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J. Am. Chem. SOC.1991, 113, 1468-1470
biradical but a singlc zwittcrion was in good accord with the chcmical reactivities described above. Further investigations of the ncw intermediate sila-m-quinomethane are in progress.
?
Acknowledgment. Wc arc gratcful for the financial support
of thc Ministry of Education, Science. and Culture of Japan (Specially Promoted Research No. 02 102004). (9) Rule, M.; Matlin. A. R.; Hilinsky. E. F.; Dougherty, D. A.; Rerson. J. A. J. Am. Chem. Soc. 1979. 101. 5098.
Configuration, Conformation, and Colligative Properties of a Phosphorus-Stabilized Anion Scott E. Denmark,* Paula C. Miller, and Scott R. Wilson'
Figure 1. Crystal structure of [Li+lb-J2.4THFa t -75 shown for clarity (35% thermal ellipsoids).
Roger Adams Laboratory, Department of Chemistry C'riirersit)* of Illinois, Urbana, Illinois 6 I80 I
THF
OC;
THF
monomer unit
b
Received October 17. I990 As part of a general program to develop chiral, phosphorusbased anionic reagents,' we required an accurate model of the structure and bonding in thcsc species to guide the design of auxiliarics.2 Apart from the pioneering spectroscopic studies on Horner-Wittig reagents by Kirilov3 and Se~den-Penne,~"'the structure of phosphorus-stabilized anions (excluding ylides) has becn 1it t le investigated .4d-h.5
de Li+i a-: A = H; B = m Li+l b-:A = B = CH3
Wc havc rcccntly rcportcd thc first X-ray crystal structure determinationbR of a phosphoryl-stabilized anion, Li+la-, which 'Correspondence author for inquiries concerning the X-ray structure determination. ( I ) (a) Corey, E. J.; Cane. D. E. J. Org. Chem. 1969.34.3053. (b) Evans, D. A.; Takacs. J. M.; Hurst. K. M. J. Am. Chem. Soc. 1979. 101. 371. (c) Hanessian. S.; Delorme. D.; Reaudoin, S.; Leblanc, Y. J. Am. Chem. Soc. 1984. 106. 5754. (d) Ahlbrecht. H.; Farnung. W. Chem. Ber. 1984. 117. I . (e) Hanessian. S.; Delorme. D.; Beaudoin. S.; Leblanc, Y. Chem. Scr. 1985, 25, NS5. (0 Hua, D. H.; Chan-Yu-King, R.; McKie, J. A.; Myer. L. J. Am. Chem. Soc. 1W7. 109, 5026. (g) Bartlett. P. A.; McLaren. K. L. Phosphorus SuIJtrr 1987.33. 1. ( h ) Schiillkopf. U.; Schiitte. R. Liebigs Ann. Chem. 1987. 45 and references to earlier work. (i) Togni. A.; Pastor, A. Tetrahedron I r t r . 1989. 30. 1071. (j)Sawamura. M.; Ito. Y.; Hayashi. T. Tetrahedron Left. 1989.30.2247. (k) Haynes. R. K.; Vonwiller. S. C.; Hambley, T. W. J. Org. C'herrr. 1989. 54. 5162 and references to earlier work. ( I ) Hancssian, S.; Bennani, Y. L.; Delorme. D. Tetrahedron Letr. 1990. 31. 6461. (m) Hancssian. S.; Rcnnani. Y. L. Tetrahedron Lett. 1990. 31, 6465. (2) (a) Dcnmark.S. E.; Marlin. J. E . J . Org. Chem. 1987.52. 5742. (b) Denmark, S. E.; Rajendra. G.;Marlin. J. E. Tetrahedron k t t . 1989.30.2469. (c) Denmark. S. E.; Dorow. R. L. J. Org. Chem. 1990, 55, 5926. (3) (a) Kirilov. 34.; Pctrov, G. Chem. Ber. 1971, 104, 3073. (b) Kirilov, M.;Pctrov. G. Monarsh. Chem. 1972. 103, 1651. (4) Rcvicws: (a) Scyden-Penne. J. Bull. Soc. Chim. Fr. 1987. 238. (b) Str7alko. T.; Scydcn-Pcnne. J.; Froment, F.; Corset, J.; Simonnin, M.-P. J. Chent. Soc.. Perkin Trans. 2 1987.783. (c) Strzalko. T.; Seyden-Penne, J.; Fromcnt. F.; Corsct. J.; Simonnin, M.-P. Can. J . ('hem. 1988, 66, 391. (d) Teuladc. M.-P.;Savignac. P.; About-Jaudet. E.; Collignon. U. Phosphorus SuIJur 1988. 40. 105. See also: (e) Corsct. J. Pure Appl. Chem. 1986, 58, 1 1 33. ( 0 Patios. C.; Ricard, L.; Savignac. P. J. Chem. Soc.. Perkin Trans. I 1990. 1577. (g) Yuan. C.; Yao. J.; Li. S.; Mo, Y.; Zhong. X . Phosphorus SuIJur 1989.46. 25. (h) Yao. C. Y. J.; Li, S. Ph0.rphoru.r Sulfur 1990.53. 21.
