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Organic Seminar Abstracts Digitized by the Internet Archive in 2012 with funding from University of Illinois Urbana-Champaign http://archive.org/details/organicsemin1971752univ / a ORGANIC SEMINAR ABSTRACTS 1971-1972 Semester II School of Chemical Sciences Department of Chemistry- University of Illinois Urbana, Illinois 3 SEMINAR TOPICS II Semester 1971-72 Reactions of Alkyl Ethers Involving n- Complexes on a Reaction Pathway 125 Jerome T. Adams New Syntheses of Helicenes 127 Alan Morrice Recent Advances in Drug Detection and Analysis I36 Ronald J. Convers The Structure of Carbonium Ions with Multiple Neighboring Groups 138 William J. Work Recent Reactions of the DMSO-DCC Reagent ll+O James A. Franz Nucleophilic Acylation 1^2 Janet Ollinger The Chemistry of Camptothecin lU^ Dale Pigott Stereoselective Syntheses and Absolute Configuration of Cecropia Juvenile Horomone 1U6 John C. Greenfield Uleine and Related Aspidosperma Alkaloids 155 Glen Tolman Strategies in Oligonucleotide Synthesis 162 Graham Walker Stable Carbocations: C-13 Nuclear Magnetic Resonance Studies 16U Moses W. McMillan Organic Chlorosulfinates 166 Steven W. Moje Recent Advances in the Chemistry of Penicillins and Cephalosporins 168 Ronald Stroshane Cerium (iv) Oxidations 175 William C. Christopfel A New Total Synthesis of Vitamin D 18 William Graham Ketone Transpositions 190 Ann L. Crumrine - 125 - REACTIONS OF ALKYL ETHERS INVOLVING n-COMPLEXES ON A REACTION PATHWAY Reported by Jerome T. Adams February 2k 1972 The n-complex (l) has been described as an intermediate on the reaction pathway for electrophilic aromatic substitution and acid catalyzed rearrange- ment of alkyl aryl ethers along with sigma (2) and pi (3) type intermediates. 1 ' 2 +xR Physical evidence for the existence of n-complexes of alkyl aryl ethers was found in the observation of methyl phenyl ononium ions by nmr 3 and ir observation of n-complexes of anisole with phenol. 4 In an effort to understand the rearrangement of alkyl aryl ethers, product studies have been used to investigate the mechanism. Two mechanistic pathways have been suggested, one involving intramolecular transfer of an alkyl group and the other intermolecular transfer. 1 ' 2 Evidence in support of the intramolecular transfer is (a.) the high ortho yield observed in the rearrangement of benzyl phenyl ether, 5 Gc-phenethyl phenyl ether, and sec-butyl phenyl ether,"7 which give respective yields of 100$, 7, and 92$ of the rearranged products, and (b) the partial retention of configuration observed in the rearrangement of optically active sec -butyl 2 ' 8 6 phenyl ether with and a-phenethyl tolyl ether with A1B ? . Evidence for intermolecular reaction is (a) the rearrangement of optically active sec -butyl phenyl ether with AlBr 3 to products with inversion of configuration8 and (b) the observation of alkyl side chain rearrangement in the products of the reaction of 2-, 3-, and 4-octyl phenyl ethers with 9 7 BF 3 . The reactions of 2-, j- , and k- octyl phenyl ethers result in product distributions of similar character with respect to both side-chain positional rearrangement and o:p ratios which are in the range of 2.2-3.0 with the ortho showing greater side-chain rearrangement. Alkylation of alkyl aryl ethers has been compared to the acid catalyzed rearrangement of alkyl aryl ethers and in the case of anisole, the n-complex has been used to explain the relatively high o:p ratio observed in the 10 alkylation of anisole with A1C1-,- methyl chloride in nitromethane. Evidence for the intermediacy of an n-complex in an electrophilic aromatic substitution is the isotope incorporation in the product and the starting material in the alkylation of anisole with methyl chloroformate-d ? and 3 -1 AgSbF6 in chlorobenzene. Scheme 1 is consistant with the formation of an n-complex (k) , and although an n-complex is involved in the mechanism a normal o:p ratio of 35:60 was observed. Recovered anisole contained 13$ anisole-0-methyl-d 3 from dealkylation of h, and methyl chloroformate-do recovered showed no incorporation of a protio-methyl. In further experiments with methyl chlor oformate-d.^, anisole, and AgSbF6 designed to investigate methyl-d 3 incorporation in the ortho and para products, the ratios of isotope incorporation for the ortho and para isomers were the same within the present limits of experimental error (Table l). 12 This suggests that the ortho and para products are derived from similar processes. If the para substituted 2 products come from intermolecular reaction as has been suggested, ' then it is likely in this case that the ortho products also come from intermolecular reaction. - 126 - Table 1 Percent of Methyl Anisoles 6 7 8 ortho 52.7 33-0 9.7 5-2 para 4-y.b 3^.0;>4.u y.q- 7.0 This recent work is an example of an electrophilic aromatic substitution with n-complex formation that seems to involve neither high ortho yields nor intramolecular reaction as a major reaction pathway. In this light it seems as though the previous criterion for the formation of n-complexes on the reaction pathway of electrophilic aromatic substitutions is not sufficient, -CD. Scheme 1 CD. + CH 3 CD3OCOCI k, ^> BIBLIOGRAPHY 1. H. J. Shine, "Aromatic Rearrangements", Elsevier Publishing Co., New York, N. Y., 1967* PP 82-89 and references cited therein. 