1850 J. Ferguson, A. R. Katritzky. and D. M. Read, J. Chem. Soc., Chem. Commun.,255 (1975). (22) S. F. Nelsen and P. J. Hintz, J. Am. Chem. Soc., 04,.7108 (1972). (23) It should be noted that a transmisslon coefficient of 1 was used for calculations of the activation parameters for 5-11, whereas Anderson and Lehn" would have used for 5-7 because they expected a Figure 1A type of barrier. If % were used, the AS* values would be 1.37 eu more positive, and Af&88 0.41 kcallmol lower. (24) (a) J. M. Lehn and J. Wagner, Chem. Commun., 414 (1970); (b) J. M. Lehn and J. Wagner, Chem. Commun., 148 (1968). (25) J. M. Lehn and J. Wagner, Tetrahedron, 26, 4227 (1970). (26) J. D. Roberts, F. J. Weigert, J. I. Kroschwitz, and H. J. Reich, J. Am. Chem. Soc., 92, 1338 (1970). (27) H. Eggert and C. Djerassi, J. Am. Chem. Soc. 05, 3710 (1973).
(28) K. Alder and M. Vogt, Justus Liebigs Ann. Chem.. 564, 109 (1949). (29) M. P. Cava, C. J. Wilkins, Jr., D. R. Dalton, and K. Bessho, J. Org. Chem., 30, 3772 (1965). (30) W. Schneider and R. Dillman, Chem. Ber., 06, 2377 (1963). (31) A. 0. Smith and W. C. Agosta, J. Org. Chem., 37, 1259 (1972). (32) (a) H. Koch, Monatsch. Chem., 03, 1343 (1962); (b) E. Pettit, M. Pecque, and M. Blanchard. Bull. SOC. Chim. Fr., 747 (1972); (c) B. A. Kazanskii and P. I. Svirskaya, Zh. Obshch. Khim., 20, 2584 (1959). (33) 0. Binsch, Top. in Stereochem., 3, 122 (1968). (34) A. J. Gordon and R. A. Ford, "The Chemist's Companion", Wiley-lnterscience, New York, N.Y., 1972, p 491. (35) (a) A. Allerhand. F. Chen. and H. S. Gutowsky, J. Chem. Phys., 42, 3040 (1965); (b) H. S. Gutowsky and C. H. Holm, J. Chem. Phys., 25 1228 (1956).
Conformations of Saturated Phosphorus Heterocycles. IV. 'H, 13C,and 31PNuclear Magnetic Resonance Studies of Geometrical Isomers of 2-Z-4-Methyl- and 4-tert -Butyl- 1,3,2-dioxaphospholanes Wesley G. Bentrude* and Han- Wan Tan Contribution from the Department of Chemistry, University of Utah, Salt Lake City, Utah 841 12. Received June 12, 1975
Abstract: The cis-trans stereoisomerism and conformations of a series of 2-Z-4-Me- and 2-Z-4-tert-butyl- 1,3,2-dioxaphospholanes were studied by 'H, 13C, and 31PN M R methods. For the 4-Me compounds ( Z = MezN MeO, t-BuO, t-Bu, PhCH2, and Ph), the trans/& ratio regardless of Z was (65 i 2)/(35 f 2) at ambient temperatures corresponding to AGO = 0.4 kcal/mol. The trans/& ratio was increased for the 4-t-Bu compounds ( Z = MeO, Me*N, t - B u ) with AGO = 0.8-1.0 kcal/mol. The greater thermodynamic stability of the trans isomer in these systems is directly opposite to the case for 2-Z4-R-1,3-dioxanes in which the cis isomer is favored. This emphasizes the importance of the presence of the phosphorus hetwhich have cis-trans thermodynamic staeroatom as found previously with the 2-2-5-tert-butyI-l,3,2-dioxaphosphorinanes bilities opposite to those found for the analogous 1,3-dioxanes. The results of this work are reasonably well interpreted for each isomer in terms of two rapidly equilibrating half-chair conformers with 4-alkyl pseudoaxial or pseudoequatorial, although possible alternative explanations cannot be excluded. The cis isomers appear to be conformationally biased toward the form with 4-alkyl pseudoequatorial. The cis or trans geometry of a given isomer is easily assigned from I3C or 31PN M R spectroscopy alone. The 31Presonance of the cis isomer appears in each case 2.4 to 7.4 ppm downfeld from the trans isomer absorption, presumably as a result of the 6 interaction of the 2 - 2 and 4-alkyl substituents. The cis and trans isomer assignments for the 4-Me compounds are made from I3C spectra on the basis of the chemical shifts of the C-4, C-5, and (2-4' as influenced by the 6 effect. The similarities of the 2-Z-4-R-1,3,2-dioxaphospholanes to other 1,3-dioxa and 1,3-dithia five-membered ring systems with P, As, or S at the 2 position are noted.
