Propargylic cyclic carbonate

Propargylic alcohol, after lithiation, reacts with CO2 to generate the lithium carbonate 243, which undergoes oxypalladation. The reaction of allyl chloride yields the cyclic carbonate 244 and PdC. By this reaction hydroxy and allyl groups are introduced into the triple bond to give the o-allyl ketone 245[129]. Also the formation of 248 from the keto alkyne 246 with CO2 via in situ formation of the carbonate 247 is catalyzed by Pd(0)[130]. 

Some single examples of this type of reaction were discussed in earlier sections, but due to the importance of these transformations, an additional overview will be provided here. One of the first transformations based on this strategy was published by Inoue and coworkers [134] using propargylic alkoxide, an arylhalide or vinyl bromide and C02 to give cyclic carbonates. The Balme group used this ap-... 

Ni(cyclam)]2+ was shown to be an efficient electrocatalyst for the intramolecular cyclization-carboxylation of allyl or propargyl-2-haloaryl ethers,200 and for the synthesis of cyclic carbonates from epoxides and carbon dioxide.201... 

Not least for the syntheses of natural products, alkoxycarbonylations with formation of allenic esters, often starting from mesylates or carbonates of type 89, are of great importance [35, 137]. In the case of carbonates, the formation of the products 96 occurs by decarboxylation of 94 to give the intermediates 95 (Scheme 7.14). The mesylates 97 are preferred to the analogous carbonates for the alkoxycarbonylation of optically active propargylic compounds in order to decrease the loss of optical purity in the products 98 [15]. In addition to the simple propargylic compounds of type 89, cyclic carbonates or epoxides such as 99 can also be used [138]. The obtained products 100 contain an additional hydroxy function. 

Synthesis of Cyclic Carbonates from Propargylic Alcohols... 

Cyclic carbonates have also been synthesized from propargylic alcohol derivatives and C02 as the starting materials. This synthetic approach (Equation 7.22) is based on cyclization of the propargylic carbonate moiety (HC=CCH20C02-) into the corresponding a-alkylidene CC, in the presence of a suitable catalyst such as ruthenium [202], cobalt [203], palladium [204, 205], copper [206-211], or phosphine [212-214],... 

Recently, Ikarya has reported the use of imidazolin-2-ylidenes with N-alkyl and N-aryl substituents and their C02 adducts as catalyst of the carboxylative cydiza-tion of internal and terminal propargylic alcohols [215], The reaction of internal propargyl alcohols with C02 has been carried out also under supercritical conditions. Ikariya et al. have developed a synthetic process to afford Z-alkylidene cyclic carbonates promoted by P( -C4H9)3 with high efficiency [216],... 

The reaction of propargylic alcohols and sc C02 in the presence of a trialkylphosphine as a catalyst gave cyclic carbonates in an excellent yield (Ikariya and Noyori, 1999). Dixneuf reported that the reaction proceeded without solvent, but not in nonpolar solvents such as toluene. The reaction efficiency in sc C02 was superior to that in solution phase (Fournier et al., 1989 Journier et al., 1991). The TON reached 1200 and the TOF exceeded 400. The sufficient concentration of C02, as well as the high reactivity of the ion-pair intermediate in sc C02, is responsible for such high efficiency. 

The intramolecular catalytic addition of propargylic carbonates to the C=CH bond to give cyclic carbonates was first discovered with a ruthenium complex [80], but appeared to be more efficiently catalyzed by a simple phosphine such as P-U-BU3 (Scheme 8.26) [81]. 

Alkynyl cyclic carbonate 13 underwent hydrogenolysis at the propargylic C—O bond along with the reduction of the alkyne to give cw-alkenol 14 (Scheme 6)1 ... 

Miscellaneous Reactions. Ionic liquids are known to promote fixation of carbon dioxide to propargyl alcohols under copper(I) chloride catalysis to give a-methylene cyclic carbonates (eq 66) or 5-methylene-l,3-oxazolidin-2-ones in excellent yields (eq 67)3 ... 

