ACCESSORIES INCREASE CAPABILITIES OF THE MODEL 2 0 2 SPECTROPHOTOMETER
By using occessories which Perkin-Elmer has de veloped specifically for its Model 202 UV-VIS Spectrophotometer, you can quickly equip this inexpensive instrument for assignments of an ad vanced nature. Among the auxiliary devices avail able are the following: Repetitive Scan Accessory (illustrated) enables the operator to re-scan any selected segment of an absorption spectrum automatically at regular time intervals-30 seconds, 5 minutes, 60 min·, utes or 10 hours full scale. Single-Beam Readout Accessory permits Model 202 to operate in the single beam mode for in vestigating sources, for flame photometry or for absolute calibration with external sources. The versatility and adjustability of the unit enable it to meet a broad range of experimental conditions. Time Drive Accessory, designed to record absorbance vs time in the Model 202, can be used to follow the kinetics of a chemical reaction at any predetermined wavelength in the normal range of the instrument. Standard motors (easily inter changeable in the laboratory) offer choice of these scan times: 2, 8 or 32 minutes; 1 or 4 hours. Other speeds are available on special order. For full details on Model 202 and its accessories, write to Instrument Division, Perkin-Elmer Corpora tion, 780 Main Avenue, Norwalk, Connecticut.
PERKIN-ELMER 44
C&EN
FEB.
2 5,
1963
Cis Olefins Obtained in Wittig Synthesis Aliphatic aldehydes used as intermediate reactants to prepare olefins by the Wittig reaction; specific halogen ion effect found Aliphatic aldehydes can be used as the carbonyl function in the Wittig method of olefin synthesis. Dr. George B. Butler of the University of Florida, Gainesville, together with Dr. Charles F. Hauser, Dr. Thomas W. Brooks, Marion L. Miles, and Dr. Maurice A. Raymond, has prepared olefins containing ethylene, butadienyl, and isoprenyl linkages. Aliphatic aldehydes were used as intermediate reactants in the Wittig synthesis. The cis isomer predominates as the steric form of the olefin products, and a specific halogen ion effect exists [J. Org. Chem., 28, 372 (1963)]. The mechanism of the Wittig olefin synthesis involves a nucleophilic attack of a phosphorane on the carbonyl car bon of an aldehyde or ketone. This forms a zwitterionic intermediate or betaine. The betaine decomposes, probably via a four-membered cyclic intermediate, to give triphenylphos phine oxide and an olefin. The trans isomer is the expected product in the Wittig reaction, since the path of least steric interference between R groups in the four-mem bered cyclic transition state leads to a trans isomer. But the Florida group finds that the synthesis of
2-octene and 4-octene gives mainly the cis isomer. Under certain condi tions, other workers have also obtained cis products with the Wittig reaction (C&EN, July 23, 1962, page 3 6 ) . In the past, however, the synthesis of olefins by the Wittig reaction has gen erally been observed to give mixtures of cis and trans isomers, with the trans isomer predominating. The syntheses carried out by the Florida workers involve condensing a phosphorane with formaldehyde or with some other aliphatic aldehyde. Gaseous formaldehyde, paraformalde hyde, acetaldehyde, propionaldehyde, n- and isobutyraldehyde, )i-hexaldehyde, 4-pentenal, acrolein, and methacrolein were successively reacted to give various olefins. The reactions are quite rapid, Dr. Butler points out. The Florida group finds that some aldehydes fail to yield the desired products when used as intermediates in the synthesis under normal Wittig reaction conditions. For example, the ylid from 5-hexenyltriphenylphosphonium bromide, when refluxed with acrolein in ethylene glycol dimethyl ether for 24 hours, doesn't yield the expected 1,3,8-nonatriene. Similar ex periments with the ylid from allyltri-
WITTIG REACTION YIELDS OLEFINS - Crt 3 (CH 1 ) 1 CH 1 U__ (C6H5)3-p+_cHR,Ra/-^ Ether
(C(,H5)3T __ C'R.,14,
+ Li/
f M 5 ) j - P = CR,*2
fc,fc.4C = 0 ,
A phosphorane
(M 5 ) 3 P + ->o
(M5)3^CR^2
/3«
ο—Cfc3R4
1^*2.0= C R * R 4
A betaine
Mechanism of the Wittig olefin synthesis involves a nucleophilic attack of a phos phorane on the carbonyl carbon of an aldehyde or ketone. The result is a zwitter ionic intermediate or betaine; the betaine then decomposes, probably via a fourmembered cyclic intermediate (with heat or long standing at room temperature), to triphenylphosphine oxide and an olefin. The phosphorus ylid (phosphorane) is prepared by reacting an ether solution of an aliphatic or aromatic triphenylphosphonium halide (chloride or bromide) with n-butyllithium
0.0
Repetitive scans at 10-minute intervals
0.1 0.2 0.3
o.o r
of the changes produced by the hy
0.1 r0.2l· 0.3 l·0.4 k 0.5 k 0.6 μ
0.4 0.5 0.6 0.7
-
08 0.9 1.0 II
-
12 1.3 14
.,
drolysis of glutethimide in K O H .
