Aryl and Heteroaryl Stannanes

Like most aryl halides, furyl halides and furyl triflates have been coupled with a variety of organostannanes including alkenyl, aryl, and heteroaryl stannanes in the presence of catalytic palladium. Carbamoylstannane 66 was prepared by treating lithiated piperidine with carbon monoxide and tributyltin chloride sequentially. The Stille reaction of 66 and 3-bromofuran then gave rise to amide 67 [61]. In another example, lithiation of 4,4-dimethyl-2-oxazoline followed by quenching with MesSnCl resulted in 2-(tributylstannyl)-4,4-dimethyl-2-oxazoline (68) in 70-80% yield [62], Subsequent Stille reaction of 68 with 3-bromofuran afforded 2-(3 -furyl)-4,4-dimethyl-2-oxazoline (69). 

If the stannane bears substituents at the position geminal to tin, the reaction requires activation by Cu1 (Section 9.6.3.2.1). The cross-coupling of aryl (or heteroaryl) stannanes with aryl (or heteroaryl) halides or triflates is a general route to various biaryls and their analogues, particularly useful in those cases when the Suzuki-Miyaura reaction is less effective for... 

The palladium-catalysed reactions with alkenyl, aryl, and heteroaryl halides or triflates as one partner ( electrophilic ) and alkenes (Heck), aryl or vinyl stannanes (Stille) or aryl, vinyl and even alkyl boronic acids (Suzuki) as the other ( nucleophilic ) partner provide a synthetic method of astonishing power and versatility. These reactions are only just starting to be explored and great things are expected of them. 

Boronic acids derived from 3- and 5-substituted indoles are useful intermediates for palladium-catalyzed vinylation, arylation, and heteroarylation (Equation (47)) <92H(34)1395, 93TL2235>. As with the stannanes, a reverse process can be utilized. 6- and 7-Bromoindole can be coupled directly with arylboronic acids to give the 6- and 7-arylindoles, respectively a feature of the reaction is that protection of the indole nitrogen is unnecessary (Equation (48)) <94SL93>. A new route to 2-substituted indoles has been developed from trialkyl-(l-methylindol-2-yl)borates. The borate (153),... 

Tin Derivatives A convenient and direct method for the preparation of metallated species in the azole series starts with lithiation, as in the case of rr-excessive systems. In thiazoles the initial lithiation is in the 2-position. Subseqnent qnenching with a stannyl halide will furnish a stannane snbstrate 100 to be used for Stille couplings with aryl or heteroaryl halides, in the present case with a bromide (Scheme 42). Stannylated oxazoles are similarly prepared and react in the same manner.The same applies to their benzannulated analogs 101. ... 

The Stille reaction of 2-chloro-3,6-diisopropylpyrazine (7) and 2-chloro-3,6-diisopropylpyrazine 4-oxide (9) with tetra(p-methoxyphenyl)stannane (readily prepared in situ from the corresponding Grignard reagent and SnCU) led to the corresponding arylation products 8 and 10, respectively [9]. Additional Stille coupling reactions of chloropyrazines and their N-oxides have been carried out with tetraphenyltin [10] and aryl-, heteroaryl-, allyl- and alkylstannanes [11]. 

---