EPR study of long-range acetyl-proton couplings in aliphatic acetoxy radicals

JOURNAL

OF MAGNETIC

RESONANCE

46,

200-212 (1982)

EPR Study of Long-Range Acetyl-Proton Couplings in Aliphatic Acetoxy Radicals* PETER SMITH AND KERRY

K. KARUKSTIS

Paul M. Gross Chemical Laboratory, Department of Chemistry, Duke University, Durham, North Carolina 27706

Received June 8, 1981 Recent liquid-phase EPR studies in our laboratory on a-formyloxy and fi-formyloxy radicals, <(X,)(X,)OCHO [ 11, and <(X,)(X,)C(X,)(X,)OCHO [2], respectively, where an (X-) group is an H- or a simple substituent such as CH,-, have demonstrated that the experimentally observed long-range y- and 6-CH formyl-proton couplings were sensitive probes of the alterations in preferred radical conformation produced by changes in the degree of substitution. W e would expect the long-range 8- and c-CH acetyl-proton couplings in simple and acetyl-substituted a-acetoxy radicals, <(X,)(X,)OC(O)CH(X,)(X,) [3], and in P-acetoxy radicals, c(X,)(X,)C(X,)(X,)OC(O)CH, [4], to show a variation with degree of substitution similar to that found for the formyl-proton couplings in the structurally analogous type-[ l] and -[2] radicals. In the present paper, we carried out a comprehensive EPR study of type[3] and -[4] radicals in aqueous solution at approximately 25°C with particular attention to the observed long-range acetyl-proton couplings. Our findings successfully confirm the expectations about the dependence of the acetyl-proton couplings on the nature of the substituents in type-[31 and -[4] radicals. Furthermore, by comparing the similar variations of the long-range couplings in type-[11 and -[3] and in type-[21 and -[4] radicals, we assigned preferred radical conformations to type-[31 and -[4] radicals. INTRODUCTION

Using measurements of long-range NMR proton spin-spin couplings, many investigators have studied the m o lecular structure and preferred conformations of simple aliphatic carboxylic acid esters (1). O n the other hand, the application of the analogous long-range EPR proton hyperfine couplings to problems of structural elucidation and conformational analysis of radicals has been relatively lim ited (2). In fact, there has been little systematic attention of long-range EPR proton couplings in radicals derived from aliphatic carboxylic acid esters (2-4). Recent (3-7) comprehensive, liquid-phase EPR studies and related INDO-MOSCF calculations (8) concerned with a-formyloxy and P-formyloxy radicals, l C(X,)(X,)OCHO [ 11, and l C(X,)(X,)C(X,)(X,)OCHO [2], respectively, where an (X-> group is an H- or a substituent such as CH3-, have demonstrated that the experimentally observed y- and 6-CH formyl-proton couplings, respectively, are sensitive probes of radical conformation. In the case of the ru-formyloxy radicals, * Supported by the National Science Foundation under Grant GP-17579. 0022-2364/82/020200-13$02.00/O Copyright 0 1982 by Academic Press, Inc. All rights of reproduction in any form reserved.

200

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201

RADICALS

the presence of the substituents Xi- and X2- influences the preferred radical conformation about the (C,-0,) and (0,-C,) bonds. For those type-[11 radicals with no more than a single (X-) substituent at C,, free rotation occurs about C,-0, with an s-trans arrangement of the C, and formyl hydrogen about 0,-C,, as typified by the two s-tram rotamers

(

*-xa c Nd’

,-

0

P

/ X2

\ Cy.H

the designation (y-X) in these and subsequent structures referring to the variable group. Accordingly, the experimentally determined y-CH formyl-proton coupling constants, acsHo values, in such radicals are all about equal, 2.40 to 2.5 1 G (3, 4) (for the sake of simplicity, throughout this paper, we will assume all experimentally determined couplings to be positive). For those radicals with two (X-) substituents at C,, rotation about the (C,-0,) bond is restricted to favor conformations such as 11(7-H), and rotation about O&, from the s-tram toward the s-&s arrangement becomes more likely. Consequently, the aF$Ho values in these disubstituted species show a marked decrease from those of the monosubstituted radicals (3). For the P-formyloxy radicals (7) enhanced rotation about Q--C, from rotamer 111(6-H) toward IV(G-H) H / .“c

\

111(6-H)

