Sterically hindered aromatic compounds. VII

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formed by spin trapping of radical intermediates by unreacted 2 (see Terabe and ..... L. R. C. BARCLAY,D. GRILLER, and K. U. INGOLD. J. 21. R. OKAZAKI et a/.

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Sterically hindered aromatic compounds. VII. Deoxygenation of 2,s-di-tert-butyland 2,4,6-tri-tert-butylnitrosobenzene L. Ross C. BARCLAY, PRABHAKER G . KHAZANIE, KENNETH A . H. ADAMS, A N D ELAINE REID Department of Chemistry, Mount Allison University, Sackville, N.B., Canada EOA 3C0 Received February 2, 1977 L. Ross C. BARCLAY, PRABHAKER G. KHAZANIE, KENNETH A. H. ADAMS,and ELAINE REID. Can. J. Chem. 55,3273 (1977). Deoxygenation of 2,5-di-tert-butylnitrosobenzene(1) by triethylphosphite gave a complex mixture containing 2,5-di-tert-butylaniline, 2,5,2',5'-tetra-tert-butylazobenzene, N-(2,5-ditert-butylphenyl)-~ert-butyl-a-2-(5-tert-butypyridyl)nitrone,and diethyl N-(2,5-di-tert-butylphenyl)phosphoramidate. In contrast deoxygenation of 2,4,6-tri-tert-butylnitrosobenzene(2) yielded mainly 3,3-dimethyl-5,7-di-tert-butyl-2,3-dihydrondoe (80%), together with small amounts of 2,4,6-tri-tert-butylanilineand a product of side chain rearrangement, 2-(2-methyl-3propeny1)-4,6-di-tert-butylaniline. Reaction pathways involving nitrenoid intermediates account for the various products. The different types of products from 1and 2 are explained in terms of differential steric and conformational effects.

L. Ross C. BARCLAY, PRABHAKER G. KHAZANIE, KENNETH A. H. ADAMSet ELAINEREID. Can. J. Chem. 55,3273 (1977). La deoxygination du di-tert-butyl-2.5 nitrosobenzene (1) par la triethylphosphite donne un melange complexe de di-tert-butyl-2,5 aniline, tetra-tert-butyl-2,5,2',5' azobenzene, N-(di-tertbutyl-2,5 phCny1)-cr tert-butyl-cr (tert-butyl-5 pyridy1)-2 nitrone et N-(di-tert-butyl-2,5 phenyl) phosphoramidate de diethyle. Par contre, la deoxygenation du trl-tert-butyl-2,4,6 nitrosobenzene (2) conduit principalement au dimethyl-3,3 di-tert-butyl-5,7 dihydro-2,3 indole (80%), avec un faible pourcentage de tri-tert-butyl-2,4,6 aniline contenant un produit de rearrangement de la chaine laterale, le (methyl-2 propenyl-3)-2 di-tert-butyl-4,6 aniline. Les sentiers reactionnels impliquant des intermediaires nitrknoi'de sont responsables des composes divers. Les differents type de produit obtenus B partir de 1 et 2 sont expliques en terme d'environnement sterique et d'effets conformationnels. [Traduit par le journal]

Introduction D e o x y g e n a t i o n o f a r o m a t i c nitro and n i t r o s o c o m p o u n d s b y t e r v a l e n t p h o s p h o r u s reagents results i n r e a c t i o n s indicative o f the presence o f n i t r e n e s as discrete i n t e r m e d i a t e s . The g e n e r a t i o n and r e a c t i o n s o f a r y l n i t r e n e s h a v e been reviewed (1-5). T h e y are o f c o n t i n u i n g i n t e r e s t i n the synthesis of heterocyclic c o m p o u n d s (3, 6-8), and the d e t a i l e d nature o f the i n t e r m e d i a t e s involved in r e a c t i o n s , s u c h as cyclizations o f 2-azidobiphenyl to c a r b a z o l e , is still an interesting q u e s t i o n (9). O r t h o tert-butyl groups are k n o w n to affect the lifetimes of v a r i o u s t y p e s of radicals (10, 11) i n a r o m a t i c c o m p o u n d s . As part of our cont i n u i n g s t u d y of s t e r i c influences on r e a c t i o n s and reactive intermediates, we h a v e examined the steric effect o f one and of two o r t h o tertbutyls on the r e a c t i o n s of a r y l nitrenes d e r i v e d

