Alkenes methoxycarbonylation

The versatile functionality pattern of bicyclic MO-acetal-y-lactams (found in conjunction with their ir-face-selective alkylation) can also be applied to Diels-Alder reactions of the corresponding alkenic methoxycarbonyl-activated derivative (427) (Scheme 102). ° Noncatalyzed addition of 2,3-dimethylbu-tadiene to dienophile (427) (60 C, 8 h) proceeded exclusively from the ir-face opposite the isopropyl substituent. The reactivities of the latent immonium and carbonyl groups in adduct (428) were exploit during transformation into [l,3,4)propellane (434). 

Methoxycarbonyl and acetyl derivatives Relative rates of decomposition into cyclopropanes and alkenes 75JCS(P2)1791... 

Hydrazine hydrate can often reduce vulnerable substituents on the pyrazolcme ring. One such example is the alkene group of the cyclopentenyl ring of pyrazol-3-one 16. It was found that heating 4-(l-methoxycarbonyl-2-oxopropyl)-2-cyclo-penten-l-ol 15 with hydrazine hydrate in ethanol afforded a mixture of pyrazol-3-one derivatives 16 and 17 in a 5 2 ratio (97JHC233) (Scheme 4). 

Interaction of a carbonyl group with an electrophilic metal carbene would be expected to lead to a carbonyl ylide. In fact, such compounds have been isolated in recent years 14) the strategy comprises intramolecular generation of a carbonyl ylide whose substituent pattern guarantees efficient stabilization of the dipolar electronic structure. The highly reactive 1,3-dipolar species are usually characterized by [3 + 2] cycloaddition to alkynes and activated alkenes. Furthermore, cycloaddition to ketones and aldehydes has been reported for l-methoxy-2-benzopyrylium-4-olate 286, which was generated by Cu(acac)2-catalyzed decomposition of o-methoxycarbonyl-m-diazoacetophenone 285 2681... 

As with carboxylic acids obtained by palladium hydroxycarbonylation, their derivatives esters, amides, anhydrides and acyl halides are synthesized from alkenes, CO and HX (X = OR, NR2 etc.). The Pd-catalyzed methoxycarbonylation is one of the most studied reactions among this type of catalyzed carbonylations and has been reviewed and included in reports of homogeneous catalysis.625, 26 The methoxycarbonylation has been applied to many different substrates to obtain intermediates in organic syntheses as well as specific products. For instance, the reaction has been applied for methoxycarbonylation of alkynes666 Highly efficient homogeneous Pd cationic catalysts have been reported and the methoxycarbonylation of alkynes has been used to develop economically attractive and environmentally benign process for the production of methyl... 

The mechanisms of the hydroxycarbonylation and methoxycarbonylation reactions are closely related and both mechanisms can be discussed in parallel (see Section 9.3.6).631 This last reaction has been extensively studied. Two possibilities have been proposed. The first starts the cycle with a hydrido-metal complex.670 In this cycle, an alkene inserts into a Pd—H bond, and then migratory insertion of CO into an alkyl-metal bond produces an acyl-metal complex. Alcoholysis of the acyl-metal species reproduces the palladium hydride and yields the ester. In the second mechanism the crucial intermediate is a carbalkoxymetal complex. Here, the insertion of the alkene into a Pd—C bond of the carbalkoxymetal species is followed by alcoholysis to produce the ester and the alkoxymetal complex. The insertion of CO into the alkoxymetal species reproduces the carbalkoxymetal complex.630 Both proposed cycles have been depicted in Scheme 11. 

Scheme 7 An example of the methoxycarbonylation of a cyclic alkene leading to a diester...

Preliminary mechanistic studies on the methoxycarbonylation of 1-octene showed that two pathways to methyl nonanoate occur, one involving the direct carbonylation of 1-octene to the linear ester, the other the alkene isomerisation in competition with the first one. Subsequently, the linear product forms by tandem isomerisation of the internal alkenes, with the terminal alkyl intermediate being trapped by migration to CO at a higher rate than any branched alkyl species. This has been confirmed by the analysis of products... 

The oxidative carbonylation reaction of enolizable ketones follows the general routes already illustrated for simple alkenes. Thus, a-methoxycarbonyl-ation may occur either by addition of a Cl - Pd - C02Me species to the enolic... 