0

IV(&H)

where the designation (6-X) in these and subsequent structures refers to the variable group, occurs as a result of progressively decreasing substitution at C, and thus reduces aFgHo while substitution at C, favors (s-tram)-to-(s-cis) rotation and also OCHoto decrease. causes a&H In simple and acetyl-substituted cu-acetoxy radicals, &(X,)(X,)OC(O)CH(X,)(X,) [ 31, we would expect the experimentally determined 6-CH acetyl-proton couplings, a6 values, to show a variation with degree of substitution at C, similar to that for their structural analogs, the type-[ l] radicals. Likewise, we would expect

202

SMITH

AND

KARUKSTIS

the dependences on substitution of the e-CH acetyl-proton couplings, a, values, in [4], to be similar to that ob/3-acetoxy radicals, l C(X,)(X,)C(X,)(X,)OC(O)CH, served for the formyl protons in the structurally analogous type-[21 radicals. However, since &H couplings in aliphatic radicals are generally small if observed (9, 10) then, by analogy with the properties of type-[21 radicals (3, 7), we would be most likely to resolve a, only for the most heavily a-substituted type-[41 radicals. The present report concerns a successful attempt to test these predictions by way of a comprehensive study of type-[31 and -[4] radicals in aqueous solution at approximately 25°C generated from saturated and unsaturated monoesters with the use of the Tic&-H202 method. Thus we have confirmed these predictions and determined preferred conformations for these radicals, verifying that long-range EPR proton couplings can be sensitive probes of radical stereochemistry and further extending the range of studies of the relation between long-range coupling constants and radical conformation. There have been a few fragmentary studies of type-[31 and -[4] radicals reported in the literature (9b, f, 11-I 7), but their focus was not on long-range couplings. In these studies, the spectroscopic resolution and the precision of the results achieved together with the wide range in the experimental conditions employed cause the data on long-range couplings, which are generally small, to be of restricted use. The extensive results of the present work and its emphasis on long-range couplings largely overcome the experimental disadvantages of these earlier, limited investigations. In the course of this present work, we observed a few radicals of structure CH,(X,)OC(O)e(X,)(X,) [5], all of which displayed 6-CH alkoxyl-proton couplings. In view of the limited amount of reliable experimental information published on such species (I 1, 12, 14, 16) we defer discussion of the relation between the 6-CH alkoxyl-proton couplings and the preferred conformations in [5] radicals until such time as more data on species of this type are available. EXPERIMENTAL

The EPR system and the associated experimental arrangements and procedures were essentially as before (4). We recorded spectra as the first derivative, all data were taken at 25 + 1“C. Potassium peroxylamine disulfonate in saturated aqueous KzC03 at 25 + 1°C was the field and g-value standard (4). The field-modulation amplitude was normally in the range 1.25 X 10-l to 4.0 X 10-l G. Peak-to-peak linewidths, AH,,, were generally approximately 0.20 G, except where stated otherwise. For those spectra consistent with more than one radical, the relative concentrations, i.e., in the steady state, were determined by the peak-height method, using lines of essentially equal M,, (I I), i.e., approximately 0.20 G. We examined all substrates using the aqueous TiC13-H202 radical-generating method (18). The reducing stream was 0.004 A4 in TiC13 and 0.1 M in HzS04. To minimize their hydrolysis (I I ), we included substrates only in the oxidizing stream and kept this free of added H&SO+ The concentration of HZ02 in the oxidizing stream was usually 0.1 A4, but, for some substrates which gave complex spectra, we varied this concentration over the range 0.1 to 0.2 M, while keeping the other reaction conditions

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IN ACETOXY

RADICALS

203

unchanged, so as to alter the relative concentrations of the radicals observed (I 1, 19) and thereby identify their spectra. The total flow rate was 4 to 6 ml set-‘, equally divided between the two streams. The substrates examined, fresh samples without further purification, were: methyl acetate (Fisher, 98%) ethyl acetate, isobutyl acetate, ally1 acetate, and ethylene diacetate (Eastman); n-propyl acetate, isopropyl acetate, vinyl acetate, and methyl acetoacetate (Eastman, practical); ethyl acetoacetate and 2-methoxyethylacetate (Pfaltz and Bauer); and ethyl L-(+)-lactate and isopropenyl acetate (Aldrich). Their concentrations in the oxidizing stream were in the range 0.04 to 0.25 M. The concentrations of the unsaturated substrates were kept low so that we would avoid complications from polymeric radicals and other possible secondary species (20, 21). Also, we studied vinyl acetate with the use of the TiC13-NH*OH radical-generating system (3, 20), the reaction conditions here being as for the TiC13-H202 work except that, in place of H202, the oxidizing stream was made 1.0 M in hydroxylamine hydrochloride (Eastman). All results refer to the TiCIs-H202 system unless stated otherwise. RESULTS