from 2,5-di-tert-butylnitrosobenzene (1) and 2,4,6-tri-tert-butylnitrosobenzene(2) b y triethyl phosphite deoxygenation. These conveniently

*

exist as the m o n o m e r s and preferentially w i t h the n i t r o s o g r o u p i n the c o p l a n a r a n t i (1) and

Out-of-~lane(2)

(I2).

v0v

@N=O

Results and Discussion (a) D e o x y g e n a t i o n s of I a n d 2. Analysis of Products D e o x y g e n a t i o n of 1 yielded f o u r identifiable products a s illustrated in Scheme 1 and T a b l e 1. The major product, diethyl N-(2,5-di-tertbutylpheny1)phosphoramidate (3) (35%) was identified from its s p e c t r o s c o p i c p r o p e r t i e s and h y d r o l y s i s t o the k n o w n amine (5). The structure o f the r e a r r a n g e m e n t p r o d u c t , t h e p y r i d i n e derivative or n i t r o n e (4) (29%) was d e t e r m i n e d from its analytical and spectroscopic proper-

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1

CAN. J. CHEM. VOL. 55. 1977

P(OC2H,), C , b . 25'C

H 0

11

1

Ar-N-P(OEt), 3

+ Ar-N=C

(Ar C

ties.' The elemental and mass spectroscopic data agreed with the formula C,,H,,N,O. A peak at m/e 407 is assigned to M - CH,, one at mle 406 to M - oxygen, and the base peak at m/e 365 to M - tert-C,H,. The structure assigned to 4 is based mainly on its nmr spectrum, especially the low field portion of the aromatic absorption. In addition to a multiplet at 7.64 6 (5-hydrogens), a weakly coupled peak at 8.70 6 (doublet, J 1. 1 Hz) is attributed to the a-H of a 2,5-disubstituted pyridine ring. Deoxygenation of 2 with triethyl phosphite required elevated temperature and longer reaction times. However, the product mixture was not so complex as in the reduction of 1 (compare Scheme 2 and Table 2), in that reduction of 2 gave a high yield of 3,3-dimethyl-5,7-di-tertbutyl-2,3-dihydroindole (7). The cyclic amine 7 was identified by its spectroscopic properties and by preparing it through hydride reduction of $7-di-tert-butyl-3,3-dimethyloxindole (101, a known photolysis product from 2,4,6-tri-tertbutylnitrobenzene (13). The rearranged unsaturated amine 8 is an unexpected product from the reduction of 7. Its infrared spectrum indicated a primary amine. The structure assigned is based mainly on the nmr spectrum, especially the bands for the re(see arranged side chain, Ar-CH2-C-CH, Experimental). The broad nature of these bands is presumably due to weak allylic coupling.

(6) Proposed Reaction Pathways A pathway (Scheme 3) for deoxygenation of 2,5-di-tert-butylnitrosobenzenethrough the dilUn]ike a aryl nitrone containing an a-methyl substituent (16), nitrone 4 failed to undergo acid-catalyzed hydrolysis. The a-tert-butyl group of 4 evidently results in steric hindrance to nucleophilic attack at the

-I?=/I 0

goup.

