Diene complexes, deprotonation

The kinetic product of deprotonation of (5) is the cis isomer (2) however, (2) is readily isomerized to the thermodynamically more stable trans isomer (3) under both acidic and basic conditions. Thus acylation reactions can often lead to a mixture of isomers and product ratios are dependent on the precise quenching conditions. Convincing evidence for endo addition of [RCO]+ to irort-diene complexes first... 

Intramolecular nucleophilic attack has been demonstrated in several cases, particularly with amine and (rj -allyl)Fp complexes an example of the former is depicted (equation 20). In such cases, the driving force for formation of five- or six-membered rings overrides the preference for attack at the more substituted alkene center. The ability of trialky-lamines to convert (> -aIkene)Fp+ complexes into (rj -allyl)Fp complexes via deprotonation allows conversion of alkadiene complexes (see Diene Complexes) (96) into carbocycles (97,98). The reasons for the stereochemical outcome of these cyclization reactions have not been fully delineated. Finally, ( ) -butadiene)Fp+ cations react in two sequential ( ) -allyl)Fp-( -alkene)Fp+ pairs to give a mixture of cyclic... 

Highly enantioselective alkylations a to acyclic diene complexes have been developed. Deprotonation of (91) with LDA to form an ester enolate, followed by reaction with iodomethane, gives the alkylated prodnct (92) in excellent yield with 82% ee (Scheme 153). Stereospecific remote alkylation was used in a synthesis toward macrolactin A (Scheme 154). In the synthetic seqnence, the primary... 

The complexes [PtR(OPh)(dmpe)] (R = Me, Et) have been prepared,566 and the X-ray structure of cis-[PtMe(OR)(PMe3)2].HOR [R = CH(CF3)2] in which the alcohol is H-bonded to the alkoxide ligand, has been determined.567 The transfer of methoxide from [PtMe(OMe)(dppe)] to a cationic molybdenum diene complex has been reported.568 The ammine complexes t/-a/w-[PtMe(NH3)L2]+ (L = phosphine) can be deprotonated to [LPtMe( i.-NH2)]2. which then react with L to form the... 

Benzo[c]furans (isobenzofurans) are reactive molecules usually employed as reactive dienes in the synthesis of more complex molecules. In the synthesis of spiro compounds related to fredericamycin A, Kumar generated the trimethylsiloxytrimethylsilylbenzo[c]furan 125 from phthalide via two consecutive deprotonations and silylations of the resulting anions. Diels-Alder reaction of the isobenzofuran as shown below with a spiroenedione leads to the formation of an endo-exo mixtures that can be smoothly converted to the dihydroxydione <00IJC(B)738>. 

An electron-rich metal can deprotonate the dicarbonyl derivative, affording the hydridopalladium intermediate 23, which can undergo a Tr-allyl 24 formation through diene insertion (which can be assimilated to a hydridopalladation of olefin) (Scheme 7). The attack of the enolate to the -jr-allyl species occurs with good enantioselectivity in the presence of the chiral ligand. The final product 21 is released and the palladium(O) complex 22 is regenerated. 

Diene)Mn(CO)3 anions are also prepared by deprotonation of the ( j3-allyl-/r-hydride) Mn(CO)3 complexes (25 MLn = Mn(CO)3) with potassium hydride23 90a c. However, since complexes 25 MLn = Mn(CO)3 are generally prepared by protonation of the corresponding anions, this method is mostly of regenerative value. 

A number of cationic (diene)metal complexes undergo a-deprotonation. Treatment of (cyclodiene)Mo(CO)2Cp+ cations (e.g. 44) with NEt3 or other non-nucleophilic bases... 

Standard organolithium reagents such as butyllithium, ec-butyllithium or tert-butyllithium deprotonate rapidly, if not instantaneously, the relatively acidic hydrocarbons of the 1,4-diene, diaryhnethane, triarylmethane, fluorene, indene and cyclopentadiene families and all terminal acetylenes (1-alkynes) as well. Butyllithium alone is ineffective toward toluene but its coordination complex with A/ ,A/ ,iV, iV-tetramethylethylenediamine does produce benzyllithium in high yield when heated to 80 To introduce metal into less reactive hydrocarbons one has either to rely on neighboring group-assistance or to employ so-called superbases. 

One of the simplest methods for preparation of an a,/J-unsaturated acyl silane is by hydrolysis of a 1-alkoxy-l-trimethylsilylbutadiene, the conjugated dienol ether of a,fi-unsaturated acyl silane, prepared by deprotonation and alkylation of the 1-alkoxydiene (Scheme 42)11. This method is generally limited in application to simple substrates, presumably due to the complexity of preparation of more highly functionalized 1-alkoxy dienes. 

The Co reagent 192, prepared by the reaction of Co2(CO)8 with sodium, is reactive, and the acylcobalt complex 193 is formed by the reaction of acyl halides. Insertion of butadiene at the Co-acyl bond generates the 7r-allylcobalt complex 194, from which the acylbutadiene 195 is formed by deprotonation with a base [82]. Based on this reaction, various acyldienes are prepared by Co2(CO)8-catalysed reaction of active alkyl halides, conjugated dienes and CO. The Co-catalysed reaction can be carried out smoothly under phase-transfer conditions. For example, 6-phenyl-3,5-hexadien-2-one (197) was prepared in 86% yield by the reaction of Mel, 1-phenylbutadiene (196) and CO in the presence of cetyltrimethylammonium bromide [83]. 

Fu and co-workers have shown that the l-chloro-boracyclohexa-2,4-diene 97 reacts with 4-phenylpyridine at room temperature to provide the />-terphenyl analog 26 in 76% yield as an early example of a borabenzene-pyridine-type complex (Equation 6). The X-ray structure of 26, discussed in Section 7.14.3, shows the three rings not to be coplanar <1997OM1501>. Exposure of the l-chloro-boracyclohexa-2,4-diene 98 to a stabilized carbene results in formation of the complex 29 in 83% yield as an air- and moisture-sensitive colorless powder (Equation 7), crystals of which were grown from toluene the X-ray structure of 29 has been discussed in Section 7.14.3. The use of bulky lithium bases to deprotonate the bis(amido) or bis(alkoxy)boranes 19 allows for the formation of the dianionic 2,2 -diboratabiphenyl derivatives 20 (X = NHPh, OBn), the solid-state structures of which have been discussed (Equation 8) <2006CJC81>. 

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