General and Synthetic Methods: Volume 10 (Specialist Periodical Reports - General and Synthetic Methods) - Hardcover

Hardcover

ISBN 10: 0851869149 ISBN 13: 9780851869148

Publisher: Royal Society of Chemistry, 1988

Synopsis

Reflecting the growing volume of published work in this field, researchers will find this book an invaluable source of information on current methods and applications.

About this title

From the Back Cover

A series of reviews by leading specialists in their fields which gives systematic and comprehensive coverage of the progress in major areas of research.

Excerpt. © Reprinted by permission. All rights reserved.

General and Synthetic Methods Volume 10

A Review of the Literature Published in 1985

By G. Pattenden

The Royal Society of Chemistry

Copyright © 1988 The Royal Society of Chemistry
All rights reserved.

ISBN: 978-0-85186-914-8

Contents

Chapter 1 Saturated and Unsaturated Hydrocarbons By N. Simpkins, 1, 
Chapter 2 Aldehydes and Ketones By K.E.B. Parkes, 32, 
Chapter 3 Carboxylic Acids and Derivatives By D.W. Knight, 75, 
Chapter 4 Alcohols, Halogeno-compounds, and Ethers By L.M. Harwood, 187, 
Chapter 5 Amines, Nitriles, and Other Nitrogen-containing Functional Groups By S.G. Lister, 230, 
Chapter 6 Organometallics in Synthesis By S.G. Davies and T. Gallagher, 320, 
Chapter 7 Saturated Carbocyclic Ring Synthesis By T.V. Lee, 416, 
Chapter 8 Saturated Heterocyclic Ring Synthesis By K. Cooper and P.J. Whittle, 457, 
Chapter 9 Highlights in Total Synthesis of Natural Products By K.E.B. Parkes and G. Pattenden, 550, 
Reviews on General and Synthetic Methods Compiled by K. Carr, D.J. Coveney, and G. Pattenden, 596, 
Author Index, 604, 

CHAPTER 1

Saturated and Unsaturated Hydrocarbons

BY N. SIMPKINS

A catalyst comprising fused iron promoted by V2O5 is extremely efficient in the gas-phase hydrodeoxygenation of ketones and alcohols at relatively low pressures. Reductive d ecyanation of a variety of nitriles can be accomplished very cleanly using potassium metal in combination with a crown ether. The use of ultrasound allows for short reaction times in the reaction of gemdihalogenopropanes with various metals to form the usual carbenoid-derived products. The reduction of C-C multiple bonds has been found to take place in the presence of platinized TiO2 under illuminated conditions. A variety of unsaturated substrates react, although the reaction times are q uite long (ca. 26 h). A useful method for conjugate reduction of α,β-unsaturated ketones and aldehydes involves reaction with diphenylsilane catalysed by Pd0 in combination with ZnCl2. Excellent yields of reduced compounds were obtained using this method, which does not affect α,β-unsaturated nitriles or esters (Scheme 1).

The use of a trialkylaluminium-alkylidene iodide mixture to effect cyclopropanation has been re-examined. The reaction was found to work well when conducted in CH2Cl2, and shows contrasting regioselectivity to the Simmons-Smith reagent in reaction with geraniol (Scheme 2).

A new method which allows enantioselective cyclopropanation of α,β-unsaturated aldehydes employs acetals derived from tartrate esters. The method appears operationally straightforward and gives good yields and enantiomeric excesses (e.e.) (Scheme 3).

2 Olefins

Sodium borohydride can now be used for the reduction of acetylenes, by employing a NaBH4-PdCl2-polyethylene glycol-CH2Cl2 system. A variety of reduced products were obtained including cis-olefins and fully reduced materials.

The use of transition metal catalysts for dehydrogenation of alkanes has received more attention. The iridium complex [(Pri3P)2IrH5] exhibits unusual selectivity for this type of reaction in that methyl groups are attacked preferentially. Similarly, a photolytic dehydrogenation reaction was observed using [IrH2(CF3CO2)(PR3)2], even in the absence of the usual hydrogen acceptor t-butylethylene.