\

= 2,5-di-tert-butylphenyl)

TABLE 1. Deoxygenation products of 2,5-di-tert-butylnitrosobenzene (1) by triethyl phosphite Compound

Isolated yieldsa

@Thepercent yields actually isolated from two separate runs by column and tlc. *The yield of 4 was increased t o 70% from a run using direct nmr observations.

polar intermediate 11 (14) should account both for products arising directly from an aryl nitrene (e.g., 3,5, and 6) and also for products of molecular rearrangement and recombination !e.g., the nitrone 4). The amine 5 and the azo compound 6 would form from the aryl nitrene 13 by hydrogen abstraction and dimerization respectively. There is also precedent for the reaction of aryl nitrenes with triethyl phosphite to form a phosphoramidate via an N-aryl phosphorimidate intermediate (15); in our case for the sequence 13 + 14 + 3. Several suggestions have been made for the reaction pathway to account for intermolecular reactions and rearrangements of aryl nitrenes leading to pyridine derivatives (16-19). Phenylnitrene and 2-pyridylcarbene are known to interconvert, at least in the gas phase (19), and one could formulate nitrone formation as simply a combination of a 2-pyridylcarbene with a nitroso compound (i.e., in this case 18 and 1). The exclusive formation of isomer 4 is an interesting example of the directional specificity of the rearrangement. This presumably occurs preferential ring migration of the unsubstituted carbon ortho to the electron deficient nitrogen. This kind of specificity has been observed before in the triethyl phosphite deoxygenation of o-nlethylnitrosobenzene (16), in the photochemical, deoxygenation of aromatic nitro-compounds in triethyl

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BARCLAY ET AL.

TABLE2. Deoxygenation products of

2,4,6-tri-tert-butylnitrosobenzene (2) by triethyl phosphite Compound

Relative yield"

Isolated yieldb

7 8 9

81.3 11.4

79 9 7

7.3

nDetermined by glc methods. bActually isolated b y tlc methods.

phosphite (17), and in the photolysis of orthosubstituted aryl azides in diethylamine (20). The specificity in the present deoxygenationrearrangement of 1 may be explained on the basis of conformational or steric control. The preferred anti conformation for 1 is presumably retained in the more bulky dipolar intermediate (11). We speculate that the preferential bonding at the ortlzo site on the same side of the benzene ring as the leaving group ((EtO),P=O) may be a result of polarization of the ortlzo position by the electrophilic phosphorus in 11. This could be formulated in terms of a cyclic intermediate such as 12 which could preferentially form the azirine (15). Alternatively 15 could be the preferred azirine simply due to steric inhibition of attack of the nitrene centre at the tert-butylated site-2. The rearrangement can be formulated as " proceeding via ring expansion of 15 in the direction indicated leading to 16 then 17 (or 18). Reaction of the aryl nitroso compound with the latter would yield the nitrone 4. This pathway has the advantage of accommodating both the directional specificity and the required C-1 to C-3 shift (17). The major product (7) from deoxygenation of

2,4,6-tri-tert-butylnitrosobenzene(2) resulted from reductive cyclization into an o-tert-butyl group. The marked difference in behaviour of 1 and 2 can be attributed to differential conformational or steric effects. The dipolar intermediate from 2 is expected to retain the out-of-plane conformation (Scheme 4, 19). This would inhibit the coplanar conformation of the charged leaving group which may be preferred for the rearrangement of the aromatic ring, as suggested above for the deoxygenation-rearrangement of 1. Alternatively, and more simply, the different products from 2 could be a result of steric inhibition of attack of the nitrene (20) at both tertbutylated ortlzo positions coupled with steric hindrance to intermolecular reactions, such as dimerization and reaction with triethyl phosphite. As a result, cyclization into a methyl group of an o-tert-butyl leading to 7 in high yield is the predominant reaction. Such cyclizations can be formulated via insertion involving an initial singlet nitrene or through hydrogen abstraction by triplet nitrene followed by radical coupling (2, 4). The latter is the suggested route to account for the cyclization yielding 7 because a radical intermediate2 can also account for the interesting side product (8), where an o-tertbutyl group has undergone skeletal rearrangement (21 -, 22 -, 8) leading to a nonconjugated Z N o ADDED ~ ~ IN PROOF:The esr spectrum measured directly on the deoxygenation reaction of 2 showed a very 10.0 G) with unresolved fine persistent 1 :1 :I triplet (a, structure. This js probably due to an anilino radical of the type Ar-N-OR (Ar = 2,4,6-tri-tert-butylphenyl) formed by spin trapping of radical intermediates by unreacted 2 (see Terabe and Konaka (22)).

