Single step transformation of a carboxylic acid into an aldehyde
$\ce{Li, RNH2 (R=Me, Et)/NH4Cl/H3O+}$


Burgstahler, A. W.; Worden, L. R.; Lewis, T. B. Direct Reduction of Carboxylic Acids to Aldehydes by Lithium in Ethylamine. J. Org. Chem. 1963, 28 (10), 2918–2919.
Bedenbaugh, A. O.; Bedenbaugh, J. H.; Bergin, W. A.; Adkins, J. D. Synthesis of aldehydes and secondary amines from carboxylic acids via imines. J. Am. Chem. Soc. 1970, 92 (19), 5774–5775.


$\ce{BH3.SMe2/PCC}$


Brown, H. C.; Rao, C. G.; Kulkarni, S. U. A Convenient Conversion of Carboxylic Acids into Aldehydes. Synthesis 1979, 1979 (9), 704–705.


$\ce{(CH3)2CHC(CH3)2BH2}$


Brown, H. C.; Heim, P.; Yoon, N. M. Selective reductions. XVII. Reaction of thexylborane in tetrahydrofuran with selected organic compounds containing representative functional groups. Comparison of the reducing characteristics of diborane and its alkyl derivatives. J. Org. Chem. 1972, 37 (19), 2942–2950.


$\ce{(CH3)2CHC(CH3)2BHCl.SMe2}$


Brown, H. C.; Cha, J. S.; Nazer, B.; Yoon, N. M. Exceptionally facile reduction of acyclic and alicyclic carboxylic acids to aldehydes by thexylchloroborane-dimethyl sulfide. J. Am. Chem. Soc. 1984, 106 (25), 8001–8002.
Brown, H. C.; Nazer, B.; Cha, J. S.; Sikorski, J. A. Selective reductions. 38. Reaction of thexylchloroborane-methyl sulfide complex in methylene chloride with selected organic compounds containing representative functional groups. Comparison of the reducing characteristics of thexylchloroborane, thexylborane, and diborane. J. Org. Chem. 1986, 51 (26), 5264–5270.
Brown, H. C.; Cha, J. S.; Yoon, N. M.; Nazer, B. Selective reductions. 39. Partial reduction of carboxylic acids with thexylchloroborane-methyl sulfide. A direct and simple aldehyde synthesis. J. Org. Chem. 1987, 52 (24), 5400–5406.


$\ce{(CH3)2CHC(CH3)2BHBr.SMe2}$


Cha, J. S.; Kim, J. E.; Lee, K. W. Facile reduction of saturated and unsaturated carboxylic acids and their salts to aldehydes by thexylbromoborane-dimethyl sulfide. J. Org. Chem. 1987, 52 (22), 5030–5032.
Cha, J. S.; Kim, J. E.; Oh, S. Y.; Lee, J. C.; Lee, K. W. Direct conversion of saturated and unsaturated carboxylic acids into aldehydes by thexylbromoborane-dimethyl sulfide. Tetrahedron Letters 1987, 28 (21), 2389–2392.


9-BBN/Li 9-BBN


Cha, J. S.; Kim, J. E.; Oh, S. Y.; Kim, J. D. One-pot conversion of carboxylic acids to aldehydes through treatment of acyloxy-9-borabicyclo[3.3.1]nonanes with lithium 9-boratabicyclo[3.3.1]nonane. Tetrahedron Letters 1987, 28 (39), 4575–4578.


9-BBN/t-BuLi/9-BBN


Cha, J. S.; Kim, J. E.; Kim, Y. S. One-pot transformation of carboxylic acids into aldehydes through stepwise treatment of acycloxy-9-borabicyclo[3.3.1]nonanes with Full-size image (&lt;1 K)-butyllithium and 9-borabicyclo [3.3.1]nonane. Tetrahedron Letters 1987, 28 (49), 6231–6234.


DIBAL-H


Zakharkin, L. I.; Khorlina, I. M. Zh. Obshch. Khim. 1964 34, 1029; J. Gen. Chem. USSR 1964 34, 1021.


bis(N-methylpiperazinyl)aluminum hydride


Muraki, M.; Mukaiyama, T. Aminoaluminum hydride as new reducing agents. I. Selective reduction of carboxylic acids to aldehydes. Chemistry Letters 1974, 3 (12), 1447–1450.
Hubert, T. D.; Eyman, D. P.; Wiemer, D. F. A convenient synthesis of bis(N-methylpiperazinyl)aluminum hydride: a reagent for the reduction of carboxylic acids to aldehydes. J. Org. Chem. 1984, 49 (12), 2279–2281.


activated silyl carboxylates


Corriu, R. J. P.; Lanneau G. F.; Perrot M. The one-pot conversion of carboxylic acids to aldehydes via activated silyl carboxylates. Tetrahedron Letters 1987, 28 (34), 3941–3944.


titanium-catalyzed Grignard


Sato, F.; Jinbo, T.; Sato, M. The Reduction of Carboxylic Acids to Aldehydes by Dichlorobis[π-cyclopentadienyl]titanium-Catalyzed Grignard Reactions. Synthesis 1981, 1981 (11), 871


N,N-dimethylchloromethyleniminium chloride and lithium tri-t-butoxyaluminum hydride


Fujisawa, T.; Mori, T.; Tsuge, S.; Sato, T. Direct and chemoselective conversion of carboxylic acids into aldehydes. Tetrahedron Letters 1983, 24 (14), 1543–1546.


$o\text{-}\ce{HSC6H4OH, POCl3, HClO4/LiAlH4/H2O, HgCl2}$


Costa, L.; Degani, I.; Fochi, R.; Tundo, P. Pentaatomie heteroaromatic cations. Note III. A Convenient Synthesis of Aldehydes from Carboxylic Acids via 2-Substituted 1,3-Benzoxathiolium Perchlorates. J. Heterocyclic Chem. 1974, 11, 943–948.


$o\text{-}\ce{(NH2)C6H4, PPA/NaOet/MeI/NaBH4}$ or $\ce{LiAlH4/H3O+}$


Craig, J. C.; Ekwurire, N. N.; Fu, C. C.; Walker, K. A. M. Conversion of Carboxylic Acids into Aldehydes and their C-1 or C-2 Deuteriated Derivatives. Synthesis 1981, 1981 (4), 303–305.