Acyclic product formation

In the presence of phosphane-free Ni(0) catalysts, substituted methylenecyclopropanes dimerize at low temperatures giving formal [2 + 2] and [3 -I- 2] cycloadducts 24 and 25, respectively. The chemoselectivity of [2 -t- 2] cycloaddition depends on the substitution pattern of the substrate and is restricted to systems without further substituents at the exocyclic double bond. [3 + 2] Cycloaddition and acyclic product formation depend on further substituents at the cyclopropane moiety. In general, the product is obtained as a mixture. The combined yield resulting from cycloaddition is higher in the presence of electron-deficient alkenes, such as dimethyl ( )-but-2-enedioate. ... 

The structures of chalcogen-nitrogen compounds are frequently unpredictable. For example, the reactions of heterocyclic systems often result in substantial reorganization of their structural frameworks, e.g. ring expansion or contraction. The formation of acyclic products from ring systems (or vice versa) is also observed. 

The primary rearranged products 71, 75,76, 77, 78 were converted to the corresponding ketones, 73, 80, 81, and the primary acyclic products 72 and 79 into their respective acyclic alcohols, 74 and 82. As the solvent polarity increased and the nucleophilicity decreased from acetic to formic to trifluoro-acetic acid, the rearranged products increased significantly from 27% to 91% for 69 and from 60% to 100% in the case of 70. The formation of the rearranged... 

Considering the facility with which dimerization products 81 and 84 are obtained, we reasoned that, in catalytic ring closure of 77, the derived dimer is perhaps initially formed as well. If the metathesis process is reversible [17b], such adducts may subsequently be converted to the desired macrocycle 76. To examine the validity of this paradigm, diene 77 was dimerized (— 85) by treatment with Ru catalyst lb. When 85 was treated with 22 mol% 2 (after pretreatment with ethylene to ensure formation of the active complex), 50-55% conversion to macrolactam 76 was detected within 7 h by 400 MHz H NMR analysis (Eq. 8). When 76 was subjected to the same reaction conditions, <2% of any of the acyclic products was detected. Although we do not as yet have a positive proof that 85 is formed in cyclization of 77, this observation suggests that if dimerization were to occur, the material can be readily converted to the desired macrolactam, which is kinetically immune to cleavage. 

Reaction of 4a with TiCl4 was carried out in the presence of siloxyalkene 3 as nucleophile and the results are summarized in Table III. In the reaction with ketene silyl acetals 3a and 3e at -78 °C, y-ketoesters 15a and 15e were obtained instead of chloride product 8 which is a major product in the absence of 3. Formation of product 15 is likely to result from trapping of alkylideneallyl cation 5 with 3 at the sp2 carbon. In contrast, the reactions with silyl enol ethers 3f and 3g gave no acyclic product 15, but gave cyclopentanone derivatives 16-18. The product distribution depends on the mode of addition of TiCl4 (entries 4-7). 

In the system nickel/L/butadiene, secondary amines can shift the cyclodimerization of butadiene to the acyclic products (7a) and (75) Its cocatalyst functfon can be visualized by the corresponding [L]-control map (Scheme 3.3-2). In the three-component system nickel/morpholine/butadiene the open-chain products are formed for log ([morpholine]o/[Ni]o) > -1. Both octatrienes (7a) and (75) are formed at the constant ratio of 1.8 over the entire range of the examined amine/nickel scale. However, the efficiency of the catalytic system is low. After a turnover of 30% butadiene, the catalytic activity ends because of the formation of stop complexes of the nickel amide type. 

The acyclic, enolic compounds 7 and 9 may exist in either cis or trans forms. Methyl ethers of 7 have been isolated in the cis form,8 but it is not known whether the trans forms, which must be acyclic, exist. The relative proportion of isomers is controlled by the geometry of the parent sugar enediol. Although the acyclic forms are readily interconvertible tautomers, it appears that, in acidic medium, further reaction occurs much more rapidly than any equilibrating reactions. Compound 7 undergoes rapid elimination of a second hydroxyl group to give 11. This acyclic product, also, may exist as either a cis or a trans isomer, both forms of which have been isolated.8 The loss of a third molecule of water per molecule occurs after, or simultaneously with, the cyclization of 11 (see Section II, 3 p. 171), and results in formation of 5-(hydroxymethyl)-2-furaldehyde (5). 

Ammonia and amines open pyran-2-one rings and the acyclic products may cyclize again on acidification to pyridones or a benzene ring, a reaction reminiscent of those of pyrylium salts (Section 2.23.2.3). Under mild conditions, unsaturated amino acids such as (275) are formed but at higher temperatures and longer reaction times, a pyridone (276) or benzamide (277) is formed. The probable course of benzamide formation is as shown in Scheme 14 (74JOU852). 