=

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CAN. J. CHEM. VOL. 55, 1977

23, although the latter involves a relative unfavourable seven-membered cyclic transition state. It is of interest to note that the nitro compounds, 2,5-di-tert-butylnitrobenzeneand 2,4,6tri-tert-butylnitrobenzene, failed to undergo triethylphosphite deoxygenations, even on prolonged treatment at elevated temperatures. Experimental The nmr spectra were recorded on a Varian A-60 nrnr spectrometer using carbon tetrachloride solvent containing tetramethylsilane as an internal reference. The ir spectra were recorded on a Perkin-Elmer model 467 grating infrared spectrophotometer using potassium bromide pellets for solids and films for liquids. Mass spectra were determined on a Varian MAT-11 I GC/MS system using a direct probe. Gas-liquid chromatography (glc) analysis were determined on a F & M model 500 gas chromatograph equipped with a dual column attachment, using 8 ft x 14 in. columns packed with 20% Carbowax on Chromosorb P and helium flow rate of 60 ml/min at 175'C. Thin layer chromatographic (tlc) preparative separations were carried out on plates (20 x 20 cm x 0.5 mm thick) prepared from Woelm Silica Gel F with 10% calcium sulfate binder added. Melting points were determined on a hot stage equipped with a microscope and are uncorrected. Elemental analyses were determined by Alfred Bernhardt, West Germany.

double bond in the side chain. This result provides an interesting contrast to results of Sundberg (16) who found a selectivity in H-abstracting for ortho alkyl aryl nitrenes derived from o-azidopropylbenzene pyrolysis and from triethyl phosphite reduction of ortho propyl or butylnitrobenzene. The formation of only the nonconjugated alkene in those cases was an argument against any mechanism involving a radical or carbonium ion at the p and y carbon. That particular concerted pathway is not open for the intermediate 20 since it lacks a y carbon. The formation of the nonconjugated double bond in 8 may be a result of kinetic control in elimination of hydrogen in the rearrangement 21 -, 22. Alternatively the second hydrogen atom might be abstracted from they position as illustrated in

( a ) Deoxygenation Procedure The deoxygenations were carried out in dry benzene under a nitrogen atmosphere. The solutions were prepared from 2-3 mmol of the nitroso compound (1 or 2) (21) and excess (20-30 mmol) triethyl phosphite (Aldrich Chemical Co.) in 15-25 ml of benzene. It was convenient t o follow the progress of the reductions by measuring the decrease in the visible absorbance readings at 790 nm for 1 and 750 nm for 2. The reduction of 1 was complete in approximately 11 h at room temperature. The reduction of 2 did not appear t o take place at room temperature but was complete after 20 h at the reflux temperature of benzene. The relative ease of reduction of 1 and 2 was also followed by direct nrnr observations of the aromatic proton region using equimolar solutions of the nitroso compound and triethyl phosphite in carbon tetrachloride. Under these conditions, the reduction of 1 was complete in 6 h. In contrast, the reduction of 2 was only 3 complete after 1 week. The reaction mixtures were worked up by first evaporating the solvent and excess triethyl phosphite under reduced pressure. The residues were dissolved in ethyl ether, washed with water, the ether extract dried over anhydrous calcium chloride, and the ether evaporated. The crudes obtained were analyzed by chromatographic methods as described below. ( b ) Product Analysis ( I ) Deoxygenation Products from 2,5-Di-tertbutylnitrosobenzene ( I ) The crude product from reduction of 1 (0.657 g) was

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BARCLAY ET A L .

