Pentanes reaction + carbenes

This reaction is virtually useless for synthetic purposes but illustrates the extreme reactivity of carbene. Treatment in the liquid phase of an alkane such as pentane with carbene formed from the photolysis of diazomethane gives the three possible products in statistical ratios227 demonstrating that carbene is displaying no selectivity. For many years, it was a generally accepted principle that the lower the selectivity the greater the reactivity however, this... 

From fenchone (783), methylmagnesium iodide and dehydration yielded Wallach s homofenchene (784), which was in turn converted to the acid chloride 785. The latter formed the ketene 786 in 19% yield when heated with triethylamine in benzene at 130-140 C, and the ketene underwent an unusual photoreaction. In pentane, the carbene insertion product cyclofenchene (787) was formed, but at -60 C in methanol, another reaction competed with the... 

Two stannenes have been synthesized by the reaction of a stannylene with a boranediylborirane (Eq. 34).85 The boranediylborirane has been shown to react toward suitable reagents as though it were the carbene,101 which is only slightly higher in energy than the boranediylborirane.102 The reaction occurs at room temperature in pentane solution. The resulting stannene has a considerable contribution from the ylide resonance structures. The carbene arising from the boranediylborirane is extremely electrophilic, and therefore the stannenes can be considered formally to be adducts of the stannylene as Lewis base and the carbene. 

Products of a so-called vinylogous Wolff rearrangement (see Sect. 9) rather than products of intramolecular cyclopropanation are generally obtained from P,y-unsaturated diazoketones I93), the formation of tricyclo[2,1.0.02 5]pentan-3-ones from 2-diazo-l-(cyclopropene-3-yl)-l-ethanones being a notable exception (see Table 10 and reference 12)). The use of Cu(OTf), does not change this situation for diazoketone 185 in the presence of an alcoholl93). With Cu(OTf)2 in nitromethane, on the other hand, A3-hydrinden-2-one 186 is formed 160). As 186 also results from the BF3 Et20-catalyzed reaction in similar yield, proton catalysis in the Cu(OTf)2-catalyzed reaction cannot be excluded, but electrophilic attack of the metal carbene on the double bond (Scheme 26) is also possible. That Rh2(OAc)4 is less efficient for the production of 186, would support the latter explanation, as the rhodium carbenes rank as less electrophilic than copper carbenes. 

Pyridine ylide/LFP studies of 83-85 in pentane or isooctane afforded carbene lifetimes of 21-24 ns (k 4 to 5 x 107 s 1), similar to the lifetime of dimethylcarbene under these conditions. Unfortunately, these lifetimes are limited by reactions with the hydrocarbon solvents the lifetime of 83 is 1.5 times longer in cyclohexane-d12 than in cyclohexane. The observation that the lifetimes of 55-CI ( 1000 ns) and 55-F (—7000 ns) are considerably longer than those of 83 and 84 could reflect the superior stabilization provided by the halogen spectator substituents of 55, but this conclusion is tentative in the absence of definitive intramolecularly controlled lifetimes for 83-85. 

In contrast, 1,2-H shift to olefin 106 is the dominant reaction of carbene 104, and this process is slow enough to be measured by LFP r = 300 ns in cyclohexane and 560 ns in pentane at 25°C.117 There is a polar solvent effect the lifetime decreases to 52 ns in acetonitrile. However, at least in the case of cyclohexane, the lifetime is solvent limited, with a KIE of 1.5 on the lifetime in cyclohexane- (460 ns). Carbene 104 is much longer-lived than dimethylcarbene (r 21 ns in pentane) or methylcarbene (<1 ns).22,89... 

In 1956, Doering et al. reported that methylene (CH2) inserted into the C H bonds of pentane, 2,3-dimethylbutane, and cyclohexene with no discrimination (other than statistical) between chemically different sites CH2 was classed as the most indiscriminate reagent known in organic chemistry. Doering and Kirmse also demonstrated that the C—H insertion reactions of CH2 in solution were direct, single barrier concerted processes with transition states that could be represented as 27 (Fig. 7.12). In particular, they did not proceed via initial H abstraction to give radical pair intermediates that subsequently recombined. (Triplet carbene C H insertions, however, do follow abstraction-recombination, radical pair mechanisms, as demonstrated in classic experiments of Closs and Closs and Roth (see Chapter 9 in this volume). 

However, the fact that dichlorocarbene reacts with methyl bicyclo[l. 1.0]butane-l-carboxylate (4) to generate exclusively pentadienes 5 and 6 in a total yield of 41 % renders this type of reaction impracticable for the synthesis of bicyclo[l.l.l]pentanes.3 Numerous examples have indeed shown that the reaction between bicyclo[1.1.0]butanes and carbenes are synthetically practical routes for pentadiene preparation4 9 and interesting evidence has been obtained to indicate that a concerted mechanism occurs.10-11... 

The first examples of a chalcogen insertion into a transition metal-carbene bond were reported in 1981. Stirring solutions of the pentacarbonyl-(diarylcarbene)tungsten complexes 39 in pentane in the presence of solid sulfur afforded the thioketone complexes 40 [Eq. (8)].171 The reaction rate increased considerably when pentane was replaced by CS2 as the solvent due to the higher solubility of elemental sulfur in CS2. 