Simple Saturated Monoacetates, HC(X,)(X,)OC(O)CH, Methyl acetate, CHjOC(0)CH,. Although the only previous study (11) of this substrate by the TiC13-H202 method gave a spectrum for but one radical, tentatively assigned to l CH20C(O)CH3 [6], we found both [6] and CH,OC(O)CH, [7] (12a), the relative concentrations of [6]:[7] being approximately l.O:O.10. Although the spectra of [6] and [7] are very similar, their assignment is unambiguous, for two reasons. First, the relative concentrations of [6] and [7] agree with the apparent reactivity of the aH radical as an electrophilic reagent toward aliphatic carboxylicacid esters (3, 4, 11, 22). Second, the measured g values, 2.0027 [6], and 2.0032 [7], accord with the idea that a-carbonyl-substituted radicals should show slightly higher g values than a-alkoxy-substituted radicals (23). Ethyl acetate, CH,CN,OC(O)CH,. As before (II), we were able to fully interpret the spectrum obtained in terms of CH&HOC(0)CH3 [8] (11, 13), present in higher concentration, and l CH2CH20C(O)CH3 [9] (1 Oa, 11,13), with no evidence for the presence of the so-far unobserved species CH3CH20C(0)CH2. n-Propyl acetate, CH,CH,CH,OC(O)CH,. This substrate gave a complex spectrum in which we could characterize only CH3CH$HOC(0)CH3 [lo], and CH$HCH20C(0)CH3 [ll], the latter being of somewhat higher concentration than the former. Both [lo] and [ 111 exhibited long-range acetyl-proton couplings, that in [ll] constituting an example of an c-CH proton coupling (9), usually not observed in aliphatic radicals (10). Because of the complexity of the overall spectrum and the relatively low concentration of [lo], we were unable to determine the /3-CH2 proton coupling in [lo]. Isopropyl acetate, (CHJ2CHOC(0)CH3. We could account for the spectrum observed in terms of (CH,),COC(O)CH, [ 121, and
204

SMITH

AND

KARUKSTIS

better resolved since, in [ 121, we observed a hitherto unreported ub coupling of 0.4 G and, in [13], we found a previously undetected y-CH3 proton coupling of 0.7 G. Zsobutyl acetate, (CH3),CHCH,0C(0)CH,. The spectrum agreed with the pres[14], (CH3)&H20Cence of only three radicals, (CH3)$HCHOC(0)CH3 (0)CH3 [15], and l CH2CH(CH3)CH20C(O)CH3 [16]. Radicals [14] and [15], present in approximately equal concentration, exhibited a6 and a, couplings, respectively, the latter constituting another rare example of an E-CH proton coupling (9, IO). On account of its relatively low concentration, the complexity and low resolution of its spectrum, and the unimportance of this radical to the chief thrust of our work, we made no attempt to determine the spectroscopic parameters of [16]. Substituted Saturated Monoacetates, HC(X,)(X,)OC(O)CH(X,)(X,) Methyl acetoacetate, CH,OC(O)CH,C(O)CH,. The spectrum was consistent with the presence of all three conceivable hydrogen-atom abstraction radicals, l CH,OC(O)CH,C(O)CH, [ 171, CH,OC(O)&HC(O)CH, [ 181 (14), and CH3OC(O)CH,C(O)CH, [ 193, their relative concentrations being 3: 1: 1. The only previous study of [ 181, in aqueous solution, reported an ru-CH proton coupling in reasonable agreement with our finding and two small (1.1 and 0.2 G) unassigned CH,-proton couplings (I4,24). However, we found only one CH,-proton coupling, 1.13 G, and could set an upper limit of approximately 0.10 G for the other such coupling. On the basis of our results for ethyl acetoacetate (see below), we are reasonably confident that the CH,-proton coupling we observed is from the 6-CH3 protons. Ethyl acetoacetate, CH,CH,OC(O)CH,C(O)CH,. Within the complex spectrum this substrate produced, we were able to assign components to CH3[21], with relCHOC(O)~H,C(O)CH, 1201, and CH,CH,OC(O)CHC(O)CH, ative concentrations of lO:l, respectively. Of these two radicals, EPR data are available only for [21], from an aqueous-solution study (12b). Our data for [21] are very different from those previously reported (126). For the a-CH proton coupling we measured 19.0 G, a more reasonable value for a radical bearing two a-carbonyl groups, X,C(O)cHC(O)X, (II, 12, 14, 25, 26), than 21.0 G (126). For the 6-CH proton interaction, we found a clear, (1:2:1)-triplet coupling of 1.1 G, whereas the earlier workers did not detect this interaction (126). On the other hand, our data indicate the y-CH proton coupling to have an upper limit of approximately 0.20 G, in contrast to the value 1.7 G reported previously (126). The difference in size we found for the 6- and -&H proton couplings is reasonable on the basis of the somewhat limited data for comparable species in the literature (1 I, 12, 27). Furthermore, our determined g value, 2.0046, is definitely less than that reported before, 2.0052 (IZb), either value being consistent with the limited literature data (23, 26). The similarity in c&H and 6-CH couplings and the g values measured for the analogous radicals [ 181 and [ 211 further support the validity of our data for [21]. 2-Methoxyethyl acetate, CH,OCH,CH,OC(O)CH,. We could resolve only the spectra of two radicals in the complex spectrum from this substrate. We assigned