I

H-C-H

Abstraction

7

20

Cyclization 4

/ I

( I ) Rearrangement (2) -H.

found to be a complex mixture by tlc analysis. It was dissolved in petroleum ether (bp 30-60°C) and separated into various fractions by column chromatography on a 2.5 x 28 cm column of neutral alumina as outlined in Table 3. Pure compounds were isolated by further separation of the column chromatograph fractions by means of preparative tlc. 2,5,2',5'-Tetra-tert-butylazobenzene(6)-Theazocompound (6) was purified by preparative tlc using cyclohexane solvent. The compound separated as a narrow orange-red band by rapid elution near the solvent front. Sublimation in vacuo gave orange crystals, mp 227°C. The nmr spectrum showed a singlet at 1.38 (18H, 2 tertbutyls) and 1.64 (18H, 2 tert-butyls) and multiplets at 7.61 (4H, aryl) and 7.94 6 (2H, aryl). The mass spectrum showed a molecular ion peak at mle 406 expected. Anal. calcd. for C28H42N2:C 82.70, H 10.41, N 6.89; found: C 82.59, H 10.30, N 6.95. N- (2,5-Di-lert-bulylphenyl) -a-tert-butyl-a-2-(5-terfbutybyridy1)nitrone (4)-Fractions 4-6 from the column chromatography (Table 3) yielded 305 mg of crystalline material. This material was chromatographed on five preparative tlc plates using benzene as the developer. There resulted two well-separated bands. The first band

Rearrangement

(near the solvent front) yielded, on extraction with ethyl ether, 197 mg (31%) of near colorless crystalline material identified as the nitrone (4). The analytical sample was obtained by sublimation in vacuo and gave a mp 189°C. The nmr spectrum showed four singlets of equal intensity at 1.39, 1.41, 1.47, and 1.536 (36H, three aryl tert-butyls

1

I

a multiplet at and tert-butyl in (CH3),C-C=N-0), 7.53-7.78 (5H, three hydrogens on benzenoid ring plus hydrogens 3,4 of pyridine ring), and a doublet at 8.70 ( J N 1 Hz, l H , a-pyridyl hydrogen). The mass sDectrum showed a molecular ion peak at mle 422 and an accurate mass measurement indicated the formula C28H42N20 (calcd. mass 422.3297; found 422.3288). Anal. calcd.: C 79.59, H 10.02, N 6.63; found: C 79.62, 79.47, H 9.96, 9.86, N 6.63, 6.56. The nitrone (4) was recovered unreacted from an attempted hydrolysis by treatment in boiling ethanol hydrochloric acid for 2 days. The second band from the above tlc separation yielded 66 mg (11%) of 2,5-di-tert-butylaniline identified by a mixture melting point and comparison of its nmr spectrum with an authentic sample. Diethyl N- (2,5- Di- tert - butylpheny1)phosphoramidate

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CAN. J. CHEM. VOL. 55, 1977

TABLE3. Column chromatography of deoxygenation products from 2,5-di-tert-butylnitrosobenzene(I)

Fraction

Solvent (ml)"

Yield (mg)

Identity of productsb

1

.PE (20)

44

Di-tert-butylazobenzene (6)

2, 3

PE (20)

19

Unidentified mixture

4-6

PE-bnz (80)

7-8

bnz (80)

9-14

bnz (300)

305 7 31 1

Di-tert-butylaniline (5), nitrone (4) Unidentified polymeric Phosphoramidate (3), traces of TEP

OPE = petroleum ether (bp 3&6OnC), bnz = benzene. bProducts were further separated and purified by preparative tlc.