( 5 ) For a rcvicw on the structure of sulfur-stabilized anions: Boche, G. Angew. Chent.. Int. Ed. EngI. 1989. 28. 277. (6) Dcnmark. S. E.; Dorow, R. L. J. Am. Chem. Soc. 1990. 112, 864.
0002-7863/9 1 / I5 13-1468$02.50/0
; *
i
80
50
60
73
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40
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8'
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10
PPU
Figure 2. I3C N M R spectra (125.8 MHz) at -100 O C . Spectrum A: l b (ncutral. nilturill abundance). Spectrum R: Li+lh- (anion, 50% I3Ccnrichcd in onc C-methyl group). Table I. Planarity Criteria for C ( 6 ) (Mean value^)^
anion
A
R
la1 + IBI. deg
1.deg
7 , deg
d. A
Li+laLi+lb-
H CH3
Ph CH,
171.1 (35) 165.4 (9)
359.4 (60) 358.5 (7)
7.3
0.06
11.6 0.1 1 "The sum of the torsion angles (la1 + 1/31); the sum of all angles about C(6) (E);the angle of the C(6)-P( 1 ) vector to the C(6)-h-B plane ( 7 ) ;the distance of C(6) to the A-P( I)-B plane ( d ) .
established, inter alia, the planarity and preferred conformation of thc carbanionic carbon. In this report we disclose crystallographic and N M R analyses of the related P-isopropyl anion LPlb(7) Two other types of phosphorus-stabilized anions have been cr stallographically defined: chelated Vg2+ and Cu2+ keto phosphonatesiSb and 1ithiophosphinomethides.'(a) Weiss. E.; Kopf, J.; Gardein, T.; Corbelin, S.; Schumann. U.;Kirilov. M.; Petrov, G. Chem. Rer. 1985, 118, 3529. (b) Vacicek. J.; Angelova, 0.; Petrov. G.; Kirilov. M. Acra C'rysrallogr.1988, C44.626. (c) Ryrne, L. T.; Engelhardt, L. M.; Jacobsen. G. E.; Leung, W.-P.; Papasergio, R. I.; Raston, C. L.; Skelton, €3. W.; Twiss, P.; White. A. H. J . Chem. Soc., Dalton Trans. 1989, 105. (d) Karsch, H. H.;Zellner, K.; Mikulcik. P.; Lachmann, J.; Miillcr, G. Organomerallics 1990, 9, 190 and references cited. (e) Fraenkel, G.; Winchester. W. R.; Willard, P. G. Organometallics 1989, 8. 2308. ( 8 ) The X-ray structure of a diethyl lithiobenzylphosphonate-DABCO complex has recently been reported. Zarges, W.; Marsch, M.; Harms, K.; Hallcr. F.; Frenking, G.; Roche. G. Chem. Ber. 1991, / 2 4 ( 4 ) . We thank Professor Roche for a preprint of this manuscript.
