Abstract:Pentacarbonyl(1-acyloxyalkylidene)chromium(0)
complexes, formed in situ by reaction of the
corresponding tetramethylammonium
pentacarbonyl(1-oxoalkyl)chromate(1−) salts with
carboxylic
acid halides, affords enol esters in moderate to good yields. In
all cases examined, the Z-enol ester
was obtained as the major or exclusive isomer. Addition of 1 equiv
of pyridine to the reaction
mixture substantially improved the Z/E ratio and, in most
cases, increased the chemical yield.
“…Bruneau reported that ruthenium-catalyzed addition of carboxylates to terminal alkynes can afford ( Z )-alk-1-en-yl esters including conjugated dienyl esters 3b. However, to the best of our knowledge, the general method for the stereodefined preparation of ( Z , E )- or ( Z , Z )-conjugated alkadienyl carboxylates has not been reported, perhaps becuase of its thermodynamic instability. 1d, So, it seems important to develop an effective method for the construction of ( Z , E )- or ( Z , Z )-conjugated alkadienyl carboxylates. In this communication, we wish to report a novel approach to ( Z , E )- or ( Z , Z )-conjugated alkadienyl carboxylates.…”
[reaction: see text] The stereocontrolled dehydrobromination of 1,2-dibromoethyl carboxylates giving (Z)-2-bromovinyl carboxylates could readily be approached by using DBU and a catalytic amount of hydroquinone as a base at -78 degrees C. The first investigation on the Suzuki-type cross-coupling of (Z)-2-bromovinyl carboxylates as electrophiles with stereodefined alkenylboronic acids provides a novel method for the construction of (Z,E)- or (Z,Z)-conjugated alkadienyl carboxylate moieties, which are often present in a range of natural products.
“…Bruneau reported that ruthenium-catalyzed addition of carboxylates to terminal alkynes can afford ( Z )-alk-1-en-yl esters including conjugated dienyl esters 3b. However, to the best of our knowledge, the general method for the stereodefined preparation of ( Z , E )- or ( Z , Z )-conjugated alkadienyl carboxylates has not been reported, perhaps becuase of its thermodynamic instability. 1d, So, it seems important to develop an effective method for the construction of ( Z , E )- or ( Z , Z )-conjugated alkadienyl carboxylates. In this communication, we wish to report a novel approach to ( Z , E )- or ( Z , Z )-conjugated alkadienyl carboxylates.…”
[reaction: see text] The stereocontrolled dehydrobromination of 1,2-dibromoethyl carboxylates giving (Z)-2-bromovinyl carboxylates could readily be approached by using DBU and a catalytic amount of hydroquinone as a base at -78 degrees C. The first investigation on the Suzuki-type cross-coupling of (Z)-2-bromovinyl carboxylates as electrophiles with stereodefined alkenylboronic acids provides a novel method for the construction of (Z,E)- or (Z,Z)-conjugated alkadienyl carboxylate moieties, which are often present in a range of natural products.
“…The present method using sulfonium salts has the following advantages: (1) While a longer reaction time (12 h) is needed than that in the known method 12 using oxonium salts 6 , a variety of functionalized alkyl groups with a secondary carbon can be effectively introduced. (2) This reaction can be carried out at room temperature, although the known method using acyloxy intermediate 8 gives enol esters at room temperature when α-hydrogens next to the carbene carbon atom are present . (3) The present method is particularly useful for preparing carbene complexes 18 with a hydroxyl group, since the known method via acyloxy intermediate 8 gives a mixture of 18 and a biscarbene complex like 20 .…”
Section: Resultsmentioning
confidence: 97%
“…However, this method is limited to the preparation of alkoxycarbene complexes containing simple methoxy and ethoxy groups. The preparation of Fischer-type carbene complexes containing functionalized alkyl groups has been reported (Scheme ). 6a,10a,, However, the methods used to prepare alkoxy carbene complexes of chromium, molybdenum, and tungsten are still limited. , 3 …”
A new and general method for preparing Fischer-type alkoxycarbene complexes 1 is reported. This method involves the alkylation of acylate complexes 7 with alkyldiphenylsulfonium salts 10 with a variety of functionalized alkyl groups. The alkylation of tetramethylammonium pentacarbonyl(1-oxyalkylidene)chromate(0) 7a-c, tetramethylammonium pentacarbonyl(1-oxymethylidene)molybdate(0) 7d, and pentacarbonyl(1-oxymethylidene)tungstate(0) 7e with alkyldiphenylsulfonium salts 10 proceeded smoothly under mild conditions to give the corresponding alkoxycarbene complexes 12-39 in good to high yields. Competitive alkylation of 7a with methyl-11 and isopropyldiphenylsulfonium tetrafluoroborate 40 shows a higher reactivity of the isopropyl group, suggesting the participation of an S-O sulfurane intermediate 41.
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