:  153 
• 4c Nielsen E. Schreiber SL. Angew. Chem. Int. Ed.  2008,  47:  48 
• 4d Galloway WRJD. Bender A. Welch M. Spring DR. Chem. Commun.  2009,  2446 
• 4e Cordier C. Morton D. Murrison S. Nelson A. O’Leary-Steele C. Nat. Prod. Rep.  2008,  25:  719 
• 5 We have previously reported the development of related methodology for the DOS of (E)-, (Z)-, and α-disubstituted alkenes from a common alkyne precursor: Sore HF. Blackwell DT. MacDonald SJF. Spring DR. Org. Lett.  2010,  12:  2806 
• 6a Aves SJ. Pike KG. Spring DR. Synlett  2010,  2839 
• 6b Kenwright JL. Galloway WRJD. Blackwell DT. Isidro-Llobet A. Hodgkinson J. Wortmann L. Bowden SD. Welch M. Spring DR. Chem. Eur. J.  2011,  17:  2981 
• 6c Su XB. Fox DJ. Blackwell DT. Tanaka K. Spring DR. Chem. Commun.  2006,  3883 
• 6d Su XB. Surry DS. Spandl RJ. Spring DR. Org. Lett.  2008,  10:  2593 
• 6e Su XB. Thomas GL. Galloway WRJD. Surry DS. Spandl RJ. Spring DR. Synthesis  2009,  3880 
• 6f Surry DS. Fox DJ. Macdonald SJF. Spring DR. Chem. Commun.  2005,  2589 
• 6g Surry DS. Spring DR. Chem. Soc. Rev.  2006,  35:  218 
• 6h Surry DS. Su XB. Fox DJ. Franckevicius V. Macdonald SJF. Spring DR. Angew. Chem. Int. Ed.  2005,  44:  1870 
• See, for example:
• 7a Shindo M. Matsumoto K. Shishido K. Synlett  2005,  176 
• 7b Taguchi H. Ghoroku K. Tadaki M. Tsubouchi A. Takeda T. J. Org. Chem.  2002,  67:  8450 
• For E-selective hydrosilylation see, for example:
• 8a Chandra G. Lo PY. Hitchcock PB. Lappert MF. Organometallics  1987,  6:  191 
• For Z-selective hydro-silylation see, for example:
• 8b Na Y. Chang S. Org. Lett.  2000,  2:  1887 
• 8c Mori A. Takahisa E. Yamamura Y. Kato T. Mudalige A. Kajiro H. Hirabayashi K. Nishihara Y. Hiyama T. Organometallics  2004,  23:  1755 
• 8d Katayama H. Taniguchi K. Kobayashi M. Sagawa T. Minami T. Ozawa F. J. Organomet. Chem.  2002,  645:  192 
• For the selective hydrosilylation of terminal alkenes to give 1,1-disubstituted alkenylsilanes (α) see, for example:
• 8e Trost BM. Ball ZT. J. Am. Chem. Soc.  2005,  127:  17644 
• 9 Itami K. Ushiogi Y. Nokami T. Ohashi Y. Yoshida JI. Org. Lett.  2004,  6:  3695 
• 10 Itami K. Mitsudo K. Nishino A. Yoshida J. J. Org. Chem.  2002,  67:  2645 
• 12 Na YG. Chang SB. Org. Lett.  2000,  2:  1887 
• 13 Mori A. Takahisa E. Yamamura Y. Kato T. Mudalige AP. Kajiro H. Hirabayashi K. Nishihara Y. Hiyama T. Organometallics  2004,  23:  1755 

11

Compound 14 was presumably formed by displacement of both the chloride and the hydride from the chlorosilane. This can be rationalised as follows: suppose that lithiation of 13 proceeds to completion. In the absence of other Lewis basic donors, it might be expected that the sulfur atom of one lithiated molecule of 13 will coordinate to the lithium of another. When the chlorosilane is added, it will be attacked by the organolithium to form 12; however, as there will be one or more molecules of the organolithium still coordinated to the sulfur atom of the newly formed 12, intra-aggregate transfer of the organic group of this coordinated organo-lithium to the proximal silicon atom may be more rapid than an intermolecular on a second molecule of the chlorosilane. If this was the case, it was reasoned that the addition of a more strongly coordinating solvent (Et₂O) should reduce the extent to which the organolithium is complexed by the thioether, and might thus prevent this second attack.

14

If either CsF or CuI are omitted from the reaction mixtures, no coupled product is observed; with extended reaction times (48 h), partial desilylation of the starting material is observed if CsF is present but not CuI.

15

Interestingly, while CuI in MeCN together with CsF promotes complete conversion of the (E)-silane 16 (Scheme  [4] ), the addition of 1 equiv of CuI did not induce any change in the spectrum of 16 in MeCN-d ₃.

16

Since thioethers are almost pure σ-donors, with no vacant low-energy orbitals suitable for metal-ligand back bonding, donation of the lone pair of sulfur to the metal might be expected to decrease the electron density at (and thus deshield) the carbons proximal to the sulfur atom, resulting in the observed increase in the chemical shift of their attached protons. Unlike the thioether, the alkene has a low-energy π* orbital suitable for metal-ligand back bonding, and so back donation from the metal to the alkene might account for the observed decrease in the chemical shift of the alkenyl protons in the presence of copper.