共老我白发less Scheme
2.
Scheme3.
F.Alonso et al./Tetrahedron61(2005)11771–11835 11778
regioselective arylations were observed in some cases.It is noteworthy that the only side products in the reaction are benzoic acid(easily recovered by extraction with hot water) and carbon monoxide(which could be transformed into carbon dioxide in industrial processes).
Further investigation on this topic by Shmidt and Smirnov showed that the use of lithium chloride instead of sodium bromide increased the catalyst activity and productivity, since chloride accelerates CO elimination from the oxidative addition product more than bromide.62
Goo b en et al.studied the decarbonylative ole?nation of aryl esters in an attempt to minimise the production of waste.63 Starting from the p-nitrophenyl ester of the carboxylic acid, the corresponding alcohol formed in the Heck reaction could be recycled back into the starting ester,with CO and water being the only by-products in the overall reaction.63a Lithium chloride and isoquinoline proved to increase the effectiveness and stability of the catalyst in such a manner that higher yields were obtained.A wide variety of benzoates of electron-de?cient phenols(e.g.,p-nitrophenol) were found to be suitable substrates,whereas both electron-rich and electron-poor ole?ns gave similar yields,with regioselectivities ranging from4:1to20:1(Scheme5).If we compare this methodology with that described above,the scope of this Heck ole?nation seems to be wider.The catalytic system,however,requires larger amounts of all of its components,as well as the presence of a substantial amount of isoquinoline that must be removed at the end of the process.The lower reactivity of the benzoic esters in comparison with the benzoic anhydrides is also re?ected in the reaction times.The above idea was extended recently to the use of isopropenyl arenecarboxylates as arylating agents in a salt-free medium.63b These arylating reagents were syn-thesised through a waste-free reaction involving ruthenium-catalysed addition of the carboxylic acids to propyne. Concerning the coupling reaction,instead of waste salts,CO and acetone are the only by-products,the volatility of which allows easy workup.Electron-rich and electron-de?cient aryl,heteroaryl and vinyl carboxylic acid esters were coupled with several ole?ns(styrene in most cases)in good to excellent yields(Scheme6).In contrast to the bene?cial features of the whole process,the reaction temperature is rather high and is probably the reason for the moderate regioselectivities generally obtained(5:1–15:1),except in the case of n-butyl acrylate(O50:1).
In the examples shown in Schemes4and5,there is the necessity to generate the starting material in an extra reaction step and separate the ole?n products from the carboxylic acids or phenols,these being practical dis-advantages for small-scale preparations.Goo b en et al. utilised mixed anhydrides of carbonic and aromatic carboxylic acids,easily prepared in situ by mixing the carboxylic acids with di-tert-butyl dicarbonate(Boc2O),as arylating agents in the Heck ole?nation.64Different aromatic and heteroaromatic carboxylic acids(including electron-rich carboxylic acids)were coupled mainly with styrene,to give the expected products in moderate to good yields(45–88%)and selectivities(5:1–28:1)(Scheme7). The reaction could be performed at a lower temperature (1208C),in comparison to the examples described above, although g-picoline was necessary to stabilise the
palladium Scheme
4.
Scheme
5.
Scheme
6.
Scheme7.
F.Alonso et al./Tetrahedron61(2005)11771–1183511779
and to avoid precipitation and loss of activity at reasonable reaction rates.
Myers et al.subjected a wide range of arenecarboxylates to ef?cient Heck-type decarboxylative coupling with ole?nic substrates under relatively mild reaction conditions and in short reaction times (Scheme 8).65The key additive,silver(I)carbonate,presumably functions both as a base and as a stoichiometric oxidant,enlarging the lifetime of the active catalyst.Good yields of the coupled products were obtained for both electron-rich and electron-withdrawing substituents on the aromatic carboxylic acid and ole?nic partners such as styrene,acrylates,(E )-ethyl crotonate and cyclohexenone.Certainly this methodology allows the direct use of aromatic carboxylic acids without any previous transformation,although the experiments suggested that at least one ortho substituent is necessary for successful decarboxylative palladation to occur.The relatively high catalyst loading and the large amount of the silver salt used are the main disadvantages of this methodology,which probably would limit its application to a laboratory scale.
In 1982,Blaser and Spencer demonstrated that aroyl chlorides could react with activated alkenes,under Pd(OAc)2catalysis in the presence of a tertiary amine at 120–1308C,to give the expected (E )-arylalkenes stereo-speci?cally and in high yields.66More recently,Miura et al.have utilised the rhodium complex [(C 2H 4)2RhCl]2for the catalytic decarbonylative Heck-type coupling of aroyl chlorides with styrene and n -butyl acrylate.67This methodology is very interesting,since the reactions are carried out in the absence of any phosphane ligand and base and with low catalyst loading,although re?uxing o -xylene (143–1458C)is needed to obtain the products with good yields at reasonable reaction times (Scheme 9).In addition,the workup seems to be signi?cantly simple,that is,?ltration,evaporation and washing with an appropriate
solvent such as methanol.A slow stream of nitrogen is,however,required to sweep away the HCl and CO evolved during the reaction,what can be a limitation for large-scale processes.Under similar conditions,cyclic alkenes such as norbornenes also reacted with aroyl chlorides accompanied by cyclisation to afford the indanone derivatives.