Over the past several years, there has been an increasing amount of interest in saturated heterocycles containing phosphorus and oxygen in five- and six-membered rings. The trivalent 1,3,2-dioxaphosphorinanes(1) and 1,3,2-dioxaphospholanes (3) are clearly analogous to the 1,3-dioxanes (2) and 1,3-dioxolanes (4). Differences in bond distances and angles imposed on such rings by substitution of phosphorus for carbon, however, might well be expected to affect the relative energies of cis or trans isomers and of their various conformers in important ways. Furthermore, in 1 and 3, the presence of an electron lone pair on phosphorus will change the steric competition between the substituents ~
2 P - Z R
3
4
1
R
~ a
Z
R
42~
3
2
3
o L $ ~R
R
O, 4
3
Journal of the American Chemical Society
-
a t the 2 position and also result in vicinal interactions (between the oxygens and the substitlients on the atom a t the 2 position) different from those present in 2 and 4. An example of the consequence of such effects is seen in the 2 - 2 - 5 tert-butyl-l,3,2-dioxaphosphorinane (1, R = 5-tBu). For Z equal C1, MeO, Ph, Me, and i-Pr, the cis isomer is the more stable form and populates almost entirely the conformer 5. By contrast, the most stable conformer of the less stable isomer in the comparable 1,3-dioxane is 6.3We have s ~ g g e s t e d 'that , ~ the strong axial preference of Z in the 1,3,2-dioxaphosphorinanesis probably a result of more favorable vicinal interactions between the lone pairs on oxyZ Z
/ 98.7 /
March 31, 1976
5
6
n
01-
00
2% m
N
.,\ .
Qd 2
d
h
h
n
h
h
W-
r-b
mm
CnG
00-
w-
'00,
r-0,
"0,
'00,
w e
'00,
h
r"0
Bentrude, Tan
/
w0,
Conformations of Saturated Phosphorus Heterocycles
1852 gen and the empty antibonding orbitals on phosphorus when Z is axial than when it is equatorial. Reduced 1,3synaxial interactions in 1 may play a role as well. In view of the findings in the 1,3,2-dioxaphosphorinane rings (l), we decided to apply to 3 the same sorts of 'H, I3C, and 3 1 PN M R techniques used to study 1. Although ' H N M R studies of certain 4-substituted-2-alkoxy- and 2C1- 1,3,2-dioxaphospholanes have been r e p ~ r t e d ,no ~ complete study of both cis and trans isomers has been published nor has an investigation of a variety of phosphorus substituents been attempted. Such studies allow a considerably more detailed understanding of conformational effects in these systems. Results Syntheses. The 2-Z-4-methyl- 1,3,2-dioxaphospholanes
(7-14) were synthesized in fairly routine fashion. The phosphonites 12 and 13 are considerably more stable than the
I
Figure 1. 'H NMR spectra of 9, methylene and methine protons: (A) 9a-9b mixture in C.&, no decoupling; (B) irradiation of ring methyl of trans isomer (9b); ( C ) simulated spectrum of 9b; (D) irradiation of ring methyl of cis isomer (sa); (E) simulated spectrum of 9a.
1
R3 R4
7, R,,R,,R,,R, = H; Z = Me,N 8, R,,R, = Me; R,,R, = H; Z = Me,N 9a, R, = Me; R,,R,,R, = H; Z = Me,N 9b, R, = Me; R,,R,,R, = H; Z = Me,N loa, R, = Me; R,,R,,R, = H; Z = Me0 lob, R, = Me; R,,R,,R, = H ; Z = Me0 l l a , R, = Me; R,,R,,R, = H; Z = t-BuO l l b , R, = Me; R,R,,R, = H ; Z = t-BuO 12a, R , = Me; R,,R,,R, = H; Z = t-Bu 12b, R, = Me; R,,R,,R, = H ; Z = t-Bu 13a, R , = Me; R,,R,,R, = H ; Z = PhCH, 13b, R, = Me; R,,R,,R, = H; Z = PhCH, 14a, R , = Me; R,,R,,R, = H; Z = Ph 14b, R, =Me; R,,R,,R, = H; Z = Ph 15a, R , = I-Bu; R,,R,,R, = H; Z = Me0 15b, R, = t-Bu, R,,R,,R, = H; Z = Me0 16a, R, = t-Bu; R,,R,,R, = H ; Z = Me,N 16b, R, = t-Bu; R,,R,,R, = H ; Z = Me,N 17a, R, = t-Bu; R,,R,,R, = H; Z = t-Bu 17b, R, = t-Bu; R,,R,,R, = H;Z = t-Bu
