Rhodium(I)-Catalyzed Carbonylative Arylation of Alkynes with Arylboronic Acids Using Formaldehyde as a Carbonyl Source

Abstract

The rhodium(I)-catalyzed reaction of alkynes with arylboronic acids in the presence of formaldehyde resulted in a carbon monoxide gas-free carbonylative arylation to yield α,β-enones. The simultaneous loading of phosphine-ligated and phosphine-free rhodium(I) complexes is required for efficient catalysis, which catalyze the abstraction of a carbonyl moiety from formaldehyde (decarbonylation) and its subsequent introduction into the substrate (carbonylation), respectively.

Key words

carbonylation - arylation - rhodium - formaldehyde - alkynes

• References and Notes

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• 8 Abbreviations: BIPHEP = 2,2′-bis(diphenylphosphino)-1,1′-biphenyl; Xantphos = 9,9-dimethyl-4,5-bis(diphenyl-phosphino)xanthene; dppe = 1,2-bis(diphenylphosphino)-ethane; dppp = 1,3-bis(diphenylphosphino)propane; dppf = 1,1′-bis(diphenylphosphino)ferrocene; BINAP = 2,2′ = bis(diphenylphosphino)-1,1′-binaphthyl.
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• 10 The role of TFA is, at present, unclear. We postulate the following two possibilities: (i) it promotes the generation of free formaldehyde from paraformaldehyde, and (ii) it accelerates the protonation of the vinylrhodium species E to give the product.

The fact that the deuteration of the β-position of the enones 3aa does not reach 99% is due to H–D exchange via 1,3- and 1,4-shift of the rhodium in the intermediate E to other Ph rings at β-position:
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• 12 Even when C₆F₅CHO, which should not generate a RhH species, was used as a carbonyl source, the reaction gave unselectively a mixture of (E)- and (Z)-3aa in 2% yield (E/Z = 37:63). We have observed the similar result when carbon monoxide was used as a carbonyl source (ref. 7).
• 13 Each of the (E)- and (Z)-3aa isolated in pure form was introduced into reactions of 4-methoxyphenylboronic acid (2b) and alkyne 1a under the standard conditions. Each reaction gave the product 3ab in almost the same yield. It is noteworthy that 3aa was recovered in the same E/Z ratio (E/Z = 32:68) in each case and that they are also the same as that of the reaction in Table 1, entry 2. Thus, E/Z isomerization of the products 3 takes place readily under the present conditions. Moreover, the reactions of (E)- and (Z)-3aa in the presence of a catalytic amount of RhH(CO)(PPh₃)₃ gave a E/Z mixture of 3aa quantitatively in a similar ratio, E/Z = 36:64 and 34:66, respectively. Therefore, we currently do not eliminate the possibility that E/Z isomerization proceeds via the hydrorhodation of RhH, generated from Rh and hydrogen (decarbonylation counterpart), to the primary product (E)-enone and the subsequent rotation of the C–C single bond, followed by the β-H elimination to (Z)-enone. See the Supporting Information.