Andrus et al.have recently applied a palladium-catalysed decarbonylative Heck-type reaction of aroyl chlorides to the synthesis of resveratrol using palladium acetate,an imida-zolium carbene-type ligand and a non-coordinating amine base,N -ethylmorpholine.68The overall yield starting from the inexpensive resorcylic acid (53%)was higher compared to the aryldiazonium or mixed anhydride approach.Boronic acids have been used as arylating agents in Heck-type reactions by several groups.4b Uemura et al.showed,in 1994,that arylboronic acids reacted with alkenes in acetic acid at 258C in the presence of a catalytic amount of palladium(II)acetate,together with sodium acetate,to give the corresponding aryl-substituted alkenes in high yields.69Alkenylboronic acids also reacted with alkenes under similar conditions to give the corresponding conjugated dienes stereospeci?cally,but the product yields were lower compared with those from the arylboronic acids.A similar treatment of sodium tetraphenylborate (NaBPh 4)with alkenes afforded the corresponding phenylated alkenes in high yields,together with biphenyl and benzene as side products.Oxidative addition of a carbon–boron bond to Pd(0),formed in situ,to give an organopalladium(II)species was assumed to be the key step of these cross-coupling reactions.More recently,Lautens et al.utilised a rhodium complex to catalyse the coupling reaction of arylboronic acids and styrenes in an aqueous media (Scheme 10).70The best results were obtained with [Rh(COD)Cl]2and TPPDS as the water-soluble ligand.It was,however,necessary to add 0.5equiv of sodium dodecyl sulfate (SDS)as a phase-transfer agent to avoid hydrolytic deboronation when the
arylboronic acid had polar functional groups.Gene
?t et al.found out that,using m -TPPTC [tris(m -carboxyphenyl)-phosphane trilithium salt]as the water-soluble ligand instead of TPPDS,no SDS was necessary,due to the inherent surfactant effect of m -TPPTC.71This ligand exhibited a similar performance in comparison with TPPDS,but was shown to be superior to TPPTS under the same conditions depicted in Scheme 10,but in the absence of 5a98184be45c3b3567ec8bfcutens’methodology was very recently extended to the coupling of a wide variety of aryl-and hetero-arylboronic acids with acrylates,acrylamides and vinyl sulfones.72The high selectivities and yields obtained
were
Scheme
8.
Scheme
9.Scheme 10.
F.Alonso et al./Tetrahedron 61(2005)11771–11835
11780
believed to result from the use of the bulky,electron-rich, water-soluble ligand tert-butyl-amphos chloride[N-2-(di-tert-butylphosphino)ethyl-N,N,N-trimethylammonium chloride].
Different catalytic systems have been developed recently for the Heck-type reaction of boronic acids and electron-de?cient ole?ns(Scheme11).Pd(OAc)2,[Ru(p-cymene)Cl2]2and RhCl3were utilised as catalysts by Mori,73Brown,74Zou,75and their corresponding co-workers,respectively.Despite the palladium and rhodium catalytic systems exhibiting better yields,higher tempera-tures were also required in comparison with the ruthenium catalytic system.Stoichiometric amounts of Cu(OAc)2were needed as a re-oxidising agent for palladium and ruthenium, the latter including3-quinuclinidone as a base in order to obtain the highest turnover.
The groups of Jung and Larhed independently introduced molecular oxygen as a re-oxidising agent in the palladium(II)-catalysed coupling of organoboron compounds and ole?ns. The optimised catalytic system reported by Jung et al.was composed of10mol%Pd(OAc)2,O2and Na2CO3in DMF at508C for3–10h.76Under these conditions,electron-rich and electron-poor ole?ns could be coupled with several boronic acids and esters,with generally good yields and stereo-and regioselectivities.The catalyst loading was diminished to1mol%by Larhed et al.in a catalytic system composed of Pd(OAc)2,O2,N-methylmorpholine and 1.2mol%2,9-dimethyl-1,10-phenantholine in MeCN at 508C for3–24h.This oxidative Heck protocol was applied to the coupling of arylboronic acids with unsubstituted acrylates77a and electron-rich ole?ns.77b The internal regioselectivity observed for the coupling with n-butyl vinyl ether and enamides was attributed to the phenanthro-line ligand,which also seemed to mediate the re-oxidation of palladium(0)with molecular oxygen,allowing a low catalyst loading.