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IN ACETOXY

205

RADICALS

these spectra to l CH20CH2CH20C(O)CH3 [22], and CH30CHCH20C(0)CH3 [23], with relative concentrations of 3:1, respectively. The rest of the spectrum presumably arose from one or both of the possible type-[31 and -[5] radicals this substrate might yield. Ethyl L-(+)-lactate, CHjCH,OC(0)CH(OH)CH,. This substrate gave a complex substrate also. We could characterize only two signals, attributed to radicals CH&HOC( O)CH(OH)CH, [ 241, and CHJZHPOC( O)c( OH)CH3 [ 251 (28 ), their relative concentrations being 0.1: 1. As expected (21, 28, 29) [25] showed both a 6-CH2 and a P-OH proton coupling, the spectrum remaining unchanged over the pH range investigated, 1.0 to 4.0. The AH,, values of the lines in the spectrum from [25] were approximately 0.40 G, suggesting the presence of more than one species, namely, presumably the anticipated s-tram and s-cis isomers about the C&,(O) bond (286, c). The likelihood that these isomers may have somewhat different a6 values is unimportant to our study. Likewise, the possible presence of intramolecular proton exchange (29b) in the s-cis isomer of [25], i.e., between the (OH)- and (C=O)-oxygen sites, is unimportant to the observation of a 6-CH2 proton coupling. Unsaturated Monoacetates, CH,=C(X)(CH2)nOC(0)CHJ,

n = 0 or 1

Vinyl acetate, CH,=CHOC(O)CH,. With the use of the TiC13-H202 and --NH*OH systems, respectively, the spectra were consistent with the presence of only HOCH$HOC(0)CH3 [26] (126, 15, 17) and H2NCH$HOC(0)CH3 [ 271 (IZb), our results being in only moderate agreement with the literature. Ally1 acetate, CH,=CHCH,OC(O)CH,. We could fully interpret the spectrum obtained in terms of the presence of both addition radicals, HOCH2CHCH20C(0)CH3 [28] (9b), and ~H,CH(OH)CH,OC(O)CH, [29] (96), their relative concentrations being 1:O.l. We made no attempt to determine the spectroscopic parameters of [29] because of its relatively low concentration and its unimportance to the main objective of this work. Isopropenyl acetate, CH,=C(CH,)OC(O)CH,. The spectrum obtained was clearly from just two radicals, considered to be HOCH,C(CH,)OC(O)CH, [30] [31], the latter previously unobserved, (15) and l CH,C(OH)(CH,)OC(O)CH, their relative concentrations being approximately 1:O.1. Our data for [30] agree only moderately with the literature (1.5). Tables 1 to 3 summarize the EPR spectroscopic data for all the radicals identified. DISCUSSION

For each ester, the apparent overall reactivity to and selectivity of radical attack, assumed to be related to the minimum substrate concentrations needed to quench the signal from any titanium complex (34) and the relative steady-state concentrations of the radicals observed (I 1, 31) respectively, are in agreement with the results of other EPR studies using the TiC13-H202 and -NH%OH systems with the same or related substrates (3, 4, 9b, I I, 35). Most of the radicals characterized fall into three general types, viz., [3] to [5]. Thus, we have arranged the data in Tables l-3 by these types, including in Table 3 all those species which do not fit in the earlier tables.

