Carbenes benzylic

Interestingly, [Ee(F20-TPP)C(Ph)CO2Et] and [Fe(p2o-TPP)CPh2] can react with cyclohexene, THF, and cumene, leading to C-H insertion products (Table 3) [22]. The carbenoid insertion reactions were found to occur at allylic C-H bond of cyclohexene, benzylic C-H bond of cumene, and ot C-H bond of THF. This is the first example of isolated iron carbene complex to undergo intermolecular carbenoid insertion to saturated C-H bonds. 

A Au mdssbauer study of reaction products of trimeric l-benzyl-2-gold(I)-imidazole leading to Au carbene or Au imidazoline complexes and trinuclear Au " imidazolyl derivatives. X-Ray crystal structure of [ (p-l-benzylimidazolato-N, C )Au 3l2j. Journal of Organometallic Chemistry, 470, 275-283. 

Hindered lithium dialkylamides can generate aryl-substituted carbenes from benzyl halides.162 Reaction of a,a-dichlorotoluene or a,a-dibromotoluene with potassium r-butoxide in the presence of 18-crown-6 generates the corresponding a-halophenylcarbene.163 The relative reactivity data for carbenes generated under these latter conditions suggest that they are free. The potassium cation would be expected to be strongly solvated by the crown ether and it is evidently not involved in the carbene-generating step. 

One such agent is prepared by NBS and peroxide bromination of ethyl 4-chiorophenylacetate (108) to give 109. This is converted by sodium hydride to the benzylic carbene, which is inserted into the double bond of ethyl acrylate to give cis-cyclopropane 110. Partial saponification cleaves the less hindered ester moiety to give 111. This is next converted to the alkoxyimide (112) on reaction with diethyl carbonate and diammonium phosphate. Stronger base (NaOEt)... 

The reaction of A-benzyl-C-imidazolyllithium with [Ag(N03)(SMe)2] affords the corresponding C-imidazolylsilver (Equation (3)), which has been proposed by molecular weight determination to be trimeric.297 A-Functionalized C-imidazolylsilver compounds or carbene-type silver complexes are obtained by reaction of the imidazolium salt with Ag20 or Ag2C03 some of these are shown in Figure 4 298,299... 

Alkenes are scavengers that are able to differentiate between carbenes (cycloaddition) and carbocations (electrophilic addition). The reactions of phenyl-carbene (117) with equimolar mixtures of methanol and alkenes afforded phenylcyclopropanes (120) and benzyl methyl ether (121) as the major products (Scheme 24).51 Electrophilic addition of the benzyl cation (118) to alkenes, leading to 122 and 123 by way of 119, was a minor route (ca. 6%). Isobutene and enol ethers gave similar results. The overall contribution of 118 must be more than 6% as (part of) the ether 121 also originates from 118. Alcohols and enol ethers react with diarylcarbenium ions at about the same rates (ca. 109 M-1 s-1), somewhat faster than alkenes (ca. 108 M-1 s-1).52 By extrapolation, diffusion-controlled rates and indiscriminate reactions are expected for the free (solvated) benzyl cation (118). In support of this notion, the product distributions in Scheme 24 only respond slightly to the nature of the n bond (alkene vs. enol ether). The formation of free benzyl cations from phenylcarbene and methanol is thus estimated to be in the range of 10-15%. However, the major route to the benzyl ether 121, whether by ion-pair collapse or by way of an ylide, cannot be identified. 

Similarly, in the acetoxycarbene series, 87-OAc rearranges to 88-OAc with kc = 8.5 x 106 s-1, 265 times faster than 17-OAc ring expands to 23-OAc (3.2 x 104 s-1).81 These rapid and dominant phenyl carbon 1,2-C shifts of 87 to 88 (bond a), rather than benzyl carbon (bond b) 1,2-C shifts to 89, are attributed to tz orbital mediation by the phenyl group in effect an electrophilic attack of the carbenic carbon on the aromatic ring. Ab initio calculations support this view.114... 

Chloro(phenyl)carbene or carbenoid generated from benzal chloride reacts with potassium salts of benzylic, allylic, and other alkoxides to produce phenyl-substituted oxiranes 9 in high yields, as an approximately 1 1 mixture of cis... 

As shown in the previous two sections, rhodium(n) dimers are superior catalysts for metal carbene C-H insertion reactions. For nitrene C-H insertion reactions, many catalysts found to be effective for carbene transfer are also effective for these reactions. Particularly, Rh2(OAc)4 has demonstrated great effectiveness in the inter- and intramolecular nitrene C-H insertions. The exploration of enantioselective C-H amination using chiral rhodium catalysts has been reported by several groups.225,244,253-255 Hashimoto s dirhodium tetrakis[A-tetrachlorophthaloyl-(A)-/ r/-leuci-nate], Rh2(derived rhodium complex, Rh2(i -BNP)4 48,244 afforded moderate enantiomeric excess for amidation of benzylic C-H bonds with NsN=IPh. 

One report of a secondary /3-deuterium KIE for a carbene insertion reaction has appeared recently. Pascal and Mischke (1991) found that the /3-deuterium KIE for the insertion of dichlorocarbene into the benzylic C—H bond of cumene (reaction (39)) was (kH/kD)p = 1.250 and 1.22 when the KIE was based on GC-MS analyses and H nmr, respectively. 

