2947
Indanyl Cations. Their Stereoselective Formation from Acyclic Phenyl-Substituted Allylic Cations and Temperature-Dependent Rearrangements Charles U. Pittman, Jr.,* and Wayne G . Miller’ Contribution from the Department of Chemistry, The University of Alabama, University, Alabama 35486. Received October 31, 1972
The 1,3-diphenylbutenylcation (2a) and the 1,1,3-tripheny1-2-propenylcation (2b) have been directly observed by nmr spectroscopy after their generation from their respective allylic alcohols [la and lb) in FSOIH at -70”. These are the first acyclic, phenyl-substituted, allylic cations to be reported. On warming the acid solutions, cyclization to indanyl cations was observed. A series of 12 phenyl-substituted allylic alcohols was prepared and studied in both FS03Hat low temperatures and HzS04 at room temperature. In all cases the acyclic allylic cations cyclized to transient indenes which protonated to give indanyl cations. Cyclization always proceeded from the least substituted end of the allylic system. Several indanyl cations underwent further temperaturedependent rearrangements to new indanyl cations. The rearrangements were followed by nmr, deuterium labeling, deuterium exchange, and temperature-dependent quenching studies. The stereoselective protonation of three transient indenes was observed. For example, the 1,2,3-tripheny1-2-butenylcation cyclizes in FSOIH at -70°,to l-methyl-2,3-diphenylindene(3d), which undergoes initial kinetic protonation to give the cis-l-methyl2,3-diphenylindanyl cation (4d). Eventually trans4d, the thermodynamic product, is formed. On continued cation (5d) occurs via a 1,3-hydride warming, further rearrangement to the trans-l,2-diphenyl-3-methylindanyl shift. Interestingly, the trans-1,2-dimethyl-3-phenylindanylcation (4c) rearranges to the trans-l-phenyl-2,3dimethylindanyl cation (5c) via two successive 1,2-hydride shifts. Abstract:
0th cyclic and acyclic alkyl-substituted allylic cat8 ions are stable in strong mineral acids concentrated HzS04, FS03H, SbF6-FS03H, etc.) and
(Le.,
have been directly observed and studied by spectroHowever, no sucscopic methods, especially pmr. 2--6 cessful observations of acyclic phenyl-substituted allylic cations have yet appeared in the literature.’ In an attempt to generate the 2-phenyl-4-methylpentenyl cation in H2S04,Deno, Pittman, and Turner8 observed only the 1,3,3-trimethylindanyl cation which was formed quantitatively by intramolecular electrophilic substitution (eq 1). The lifetime of the acyclic cation was so short that it could not be observed. One expects that a phenyl group should stabilize an allylic cation. However, the availability of intramolecular cyclization as a reaction path causes the acyclic cation to be “chemically unstable.” Contributing to this chemical instability is the easy equilibration between conformations a and b which are both expected to be rotated significantly out-of-plane7 due to the 1,3 methyl or 1,3 methyl-phenyl nonbonded repulsions. (1) This work is a portion of the Ph.D. Thesis of W. G. M., University of Alabama, University, Ala., 1971. The authors would like to acknowledge Mr. William Jones, an undergraduate research participant, for extensive assistance in the experimental work and to the College Work Study Program for the support of his contributions. (2) N. C. Deno, H. G. Richey, Jr., N. Friedman, J. D. Hodges, J. J. Houser, and C. U. Pittman, Jr., J . Amer. Chem. Soc., 85, 2991 (1963). (3) (a) G. A. Olah, M. B. Comisarow, C. A. Cupas, and C. U. Pittman, Jr., ibid., 87, 2997 (1965): (b) G. A. Olah and M. B. Comisarow, ibid., 86, 5682 (1964); (c) G. A. Olah and J. M. Bollinger, ibid., 90, 6083 (1968). (4) N. C. Deno and R. R. Lastomirsky, ibid., 90, 4085 (1968). ( 5 ) P. v. R. Schleyer, T. M. Su, M. Saunders, and J. C. Rosenfeld, ibid., 91, 5174 (1969). (6) C. U. Pittman, Jr., Chem. Commun., 122 (1969). (7) See, for example, a review by N. C. Deno, “Carbonium Ions,” Vol. 11, G . A. Olah and P. v. R. Schleyer, Ed., Interscience, New York, N. Y.,1970, Chapter 18, p 789. (8) (a) N. C. Deno, C. U. Pittman, Jr., and J. 0. Turner, J . Amer. Chem. Soc., 87, 2153 (1965); (b) C. U. Pittman, Jr., Ph.D. Thesis, Pennsylvania State University, University Park, Pa., 1964.
