Stereochemistry decarbonylation

A complete description of stereochemistry of the carbon monoxide insertion and decarbonylation requires knowledge of configurational changes at the metal and a-carbon. Calderazzo and Noack (54) showed that the optical activity of the equilibrium mixture... 

Entry 5 is an example of the use of fra-(trimethylsilyl)silane as the chain carrier. Entries 6 to 11 show additions of radicals from organomercury reagents to substituted alkenes. In general, the stereochemistry of these reactions is determined by reactant conformation and steric approach control. In Entry 9, for example, addition is from the exo face of the norbornyl ring. Entry 12 is an example of addition of an acyl radical from a selenide. These reactions are subject to competition from decarbonylation, but the relatively slow decarbonylation of aroyl radicals (see Part A, Table 11.3) favors addition in this case. 

Compound 146 was converted to 147 (relative stereochemistry is shown in Scheme 68) by addition of a series of aldehydes, and other isomers were also obtained which were easily purified. But attempts to liberate a dihydrofuran from 147 were unsuccessful, which is believed to be due to decarbonylation by rhenium.306 Alternative approaches can be envisaged, although the authors noted the difficulty in the preparation of the starting material as a serious barrier to this.306... 

Rhodium(II) acetate catalyzes C—H insertion, olefin addition, heteroatom-H insertion, and ylide formation of a-diazocarbonyls via a rhodium carbenoid species (144—147). Intramolecular cyclopentane formation via C—H insertion occurs with retention of stereochemistry (143). Chiral rhodium (TT) carboxamides catalyze enantioselective cyclopropanation and intramolecular C—N insertions of CC-diazoketones (148). Other reactions catalyzed by rhodium complexes include double-bond migration (140), hydrogenation of aromatic aldehydes and ketones to hydrocarbons (150), homologation of esters (151), carbonylation of formaldehyde (152) and amines (140), reductive carbonylation of dimethyl ether or methyl acetate to 1,1-diacetoxy ethane (153), decarbonylation of aldehydes (140), water gas shift reaction (69,154), C—C skeletal rearrangements (132,140), oxidation of olefins to ketones (155) and aldehydes (156), and oxidation of substituted anthracenes to anthraquinones (157). Rhodium-catalyzed hydrosilation of olefins, alkynes, carbonyls, alcohols, and imines is facile and may also be accomplished enantioselectively (140). Rhodium complexes are moderately active alkene and alkyne polymerization catalysts (140). In some cases polymer-supported versions of homogeneous rhodium catalysts have improved activity, compared to their homogenous counterparts. This is the case for the conversion of alkenes direcdy to alcohols under oxo conditions by rhodium—amine polymer catalysts... 

In an attempt to resolve this question of stereochemistry and also to determine whether or not the decarbonylation of an acid chloride containing a f3 hydrogen takes place stereospecifically, erythro- (XI) and fhreo-2,3-diphenylbutanoyl chlorides (XII), obtained by the reaction of the known acids (13, 14) with oxalyl chloride, were synthesized. The reaction of these acid chlorides (see Reaction 8) with chlorotris( triphenyl-phosphine) rhodium gave the corresponding acyl complexes of type lib [R = C6H5CH(CH3)CH(C6H5)]. Decarbonylation of the erythro- cy complex in benzene at 30 °C gave a 90% yield of frans-a-methylstilbene while decarbonylation of the threo-acyl complex under similar reaction... 

The chiral aldehyde 88 was decarbonylated to give 89 with overall retention of the stereochemistry [45]. The unsaturated aldehyde in the polyfunctionalized molecule 90 was decarbonylated smoothly to afford alkene 91 [46]. The decarbonylation of aldehydes catalysed by a supported Pd or Rh complex is carried out at high... 