0 1991 American Chemical Society
J . Am. Chem. SOC.,Vol. 113, No. 4, 1991
Communications to the Editor which demonstrate that (1) the planarity of the carbanion in Li'la- is not primarily due to phenyl conjugation, (2) the barrier to rotation about the P( I)-C(6) bond is very low, (3) the preferred conformation of the anion is parallel, and (4) the anion exists as a dimer in T H F solution. The lithio anion Li'lb- (derived from 1,3-dimethyl-2-isopropyl- 1,3,2-diazaphosphorinane 2-oxide, l b ) readily crystallized from T H F as a dimcr with approximate C, symmetry.' Each lithium atom is bonded to both phosphonyl oxygens and is solvated by two T H F inolcculcs. Half of the dimer unit is presented for clarity in Figurc I . Most of the important structural features of Li'lb- are very similar to those from Li+la-, e.g., the lack of an Li(2)-C(6) bond (3.778 ( I O ) A), the anion conformation (torsional angle O( l)-P(l)-C(6)-C(8) 164.4 (IS)', and the pyramidality of thc nitrogens (Eangles N(1) 344.7 (7)O, N(2) 345.1 (20)'). Most striking, however, is the near planarity of the carbanionic carbon. The comparison between Li'la- and Li'lbby four critcria of planarity is summarized in Table I. While no singlc mcasurc of planarity is best, the correlation between the two anions is rcmarkable and stands in striking contrast to the corresponding lithio sulfones.I0 In these structures (RS02CAB)-, the carbanion is planar in the a seriesloa (Icy1 IpI = 180O) while + 101 = l39').lob in thc b scrics it is highly pyramidalized 1.( The planarity of Li'la- and Li'lb- persists in solution as indicated by the largc incrcasc in 'JPcl1 upon lithiation (AiJpc:la, 105.7 Hz; I b , 125.8 Hz). Both Li'la- and Li'lb- display the same conformation of the anion, whcrcin the P( I)-O( I ) bond is nearly coincident with the A-C(6)-B plane (torsion angles 12.9' and 15.2', respectively). The deviation from ideality in Li'lb- is noteworthy. Both the slight pyramidalimtion of C(6) and the torsion of the anionic plane arc consistcnt with a hyperconjugative interaction with a*(P(I)-N( I ) ) . ' * The barrier to conformational change in solution could not bc cstablishcd in Li'la- since single resonances both for thc N-mcthyl groups and for HC(6) were observed down to -76 'C. This has two limiting interpretations: ( I ) a high barrier in an anacomeric Cy-symmetric,parallel anion or (2) a low barrier. To distinguish these possibilities, a sample of l b labeled with 13C (50%) in one C-methyl group was lithiated and I3C N M R spectra werc rccordcd at low temperature, Figure 2. Upon lithiation of l b (n-BuLi, T H F , final concentration 0.21 M)I3 the carbanionic carbon (C(6)) moved dramatically upfield (A6 -15.6 ppm). Further, a t -100 OC, the methyl groups bonded to C(6) appeared as a single broadened resonance ( Wl,, = 75 Hz). This broadening confirms that the preferred conformation in solution is as shown in Figure 1 wherein the diastereotopic C-methyl groups should be a n i s o c h r o n ~ u s . ~Although ~ we cannot unambiguously rule out the possibility that the broadening is due to aggregation effects (vide infra). similar broadening was observed at -100 'C at lower concentration (0.03 M, W,,, = 74 Hz).The coalesced resonance for these nuclei indicates a small barrier to rotation with an estimated upper limit of ca. 8 k ~ a l / m o l . ' ~
+