d-a, Conformer b should undergo ready cyclization, and
a
b
one might then expect that phenyl-substituted acyclic allylic cations would only be observable at low temperatures. A few indanyl cations have been generated by protonating indenes in strong acid^.^^^ The l-methylindanyl cation underwent complete exchange of the C-2 hydrogens in less than 1 min at 25” in concentrated DzS04.* Thus, indanyl cations are in equilibrium with the corresponding indene, but the position of equilibrium greatly favors the cation. In this paper, we report the first observations of acyclic phenyl-substituted allylic cations. Also, the stereoselective protonation of transient indenes, formed by cyclization of phenyl-substituted allylic cations, is reported. Furthermore, the temperature-dependent hydride shifts of several indanyl cations were observed. A series of phenyl-substituted allylic cations was (9) V. Bertoli and P. H. Plesch, Specrrochim. Acta, Part A , 25, 447 (1969).
Pittman, Miller
Indanyl Cations
2948 2.15
i
1.0
2.0
3.0
5.0
I
i
8.0
i
i
7.0
i
i
8.0
1
Figure 1. Pmr spectra of (a) an FS03H-S02 solution of predominately the 1,3-diphenylbutenyl cation (2a) in the process of cyclizing to the 1-methyl-3-phenylindanylcation (4a); (b) the resulting 1methyl-3-phenylindanylcation at - 22 '. 2 .o
1.0
generated from allylic alcohols, and one diene, in the strongly acidic media (FS03H-S02). The primary mode of investigation was pmr spectroscopy, but deuterium labeling, deuterium exchange studies, and product analysis were also employed. All the allylic cations studied eventually cyclized to indanyl cations. These reactions fall into two categories: (1) cyclization to give an initial indanyl cation which does not rearrange further, and ( 2 ) cyclization to give an initial indanyl cation which rearranges to give a more stable indanyl cation (Scheme 11). In those cases where subsequent rearrangement follows, variable-temperature pmr studies permitted the cyclization mechanism to be followed stepwise by observing each intermediate ion as the temperature was raised.
Results When 2,4-diphenyl-3-buten-2-01(la) was dissolved into FS03H-S02, at temperatures from -75 to - 5 O " , the 1,3-diphenylbutenyl cation (2a) was generated and observed by pmr (see Scheme I). At -50" the pmr Scheme I OH Ph-
I
C-
I
2'
3.0
Figure 2. "r spectrum of the 1,1,3-triphenylpropenyl cation (2b) in FS03H-S02 at - 50".
spectra of these solutions show the acyclic cation 2a in the process of cyclizing to the I-methyl-3-phenylindanyl cation (4a) (see Figure 1). At -70" almost pure solutions of 2a are obtained. Acyclic ion 2a readily cyclizes on warming, and at -22" only the cyclic indanyl cation 4a is observed. lo However, when alcohol l a was added to 96z HzS04neither 2a nor 4a was observed. Instead, the 3-methyl-1-phenylindanyl cation (sa) was quantitatively formed. lo Thus, in both FS03H-S02 and H2S04,l a dehydrates to give the acyclic cation 2a which cyclizes to produce indanyl cation 4a. However, in sulfuric acid at room temperature the reaction sequence proceeds farther. Cation 4a rearranges to give the more stable cation 5a where two phenyl groups are now in conjugation with charged carbon. Upon quenching the sulfuric acid solution of 5a into cold, aqueous sodium hydroxide, 3-methyl-1-phenylindene (7) was isolated in 87 yield. Similarly, careful
z
F S0 H -S0
CH=CH-
Ph
- 50'
FS03H-SOz
P
h OH lb
w Ph
- 60'
I
Ph 9
Ph
5a
7
Journal of the American Chemical Society / 95:9 / May 2, 1973
6b (10) These cyclizations are quantitative. No other decomposition, alkylation, or other products could be detected. In the strongly acidic FS03H-S02 or H2SO4 media, the concentrations of diene corresponding to l a or the indenes corresponding to 4a or Sa are so low that second-order alkylation reactions are not observed.