In contrast to the failure to observe an intramolecular reaction of (allyldimethylsilyl)car-bene56, photolysis of the (allyldiisopropyl)diazoacetate 107 furnishes l-silabicyclo[2.1.0]-pentane 109 in good yield61 (equation 27). Obviously, the intramolecular cyclopropanation of carbene 108 accounts for this result. Altogether, the chemistry of (alkenylsilyl)carbenes (equations 24, 26 and 27) exhibits close similarities to that of the all-carbon analogues1. 

In a Schlenk tube with an argon atmosphere at r.t., precatalyst (20.6 mg, 0.055 mmol, 20 mol%) was suspended with absolute THF (1.7 mL). A solution of freshly sublimed KOt-Bu (5.9 mg, 0.052 mmol, 19 mol%) in absolute THF (0.6 mL) was added slowly, and the solution stirred for 5 min. An aldehyde ketone (0.275 mmol) was dissolved in absolute THF (0.5 mL) and added to the carbene solution. The reaction mixture was stirred for 48 h, diluted with DCM, quenched with water, extracted twice with DCM, and dried over MgSC>4. The solvent was evaporated and the crude product purified by FC on silica gel (n-pentane DCM, 1 2) to yield the corresponding acyloin. 

Methyllithium (and likewise BuLi and allyllithium) also adds to the carbene ligand of (CO)5W[C(OMe)Ph]. However, the reaction of the resulting anionic adduct with Si02/pentane at —40°C yields pentacarbonyl(n -olefin)-W complexes, probably via the intermediary formation of the methyl(phenyl)carbene complex and following rearrangement via 1,2-hydrogen shift ... 

The high selectivity of the reaction of cobalt-carbene complexes with alkynes for furan products was taken advantage of in the synthesis of bovolide, a natural flavor constituent of butter. The carbene complex (230) was prepared in two steps from n-pentanal and was treated with 3 equiv. of 2-butyne. The crude reaction mixture, which presumably contained the furan (231), was treated directly with 3 equiv. of trimethylsilyl iodide to give bovolide in - 50% yield from carbene complex (230). 

The nucleophilic character of triphenylphosphine and the electrophilic character of carbenes suggested to two groups of investigators that the two reagents should combine to form ylides of a new type to use in expansion of the Wittig reaction. Speziale et ai. found that triphenylphosphine on treatment in pentane with chloroform and potassium t-butoxide at 0° gives yellow triphenylphosphine dichloromethylene, which reacts with benzophenone to yield I,l-diphenyl-2,2-dichloroethylene (1). 

On the other hand, cycloaddition of cyclobutene by bis(methoxycarbonyl)carbene yielded dimethyl bicyclo[2.1.0]pentane-5,5-dicarboxylate (10) as the sole product (55% yield) in the direct photolysis, whilst the triplet carbene reaction in addition to 10 produced two vinylcy-clopropanes 11 and 12 which probably arise from the same intermediate 1,3-diradical. 

Electrophilic addition reactions of carbenes to cyclopropane structures are rare and not synthetically important. Reaction of carbenes with bicyclo[2.1.0]pentane generally gave products arising from C —H insertion at one of the methylene groups of the four-membered ring. ° An exception was difiuorocarbene which gave 1,1-difluorohexa-l,5-diene (2) in very low yield by cleavage of two C —C bonds. - ... 

Group Migration Reactions.- Direct irradiation of cyclooctene (la) in pentane brings about CIS -trans isomerization as well as the formation of the bicyclic products (2a) and (3). These are formed via the carbene intermediates (4) and (5). The latter carbene (5) also affords methylenecycloheptene. cis -trans -Isomerization also arises with the cycloalkenes (lb) and (Ic). Product (2b) is formed from cyclodecene (lb) and... 

Pr2Im and Pr2Im are very moisture sensitive, thermally labile, and decompose at room temperature after a few days with the formation of black oils. However, these carbenes can be stored under an inert atmosphere at 40°C for months without recognizable decomposition. For further reactions, the carbenes can be used as liquids at room temperature (density at 20°C approximately 1 g/cm3), but they should be restored at —40°C immediately after use. These carbenes are highly soluble in THF, diethylether, toluene, and benzene, and are moderately soluble in hexane and pentane. They are strong bases and decompose in solvents with acidic C—H bonds such as acetone or acetonitrile. With halogenated solvents such as chloroform, they decompose exothermically. 

The irradiation of the methyl substituted cyclic allene (163) has been studied to evaluate the influence of substitution on the reactions encountered. This work follows on an earlier study of the photoreactivity of the unsubstituted allene. Direct irradiation using light of wavelength > 220nm in pentane brought about the formation of the products shown in scheme 7 in the yields shown below the appropriate structure. The products obtained from this reaction were compared with those from a thermally generated carbene intermediate and the conclusion reached is that carbene intermediates are involved in the photochemical reaction. This is different from the unsubstituted case 80... 

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