(9) Space group Pi, a = 10.227 (3) A, b = 11.852 (4) A, c = 17.840 (8) (Y = 80.74 (4)O, (3 = 77.76 (3)O, y = 69.34 (3)O, 2 = 2 (one dimer/ asymmetric unit); R = 0.048 and R, = 0.060 for 41 52 reflections with I > 2.58u(I). Selected distances (mean values) (A) and bond angles (mean values) (dcg) (the asymmetric unit has approximate C, symmetry (Li(2), 0 ( 5 ) , O(6) plane)): P(1)-C(6) 1.657 (3); P(I)-O(1) 1.525 (2); Li(2)-0(1) 1.922 (5); C(6)-CH, 1.510 (5); P(I)-N(I) 1.708 (7); P(l)-N(2) 1.697 (3); P(1)-C(6)-C(7) 120.8 (2); P(I)-C(6)-C(8) 122.9 (8); C(6)-P(l)-O(l) 115.7 (2); P(l)-O(l)-Li(2) 134.5 ( 1 1 ) ; N(l)-P(l)-N(2) 99.6 (6). Torsional angles (mean absolute value) (deg): O(I)-P( I)-C(6)-C(8) f164.4 (18); Li(2)-0(l)-P(l)-C(6) f48.6 (33). (IO) (a) Boche, G.; Marsch, M.; Harms, K.; Sheldrick, G. M. Angew. Chem., Int. Ed. Engl. 1985, 24, 573. (b) Gais, H.-J.; Vollhardt, J.; Hellmann, G.; Paulus, H.; Lindner, H. J. Tetrahedron Lett. 1988, 29, 1259. (I I ) (a) Albright. T. A . Org. Magn. Reson. 1976.8, 489. (b) Duangthai, S.; Webb, G. A . Org. Magn. Reson. 1983, 21, 125. (c) Webb, G. A,; Simonnin, M.-P.; Scyden-Penne, J.; Bottin-Strzalko, T. Magn. Reson. Chem. 1985, 23, 48. (12) Denmark, S. E.; Cramer, C. J. J . Org. Chem. 1990, 55, 1806. ( 13) The spectrum was taken on a sample of recrystallized anion. The final concentration was determined by integration against benzene as an internal standard. (14) This situation would not obtain if the angle O( I)-P(l)-C(6)-C(8) were 90' in Li+lb-. i.e.. a C, orthogonal anion.
A,
1469
A low rotational barrier in these anions is consistent with the e/kracemiza,ion observed for (+)-diethyl 2-octylsmall kexchan phosphonatell and is again in contrast to the large ratio observed for sulfone^.'^ Moreover, Gais has recently reported the rotational barriers for lithio sulfones (AG*215 9.6-9.7 kcal/mol) and triflones (1Ctzl5 16.0-17.8 kcal/mol).i8 A low barrier would appear to be inconsistent with the significant double-bond character of the P( l)-C(6) bond (1.657 (3) ,&).I9 However, the barrier to rotation will depend on the splitting between the orthogonal *-acceptor orbitals on the phosphorus unit.20 This splitting is expected to be small for Li'la- and Li'lb- where the ligands on phosphorus are all electronegative, resulting in effective overlap a t all angles. Finally, we have been able to establish the aggregation state of the anion in T H F solution by the use of 6Li N M R spectroscopy.21 The lithio anion 6Li'lb- was generated in T H F (0.15 M)I3 by deprotonation of l b with sec-butyllithium-6Li.22 The appearance of a distinct triplet a t -0.62 ppm ( J = 1 . 1 Hz) in the 6Li N M R spectrum a t -100 'C requires that each lithium nucleus be coupled to two (S = I/*) 31Pnuclei, thus implicating a dimeric structure. The P-decoupled 6Li N M R spectrum23showed the expected singlet, thus confirming the two-bond 6Li-31P coupling. Two-bond coupling between lithium and phosphorus has recently been observed for H M P A solvates of lithium halides (7Li),24aate complexes (7Li),24b3cphenyllithium (6Li),24cand lithium amide bases (6Li).24dThe narrow 3 i Presonance (202.5 MHz, -100 "C, W ! / 2= 8.6 Hz) and the ability to resolve a I-Hz coupling in the 6 L spectrum ~ further support the conclusion that the broadening of the C(6)-methyl groups in the I3C spectrum is not due to a bimolecular process. The assignment of a dimeric solution structure for Li'lb- by the 6Li N M R method was corroborated by cryoscopic measurements in THF a t -108 0C.25 The degree of association ( n ) was found to be 2.00 i 0.1 7 (1 5 I .6 mmol/kg).26 A dimeric aggre( 1 5) This limit is based on the assumption that the observed linewidth is the minimum difference in chemical shift at the decoalescence limit ( W ,