The treatment of carbonyl compounds with Sml2 and methyl chloroformate in the presence of molecular sieves affords the cyclic carbonates or biscarbonates of pinacols. This one-pot reaction proceeds rapidly even with aliphatic ketones. The stereochemistry of the reaction run by this procedure is different from that of conventional pinacolic couplings. Trifluoromethylated five-membered cyclic carbonates were prepared through the palladium-promoted reaction of tertiary trifluoromethylated propargylic alcohols and sodium carbonate (Scheme 6). ... 

Scheme 6 Synthesis of trifluoromethylated five-membered cyclic carbonate from appropriate propargylic alcohol and Na2C03...

Ionic liquids have also been found to be efficient reaction media for the activation of CO2 [82-84]. Diurea derivatives are easily obtained from amines and carbon dioxide with a CsOH/ionic liquid catalyst system [83]. N-Substituted 4-methylene-2-oxazolidinones are also efficiently synthesized in IL media by reaction of carbon dioxide with propargylic alcohol and different amines [82]. Other examples include the cycloaddition of CO2 to epoxides to produce cyclic carbonates [85] and synthesis of cyclic urethanes [86,87]. 

The selective formation of Z-isomer of cyclic carbonate 9 from alcohol 8 was achieved using supercritical CO2 and 7-methyl-l,5,7-triazabicyclo[4.4.0]dec-5-ene (MTBD) catalysis (Scheme 5) [20]. It should be noted that similar incorporation of CO2 and formation of substituted cyclic carbonates was also successfully carried out with variety of propargylic alcohols in toluene at room temperature and 1 MPa pressure using catalysis by silver acetate and l,8-diazabicycloundec-7-ene (DBU) [21]. 

Yoshida et al. uncovered a novel palladium-catalyzed cascade reaction of propar-gylic carbonates with phenols which involves a CO elimination/fixation step and affords phenoxy-substituted cyclic carbonates. They discovered that this reaction proceeds in a highly enantiospecific manner to give the chiral cyclic carbonates 276 and 277 via an overall cascade chirality transfer process through use of the chiral propargylic carbonates 275 as the substrates [98] (Scheme 6.73). 

Several groups also used NHC-Ag catalysis in order to promote the reaction of a terminal alkyne with an electrophile. Following this process, Sono-gashira couplings and the addition of alkynes to isatins or C02 were described. In the last case, supported NHC-Ag nanoparticles were used. The efficiency of the process was attributed to both the activation of the alkyne by the NHC-Ag complex and the activation of CO2 by a free NHC moiety in a cooperative manner. CO2 insertion has also been exploited in order to synthesize cyclic carbonates and carbamates by NHC-Ag catalysis starting from epoxides, propargyl alcohols or allenyl amines [eqn (11.4)]. ... 

In general, allenyl cations 38 attack at the sp2-carbon atom of 1,3-dienes and form vinyl cations 39 and 40 (R = H, alkyl) or (R = aryl). Although a concerted cycloaddition mechanism is possible, a stepwise mechanism is preferred34. If a nucleophilic attack at the sp-carbon atom of the allenyl cation takes place, then cation 41 and the resulting cations 42 and 43 are formed. Some examples of bicyclic products obtained from cyclic 1,3-dienes and propargyl chlorides are given in equation 1534. 

A similar process involving an all-carbon cyclic system has also been investigated [59]. Other examples involving the prototropic rearrangement of enediynes having an imino or a keto substituent at the propargylic position to form the corresponding enyne-allenes have also been observed [60, 61]. 

Carbonylation of propargyl carbonates bearing an amino group yields lactams. The a-vinylidene /1-lactams 82 are prepared by the carbonylation of 4-benzylamino-2-alkynyl methyl carbonates 81 [20], The best results are obtained by using the cyclic phosphite (4-ethyl-2,6,7-trioxa-l-phosphabicyclo[2,2,2]octane) (83). The lactam formation is carried out in THF or MeCN as solvents at 50 °C under 1-10 atm of CO. 

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