2. MARCO TEORICO CONCEPTUAL
2.1. ANTECEDENTES
2.2.5. FACTORES AMBIENTALES PROTECTORES:
288K. 268K 228K 'W >> V s V ' V V ViL 218K K*t !.*< I. It I,7» L
13C NMR spectra
The 13C spectrum at 308K shows single peaks for each of the alkyl carbons
(O-CH
2-CH
3, O-CH
2-CH
3, S-CH
2-CH
3 andS-CH
2-CH
3).
(There are alsospurious peaks at 14 p.p.m. on this spectrum and the spectra taken at 294K and 268K.) By 268K, the signal for the
O-CH
2-CH
3 carbon (58 p.p.m.)has begun to broaden and by 248K it has split into two well-defined singlets.
Similarly, the signal for the O-CH2-CH3 carbon (13 p.p.m.) shows its
coalescence at 268K. By 248K, it too shows a doublet. In contrast to this, the S-CH2-CH3 stays a sharp singlet all through the range of temperatures.
The
S-CH
2-CH
3 carbon shows the most interesting pattern of all. Inthe spectra taken at 308 and 294K, the signal is a singlet (33 p.p.m.). The signal is a broad peak at 268K yet by 248K it appears as four peaks. This would imply that there may be four conformers “observable” at this temperature. This notion was supported by the evidence of the spectra. iH NM R spectra
Considering first the O-CH2-CH3 signals (4.1 p.p.m.); at 328K, the signal
is a quartet for the “averaged” behaviour of the molecules. This is broadening by 308K and has split into two by 268K. The high frequency signal begins to be resolved by 228K and at 218K, both sets of signals are showing fine structure. It appears at this temperature that the high frequency portion is about to resolve into two quartets. This provides further evidence for the involvement of four separate conformers, consistent with the signals of SCH2.
The S-CH2-CH3 signal (2.3 p.p.m.) at 328K is a quartet of doublets
showing coupling to phosphorus (^Jp_H 1.8Hz). By 288K this quartet is becoming broad and ill-defined. It shows its coalescence temperature at 268K and has split into two by 228K. At 218K, the low-frequency portion shows some fine structure
The S-CH2-C H3 signal (1.1 p.p.m.) is a sharp triplet at 328K and
remains so down to 268K. By 228K it has split into a multiplet which appears to be two superimposed triplets. The O-CH2-C H3 signal (1.0
p.p.m.) appears as a slightly broadened triplet at 328K. By 288K the signal has broadened so far it is almost invisible. By 268K, it appears as two widely spaced humps. These have become well-defined triplets by 228K.
This somewhat complex pattern indicates the involvement of four conformers 258-261 from rotation of both the ylide C-SOEt and the ylide C-COOEt bonds.
O - O E t j P h a P ^ Y ^ O E ,^ Phal-Y^Q. I 2 5 8 2 5 9 p - O E t P h a l - y ^ O E t ^ PhaP^yVo- 2 6 0 2 6 1
The occurrence of up to four separate signals at the lowest temperature is consistent with these each giving separate signals at least for some of the atoms present. Careful consideration of the data leads to the
conclusion that the rotation of the sulphoxide group has a low activation energy and so still occurs rapidly at temperatures where the ester rotation has been “frozen out”. Attempts to quantify the conclusions by calculating energy barriers were complicated by the ambiguity over which signal in the low temperature spectra corresponded to each of the four conformers. Similarly, rough calculations based on the observed coalescence temperatures gave values of AG* in the range 10-15kcal mol-i for most of the processes involved but these could not be assigned to specific interconversion processes. These values do correspond well with that of 13 Skcal mol-i previously obtained by D r y sd a le ^ ^ s for [(4-methylbenzene sulphinyl)ethoxycarbonylmethylene]triphenylphosphorane, where the larger S-Ar group prevents rotation of the sulphoxide (or possibly the rotations are always too fast to observe). Clearly, further detailed study of this problem would be required to fully understand and quantify the processes occurring.
2 FV P
a FVP of ylides of type 222
As already noted, previous p y r o l y s e s^85 of ylides 222 (Ri = H, Me and Et) gave, for aromatic R^, Ph3?=0 , vinyl sulphides 223 and sulphides 224.
For aliphatic R^, the products were Ph3P and 224. Ph3P5s^C02CH2R^ FVP _ r1
T -Ph3P=o If
"r2 -PhsP R2g^ SR2
Formation of the vinyl sulphide 223 had been rationalised by assuming extrusion of Ph3P= 0 to give carbene 262, followed by intramolecular C-H
insertion to give P-lactone 225 (route A). It was already known that such P-lactones can extrude CO2 to give alkenes^^^^ However, a variety of routes
were envisaged for the formation of 224. R^'^O •r^O -Ph^3P= 0 P h j P ^ ^ C O j C H j R ' - E ^ ^ ^ .•f^OL __C 0 225 R' SR^ 223 o''®'r2 266 o 265 [1,2-0 ] 264 o R' " ^ 0 SR^ 267. b R^'^O'^^SR^ rI-^O'^SR^ R^'^SR^ 224 O -C O o -C0 2 -CO -C O - C O : / [1,2-C] O R [1,2-C] R" R‘\ . A q . s r2 269
Comparison with authentic samples had shown that aryl thiopropionate 263 was not present in the pyrolysate of ylide 222 (Ri = Me, = /7-CI-C6H4).
This seems to rule out the potential alternative reaction shown for carbene 262, (route B) which would have given the ketene 264, since aryl thioesters 263 have been shown to pass unchanged through the FVP
was also not present in the pyrolysate of ylide 222 (Ri = Me, R2 = p-Cl-
C6H4) and so it too cannot be formed by pyrolysis of 2 2 2; otherwise at
least a trace of 265 would be in the pyrolysate.
An alternative route to 224 was extrusion of PhaP from 222 to give carbene 266, which might rearrange to alkyl arylthiooxalate 267. This could then successively lose CO and CO2 to give sulphide 224. FVP of
alkyl arylthiooxalate 267 (Ri = Me, R^ = Ph, p-M e-C6H4) at 600°C had
given alkyl arylthiocarbonate 268 and at 750°C had given 268 and sulphide 224 (Ri = Me, R2 = Ph, /?-Me-C6H4). Similarly, FVP of 267 (Ri
= Me, R2 = J2-CI-C6H4 and Ri = H, R2 = Ph) at both 600°C and 750°C had
given 224. The final mechanism to check in this potential route was whether 266 itself would actually give 267. Attempts were made to generate examples of 266 via the corresponding diazo precursors but these were not successful {vide infra).
The problem remained that unidentified carbonyl compounds had been detected in the pyrolysates. The masses of these compounds corresponded with the masses of the isomeric thioesters 263 and 265, yet as stated previously, these would have been detected if they were formed. Early in the course of this work, a crude sample of 269 was prepared. Its NMR spectrum was obtained but the carbonyl signal was different to that of the unknown carbonyl compound in the pyrolysate of 222 (Ri = Me, R2
= /7-CI-C6H4).
- I —
This is some slight evidence against the route C. However the only way of disproving this route would have been to prepare 266 and prove (i) that 266 was formed by FVP of 222 and (ii) that 266 was the source of 224. Further useful evidence would have been obtained from the pyrolysis of 269 and its presumed precursor on route C. Hence a fresh attempt to discover the identity of these mystery carbonyl compounds was needed.
The results obtained for pyrolysis of ylides 222 at 600°C in this work are as follows;
ylide ratio of PhgP: Ph3P=0 :Ph3P=S
% yields of other products
Ph3P c y ,C0 2CH2CH3