2949 Table I. Pmr (100 MHz) Parameters for Phenyl-Substituted Allylic and Indanyl Cations'fb Ion 2a -50"
Ri Me, 6.74, s
R2
R1
H (coupled to R4 = H)
4a -22"
Me, 6.45, s
4c
Me, 6.47, s
0.83 and 0.69 low-field members, JAB = 14 1.37 (calcd). H (coupled to R 4 = H) 1.81, 1.67, 1.43, and 1.29, JAB = 14 1.73 (calcdy H (coupled to H') 6.28, 6.02, 5.62, and 5.36, JAB = 26 Me, 8.31
-40" 4d (cis)
Me, 6.65, s Me, 6.75, s
2b -50'
-70' 4d (trans) -70" Sa RTd
6e -40" 6f -15" 6g - 10"
6h -40" 6i RT 6j (cis) -30" 6j (trans) -30" 6k (cis) -10" 6k (trans) - 10" 61 $10"
1.37 (calcd).
Ph, 2.65, m
H, 5.55, s
Ph 3.13, m
Ph, 2.93, m
H , -4.66
6.18, q J =7 4.66
Ph, 2.74, m
Ph, 2.93, m
H, 5.17, s
5.10, s
Me, 8.01, d J = 7
H, -5.74
(see R2)
Me, 8.30, t J = 7 Me, 7.97, t J=7 Me, 8.91, d J=7 Me, 8.34, d J =7 Ph, 2.41, m
H, 6.46, q J = 7 H, 5.16, q J = 7 H, 5.92, m H, 6.28, q
5.89, q J=7 5.89, q J = 7 4.35, d J = 4 4.99, s
J = 7 H, 4.77, s
(see Rz)
Ph, 2.76, m
H (coupled to H')
6b RT
H (coupled to Rz = H )
(see Rz)
Me, 8.28, t J = 7 Me, 7.97, t J = 7 Ph, 2.76, m
5d (cis) - 30" 5d (trans) - 30"
H (coupled to R? = H) 0.77 (calcdy
H, 5.07, s
5.98, 5.74, 5.32, and 5.08,J.m = 24
-loo RT
H'
Ph, 2.75, m
H (coupled to H')
5c (trans)
R4
5.93, 5.59, 5.21, and 4.97, JAB = 24 Me, 6.54, s Me, 6.42, s Me, 6.65, s
Me, 8.44, d J=7 H, 6.4 RZ = R s = Me are contained in a multiplet centered at 8.13 Rz = R 3 = R 4 = Me are all contained in a large multiplet centered at 8.53
Me, 6.69, s
Ph, 2.57, m
Me, 6.14, s
H, 5.50, s (coincident with H') Me, 8.81, d J = 7 Me, 8.29, d J = 7 Me, 8.98, d J = 7 Me, 8.42, d J=7 H , H', AB pattern 5.94, 5.58, 5.22, and 4.98, JAB = 24
5.95, m ~ 6 . 5 1
Me, 8.40, s (cis to Rz) Me, 7.70, s
Me, 9.00, s (trans to Rz) Ph. 2.39, m
5.28, m, s
Ph, 2.65, m
H, 4.83, d J = 4 H, 5.45, s
5.17, m
Ph, 2.65, m
5.50
Me, 8.04, s
5.57, q J = 7 5.55
Me, 8.19, s
5.55
Ph, 2.44 A&
H, 4.79, s (broadened)
(see Rz) ~~~~~
Chemical shifts in 7 from TMS and coupling constants in Hz; s = singlet, d = doublet, t = triplet, and q = quartet. * All data for temperatures below room temperature were obtained with FSOsH-SOc, and all room-temperature data were obtained with 96% H2SO4. c J. A. Pople, W. G. Schneider, and H. J. Bernstein, "High-Resolution Nuclear Magnetic Resonance," McGraw-Hill, New York, N. Y., 1959, p 199. R T = room temperature. 01