From Chapter 7 it is apparent that the trichloromethyl anion is formed under basic conditions from chloroform, as a precursor of the carbene. The anion can also react with Jt-deficient alkenes (see Section 7.3) and participate in nucleophilic substitution reactions, e.g. 1,1-diacyloxy compounds are converted into 1,1,1-trichloroalkan-2-ols [58] (Scheme 6.35). Similarly, benzyl bromides are converted into (2-bromoethynyl)arenes via an initial nucleophilic displacement followed by elimination of hydrogen bromide [59] (Scheme 6.35). 

Evident cases of abstraction/recombination mechanism are observed with phenylsubstituted carbenes. Diphenyl-diazomethane, which is photolyzed to give the triplet diphenyl-carbene, very readily abstracts a hydrogen atom from the benzyl group of toluene. The primarily formed radicals can now recombine to give a formal "insertion product — 1,1,2-triphenylethane — or they can recombine to form 1,1,2,2-tetraphenylethane and 1,2-diphenylethane... 

Experimental Procedure 2.2.1. Photolysis of a Chromium Carbene Complex 2-Benzyl-4-benzyloxy-4-methyl-2,3,4,4a,7,7a-hexahydro-li/-cyclopenta[c]pyri-din-3-one [294]... 

In many of the reported preparations of stable carbene complexes from alkyl complexes, alkyl groups without p-hydrogen (e.g. neopentyl, 2,2,2-trifluoroethyl, trimethylsilylmethyl, methyl, benzyl) were chosen in order to avoid p-elimination. There are, however, also examples of moderately stable, non-heteroatom-substituted alkylidene complexes with hydrogen in the -position to the metal (see, e.g.. Figure 3.8). 

The different synthetic applications of acceptor-substituted carbene complexes will be discussed in the following sections. The reactions have been ordered according to their mechanism. Because electrophilic carbene complexes can undergo several different types of reaction, elaborate substrates might be transformed with little chemoselectivity. For instance, the phenylalanine-derived diazoamide shown in Figure 4.5 undergoes simultaneous intramolecular C-H insertion into both benzylic positions, intramolecular cyclopropanation of one phenyl group, and hydride abstraction when treated with rhodium(II) acetate. 

Complexation of [Cp IrCl2]2 with iV-heterocyclic carbenes has led to complexes such as 25, developed by Peris and coworkers [107, 108], and 133, developed by Crabtree and coworkers [12]. Complex 24 is activated by the addition of silver triflate and is effective for the iV-alkylation of amines with alcohols and for the iV-alkylation of anilines with primary amines. Complex 25 has also been shown to couple benzyl alcohol 15 with a range of alcohols, including ethanol 134, to give ether products such as ether 135 (Scheme 31). Complex 133 was an active hydrogen transfer catalyst for the reduction of ketones and imines, using 2-propanol as the hydrogen source. It was also an effective catalyst for the iV-alkylation of amines... 

Chromium(II) sulfate is a versatile reagent for the mild reduction of a variety of bonds. Thus aqueous dimethylformamide solutions of this reagent at room temperature couple benzylic halides, reduce aliphatic monohalides to alkanes, convert vicinal dihalides to olefins, convert geminal halides to carben-oids, reduce acetylenes to /raw5-olefins, and reduce a,j3-unsatu-rated esters, acids, and nitriles to the corresponding saturated derivatives. These conditions also reduce aldehydes to alcohols. The reduction of diethyl fumarate described in this preparation illustrates the mildness of the reaction conditions for the reduction of acetylenes and o ,j8-unsaturated esters, acids, and nitriles. 

When 1,3-dimethylorotic acid (233) was heated at 198 °C in benzyl bromide for 3h, 6-benzyl-1,3-dimethyluracil (236) was formed in 10% yield together with the product of decarboxylation, 1,3-dimethyluracil (235). This finding supports the involvement of a carbon-6-centred nucleophilic intermediate in the decarboxylation reaction a carbanion (234) could be involved or a carbene (237)." ... 

Decarboxylation of 1,3-dimethylorotic acid in the presence of benzyl bromide yields 6-benzyl-1,3-dimethyluracil and presumably involves a C(6) centered nucleophilic intermediate which could nonetheless have either a carbene or ylide structure. Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry has been used to explore the gas-phase reactions of methyl nitrate with anions from active methylene compounds anions of aliphatic ketones and nitriles react by the 5n2 mechanism and Fco reactions yielding N02 ions are also observed nitronate ions are formed on reaction with the carbanions derived from toluenes and methylpyridines. 

Iridium(l) precursors [lr(cod)(L)] with bidentate N-heterocyclic carbene ligands L3 appeared slightly less active in the hydrosilylation of acetophenone with diphe-nylsilane than did the similar rhodium complexes, giving respectively yields of 85% of I and 15% of II for the Pr substituent, and 83% of I and 17% of II for the benzyl moiety, after 2 h reaction at room temperature [47]. However, when carbene ligands of type L3 were used a significant increase in the ee-value of the sec-phenethyl alcohol R isomer of up to 60% was observed. 

In this transformation, manganese(IV) oxide oxidizes allylic or benzylic alcohols to aldehydes followed by nucleophilic attack of the in situ formed triazolinyli-dene carbene (Scheme 41). The authors suggest the formation of an acyl anion equivalent LX is slow in MeOH compared to oxidation to allow for an activated carboxylate LXII. 

Nolan and co-workers have extended the scope of transesterification reactions to include phosphonate esters as phosphorylating agents [137]. In this publication the authors use dimethyl methylphosphonate 273 and benzyl alcohol with a variety of imidazolylidene carbenes (Table 23). The nse of molecnlar sieves to absorb methanol leads to increased conversion however, longer reaction times lead to decreased... 

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