The reductive removal of allylic oxygenated functions can be carried out effectively using nickel boride. Allylic alcohols and their silyl ethers react , although they require much longer reaction times than the corresponding acetates (Scheme 4). Another new deoxygenation procedure constitutes the latest conversion of epoxides into the corresponding olefins, and utilizes arylseleno-carboxamides. The method is stereospecific (retention) although it requires the presence of a strong acid (CF3CO2H) and does not convert more sterically hindered epoxides such as norbornene oxide.

Luche has reported the reaction of carbonyl compounds with allylic halides in aqueous media. The reaction can be performed using either zinc or tin, and displays good chemoselectivity between aldehydes and ketones (Scheme 5).

Asymmetric coupling of aryl Grignards with allylic pivalates is possible in good e.e. by use of NiCl2[(S,S)-chiraphos] in only catalytic (1 mol%) amounts. Another allylic coupling reaction uses palladium to mediate displacement of an acetoxy-group from an allylic geminal diacetate by a sta bilized nucleophile, e.g. Scheme 6. Depending upon the substituents present on the reacting partners, the regioselectivity alters and a variety of products can be prepared.

A number of allylated and related products having quaternary carbon atoms may be prepared by radical chemistry starting from tertiary alcohols.

Allylstannanes have been prepared in a regioselective fashion by a selenoxide elimination route, and also via direct metallation of hydrocarbons. The latter procedure when combined with a protodestannylation step enables isomerization of various terpenes, e.g. Scheme 7.

A number of reports have focused interest on the synthesis of various allylic sulphur compounds. A one-pot procedure for the preparation of allylic sulphides from the corresponding alcohols involves initial rearrangement of the xanthate followed by extrusion of COS (Scheme 8). Allylic sulphides and sulphones are available from the corresponding nitro-compounds. Thus (1) on treatment with NaSPh in HMPA gave the sulphide (2), whereas the sulphone (3) was produced by reaction of (1) with PhSO2Na in DMF in the presence of [Pd(PPh3)4] (Scheme 9). Although the contrasting regioselectivity of the reactions is interesting, the products are perhaps more readily available by other methods [e.g. in the case of (3) by alkylation of the allylic sulphone anion]. Another research group has published similar chemistry starting from vinyl nitro-compounds. Warren et al. have published more chemistry leading to allylic (and also vinylic) sulphides, utilizing both β-hydroxy-sulphides and allylic phosphine oxides. Other applications of the phosphine oxide chemistry to the preparation of allylic products have also appeared. Vinyl sulphides have also been prepared by benzyne-induced fragmentation of 1,3-oxathiolanes and via hydroboration of 1-iodoalkynes (Scheme 10).

Vinyl alkyl selenid es can be prepared from the more readily obtainable vinyl methyl selenides by a d emethylation/alkylation sequence which retains the stereochemistry of the starting materia1. The chemistry of vinylic compounds containing silicon groups have received considerable attention. Acetylenes can be disilylated using a reagent derived from Me3SiLi, MeMgI, and MnCl2. Distannylation can also be achieved. Addition of (trimethylsilyl) trimethylstannane across the triple bond of alk-1-ynes gives products of type (4) in regio- and stereo-specific fashion.

Vinyl nitriles containing silicon groups have been obtained by palladium-catalysed ad dition of TMSCN to acetylenes, and by the addition of HCN to silylated acetylenes mediated by nicke1. The regioselectivity of the copper-catalysed silylzincation of terminal acetylenes described by Oshima can be very effectively controlled by the correct choice of reagent (Scheme 11).

Corey has now published additional details concerning the chemistry of the reagent derived by treatment of methylenetriphenyl-phosphorane with an additional equivalent of alkyl-lithium. The reagent formulated as (7) methylenates even very sterically hindered ketones, and also opens epoxides (Scheme 12). In contrast to this report, Schlosser has provided good evidence for formation of (7) only by halogen-metal exchange of (8), whereas base treatment of methylenetriphenylphosphorane results in ortholithiated species (9) (Scheme 13). This disparity is probably due to differences evident in the reaction conditions used by each group and particularly the temperatures used for the second metallation.