analogues unsubstituted at carbon, which we find to be rapidly converted to higher molecular weight products.6 The formation of dimers and trimers from the 2-phenyl-l,3,2dioxaphospholane has been d ~ c u m e n t e d .The ~ monomeric nature of 12 and 13 was affirmed by their low GLC retention times and boiling points (comparable to other 3). Furthermore, on standing neat for several days at room temperature, they remain as mobile liquids unlike their unsubstituted counterparts. Reaction with S-8 converts 12 and 13 to the sulfides which also are readily analyzed by GLC with retention times similar to those for other 3-sulfides. Additionally, only two isomeric forms of 12 and 13 were found by 3 1 PNMR. The dimer and trimers of 3 would be expected to show the presence of several isomers. Also, the conformational bias indicated by the PMR parameters for the cis isomers of 1 and 13 (to be discussed later) would not be expected of a 12- or 18-membered ring, which should be highly flexible. 'H NMR Analyses. Compounds 7-14 were analyzed at 100 MHz (Table I). Conjugate solutions were resolved by analysis at 60 MHz or by analysis of the phosphorus-decoupled spectrum. In all cases, hand-calculated spectral parameters were refined using the LAOCN3 program. Error limits are given in Table I. In the ' H N M R analyses of 7-17, the maximum errors in line assignments were generally less than twice the root mean square error. It was possiJournal of the American Chemical Society
/ 98:7 /
ble to assign 70-95% of the transitions. Since for the AA'BB'X spin systems J A A ~cannot be distinguished from J B B , the , values for 312 and 5 3 4 for 7 in Table I may be reversed. In all previous studies, the couplings J12 and J34 for such AA'BB'X systems have been assumed or found to be The nonequivalence of these couplings has been notedg with 2-phenyl- 1,3,2-dithiaphospholane but not for 1,3,2-dioxaphospholanes. The 4,4'-dimethyl compound 7 was analyzed in the ABX form and then refined as an ABK3X system since one ring methyl is coupled to a methylene hydrogen. In Table I, the coupling constant designations Jzs, J35, etc., refer to the protons at positions 1-4 as indicated in the structure of phosphorus, designated as nucleus 5. The analyses of the 4-substituted- 1,3,2-dioxaphospholanes are typified by the Me2N compound 9 . With the exception of 14, the ring methyls for cis and trans isomers (doublets) absorb at different frequencies. The major isomer with methyl at higher field is assigned the trans geometry (to be discussed later). Selective homonuclear decoupling was used to simplify various portions of the ' H N M R spectrum (Figure l a ) of the mixture of stereoisomers. Thus, on radiation of the high-field methyl, the spectrum of the adjacent methine proton, which appears farthest downfield in Figure la, is simplified to eight lines (left-hand portion of Figure lb). From the spacings of the methine signals and the knowledge that Jgeme -8.5 Hz in 1,3,2-dioxaphospholanes, it was possible to pick out all the transitions for the AKMX system represented by the trans isomer 9b. The simulated spectrum for 9b is shown in Figure IC. The transitions for the minor isomer 9a were assigned in similar fashion by radiation of the low-field methyl signal (Figure Id). The very low-field methine proton resonance of the trans isomer is now very complex because of partial (offresonance) decoupling. Fortunately, the methine proton of cis-9 is upfield of that region, although it overlaps the downfield methylene hydrogen of trans-9. The more upfield methylene hydrogen of cis-9 ( H 4 , 6 366 Hz) is well separated in Figure Id from the other methylene resonances. By expansion of the region (50-MHz sweep width), the methine transitions of cis-9 were assigned without difficulty. The simulated spectrum for 9a is given in Figure le.
March 31, 1976
1853 Table 11. 13C and 31PSpectral Parameters for 2-Z-4-R-1,3,2-Dioxaphospholanesa
-Z
6 13CC
Compound
Z
R
s 3lPb
c-4
72.78 (7.5)d 71.78 (8.1) 74.23 (9.8) 72.84 (9.4) 73.82 (9.0) 71.21 (7.8) 74.07 (6.4) 73.58 (8.8) 74.44 (9.7) 72.45 (9.8) 75.08 (9.5) 72.52 (8.9) 85.94 (9.9) 84.45 (9.7) -83.7
9a
cis-Me
Me,N
-147.34
9b
trans-Me
Me,N
-142.51
1Oa
cis-Me
CH,O
-139.62
10b
trans-Me
CH,O
-135.25
1 la
cis-Me
t-BuO
-140.22
llb
trans-Me
t-BuO
-137.84
12a
cis-Me
t-Bu
-207.37
12b
trans-Me
t-Bu
-201.48
13a
cis-Me
PhCH,
-190.36
13b
trans-Me
PhCH,
-185.44
14a
cis-Me
Ph
-169.00
14b
trans-Me
Ph
-165.19
15a
cis-t-Bu
OMe
-143.10
c-4"
(2-4'
17.41 (3.0) 20.14 (4.1) 18.99 (
