Dotz Reaction Cr

In the Dotz reaction, an unsaturated chromium carbene complex is combined with an alkyne to give a substituted phenol. The unsaturated group may be an alkenyl, cycloalkenyl, or aryl group. There is almost always a methoxy or other alkoxy group attached to the carbene. 

By numbering the atoms, we see that the alkylidene portion of the Cr complex contributes three C atoms to the new aromatic ring, and the alkyne contributes two. The sixth C atom with its associated O must come from a CO ligand. Make C2-C7, C4-C5, C5-C8. Break Cr-C2. 

Compounds with multiple M-C bonds are prone to undergo [2 + 2] cycloadditions. Such a reaction in this system makes the C2-C7 bond and generates a met-allacyclobutene. In this system, the [2 + 2] cycloaddition is preceded by loss of CO from the unreactive, 18-electron carbene complex and coordination of the alkyne. 

When the Dotz reaction is executed in EtOH, products derived from addition of EtOH to a dienylketene are obtained, suggesting that either of the latter two mechanisms is correct. 

The carbene complex (CO)5Cr=C(CH3)OMe, prepared from Cr(CO)6 and MeLi, can also be used to prepare Cr carbene complexes. The five CO groups attached to Cr in this compound exert such a strong electron-withdrawing effect on the Cr=C 77 bond that the latter becomes polarized toward Cr. In fact, the Cr(CO)5 fragment is so electron-attracting that the CH3 group becomes as acidic as a methyl ketone Addition of an aldehyde and a Lewis acid causes an aldol reaction to occur, thereby producing an unsaturated Cr carbene complex. 

---