A very direct electrochemical method for the preparation of 1-cycloalkenyltriphenylphosphonium salts has been reported, which uses simple cyclic alkenes and triphenylphosphine as starting materials. Although yields are only moderate, this route should prove the method of choice for preparation of these valuable inter mediates. Salt-free Wittig reaction of 2-oxygenated cyclohexanones exhibits good to excellent Z-selectivity depending on the exact nature of the 2-substituent (Scheme 14).

Amongst the alternatives to phosphorus-based olefination procedures, the use of sulphones remains popular. Thus, fluoromethyl phenyl sulphone has been used to prepare vinyl fluorides via fluoro-α, β-unsaturated sulphones, and an improvement on an earlier methylenation procedure involves alkylation of sulphone anions with R3SnCH2I (Scheme 15). Use of R3SnCH2I rather than its silicon analogue results in a d ramatic increase in the rate of both the alkylation and fragmentation steps. The method was also extended to methylenation of nitriles although somewhat harsher conditions (MeLi, -20 °C) were required for the second fragmentation step as Bu4NF was found to be ineffectual. Sulphones are also used in a new method for the stereoselective preparation of α,β-unsaturated amides. The dianion of the sulphone (10) was sequentially alkylated, and then reacted with NaBH4 to furnish the desired amides (Scheme 16). Reaction of (10) with epoxides was also possible, giving adducts which could be cyclized using KOBut leading to substituted dihydropyrans.

Unsaturated amides and esters are also available by a novel palladium-catalysed carbonylation of enol triflates. This method gave uniformly high yields on a number of steroidal substrates, e.g. Scheme 17.

Wittig and Peterson methodologies have been used for the preparation of α,β-unsaturated thioesters and α-silyl- α,β-unsaturated esters respectively. Selenoxide elimination is well established as a mild method for olefination, selenium usually being introduced into the substrate molecule in its divalent state. The use of phenylselenium trichloride now allows direct introduction of tervalent selenium and enables subsequent conversion into the selenoxide and elimination without the use of an oxidant (Scheme 18). Reduction of the intermediate dichloroselenides to the corresponding selenides was also achieved by reaction with thiourea.

A number of papers have appeared detailing new developments of existing annulation methods which yield cyclic olefins. Danheiser has modified his stereocontrolled [4+1] annulation approach to cyclopentene d erivatives, to accommodate carbon rather than oxygen substitution at C-3 (Scheme 1 9). The key step in the sequence is the carbanion-accelerated vinylcyclopropane-cyclopentene rearrangement which appears quite efficient. Unfortunately the rather poor yields in the initial steps of the sequence and the lengthy nature of the overall procedure detract somewhat from its appeal. Posner has developed a convenient one-pot, three-component construction of cyclohexenes which involves two consecutive Michael additions followed by a ring closure reaction, e.g. Scheme 20.

Fragmentation reactions of cyclic substrates containing silicon or tin provide a useful route into functionalized acyclic olefins. Wilson has developed the CeIV-mediated oxidative fragmentation of γ-hydroxy-silanes which affords fair yields of δ,ε-unsaturated aldehydes or ketones. Cyclic β-stannyl-oximes fragment similarly when treated with lead tetra-acetate, leading to either acyclic or ring-contracted products (Scheme 21). Both this and another study indicate that such fragmentations occur with efficient translation of stereochemistry into the olefinic products. A cyclopropane-opening carbonylation reaction gives good yields of γ,δ-unsaturated carboxylic acid derivatives (Scheme 22). The reaction offers a method of regioselective carboxylation of an allylic alcohol or halide [the precursors to cyclopropanes (11)], but has the disadvantage of using 3-6 equivalents of [Ni(CO)4].

3 Conjugated 1,3-dienes

Taylor has nicely controlled the double carbocupration reaction of organocuprates with acetylenes to provide a general entry to Z,Z-dienes, e.g. Scheme 23. After treatment of the dialkylcuprate with acetylene (initially at -50 °C and then at 0 °C) the reaction can be quenched with a variety of electrophiles (RX, enones, CO2, etc.) to give the Z,Z-products stereospecifically.