The corresponding esters are much less informative because the centers of chirality in their acyl radicals are structurally protected from racemization like that experienced by translational or rotational motions of prochiral alkyl radicals. In addition, the decarbonylated radicals derived from them are formed long after their acyl precursors have moved to orientations with respect to their aryloxy partners that result in a loss of the memory of their host stereochemistry within a cage see above. Thus, of the Claisen-like photoproducts from irradiation of (7 )-lb, only the BzON (i.e., 3b) retains a measurable amount of optical activity even in the solid phases of long -aIkane. However, in polyethylene hlms, all of the Claisen products from irradiation of (7 )-lb—2-BN, 4-BN, and 3b—exhibit signihcant ee values. In the same media, the photo-Fries products from lb retain virtually all of the enantiomeric purity of the... 

Walborsky, H. M., Allen, L. E. Stereochemistry of tris(triphenylphosphine)rhodium chloride decarbonylation of aldehydes. J. Am. Chem. 

Stille, J. K., Huang, F., Regan, M. T. Mechanism of acid chloride decarbonylation with chlotetris(triphenylphosphine)rhodium(l). Stereochemistry and direction of elimination. J. Am. Chem. Soc. 1974, 96,1518-1522. 

The rhodium complex [RhCl(PPh3)3] readily brings about stoichiometric decarbonylation of aldehydes, acyl halides and diketones. A typical aldehyde decarbonylation is illustrated by equation (69). a,3-Unsaturated aldehydes are decarbonylated stereospecifically (equation 70), while with chiral aldehydes the stereochemistry is largely retained (equation 71). ° ... 

Disubstituted cyclopropenes undergo addition to tetraarylcyclopentadienones to produce mixtures of exo- and emfo-adducts in which the endo-isomer predominates (Table 12).187188 The predominance of enr/o-product has been explained in terms of secondary orbital interactions, involving the methylene group of the cyclopropene.147 3-Methyl-3-vi-nylcyclopropene reacted with 2,3,4,5-tetraphenylcyclopenta-2,4-dien-l-one at the cyclopropene double bond and a [4+2] cycloadduct of unspecified stereochemistry was isolated (41% mp 231-234 °C) together with decarbonylated products reaction of 2-oxo-4,5-diphenyl-l,3-bis(methoxycarbonyl)cyclopentadiene with 3-methyl-3-vinylcyclopropene was more complicated and apparently attack occurred at both 71-bonds.1... 

A related example involving a more complex substrate is found in Rapoport and Luly s formal synthesis of the 7-methoxymitosene (24). Again, decarbonylation of an a-tertiary amino acid chloride is employed to generate the iminium ion. The indoline acetal (23) produced in this way is essentially one diastereomer however, its stereochemistry was not established (Scheme 13). 

Treatment of 12 with (Me3Si)3SiH and Et3B predominantly alforded cyclic ethers with cis stereochemistry [17], Construction of oxepines (n = 2) proceeded at low concentration. Additionally, the reaction at low temperature was effective to suppress decarbonylation of the intermediary acyl radical derived from acyl selenide (Scheme 10). 

The stereochemistry of aldehyde decarbonylation has received much attention. Walborski and Allen have shown that the decarbonylation of optically active aldehydes proceeds with 93% retention of configuration,as shown in Equation 11 ... 

CO insertion into alkenyl-metal bonds also occurs stereospecifically in many cases. An example of insertion of CO into an alkenyl complex is shown in Equation 9.23. Complexes containing either cis- or frflns-CH=CHPh groups insert CO with retention of configuration. Similar stereochemical results have been observed for decarbonylation. For example, the reaction of RhCl(PPh3)3 with E-PhCH=C(Et)CHO induces decarbonylation with retention of the double-bond stereochemistry. °... 

Manganese.—Elimination of transition-metal hydride from metal alkyls and addition of metal hydrides to alkenes are usually considered to be cA-processes. Since acylmanganese compounds undergo stereospecific reversible decarbonylation, thermal decomposition of (eryrAro-2,3-dimethylpentanoyl)(pentacarbonyl)manga-nese(i) should allow the determination of the stereochemistry of elimination of [MnH(CO)8] (Scheme 4). However, both the erythro and a mixture of the erythro and threo acyl complexes decompose thermally to give the same mixture of cis- and trans-3-methylpent-2-ene and 3-methylpent-l-ene under conditions which do not isomerize these alkenes. It is suggested that the mechanism involves interconversion of... 

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