Room Temperature, Endo-Specific 1-Aza-1,3-butadiene Diels-Alder Reactions: Acceleration of the LUMOdiene-Controlled [4 + 2] Cycloaddition Reactions through Noncomplementary Azadiene Substitution

Journal of Organic Chemistry

Volumn 55, Issue 10, 1990, Pages 2999-3000

  Boger, Dale L ^a   Corbett, Wendy L ^a   Wiggins, J Mark ^a  

^a PURDUE UNIVERSITY   (United States)

Author keywords

[No Author keywords available]

Indexed keywords

EID: 0000095896     PISSN: 00223263     EISSN: 15206904     Source Type: Journal    
DOI: 10.1021/jo00297a006     Document Type: Article

References (17)

1. Alfred, P. Sloan fellow, 1985–1989.
2. Boger, D. L.; Weinreb, S. M. Hetero Diels-Alder Methodology in Organic Synthesis; Academic Press: San Diego, 1987.
3. Boger, D. L. Tetrahedron 1983. 39. 2869.
4. Boeger, D. L.; Kasner, A. M. J. Am. Chem. Soc. 1989, 111. 1517.
5. Phosphonium salt 1 was prepared from ethyl 3-bromopyruvate by the following sequence: (1) NH2OH-HCl, CH3OH-CHC13, 23 °C, 18 h, 92%, (2) DHP, PPTS, CH2C12, 23 °C, 20 h, 94%, (3) Ph3P, THF-C6H6, 80 °C, 18 h, 91%. For the related preparation and Wittig reactions of EtO2CC(NOCH3)CH2PPh3+Br-, see: Bicknell, A. J.; Burton, G.; Elder, J. S. Tetrahedron Lett. 1988, 29, 3361.
6. For a brief discussion of complementary versus noncomplementary substitution see: Boger, D. L.; Robarge, K. D. J. Org. Chem. 1988, 53, 3373–5793.
7. Teng, M.; Fowler, F. W. Tetrahedron Lett. 1989, 30, 2481.
8. Teng, M.; Fowler, F. W., submitted
9. Kim, J.-B.; Hall, H. K., Jr. Macromolecules 1988, 21, 1547.
10. Hudson, R. F.; Brown, C.; Record, K. A. F. J. Chem. Soc., Perkin Trans. 2 1978. 822.
11. All new products exhibited 1H NMR, 13C NMR, IR, MS, and HRMS or CHN analyses consistent with the assigned structure. All yields are based on pure material isolated by chromatography (Si02, Florisil). Consistent with intuitive expectations, the N-sulfonylimines 5–6 proved more sensitive to hydrolysis by adventitious water than N-sulfonyl azadienes lacking the C-2 ethoxycarbonyl group3 but may be purified by rapid chromatography with partial but not extensive loss of material.
12. Characteristic coupling constants (C2-OR axial): jc2-he/c3-he=2.7-5,0 Hz, jc2-he/c3-hax=2.5-4.4 Hz, Jc3-He/C4-He= 1.7-3.3 Hz, JC3-Hax/C4-He = 8.9-9.3 Hz, JC4-H/C5-H=3.2-3.6 Hz, JC2/H2= 163-158 Hz, The exceptions (20, 22, 24) may exist in the all-equatorial conformation; for 20: JC2-Hax/C3-Hax= 4.4 Hz, JC3-Hax/C4-Hax=13 Hz, JC4-Hax/C5-H= 3.6Hz, JC2/H2 = 156.6 Hz; for 22 JC2/H2 = 153.7 Hz; for 24 JC2/H2 =140-145 Hz.
13. The single-crystal X-ray structure determinations of 7a and 19a established the C-2/C-4 and C-2/C-3/C-4 cis relative stereochemistry that must arise through endo [4 + 2] cycloaddition and proved consistent with the 1H NMR spectroscopically assigned structures and stereochemistry. For 7a: JC2-He/C3-He=2.5 Hz, JC2-He/C3-Hax =4.1 Hz, JC3-He/C4-He = 3.0 Hz, JC3-Hax/C4-He = 9.3 Hz, JC4-He/C5-H= 3.6 Hz, JC2/H2 = 158.5 Hz; for 19a: Jc2-he/C3-Hax=2.8hz,jc4-he/c5-heq=8.9 Hz, = JC4-Hax/C5= 3.4 Hz, JC2/H2 = 159.9 Hz.
14. Computational studies (AMl, MNDO, supplementary material) support the diene C-2/dienophile OR secondary orbital interaction derived endo diastereoselectivity (sign and magnitude of diene C-2 (LUMO)/dienophile OR (HOMO) coefficient) and suggested a predictable rate acceleration with introduction of a N-sulfonyl-l-aza-l,3-butadiene C-2(CO2Et) electron-withdrawing substituent [AAE (HOMOdienophil/LUMOdiene) = -0.2 eV (AM1)].
15. No trace of 26 was detected in the reaction mixture of 25/5a with ethyl vinyl ether.
16. The relative rates of [4+2] cycloaddition were derived from product distributions obtained from the reaction of a mixture of (Z)- and (E)-l-ethoxypropene (2.8:1,10 equiv) with 5a: 25 °C, 96 h, CH2Cl2, 1 atm, 19a:20a (1.0:3.3), 54% and 25 °C, 96 h, CH2C12, 6.2 kbar, 19a:20a (1.0:2.0), 65%.
17. Diene 5a failed to participate in a [4 + 2] cycloaddition reaction with 1-octene, methyl acrylate, and p-benzoquinone under reaction conditions detailed herein.