(3)-Fractions 9-14 from the column chromatography (Table 3) contained 311 mg (31%) of the crude phosphoramidate (3). It was separated from traces of a colored (violet) impurity by preparative tlc on four plates using 20% ethyl ether in benzene as developer. The ester 3 developed very slowly and was readily separated in this way. It was isolated as a colorless, thick, oily material. The nmr spectrum showed a singlet at 1.32 superimposed on a triplet between 1.47-1.22 ( J in Hz) (15H, a tert-butyl and two CH3-Qf the CH,CH,O- groups), a singlet at 1.50 (9H, a tert-butyl), a multiplet at 3.9-4.5 (4H, J = 7 Hz, two -CH2-s of CH,CH,O- groups), a doublet,

I

exchangeable with D,O at 5.28 ( l H , -N-H), and an abc type of multiplet at 7.2-7.7 6 (3H, Jab= 9 Hz, Jbc= 2 HZ, aryl). The ir spectrum showed a band at 3480 cm-' attributed to N-H. A strong band at 1250 cm-' is assigned to P=O and very strong absorption at 970-1060 to P-0-ethyl. The mass spectrum showed a molecular ion peak at mle 341 as expected. Anal. calcd. for ClsH3,N P 0 3 : C 63.32, H 9.45, N 4.10, P 9.07; found: C 63.13, H 9.44, N 4.15, P. 8.99. A sample (39 mg) of the phosphoramidate (3) was hydrolyzed by refluxing in a mixture of ethanol (10 ml) and concentrated hydrochloric acid (5 ml) for 2 days. The alcohol was distilled, the solution neutralized with sodium carbonate and the product extracted with ether. There was obtained 19 mg (81%) of 2,5-di-tert-butylaniline, identified by its nmr spectrum and melting point. The deoxygenation of 1 with triethylphosphite was repeated in order to test the reproducibility of the rather complex reaction. The work-up and separation of products by a combination of column chromatography and tlc was carried out in a similar manner to the first run. The yields of the main products from this second run were azo compound 6 3%, nitrone 4 29%, amine 5 13%, and the phosphoramidate 3 35%. These results are in reasonably good agreement with those described above. When an equimolar mixture of 1 and triethylphosphite was used as described in the direct nmr observation, the yield of 4 (tlc) was increased to 70%. (2) Deoxygenation Products from 2,4,6-Tri-tertbutylnitrosobenzene (2) The crude product from reduction of 2 (0.550 g) was

separated into three main bands by preparative tlc using benzene as developer. The first band gave 37 mg (6.9%) of 2,4,6-tri-tert-butylaniline (9) identified by its ir and nmr spectra. The second band yielded the major product, a colorless solid material (482 mg, 90%) which was shown by the nmr spectrum to be a mixture of the two amines 7 (mainly) and 8 described below. This mixture was separated on preparative tlc plates prepared from acidtreated silica gel. For this purpose, the plates were prepared from a slurry of the silica gel and binder in 1: 1 ethanol-acetone containing 5% concentrated hydrochloric acid. The minor component (8) (48 mg, 9%) of this mixture eluted first with benzene followed by the major reaction product (7,424 mg, 79%). This deoxygenation of nitroso compound 2 was repeated in order to determine more precisely the percent composition of the products by glc. The compounds eluted from the column in the order 7 , 9 , and 8 and in the relative yields of 81.3%, 7.3%, and 11.4% respectively. 3,3-Dimethyl-5,7-di-tert-butyl-2,3-dihydroidol (7)The major product 7 from the deoxygenation of 2 was sublimed in vacuo for analysis. It was a colorless crystalline compound, mp 105-106°C. The nmr and ir spectra established this compound to be identical to 7 obtained earlier (13) from hydride reduction of 5,7-di-tert-butyl3,3-dimethyloxindole. It gave a parent mass at mle 259 as expected. Anal. calcd. for C18H29N: C 83.33, H 11.26, N 5.40; found: C 83.52, 83.48, H 11.27, 11.25, N 5.20, 5.16. 2- (2-Methyl-3-propyl) -4,6-di-tert-butylaniline (8)The amine 8 from the deoxygenation was isolated as an oil and distilled in uacuo for analysis. The nmr spectrum showed sharp singlets at 1.32 and 1.47 (9H each, tert-