Anodic regioselective acetamidosulfeny-lation of alkenes is similarly achieved by oxidation of diphenyldisulfide in acetonitrile [81]. Cyclic enamines, which are intermediates in the oxidation of cyclic N-methoxycarbonyl amines, react in aqueous acetonitrile that contains chloride ions to a-hydroxy- 8-chloro compounds via intermediate chloronium ions [82]. Enolethers undergo a regioselective azidomethoxyla-tion to yield acetals of a-azido carbonyl compounds upon electrolysis in methanol containing sodium azide [83]. The reaction proceeds possibly via addition of an anodicaUy generated azide radical. 

Alkenes reacted with RuCyaq. CH3CO3H/CH3CN-CH2CI2 giving a-ketols thus cix-5-(methoxycarbonyl)-2-cyclohexenyl acetate (1) gave (2R, 35, 5R )-3-acetoxy-2-hydroxy-5-(methoxycarbonyl)-l-cyclohexanone (2) (Fig. 3.9, Table 3.2). Cortisone acetate was isolated in this way from epiandrosterone after a number of steps [179]. 

The stereospecific formation of 5,6-dihydro-l,4-dithiins from the reaction of the 13-dithiete 60 with alkenes has been shown to proceed through its valence isomer, l,2-bis(methoxycarbonyl)ethane-13-dithione (Scheme 42) <99JOC8489>. 

Similarly, Uneyama and Watanabe (91TL1459) have reported the synthesis of trifluoromethylated AI-aryl-1-azabutadienes by palladium-catalyzed coupling of trifluoroacetimidoyl iodides with alkenes as well as the transformation of the azadiene derived from methyl acrylate into the corresponding 4-methoxycarbonyl-2-trifluoromethylquinoline in quantitative yield. 

An interesting new method for the preparation of the methoxycarbonyl (and related) compound (35) involves the hydroboration-carbon monoxide insertion in bis-alkenic amines (34). The best reagent for this process is thexylborane, followed by cyanidation (82JOC1494). The yields are not very good and seven-membered ring compounds (36) can also be formed, but the procedure is short and simple. 

Similar products are formed from the cycloaddilions of alkenes with methyl penta-2,3-dienoate, methyl 4-methylpenta-2,3-dienoate and methyl 2-methy penta-2,3-dienoate, with the cycloadditions occurring at the double bonds that do not bear the methoxycarbonyl group.5... 

By heating 32 in aqueous dimethyl sulfoxide (DMSO), containing sodium chloride, up to 170°C, loss of a methoxycarbonyl group and then -elimination of acetic acid occur to give alkene 33 in 70% yield. From this, carba-pl-L-mannopyranose 34 has been produced by conversion of the ester group to the hydroxymethyl function, hydroboration, and de-O-protection. Thus, the hydroboration step proceeded by cis-addition anti- to the allylic benzyloxy group. 

A sequential use of two pericyclic reactions involving sulfur compounds is also implied in a general allylic amination of alkenes with Kresze s reagent, 7V,A -bis(methoxycarbonyl)suIfurdiimide (8). A [2,3] sigmatropy, following an ene reaction of the S-allyl sulfinamidine (9) intermediary formed, affords the diamino sulfane (10), easily converted to the carbamate (11) and then to the amine (12) or (13) [511]. The reactions are featured with 2-methyl-2-butene as the alkene to be functionalized. They are described in Organic Syntheses [512]. 

Intramolecular insertion of alkenes into ir-allylpalladium complexes has been shown to proceed regio-selectively to yield five-membered rings.2621263 TTiis transformation is equivalent to a palladium ene reaction and is completed by a (3-hydride elimination. The C==C bond of ketenes is also capable of undergoing insertion into ir-allylpalladium intermediates.264 The final course of this reaction is dependent on the leaving group in the initial allyl-X precursor. For X = OAc, dienes are observed for X = Me0C02, a methoxycarbonyl adduct is obtained (equation 303).264... 

A highly efficient and regioselective methoxycarbonylation of terminal alkyl- and aryl-alkenes (RCH=CH2) can be attained via a palladium-salicylic borate-catalysed protocol. The substrates include aliphatic alkenes, allylbenzenes, and styrene derivatives. The regioselectivity, in favour of the linear esters RCH2CH2C02Me, is up to quantitative, which is unprecedented in the case of styrenes.88... 

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