-
quenching of a FSO3H-SO2 solution of 4a into sodium - 60" produced l-methyl-3methoxide-methanol at phenylindene (8) in 71 yield. The acyclic 1,1,3-triphenyl-2-propenyl cation (2b) was the other acyclic cation to be directly observed. (lb) in It was formed from 1,1,3-triphenyl-2-propenol FSO,H-SO, at - 60", but above - 30" cyclization occurred. The pmr spectrum of 2b is shown in Figure 2. At room temperature in H2S04the 1,3-diphenylindanyl cation (6b) was generated directly from l b without observation of the open ion 2b, and upon quenching this solution of 1,3-diphenylindene (9) was isolated in 84% yield (see eq 2). Cations 2a and 2b are the first acyclic phenyl-substituted allylic cations to be observed,
and they are only detected at low temperatures. Their pmr spectra are summarized in Table I. A common characteristic of the pmr spectra of both 2a and 2b is the clean AB pattern exhibited by the terminal and central protons on the allylic backbone. The large, 14 Hz, coupling of this pattern indicates these ions possess a fixed, trans, and reasonably coplanar relation to one another. This conclusion is strongly supported by examining aliphatic acyclic allylic cations. Schleyer, et aZ.,ll reported a 9-Hz coupling in the cis,cis-1,3dimethylallyl cation (lo), while the trans,trans-1,3-dimethylallyl cation (11) exhibited a 14-Hz coupling. (11) P. v. R. Schleyer, T. M. Su, M. Saunders, and J. C. Rosenfeld, J . Amer. Chem. SOC.,91, 5174 (1969).
Pittman, Miller I Indanyl Cations
2950
vcH3
H3C CH, CH3 10
14 Hz
'.$,
H
C-1 and C-3 phenyl groups would equilibrate on the nmr time scale. Even at +50° this was not observed. In the second case, the I-phenyl-3-p-tolylindanylcation (61) (quantitatively formed from 21 in FS03H at IO"
11
A total of 11 phenyl-substituted allylic alcohols (la-1) were prepared and their reactions studied in strongly acid media (summarized in Scheme 11). The acyclic Scheme I1
Ph H
Ph 61 2
5
1
rearrangement
or H2S04at 25") does not undergo rearrangement to the I-p-tolyl-3-phenylindanylcation at room temperature in HzS04. This is significant since the latter would be more stable than 61. Finally, it should be mentioned that the 1,3-dimethyl-3-phenylindanylcation (6i) was not observed to rearrange to either the l-phenyl-3,3dimethyl- or the I-phenyl-2,3-dimethylindanylcations (eq 3).
