Synthesis and Reactivity of Silylformylation Products Derived from Alkynes

The silylformylation reaction of unsaturated compounds consists of the rhodium‐catalysed addition of a R 3 Si and a CHO moiety to a carbon–carbon multiple bond. The silylformylation process is applicable to both terminal and internal alkynes and is tolerant towards several functional groups. Many factors can influence the chemoselectivity of the reaction that generally affords β‐silylalkenals with high yields and regioselectivity. The β‐silylalkenals are polyfunctionalised substrates that can be submitted to several chemical transformations and in particular can undergo fluoride‐promoted (TBAF) aryl migration from an aryldimethylsilyl group to the adjacent carbon atom thus generating 2‐(arylmethyl)aldehydes, important industrial precursors of perfumes. This silylformylation‐aryl migration two‐steps sequence can be applied to functionalised alkyne precursors and chemo‐, regio‐ and stereoselectivity of the products can be modulated according to the nature and the position of the functional group. When a good leaving group is present in the ω position of the aliphatic chain of the alkyne cyclisation products are obtained, while α,β‐unsaturated aldehydes are generated from propargyl tosylamides, acetates and benzoates. α‐Silylmethylene‐β‐lactams, prepared from p ‐tosylamides according to silylcarbocyclisation reaction, can be transformed into the corresponding arylmethyl‐β‐lactams by means of TBAF‐induced rearrangement that takes place without involving the lactam ring. A plausible mechanism of the rearrangement reaction is reported that suggests the addition of the fluoride ion to the arylsilicon moiety and the consequent migration of the aryl group to the adjacent carbon atom. Both aryl and heteroaryl substituents can migrate without any loss of configuration further enhancing the high synthetic value of this protocol. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)