HHHCH,Hm HHHHHCHj-

HCH,CH,CHZCH,(CHWHHCH,CH,HOCH>HZNCHIHOCH2-

161 PI

X4HHHHHHHHHHm H-

X3HHHHHCH,C(O)CH,C(O)HOHHH18.10 20.46 19.10 18.83 20.06 20.38

(2) (2) (1) (4) (1) (4)

20.21 (1) 18.81 (2) 18.51 (4)

a

CH-proton

24.11 23.96 12.06 14.36 12.64 22.92

1.420 1.438 1.40 0.396 1.422 1.038 1.056 1.474 1.286 1.18 0.415

6 (0.6) (0.4) (4) (0.6) (1.0) (1.0) (0.2) (0.5) (0.4) (l)h (0.1)

constants’.’ (G)

(1) (4) (1) (4) (1) (1)’

22.0:: (2) 22.75 (2)

23.96 (2)

P

coupling

RADICALS’

2.00248 2.0026 1 2.00266

60 58 58

46

2.00266 2.00266 2.00262 2.00265 2.00252 2.00266 2.00272 2.00275

g-value””

(15)

(12b) Wb)

(15)

(II) (II, 13)

Ref.

0 At 25 f 1 “C. From the reported (30) error limits for aN and the g-value of the standard, the maximum error limits are: a values, f approximately 20,, where 0,) is the standard deviation of the mean; g values, +0.00006. ’ All protons at each site are magnetically equivalent. The number in parentheses is 6, X 102. ’ In general, the c~-CH D values are in reasonable agreement with the literature (21, 126, 13. 15, 18, 31), and the other a values are either in only moderate to poor agreement (If, IZb, 13. 15, 18, 31), or are not even reported (15). dUsing Eq. [I], with B = 58.60 G (32) and previously reported values for A (-CH,), A(-CHZOH), and A(-CHZNH,), viz., 0.081, 0.067, and 0.043 (3, 4) respectively. In addition, we calculated A(-CHICHj) and A(-OC(O)CH,) to be 0.090 and 0.1 10, respectively, from the D value data for CH,CH$HCH, (33), [8], and [12] (this table), setting A(-CH(CH&) = A(-CH,CH,). (’IJ”,, 0.00001. ‘The only comparable literature values, for [26] and [27], are in wide disagreement with our data (I2b) and general expectation (23). g Unresolved, see Results. Estimated to be 22.70 G (4) in allowing for second-order effects when not in calculating the g value. haT-N = 7.17 (2.0) G. ’fl-CHIOH. ’@CH,.

v71 1301

PI

1241

PO1

1141 [I71

[lOI 1121

X*-

X1-

Radical

1

EPR SPECTROSCOPIC DATA FOR ~(X,)(X,)OC(O)CH(X,)(X,)

TABLE

;d

i

X

HH-

HCH,-

H-

[111

iI31

HH-

H-

CH,OHOCH,-

H-

1231

t311

CHj-

HH-

HH-

HO-

HH-

HH-

X4-

CH,H-

HH-

X3-

14.19 (1)

17.57 (1) 21.54 (1)

21.97 (2)

21.96 (1) 21.44 (4)

a 25.01 (4) 17.48 (2)R 25.26 (4)* 21.97 (2) 13.37 (2)R 23.08 (4)h 8.00 (1) 18.42 (1)’ 22.95 (1)”

B

0.812 (0.7)

1.955 (0.5)

0.70 (2)’

Y

k

k

f

f 0.314 (1.4)

f 0.254 (1 .O)

t

a See Table 1, Footnote a. h See Table 1, Footnote b. ‘ In genera], the a values are in only moderate agreement with the literature (96, 13, 1.5). dSee Table 1, Footnote d. In addition, we assumed A(-OCHJ) and A(-CH,OC(O)CH,) = A(-CH(CH,)OC(O)CH,), 0.065, basing this latter value on the (I value data for Ill] and [ 151. ” CT,,,, 0.00001. These values are as expected (96, 23). r Since AH,, = approximately 0.40 G, a $ approximately 0.15 G. g@-CH20C(0)CH+ h &CH,. ’+H,. ’ @-CH,OH. Ir Since AH,, = approximately 0.20 G, a % approximately 0.07 G.