CH3
Rl
6
3 a,
R1 =
b, R1 = c, R1 = d. R1 = e,' RI = f, R1 = g, R1 = h, R1 = i, R1 = j, R1 = k,R1 = I, RI = a,
R1 =
c, R1 = d,Rl =
4
1. 2. 3. -,-, - , 4., 6 Me; Rz = R4 = H; R3 = Ph R3 = Phi Rz = R4 H Rz = Me; R 3 = Ph; R4 = H Me, Rz = R 3 = Ph; R4 = H R 3 '= Me; RP = RI = H Rz = R3 = Me; R 4 = H Rz = R 3 = R4 = Me R3 = R 4 = Me; Rz = Ph R 3 = Me; Rz = H; R4 = Ph R3 = Ph; Rz = Me; R 4 = H R 4 = Me; Rz = R3 = Ph Ph; Rz = Rc H; R3 = p-tolyl 4, 5 Me; Rz = R 4 = H; R 3 = Ph Rz = Me; R3 = Ph; R 4 = H Me; Rz = R3 = Ph; R 4 = H
allylic cations 2c-1 were not observed even in the -60 to -70" temperature range. Instead they rapidly cyclized to give indanyl cations. One group, alcohols lb,e-1, cyclizes ljia open cations 2b,e-1 to produce unrearranged indanyl cations 2b,e-1. The second group of alcohols, la,c,d, initially cyclizes via open cations2a,c,dto indanyl cations 4a,c,d. However, these cations undergo further rearrangement to ions 5a,c,d. The three indanyl cations which further rearrange (4a,c,d) all possess a methyl group at C-1 and both a phenyl and a hydrogen at C-3. Furthermore, all rearrangements proceed from a I-methyl- to a I-phenylindanyl cation. No further rearrangement takes place whenever an indanyl cation is initially formed with R1 = Ph (for example, see 6b,j,l). This is most clearly illustrated in two cases. First, the cyclization of 2j produces the 1,3diphenyl-2-methylindanyl cation (6j) quantitatively in Ph
Ph
6i
both H2S04at 25" and FS0,H-SO, at -20 to -70". If a rearrangement was taking place it would be degenerate, and if the rate were sufficiently fast, both the Journal of the American Chemical Society 1 95:9 1 May 2, 1973
6i
CH, 'CH,
A common feature observed for all the cyclization reactions where a phenyl is present at both ends of the allylic system is the preference for the end of the allylic cation which is least stabilized to effect electrophilic attack. In this circumstance the more thermodynamically stable indanyl cation, of the two that are initially possible, is formed as the kinetic product.12 Equations 4 and 5 summarize the examples which illustrate this distinction. Note that neither conformation A nor B in eq 4 is the most stable one, but they equilibrate through it. The observed path could result from (1) a higher concentration of conformer B, (2) an intrinsically greater rate constant for electrophilic attack in conformer B where the less stabilized end of the allylic system attacks the ring, or (3) a combination of both (1) and (2). Quenching sulfuric acid solutions of the indanyl cations formed above into cold aqueous sodium hydroxide provides a general indene synthesis. The indenes corresponding to the stable indanyl cation end product in the acid (structures 5 and 6, Scheme 11) were isolated in yields of 57 to 97z, with an average yield of 8 5 % . (12) Prior reversible formation of the less stable indanyl cation appears to be ruled out by several observations. For example, the protonation of several indenes, with a hydrogen at C-1, did not give either their corresponding open allylic cations or the observed indanyl cation cyclization products of the corresponding open allylic cations. For example, protonation of 3-methyl-3-phenylindene did not give either 2a or 4a under any of the conditions studied (FSQaH-SQz from -70 to +20° or &SO4 at room temperature). Similarly, protonation of 2-methyl-3,3-diphenylindenegave neither 2j nor 6j. Furthermore, indanyl cations with hydrogen at C-1 gave dimeric and polymeric products rapidly in FSOaH-SO2 at -50".