In this chapter, attention is primarily focused on simple single-stage catalytic enantioselective carbometallation reactions leading to the formation of acyclic products in most cases regardless of their actual mechanisms. However, some closely related cyclic processes are also discussed. At present, the scope of such processes appears to be largely limited to those involving a few early transition metals, especially Zr. 

Activation of two Si—Si bonds in bis(disilanyl)alkanes with palladium(O) bis(tert-alkyl isocyanide) induced the formation of the cyclic bis(silyl)palladium(II) bis(terf-alkyl isocyanide) complexes (100) and disilanes described schematically in Scheme 42. These complexes were found to react with phenylacetylene, affording different amounts of five-membered cyclic products and acyclic products which are derived from the insertion of the alkyne into the general intermediate complex 101 (Scheme 42, equation 54). The bis(silanyl)dithiane palladium complex (102) was isolated and characterized in the solid state the two silicon atoms, the two isocyano carbons and the palladium atom are nearly in a plane with a short cross-ring Si—Si distance of 2.613(2) A, suggesting the possibility of covalently bonded two Si—Si atoms in the four-membered ring. Similar reaction with cyclic disilanes afforded oligomers, and cyclic 20-membered compounds have been prepared in the presence of nitriles248,249. 

Structurally related dienols and acyclic trienols, when reacted in fluorosulfuric acid, give tricyclic ether derivatives in kinetically controlled cyclization.810,811 The stereospecific product formation is rationalized by synchronous internal anti-addition via chair-like conformations of the protonated cyclohexene ring, resulting in ring closure with equatorial C-C bond formation and concomitant internal nucleophilic termination by anti-addition of the OH group [Eq. (5.294)]. Z/E isomerization may be competitive with cyclization. 

Other polyhalogenated compounds can be used with similar success. CpFe(CO)2 dimer leads to the formation of mixtures of lactones and esters when reacted with an alkene and methyl trichloroacetate [79] with the lactone being the major product (Scheme 3.10). Similar results were reported earlier by Freidlina and Velichko [80]. FeCl3 and Fe(CO)5 are both suitable for catalyzing the addition of methyl dibromoace-tate to electron-deficient alkenes such as methyl propenoate. It was observed that the ratio of products (acyclic vs. lactone) could be tuned by varying the reaction conditions. In all cases, the acyclic product is predominantly formed. Only in the presence of a co-catalyst such as N,N-dimethylaniline are small amounts of lactone observed. Noteworthy, elevated temperatures (above 100 °C) are necessary for this transformation. 

Similar product ratios were reported for the methyl pyruvate/2,3-dimethyl-2-butene photoreaction. In this case, however, a state selectivity effect is responsible for the formation of the different ether and alcohol products [31]. Obviously the existence of allylic hydrogens favors the formation of unsaturated acyclic products via hydrogen migration steps at... 

Dehydration of primary nitro compounds (Mukaiyama reaction)12 affords nitrile oxides, which may dimerize to yield furoxans, or otherwise be trapped by suitable dipolarophiles such as double or triple bond systems, leading to the formation of various heterocyclic systems, 5.13 The latter have been used for further derivatization in the heterocyclic series, or in "return" as precursors of acyclic products after ring cleavage,7d-14 for example, 1,3-amino alcohols 6 or (J-hydroxycarbonyl compounds, 9. 

Acyclic C-acyl imines have recently been studied as dienophiles.32-34 p j example, Prato and coworkers examined the reaction of imines (41) (equation 13) with several cyclic and acyclic 1,3-dienes. Under neutral conditions, (41) is unreactive as a dienophile. However, under Lewis acid catalysis these imines react to afford mixtures of adducts. With 1,3-cyclohexadiene, bicyclic adducts (42) and (43) are produced along with (44) in which the imine has acted as an azadiene. The ratios of these sorts of products are dependent upon the particular imine and diene used. The formation of adducts of type (43) proved to be both regio- and stereo-selective. Product formation in these cases can be rationalized lx>th by concerted and by stepwise ionic mechanisms. ... 

Because typical additions have already been treated, we merely point out here that Table 19 provides further acyclic products. Likewise, Table 20 catalogues the formation of some heterocyclic families. What follows, therefore, are several nitrogen examples which are in some sense special. 

Carbalumination of 271b with tri-isopropylaluminium affords acyclic products, the formation of which can be attributed to rearrangement of the initially formed cyclopropyl carbocation (equation 84). Ring-opened products also result from reactions with trialkylboranes, but only cyclopropanes, from addition to the n bond, are seen in reactions with organomagnesium reagents . 

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