2. a) M. J. S. Dewar, "Molecular Rearrangements", P. deMayo Ed., John Wiley and Sons, Inc., New York, N. Y., 1963* PP 313-318 and references cited therein; b) idem, p 31^- 3. W. P. Meyer, University of Illinois Seminar, p 262, Summer 1971 and references cited therein; C. M. Brouwer, E. L. Mackor, and C. Maclean, Rec. Trav. Chem. Pays-Bas, 85, 109, 11^ (1966). k. B. B. Wayland and R. S. Drago, J. Amer. Chem. Soc, Qk, 3817 (1962). 5. C. S. Tarbell and J. C. Petropoulos, ibid., jk, 2^"[l952). 6. H. Hart and R. J. Elia, 76, 3031 (195*0- 7. M. J. S. Dewar and N. A. Puttnam, J. Chem. Soc, ^090 (1959). 8. P. A. Spanninger and J. L. von Rosenberg, Chem. Commun., 795 (1970). 9. H. R. Sonawane, M. S. Wadia, and B. C. Subba Rao, Indian J. Chem., 6, 19^ (1968). 10. P. Kovacic and J. J. Hiller, Jr., J. Org. Chem., 30, 1581 (1965). 11. D. A. Simpson, Ph. D. Thesis, University of Illinois, Urbana, 111., 1970; D. A. Simpson, S. G. Smith, and P. Beak, J. Amer. Chem. Soc, 92, 1071 (1970). 12. P. Beak and J. T. Adams, unpublished data. - 127 - NEW SYNTHESES OF HELICENES Reported by Alan Morrice February 28, 1972 INTRODUCTION Newman introduced the name "helicenes" to denote the series of ortho- condensed aromatic hydrocarbons where the benzene rings are angularly annelated in such a manner as to give a helically shaped molecule. The prefixes penta, r hexa, hepta are used to denote the ), 6, 7 ring compounds, respectively, and a shorthand way of representing this feature is, for example, [6]helicene for hexahelicene (l). Wynberg has introduced the term "heterohelicenes" in referring to those helicenes where one or more of the benzene rings have been replaced by heterocyclic rings. As examples of inherently dissymmetric OIQ chromophores these compounds are of great interest to chemists since their optical properties can O provide the basis for comparison between different theoretical treatments. We will consider primarily the syntheses that have been developed which utilize OIQ a photoinduced cyclodehydrogenation of an arylolefin to give the helicene framework; however, more Hexahelicene 1 classical methods of synthesis will also be considered. HELICENES Although pentahelicene was synthesized as early as 1938? 1 stereochemical appreciation of the helicenes can be said to date from Newman's classical synthesis of hexahelicene in 1956. 2 The long sequences involved, however, did not lend themselves easily to the synthesis of the higher homologs. Wood and Mallory3 showed that the well known photocyclization of cis- stilbenes to dihydrophenanthrenes could be used to prepare the fully aromatic phenanthrene if carried out in the presence of an oxidant, e_. g. , iodine. R. H. Martin and his colleagues extended this procedure to the synthesis of helicenes by the cyclization of appropriate arylethylenes. Acylation of phenanthrene 4 with 2 under Friedel- Crafts conditions gave the ketone _3 and presumably the 2- isomer as well, 5 although no mention was made of the latter. Reduction (LiAlH4 ) followed by dehydration (HCOgH) gave the diphenanthrylethylene k (Scheme l), without statement of the yields. Scheme 1 CH 2CO CH=CH CH2C0C1 6 A much more general approach is shown in Scheme 2. The phosphonium salt 8 could be condensed with benzaldehyde, 2-naphthaldehyde, and 3- phenanthraldehyde to give, each in good yield, the arylalkenes 9, 10, and 11, respectively. Alternatively, advantage could be taken of some interesting work recently reported by Bestmann and co-workers. 7 Treatment of compound 8 with sodamide in liquid ammonia gave the corresponding alkylidenephosphorane - 128 - Scheme 2 ( + © CH2PPh 3Br (a) R = CH3 (b) R = C0 2 CH3 H" H (a) 1. NBS Of lQ) (b) 1. LiAlH4 2. PPh3 2. PBr3 3- PPh3 ©0 ^ CH PPh Br 2 3 s 11 12 which, in the presence of molecular oxygen, underwent self condensation to the alkene 12. The formal mechanism is believed to involve initially oxidation of the ylid to the aldehyde which then condenses with another molecule of the ylid to give the alkene in a normal Wittig reaction. Diarylolefins could be synthesized either via a double Wittig reaction8 or by a Siegrist 9 ' 10 reaction (Scheme ~y). This latter reaction proceeded in good yield (89/ ) and could be extended to the synthesis of heterocyclic stilbenes. The cyclizations were carried out using the c is/trans mixtures obtained from the above reaction schemes in benzene solution containing a few crystals of iodine.
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