2951
R H
CH3 more stable
less stable
f
-
L
2
4a-D
2a-D
w:
(4)
CH3 D
5a-D
A
2a,R=H e, R = M e d , R-Ph
shifts, since such a mechanism would predict replacing at least 50% of the C-2 protons by deuterium and incorporating hydrogen at C-3 in Sa-D. Further support was obtained by generating Sa from l a in 96% DzS04. In this experiment 5a was deuterated only at (2-2. No deuterium was found at C-3 as required by a mechanism involving two consecutive 1,2-hydride shifts (see Scheme 111). Similarly, the cyclization of Id to
B
@ + H
p $ +H
Scheme III
Ph
H less stable
more stable
f
k
(5)
1
X
J 1 1 2 shift
A
2b,R=X=H j, R=CH,; e,
B
Ph
1,3 shift
Ph
X =H
R-HX-CH,
Furthermore, the low-temperature (-70') quenching of FS0,H-SO, solutions of intermediate indanyl cations 4a-c into sodium methoxide-methanol resulted in the isolation of the corresponding indenes in yields between 48 and 71 %.13 Mechanism of the 4a,c,d Sa,c,d Rearrangements. The rearrangements of 4a,c,d to Sa,c,d can be envisioned as taking place via a single 1,3-hydride shift or by two successive 1,2-hydride shifts. l 4 The rearrangements of 4a and 4d to Sa and Sd, respectively, took place by single 1,3 shifts. Conversely, the rearrangement of 4c to Sc occurs via two successive 1,2-hydride shifts. These conclusions were demonstrated by deuterium labeling and exchange studies. For example, 2,4diphenyl-3-buten-2-01-4-dl (la-D), 90 %, was prepared and quantitatively converted, in H,S04 at room temperature, to the 3-methyl- 1-phenylindanyl-3-dl cation (Sa-D). The pmr spectra of cation 5a-D exhibited a sharp singlet for its C-3 methyl group and showed no deuterium at C-2. Exclusive formation of Sa-D rules out a mechanism involving two successive 1,2-hydride
-
(13) The synthesis of indenes from phenyl-substituted allylic alcohols appears general and will be the subject of a subsequent paper. (14) Intramolecular hydride shifts have been extensively investigated. For a recent review, see G . J. Karabatsos and J. L. Fry, in ref 7, pp 521-572.
not observed
Sa-D observed
give Sd in 9 6 z DzS04did not result in any deuterium incorporation at C-3, and this confirms the 1,3-hydride shift. This is particularly interesting since successive 1,2-hydride shifts would seem favored in the 4d + Sd rearrangement, as opposed to the 4a -+ Sa case, due to the availability of the stable tertiary benzyl cation intermediate 12a. The uncertainty (due to the
12 1 2b, a ,RR=- =CH, Ph
&$ Me
R
H
nmr technique both in acid and of the drown products in CCL) is estimated at f 2-3 The rearrangements of 4a and 4d contrast sharply with the 4c to Sc rearrangement which does proceed by successive 1,2-hydride shifts. Thus the 4c to 5c rearrangement is proceeding through tertiary (but not benzylic) cation 12b. Generation of 5c from IC in 96 DzS04 or FS03D resulted in complete deuteration
z.
Pittman, Milter
Inrianyl Cations
2952
Scheme N
I
8.11 n.29
1.n
2 n
3.0
4.0
1.0
6.0
7.0
1.0
1
9.0
D
1
Figure 3. "r spectrum showing the cis-l,3-diphenyl-2-methylindanyl cation (6j) converting to the trans-1,3-diphenyl-Z-methylindanyl cation (6j) in FSOsH-SO2 at - 30".
