Allenes monosubstituted

From monosubstituted allenes, RCH=C=CH2, two different anions or organo-... 

The heat of hydrogenation for allene (Problem 6.37) to yield propane is -295 kj/mol, and the heat of hydrogenation for a typical monosubstituted alkene such as pcopene is - 126 kj/mol. Is allene more stable or less stable than you might expect for a diene Explain. 

Allen CCR, DR Boyd, H Dalton, ND Sharma, I Brannigan, NA Kerley, GN Sheldrake, SC Taylor (1995) Enantioselective bacterial biotransformation routes to cw-diol metabolites of monosubstituted benzenes, naphthalene and benzocycloalkenes of either absolute configuration. J Chem Soc Chem Commun 117-118. 

Intermolecular hydroalkoxylation of 1,1- and 1,3-di-substituted, tri-substituted and tetra-substituted allenes with a range of primary and secondary alcohols, methanol, phenol and propionic acid was catalysed by the system [AuCl(IPr)]/ AgOTf (1 1, 5 mol% each component) at room temperature in toluene, giving excellent conversions to the allylic ethers. Hydroalkoxylation of monosubstituted or trisubstituted allenes led to the selective addition of the alcohol to the less hindered allene terminus and the formation of allylic ethers. A plausible mechanism involves the reaction of the in situ formed cationic (IPr)Au" with the substituted allene to form the tt-allenyl complex 105, which after nucleophilic attack of the alcohol gives the o-alkenyl complex 106, which, in turn, is converted to the product by protonolysis and concomitant regeneration of the cationic active species (IPr)-Au" (Scheme 2.18) [86]. 

Epoxidation of allenes.1 The spirodioxides formed by epoxidation of allenes are unstable to acids, and only hindered ones have been obtained on epoxidation with peracids. They can be obtained, however, in 90-95% yield by epoxidation of allenes (even monosubstituted ones) with dimethyldioxirane in acetone buffered with solid K2C03. 

Himbert and co-workers discovered the interesting intramolecular [4 + 2]-cycload-dition of allenecarboxanilides 353, which is possible even with monosubstituted benzenes (R H, Scheme 7.49) [25, 340]. During heating, partially an equilibrium between the allene 353 and the cycloadduct 354 is established. This Diels-Alder reaction can be applied to the corresponding N-(3-pyridyl) [335] or N-(l-naphthyl)... 

The first investigations in the 1960s [11,12] established the base-induced isomerization of alkyne precursors as the most practical and general route for the synthesis of alkoxy-and aryloxyallenes. In the meantime, a number of monosubstituted allenes 8 bearing an achiral or a chiral group R is smoothly accessible by this efficient procedure (Scheme 8.5) [1, 2,13-19]. Beside the most commonly used base potassium tert-butoxide, other bases, e.g. n-butyllithium, are also applicable for this isomerization. Recently, the yields of alkyne-allene isomerizations could be significantly increased, in particular with aryloxy-substituted allenes, by using microwave irradiation (Eq. 8.1) [20]. 

An early synthesis of allenylzinc reagents employed a two-step procedure in which monosubstituted allenes were subjected to lithiation in THF with tBuLi at -90 °C and the resulting allenyllithium intermediates were treated with ZnCl2. The allenylzinc reagents thus generated react in situ with aldehydes to afford mainly anti homopropargyl alcohols (Table 9.46) [98],... 

Boyes and Wild reported the manganese-mediated regioselective chlorination of allenes [14]. For the dichlorination of cyclo-l,2-nonadiene, the dichloride was obtained as the major product whereas the reaction of monosubstituted allene afforded a mixture of regiosomers of the dichlorination products with 2,3-dichloro-1-alkene 12 being the major one. The yield with oxalyl chloride is better than that with TMSC1. 

Allenic halides can also react with monosubstituted malonates to afford 2-(l, 2 -allenyl)malonates 128 and 131. With an unsubstituted malonate the migration of the C=C bond was observed to form 1,3-dienes 129 and 130 [65]. 

Limitations of the Pd(II)-catalyzed cydoisomerization/dimerization are strongly coordinating groups such as pyridyl groups, amines, thioethers or halogens on the allene [61]. With 96, AuC13 delivered products 102, which are constitutional isomers of the dimers of the Pd(II)-catalyzed reactions (Scheme 15.27) [62]. Compounds 102 are always accompanied by the monosubstituted furans 99. Furthermore, these catalysts were highly reactive and allowed a much faster reaction than Ag(I) or Pd(II) under the same conditions. 

Miyaura and co-workers reported the platinum-catalyzed diboration of allenes with bis(pinacolato)diboron (Scheme 16.52) [57]. The catalytic cycle involves a sequence of oxidative addition of bis(pinacolato)diboron to Pt(0), insertion of an allene into the B-Pt bond and reductive elimination of an allylic boronate, re-producing the Pt(0) species. (Z)-Allylic boronates are formed stereoselectively in the reaction with monosubstituted allenes, which strongly suggests a pathway via a vinylplatinum species rather than a Jt-allylplatinum species. 

Hydrozirconation of monosubstituted allenes offers easy access to allylzircono-cene chlorides, which react with carbonyl compounds to afford homoallylic alcohols in a highly regio- and stereoselective manner (Scheme 16.68) [73-75],... 