0.7 L 0.8 I 0.9 l·
i.o I
1 2O0
L
250
WAVELENGTH (MILLIMICRONS)
Time vs. absorbance analysis of the I.I 1.2 I same hydrolysis, p e r f o r m e d on a 1.3 l· Model 202 with Time Drive. 1.4 f1-5 t "
10 20 TIME (MINUTES)
30
performance-proved instruments for SPECTROSCOPY
GET BIG-INSTRUMENT PERFORMANCE FROM A COMPACT, LOW-COST UV-VIS SPECTROPHOTOMETERTHE P-E MODEL 202 There are three ways to assess the Model 202 UV-VIS Spectrophotometer: (1) as a
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broad range—190 to 390 π\μ in the ultra
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violet and 350 to 750 ντ\μ in the visible.
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A complete range of accessories is avail
ficult differential analyses—a technique of
able. Time drive, repetitive scanning, ac
growing significance in the ultraviolet and
cessory readout, and other approaches to
(3) as an all-around tool, enhanced in
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versatility by a wide range of hard-work
within any laboratory budget.
ing accessories.
For brochure presenting full details a n d
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PERKIN-ELMER r\4éA
phenylphosphonium bromide and. 5hexenal and with the ylid from allyltriphenylphosphonium bromide and nbutyraldehyde also were unsuccessful. Dr. Butler attributes these failures to the pronounced tendency for alde hydes containing one or more alpha hydrogens to undergo aldol (or re lated) reactions in the presence of strong bases. Butyllithium, used in excess to generate the phosphorus ylid, and the ylid itself are both strong bases. Moreover, alpha,beta-unsaturated aldehydes such as acrolein polymerize easily in the presence of strong bases. However, Dr. Butler's group as sumed that a phosphorane could react with an aldehyde in a fast reaction to form a reactive complex: R,R,C-P+(CoH.03 I R ;J R.C-0-
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C&EN
F E B . 2 5, 1 9 6 3
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Immediate quenching, resulting in col lapse of the intermediate, would then give the olefin and triphenylphosphine oxide. Using this approach, the Florida scientists were able to prepare a num ber of olefins. Mild reaction condi tions (about 10° C.) and short re action times (about five minutes) were generally found to be successful. Good Yields. Olefins possessing only one double bond are isolated in good yields, ranging from a high of 82% for 4-octene to a low of 67% for 2-methyl-3-heptene. The yields of polyenes are somewhat lower. A low of 6% is obtained for 1,3,6,8-nonatetraene and 1,3,9,11-dodecatetraene and a high of 52% for 1,3-heptadiene. In all cases, better yields result with a 2:1 ratio of aldehyde to phos phorane, rather than a 1:1 ratio. Four olefins having a single terminal methylene group were prepared from readily accessible organic halides and formaldehyde. The yields range from a high of 7 5 % for styrene to a low of 4 2 % for 1-octene. The use of paraformaldehyde (vs. gaseous formal dehyde) doesn't affect the yields, but offers a simplified experimental pro cedure. A halogen ion effect was noted in the work. Dr. Butler's group observes that the kind of halogen anion (chlo ride or bromide) present in the benzyltriphenylphosphonium compounds— the compounds used to prepare the starting benzylidenetriphenylphosphoranes—appears to play a significant role in the rate with which the phos-
phorane reacts with formaldehyde. I Although sufficient data are not yet | available, Dr. Butler feels that a salt effect (LiCl vs. LiBr) may be operating. Dr. Butler's group has carried out auxiliary experiments which indicate that the phosphorane derived from the chloride salt reacts with formaldehyde 10 times faster than the phosphorane derived from the corresponding bromide salt does. The yield of 1,3-heptadiene appears to depend on a halogen effect. The phosphorane derived from allyltriphenylphosphonium chloride gives the diene in 52% yield. Only a 2 8 % yield is obtained when the phosphorane is derived from allyltriphenylphosphonium bromide. Dr. Butler and his group have also used the Wittig synthesis to add two formaldehyde units to the biphosphoranes:
GOT A
CRACKING PROBLEM DUE TO THERMAL SHOCK ?
( C«H.,)aP=CH( CTUnCH-Pi C«H,)« and ( CoH.-,)aP=CM--CI l=P( CoH.-, h
This gives the corresponding symmetrical dienes. The two di-terminal olefins— 1,6-heptadiene and p-divinylbenzene—were prepared this way in yields of 45% and 42%, respectively. Decomposition. Decomposition of the intermediate species to give an olefin and triphenylphosphine oxide is speeded by adding a quenching solution (water) to the condensation mixture. It is added to the reaction mixture after the reactants have been allowed to undergo condensation for five minutes at 10° C. Clear water and ether layers are obtained from which the olefin can easily be isolated. Reaction times of considerably longer than Rve minutes don't affect the yields of olefins obtained from the aliphatic aldehydes. But long reaction times decrease the yields of olefins derived from acrolein and methacrolein. The presence of mostly cis isomers in the preparation of both 2-octene and 4-octene was shown by comparative gas-liquid chromatography and by infrared. Lower temperatures lessen the predominance of the cis isomer product, Dr. Butler says. The predominance of the cis isomer with 2-methyl-3-heptene and 3-methyl3-heptene was indicated by IR and gas chromatographic data. But, Dr. Butler says, sufficient data aren't yet available to verify conclusively the predominance or exclusive presence of either the cis or trans isomer.
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2 5, 196 3 C & E N
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