at C-3 but no deuterium incorporation at (2-2. l 5 These results are only readily explained by a mechanism involving two 1,2-hydride shifts. This was further confirmed by generating 5c-D from 2,4-diphenyl-3-methy1-3 buten-2-01-4-d1 (lc-D), in FS03H-S02 at - 10". In this case all the deuterium in 5c-D was found at C-2 and none was found at C-3 (see eq 7).16
trans. 6 j
changes occurred. cis- and trans-6j are readily differentiated by pmr since the C-2 methyl group of the cis ion lies in the face of the phenyl ring at C-3. Thus, it is abnormally shielded (sharp doublet at T 8.81), whereas the C-2 methyl resonance in the trans ion is found at T 8.29. Moreover, the coupling between protons on C-2 and C-3 is 4 Hz in the case of cis-6j, but is -0 Hz in trans-6j. The magnitudes of these coupling conH stants are completely in accord with the dihedral angles between these two sets of protons.'* Thus, stereoselective protonation of 3j is occurring trans to the C-3 (7) phenyl substituent (path A, Scheme IV) producing the Me H less stable ion, cis-6j. Clearly, path A is kinetically 5c-D favored, and this is probably dictated by the fact that this path results in less steric hindrance, from the C-3 Stereoselective Protonation of Transient Indenes. phenyl group, to the approach of the proton donor.Ig The cyclizations of phenyl-substituted allylic cations trans-6j does not have a cis 1,2 phenyl-methyl eclipsing (2a-I) to indanyl cations proceed by an intramolecular interaction. Thus, as the temperature is raised, cis-6j SEAr mechanism. This requires the formation of an can convert to the more stable thermodynamic product intermediate indene (see 3, Scheme 11) which is subtrans-6j via the equilibrium with indene 3j. The pmr sequently protonated in the acidic media. Several of spectrum of a FS03H-S02 solution of cis-6j in the the transient indenes are capable of stereoselective proprocess of equilibrating to trans-6j is shown in Figure 3. tonation, and this has now been demonstrated in four The product obtained when 2,3,4-triphenyl-3-butenexamples (3c,d,j,k). Two of these cases, 3j and 3d, 2-01 ( l a ) is dissolved in FS03H-S02 at -70" is a will be discussed in detail. Upon dissolving 3-methyl-2,4-dipheny1-3-buten-2-01 mixture of the cis- and trans-2,3-diphenyl-l-methylindanyl cations (4d). Depending upon the care taken (lj) into FS08H-S02 at -70°, one obtains a solution in the preparation of these solutions, the cis isomer consisting predominantly (>80%) of the cis-1,3-dipredominates from 90 to >65%.20 Even at -70" phenyl-2-methylindanyl cation (6j). l7 However, ciscis4d is completely converted to trans-4d in 70 min 6j equilibrates readily to the more stable trans-6j (see (18) For a summary of vicinal coupling constants in ring systems, Scheme IV). On standing or warming, " 2 6 j is see L. M. Jackman and S. Sternhell, "Applications of NMR Specprogressively formed (at - 30" after 0.5 hr about 50% troscopy in Organic Chemistry," 2nd ed, Pergamon Press, Elmsford, cis and 50z trans were present) until at - 15" only N. Y.,1969, Chapters 4-2 and 4-3. (19) The proton donor is presumably the FSOaH molecule which trans-6j remains. On recooling to -70" no further
r
qpe
(15) The pmr spectrum of 5c generated in DzSO4 exhibited a clean singlet for the C-3 methyl group at T 7.97. When 5c was generated from IC in FSO~D-SOZat - 1 5 " the methyl group at C-2 appears as a clean doublet at T 8.27, J = 7 Hz,and the proton at C-2 as a quartet at I 5.87, J = 7 Hz. (16) The C-3 methyl appeared at T 8.30 and the C-2 methyl group was a singlet T 8.28. The proton at C-3 appeared at 6.46, q, J = 7.0 Hz, but the C-2 proton at T 5.89 was gone. (17) In two such experiments, where special care was taken to precool an SO2 solution of l j before addition to the acid and where the addition was done very slowly with vortex action stirrer, cis-6j was produced in >90% yield.
Journal oj'the American Chemical Society / 95:9 1 May 2, 1973
has considerable steric bulk. If the transition state for the protonation of indenes closely resembles the cation being formed, it seems likely that the bulk of the FSOaH molecule, with its solvent shell, would make itself felt in a competition between paths A and B. Stereospecific protonation of cyclopentadienes has been reported by T. S. Sorensen, I. J. Miller, and C. M. Urness, Can. J . Chem., 48, 3374 (1970). (20) In most experiments a very small, constant concentration of the cis-l,2-diphenyl-3-methylindanylcation (5d) was present from the beginning, even at +70". This is presumably due to hot-spots during the preparation of the solutions. As the temperature increases above - 40", the &/trans ratio continuously decreases from the initial ratio of -1 at -40". When the ratio is ~ 1not, all the rrans-4d has been consumed.
2953 Scheme V
Id
- 70'
FSOaH
e.&.
Ph
Me
Ph
1
[rhp ] pz
Ph
cis4d
e
AI3