Exposure of a monosubstituted allene to a Grubbs carbene complex allows the formation of a 1,3-disubstituted allene accompanied with polymers (Scheme 16.93) [99]. The product distribution depends considerably on the alkyl substituents on the allene moiety and particularly the reaction of a series of phenylallenes undergoes complete conversion to polymers. 

The homo-coupling reaction of 1,1-dimethylallene proceeds at -50 °C to give a symmetrically substituted diene in good yield (Scheme 16.99) [105], In sharp contrast, the reaction of a monosubstituted allene proceeds via a titanabicycle to give an unsymmetrical diene. 

When allenyl aldehydes are allowed to react with DMDO, the aldehyde moiety is not oxidized to the acid except for monosubstituted allenes [21]. In all other cases, the carbonyl oxygen participates as a nucleophile in the opening of the intermediate epoxide. From 2,2,5-trimethy]-3,4-hexadienal 67, for example, five different products can be synthesized selectively under different reaction conditions (Scheme 17.22). When p-toluenesulfonic acid (TsOH) is present or DMDO is formed in situ, then the initially formed allene (mono)oxide reacts with the aldehyde moiety to give 68 or 69. In the presence of excess DMDO and the absence of acid, three other products (70-72) can be formed via the spirodioxide intermediate. These reactions, however, seem to be less general compared with similar reactions of allenyl acids and allenyl alcohols. y-Allenylaldehydes 73 can be cyclized to five-membered hemiacetals 74 via the spirodioxide intermediate. 

Hydration of allene and mono- and disubstituted allenes leads to ketones through the rearrangement of the intermediate enols.29 Further details of the mechanism are not known, but protonation of the terminal carbon in monosubstituted allenes is probable. Although the formation of isomeric ketones may be expected, only ketones possessing the keto function on the central carbon of the allene bond system were found to form.30 When alcohols add to allenes, enol ethers of the corresponding ketones are usually the products. They may further react to form acetals. Hg2+ salts may be used as catalysts.31... 

Monosubstituted allenes afford predominantly 2-chloro-2-alkenes (equation 47),73 75,76,78 although 1-phenyl-l,2-propadiene forms exclusively cinnamyl chloride (equation 48).79... 

Terminal monosubstituted allenes afford primarily ( )- and (Z)-2-bromo-2-alkenes using either approach (equation 104).153-155 However, terminal disubstituted allenes give quite different products (equation 105).82-133-135... 

Boyd DR, Sharma ND, Llamas NM, O Dowd CR, Allen CCR (2006) Chemoenzymatic Synthesis of the trans-Dihydrodiol Isomers of Monosubstituted Benzenes via anti-Benzene Dioxides. Org Biomol Chem 4 2208... 

Monosubstituted allenes 510 can be successfully inserted into the oxapalladacycles 511 to afford regioisomeric chromans 512 and 513. Oxapalladacycles 511 bearing no substitution on the aromatic ring give a mixture of regioisomeric products, but w >-methylene chromans 512 are exclusively formed upon introduction of a substituent at C7 (Equation 212, Table 25) <2004JOC8266>. 

The diboration of allenes afforded another series of allylboron compounds that each have a boryl group at the vinyl carbon. The addition had a strong tendency to occur at the internal double bond of terminal allenes such as 1,2-heptadiene (Equation (33)).237 However, steric hindrance in both allenes and phosphine ligands forced the addition toward the terminal double bond of dimethylallene (Equation (34)).237 On the other hand, addition to the terminal double bond occurred selectively for both monosubstituted and 1,1-disubstituted allenes when Pd(dba)2 was used in the presence of a co-catalyst (RI) such as I2, Arl, and iodoalkenes (Equation (35)).238 The role of co-catalyst was attributed to in situ formation of I-Bpin intermediate, which undergoes oxidative addition and insertion leading to 2-boryl-7r-allylpalladium intermediate. 

The palladium-catalyzed silylboration of typical allenes exhibited a strong tendency to occur at the internal double bond giving allylsilanes, which undergo allylation of aldehydes in the presence of a Lewis acid (Equation (44)).254 257 The selectivity can be varied by changing the substituents, but the boron atom always added to the central carbon. Enantioselective silylboration of terminal monosubstituted allenes was demonstrated by double asymmetric induction using chiral silylboranes and chiral catalysts (Equation (45)). 

Monosubstituted or 1,1-disubstituted allenes are the simplest cases where at least some stereochemical information on dimerizations may be gained. 

Allenylzinc reagents. These reagents (1) are obtained by lithiation (t-BuLi) of monosubstituted allenes followed by transmetallation with ZnCU. They react with alde-... 

A synthetically useful and convenient method for the B(C6F5)3-catalyzed hydro-stannylation of alkynes with tributyltin hydride, prepared in situ from easily handled and inexpensive chlorostannane and hydrosilane, has been developed by Yamamoto and his colleagues [155]. The hydrostannylation of monosubstituted alkynes proceeds in a regiospecific manner to give the /3-hydrostannylation products exclusively (Eq. 100). The reaction is trans stereoselective. This method can also be applied to the hydrostannylation of allenes and alkenes (Eqs 101 and 102). 

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