Experiment IV-143B-SP 09

Experiment IV-143B-SP 09 - Experiment IV: Multistep...

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Experiment IV: Multistep Convergent Synthesis: Synthesis of Hexaphenylbenzene 1) Synthesis Ph Ph O O Cu(OAc) 2 NH 4 NO 3 HOAc Ph NMe 3 + OH - Ph Ph OH O Ph Ph O CHO Ph Ph O Ph Ph Ph Ph KOH Ph Ph Br Br Ph Ph IV-03 IV-04 IV-01 IV-07 IV-05 IV-06 Thiamine HCl (IV-02) NaOH (aq) nBu 4 N + Br 3 - Tetraphenylcyclopentadieneone Diphenylacetylene IV-08 IV-10 IV-11 IV-09 Linear synthesis I: Tetraphenylcyclopentadieneone Linear synthesis II: Diphenylacetylene
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Ph Ph Ph Ph Ph Ph O Ph Ph O Ph Ph Ph Ph IV-07 IV-11 + + CO (g) IV-12 Synthesis convergence: The synthesis of hexaphenylbenzene represents what is known as a “convergent synthesis”. That is, two independent linear synthetic pathways will be pursued which will ultimately “converge” at the final step. Both linear syntheses can be run simultaneously and either can be initiated first (they do not have to be conducted in the order presented here). The advantage of a convergent synthesis is not overall yield but rather material throughput. The first linear synthesis shown in the scheme above begins with the conversion of benzaldehyde ( IV-01 ) to benzoin ( IV-03 ) which is known as the benzoin condensation. The traditional reaction conditions for such a condensation utilize cyanide as nucleophile. However, due to the hazards involved with the handling of cyanide, a less toxic reagent (thiamine hydrochloride ( IV-02 ) is utilized. The second step is a simple and very mild redox reaction of IV-03 to benzil ( IV-04 ) in which copper (II) is reduced to copper (I)
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and nitrogen gas is evolved. The reaction between IV-04 and carbonyl compound IV-05 in the presence of a base is a double aldol condensation with elimination of two equivalents of water to give enone IV-07 . The second linear synthesis shown above involves the preparation of diphenylacetylene ( IV-11 ). Addition of bromine to trans -stilbene yields the meso dibromide, IV-10 . Rather than using elemental bromine for this step, which is a liquid and difficult to handle, we employ a reagent ( IV-09 ) which is a solid and easier to handle. Double elimination of IV-10 yields the desired product. The final convergent step is a Diels Alder reaction between diphenylacetylene ( IV-11 ) and tetraphenylcyclopentadienone ( IV-07 ) followed by loss of CO to give hexaphenylbenzene ( IV-12 ). 2) Synthetic Procedures 2.1 Synthesis of Benzoin ( IV-03 ) Vitamin B 1 , also known as thiamine pyrophosphate, is a coenzyme universally present in all living systems. It was discovered as a required nutritional factor in humans by its link to beriberi, a disease of the peripheral nervous system caused by a lack of vitamin B 1 , in the diet. This substance catalyses several important biochemical reactions, including the conversion of aldehydes to α -hydroxyketones (acyloins). The related compound thiamine hydrochloride ( IV-02 ) can effect the same transformation in the absence of other biological substances. We will take the advantage of this reactivity to produce benzoin from benzaldehyde. You must understand the mechanism of this transformation. It can be
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This note was uploaded on 01/24/2012 for the course CHEM CHEM143B taught by Professor Ternansky during the Spring '11 term at UCSD.

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Experiment IV-143B-SP 09 - Experiment IV: Multistep...

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