[=I

HCH,-

CH,-

1151

[91

X*-

X1-

Radical

RADICALS

CH-proton coupling constant?. (G)

EPR SPECTROSCOPICDATA FOR <(X,)(X,)C(X,)(X4)0C(0)CH3

TABLE 2

2.00289 2.00230

65 48

(9b)

(15)

(13)

Ref.

respectively, to be 0.172 (32) and

2.00262

2.00246 2.00261

2.00226 2.00238

51 57

47 54

g Value’

Liz 0

F

s

b2

8 c ;r! 2 cl 2

CH,OC(O)CH, CH,OC(O)CHC(O)CH, CH,OC(O)CH,C(O)CH, CHKHIOC(O$HC(O)CH, .CH,OCH,CH,OC(O)CH, CH,CH,OC(O)C(OH)CH,

21.58 19.05 19.62 18.96 16.93

a (14) (2) (3) (1) (1) 16.73 (2)

B

CH-proton

coupling

1.54 1.126 0.66 1.13 1.934 1.605

6 (2) (0.9) (2) (1) (0.3) (0.5)

constan@

(G)

1.862 (0.7)’

e

Other

ANDALLOTHERRADICALS'

a See Table 1, Footnote a. b See Table 1, Footnote b. ’ Except for [7], our results are only in fair to poor agreement with the literature (12, 14, 28). d (T,,,,0.00001. In general, these values seem as expected (12, 14, 23, 28); however, see text. ’ Since AHpp = approximately 0.30 G, u, 5 approximately 0.10 G. /@OH. g The spectrum remained unchanged over the pH range 1 .O to 4.0.

[7] [18] [ 191 [21] [22] [25]

Radical

3

EPR SPECTROSCOPICDATAFOR CH,(X,)OC(O)C(X,)(X,)

TABLE

2.00317 2.00438 2.00422 2.00457 2.00307 2.00384

g Valued

(2SY

(12b)

(Z2a) (14)

Ref.

E

$

ii

z

COUPLING

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IN ACETOXY

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209

[ 31 Radicals

From the viewpoint of our investigation the two couplings of interest are those of the p- and 6-CH protons. The a@-u data reveal partial information about the preferred radical conformations using the well-known relationship (32, 36, 37) aSPH = pr;-cB(cos2 [),

[iI

where p;-c denotes the unpaired spin density at C,; B, 58.60 G (32); 4, the dihedral angle between the (C,-H) bond and the axis of the 2p, orbital at C,; and ( ), the time average. Table 1 includes the applicable .$ values, from which we may infer: (1) the most likely conformation of [ 141 is intermediate between the eclipsed and staggered arrangements, I(?-CH(X,)(X,)) and II(y-CH(X,)(X,)), respectively, of the (Y-(-OC(0)CHS) group and the a-C 2p, orbital; (2) the most likely conformations for [26] and [30] are probably those where the (-OH)-group 0 is in the plane defined by the C of the (CBH20H) group and the axis of the 2p, orbital at C,; (3) the most likely conformation for [27] probably has the (-NH2)-group N in the plane through the C of the (-CBH2NH2) group and the axis of the 2p, orbital at C,; and (4) the most likely conformations about the (C,-0,) bond for [26], [27], and [31] are as in II(-pCH(X,)(X,)). The a6 data show trends as predicted on the basis of the a-formyloxy radical studies (3-7). First, there is a significant change in the preferred radical conformation and, thus, in the size of a6 as the degree of substitution at C, changes from mono- to dialkyl substitution: al for [ 121 and [30] are about equal but considerably smaller than a6 for either of the a-unsubstituted radicals, [5] and [ 171, or for any of the singly a-substituted species, [7], [lo], [14], [20], [24], and [26]. Second, the data in Table 1 also indicate that, in general, the identity of the substituent, (X-) has little effect on a6. We do observe small decreases in a6 when the substituent Xi- contains an oxygen or nitrogen atom and X2- is H-, [26] and [27]. This behavior is presumably attributable to an enhanced homoconjugative interaction between the p orbitals on the heteroatom and at the radical site, these radicals preferring an eclipsed arrangement of the 2p, orbital at C, (e.g., (38)). This interaction transfers some spin density to the heteroatom and, consequently, reduces the potential size of a6. Also, there is a small decrease in a6 for [17] and [20], where X3- is CH&(O)- and X4- is H-, this being indicative of a lack of free rotation about the (C(O)-CH(X,)(X,)) bond. The steric bulk and electronic interactions of the (X3-) substituent result in a tendency toward a preferred conformation which does not allow the acetyl protons to maintain a W-plan arrangement (39) with the 2p, orbital at C,. However, for the remaining radicals in Table 1, [6], [8], [lo], [12], [14], and [24], the substituents do not differ appreciably in their ability to alter the unpaired electron-spin distribution and therefore to affect a6. The analogous dependence on substitution at C, of the T-CH formylproton couplings in type-[11 radicals (3, 4) and the 6-CH acetyl-proton couplings in type-[31 radicals reasonably suggests that these two series of radicals exist in similar preferred conformations.

