Mechanochemical process for synthesising a cyanine compound

A mechanochemical process for synthesising a cyanine compound of Formula (I) or a salt thereof:

• wherein:
• R1 and R3 are each independently selected from the group consisting of H, OH, C1-3 methyl, halo, phenyl;
• or R1 and R3 are joined together to form —CH2—CH2— or —CH2—CR2—CH2—, where R is H or CH3;
• R2 is H, halo, —CN, —OH, —COOH, —COOCH3, C1-6 alkyl, C5-6 aryl, or N(R6)2, where R6 is H, C1-6 methyl, or phenyl;
• each R4 is independently selected from the group consisting of —C1-5 alkyl, which may be optionally substituted by —COOH or —SO3H;
• each R5 is independently selected from the group consisting of H, halo, —OH, —COOH, —SO3H, or C1-6 methyl, or two R5 groups on adjacent carbons together form a phenyl ring;
• each n is independently 0, 1 or 2;
• and
• each X is independently selected from the group consisting of S, CH2 or C(CH3)2;
• the process comprising mechanochemically reacting a compound of Formula (II) or a salt thereof with one or more compounds of Formula (III) in the presence of a catalyst of Formula (IV) and a base,
• wherein the compounds of Formulae (II) and (III) and the catalyst of Formula (IV) have the following structures:
• wherein R1, R2 and R3 are as defined for Formula (I);
• wherein n, R4, R5, and X are as defined for Formula (I);
• wherein R7 is selected from H, a halogen, C1-C6 alkyl, C1-C6 alkoxy and phenoxy;
• and
• wherein the process is carried out in the absence of solvent.