I

butyls), a broad singlet at 1.78 (3H, methyl of CH,=CCH,), a broad singlet at 3.37 (2H, benzylic methylene of CH2

II

Ar-CH,-C-CH,),

two broad singlets at 4.92-5.00

(2H, nonequivalent olefinic Hi of 'c=cH,), a singlet at / 3.80 (2H exchangeable, -NH,), and doublets at 7.11 and 7.37 6 ( J 3 Hz) ( I H each, meta aryl hydrogens). The ir spectrum showed bands at 3520 and 3420 cm-' attributed to N-H stretching absorption of a primary amine and a band at 3090 cm-' to the C-H stretching

--

\

absorption of the 'C=CH, group. The mass spectrum / showed a parent'mass at m/e 259 as expected. Anal. calcd. for Cl8HZgN: C 83.01, H 11.22, N 5.38; found: C 83.21, H 11.09, N 5.53.

Acknowledgments The authors are grateful to the National Research Council of Canada for financial support of this research. We thank Donald Embree of the National Research Council, Atlantic Regional Laboratory, for the accurate mass determination. 1. J. I. G. CADOCAN and R. K . MACKIE.Chem. Soc. Rev. 3,86 (1974). 2. R. A. ABRAMOVITCH. Organic reactive intermediates.

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BARCLAY ET AL.

Edited by S. P. McManus. Academic Press. 1973. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12.

Chapt. 3. ACC.Chem. Res. 5, 303 (1972). J. I. G. CADOGAN. W. LWOWSKI.Nitrenes. J. Wiley and Sons, New York. 1970. R. A. ABRAMOVITCH and B. A. DAVIS.Chem. Rev. 64, 149 (1964). J. I. G. CADOGAN, D. S. B. GRACE,P. K . K.LIM,and B. S. TAIT.J. Chem. Soc. P e r k ~ nTrans. I, 2376 (1975). D . N. JOHNSTON, and P. V. R. SHANA. H. JACKSON, NON.J. Chem. Soc. Chem. Commun. 911 (1975). R. N. CARDEand G. JONES.J . Chem. Soc. Perkin Trans. I , 510 (1975). D . W. GILLESPIE,and B. A. DER. J. SUNDBERG, FRAFF.J . Am. Chem. Soc. 97,6193 (1975). L . R. C. BARCLAY,D. GRILLER, and K. U. INGOLD. J. Am. Chem. Soc. 96,3011 (1974). and K . U. INGOLD. J. D. GRILLER, L . R. C. BARCLAY, Am. Chem. Soc. 97,6151 (1975). R. OKAZAKI and N. INAMOTO. J Chem. Soc. B, 1583 (1970).

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13. L. R. C. BARCLAY and I. T. MCMASTER.Can. J. Chem. 49,676 (1971). 14. R. J. SUNDBERG and R. H. SMITH.J. Org. Chem. 36, 295 (1971). D. J. SEARS,D . M. SMITH,and M. 15. J. I. G. CADOGAN, J. TODD.J . Chem. Soc. C, 2813 (1969). J . Am. Chem. Soc. 88,3781 (1966). 16. R. J. SUNDBERG. 17. R. J. SUNDBERG, B . P. DAS,and R. H.SMITH.J. Am. Chem. Soc. 91,658 (1969). 18. J. I. G. CADOGAN. Q. Rev. 22,222 (1968). 19. C. WENTRUP.Tetrahedron, 30, 1301 (1974); C. THBTAZand C. WENTRUP.J . Am. Chem. Soc. 98, 1258 (1976). 20. R. J. SUNDBERG, S. R. SUTER,and M. BRENNER. J. Am. Chem. Soc. 94,513 (1972). 21. R. OKAZAKIe t a / . Bull. Chem. Soc. Jpn. 42, 3559 (1969); 42,361 1 (1969). 22. S. TERABEand R. KONAKA.J. Chem. Soc. Perkin Trans. 11,369 (1973).

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