210

SMITH

l C(X,)(X2)C(X3)(X~)OC(0)CH3

AND

KARUKSTIS

[ 41 Radicals

The /3- and PCH proton couplings in these radicals are those of chief concern in this present study. Table 2 includes the applicable ,$ values calculated as before in the case of the type-[31 radicals. From these [ values we may infer: (1) the preferred conformation of [15] is III(G-CH3), with the C, 2p, orbital and -OC(O)- part of the p(OC(O)CH,) group eclipsed; (2) the sequence [ 111, [28], and [ 91 exhibits an increased tendency for the p-(OC(O)CH,) group to move from III(G-CH3) toward staggered position IV(G-CH3), to the limit of essentially free motion between III(GCH3) and IV(G-CH3) in [9]; (3) radical [ 131, with substitution only at Cg, shows a slight preference toward III(iXH3); (4) radical [23] shows a strong preference for III(G-CH3); and (5) the (-OH)-group 0 in [28] is in the plane defined by the C of the (-CBH20H) group and the axis of the 2p, orbital at C,. The a, data show the trends expected on the basis of the results for the [2]radical series (3-5, 7). Specifically, we would expect the a, values to follow the order: [15] > [ll] - [23] - [28] > [8] > [13], and we find [15] > [ll] and the rest all less than [ll] but too small to be exactly determinable. The similarity in the dependence on substitution at C, of a?y” values for type-[21 radicals and a, values for type-[41 radicals suggests similar preferred conformations for the two series. The a, data indicate that, in [8], [ll], [15], [23], and [28], the preferred conformation appears to be s-tram about 0,-C, with [ 151 restricted in an eclipsed arrangement of the C, 2p, orbital and the -OC(O)- part of the ,&(OC(O)CH,) group; for the sequence [ 111, [23], and [28], there is an increased tendency toward conformation IV(G-CH3) as (Y substitution decreases, to the limit of free rotation about C-C0 in [ 81, and, for [ 131, with p substitution, the s-cis conformation about the (0,-C,) bond becomes more favored. CH,(X,)OC(O)?(X,)(X,i

[ 51 Radicals

The 6-CH alkoxyl-proton couplings in these radicals are of chief interest here. Although the data for [5] radicals resulting from this present work are limited in number, our findings and the fragmentary results for [5] radicals available in the literature (11, 12, 14, 16) suggest that the degree of substitution at C, does not C(o)oCH2(X~). In a later study, we hope to report more data on significantly affect a&H type-[51 radicals (40) to conclusively determine the factors affecting the size of the 6-CH alkoxyl-proton couplings and the preferred conformations in such species. ACKNOWLEDGMENT The substrate ally1 acetate was kindly provided by Dr. Anton Schindler, Research Triangle Institute, Research Triangle Park, North Carolina.

Camille Dreyfus Laboratory,

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COUPLING 5. 6. 7. X. Y.

10.

II. 12.

13. 14. 15. 16. 17. IR. 19. 20. 21. 22. 23.

24.

25. 26. 27. 28.

29.

30. 31.

IN ACETOXY

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