Terminal conjugated (E) dienes and trienes are available by SnCl2-mediated reaction of an aldehyde with 1-bromo-3-iodo-propane, e.g. Scheme 24. The procedure is operationally simple, and is chemoselective in that ketones are unreactive. Corey has used the amino-ylide (12) to convert the hindered aldehyde (13) into the Z-homoallylic amine (14). Subsequent Cope elimination then furnished the Z,E diene (Scheme 25).

Cyclic dienes are available by tellurolate 1,4-elimination of 1,4-dibromo-2-enes, and also by a new annulation sequence involving intra molecular Pd-catalysed reaction of an enol triflate with a vinylstannane appendage (Scheme 26).

A variety of polysubstituteq dienes have been synthesized using a very high yielding sequence starting from α,α'-diketo-sulphides (Scheme 27). Even very heavily substituted dienes can be made in this way, the products being formed stereoselectively in certain cases.

Trost has described a new palladium(2+)-catalysed ene-type cyclization which furnishes either 1, 3- or 1 ,4-dienes. This method utilizes enynes such as (15) as starting materials, which themselves are readily available by previous Pd technology (Scheme 28). The method tolerates a variety of other functionality in the molecule, and both the enyne synthesis and subsequent cyclization can be combined in a one-pot procedure if desired.

Cyclization of aromatic enynes such as (16) to give vinyl-phenanthrenes occurs on exposure to metal carbenes, e.g. R1R2=W(CO)5 (Scheme 29). The intermediate metallocenes of type (17) are proposed to undergo highly selective ring opening to give the products of cis stereochemistry. A number of papers report developments in the palladium- or ruthenium-catalysed coupling of various vinylic substrates with a variety of partners to give dienyl products; some examples are outlined in Scheme 30.

A one-pot synthesis of 1,1-bis(methylthio)alka-1, 3-dienes has appeared, and Wallace has outlined some useful stereoselective routes to various functionalized hexa-2, 4-dienals starting from a readily available cyclobutene. Phenylsulphonylmercuration of 1, 3-dienes, followed by base-promoted demercuration, constitutes a regioselective route to dienyl-sulphones (Scheme 31).

4 Non-conjugated Dienes

Allylic sulphones feature in the oxidative dimerization described by Büchi whereby lithio anions derived from allylic sulphones were treated with either iodine or FeCl3-DMF complex, to provide 1,5-dienyl bis-sulphones, e.g. Scheme 32. As can be seen, complementary isomer distributions were observed for each oxidant.

1,5-Disubstituted Z,Z-penta-1,4-dienes (21) were prepared by the sequence shown in Scheme 33. The method is conceptually similar to the sequence described above (Scheme 23) for Z,Z-1, 3-dienes, although the yields and stereoselectivies are somewhat more modest.

Hexa-1,5-dien-3-ols are obtained (albeit with moderate regio-selectivity) from the reaction between allylic epoxides and tri-alkylalkynylborates (Scheme 34). This selectivity complements the behaviour of various other organometallics (M = Li, MgBr, Zn, etc.) which give predominantly products of type (23). The use of allyloxybenzothiazoles as electrophiles in organometallic coupling reactions has recently been extended to reactions involving allylic Grignards as reaction partners. These reactions show high regio-selectivity, which can be controlled to give 1,5-dienes of type (24) or (25) by appropriate choice of reaction conditions (Scheme 35).

Finally, a contribution from the Trost group describes the intramolecular coupling of an allylic acetate with an in situ generated allylstannane, inevitably catalysed by palladium (Scheme 36 ).

5 Allenes

A series of simple allenes has been prepared by a very straightforward multistep procedure, starting from crotonaldehyde (Scheme 37). Another route to such compounds utilizes the reaction Bu3SnLi and ethers derived from β-phenylsulphinyl-β,γ-unsaturated alcohols (Scheme 38).

Allenic ketones are available by a simple and high-yielding sequence starting from the acetal-aldehyde (26), itself readily available from ethyl pyruvate (Scheme 39).

(Continues...)

Excerpted from General and Synthetic Methods Volume 10 by G. Pattenden. Copyright © 1988 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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Edition details

• Publisher: Royal Society of Chemistry
• Publication date: 1988
• Language: English
• ISBN 10: 0851869149
• ISBN 13: 9780851869148
• Binding: Hardcover
• Number of pages: 648
• Editor: G Pattenden