Stannanes, allyl cycloaddition reactions

Cycloaddition reactions of a,/3-unsaturated chromium and tungsten complexes have been studied to a great extent and have been reviewed.3 -6 Our report on cycloaddition of (l-alkynyl)carbene complexes is restricted to a short abstract and an update including more recent results. A most remarkable feature of [4+2] cycloadditions of 1,3 dienes to C=C bonds of (l-alkynyl)carbene complexes, e.g., li, is that such reactions proceed under very mild conditions, compared to those for reactions of propargylic esters, e.g., 41. Thus, formation of a Diels-Alder adduct, e.g., a norbornadiene derivative 42, can be achieved at 25°C via carbene complexes instead of at 190°C via the direct route (Scheme 15).68 Ligand disengagement from compound 40 can be achieved in various ways, e.g., by formation of an ester 43 through oxidation of the Cr=C bond, or by formation of an allyl silane 4369 or a stannane.70 71... 

The singlet oxygen ene reaction of allylic stannane, 16, is a synthetically useful procedure that leads to quantitative formation of the metal ene (M-ene) product [61-64]. However, other allylic stannanes, (e.g., 20 and 21) with less electropositive tin centers generate both hydrogen ene (H-ene) and novel cycloaddition products along with the M-ene product. The complete absence of a M-ene product in the photo-oxygenation of 22 has also led to the speculation that the M-ene reaction is less important for allylic stannanes in which the tin is bonded to a 2° rather than to a 1° carbon [65-67]. On the other hand, the use of more polar solvents can be used to enhance M-ene product formation [68]. 

Jorgensen et al. reported that C2-symmetric bis(oxazoline)-copper(II) complex 25 also acts as chiral Lewis acid catalyst for a reaction of allylic stannane with ethyl glyoxylate [37]. Meanwhile, p-Tol-BINAP-CuCl complex 26 was shown to be a promising chiral catalyst for a catalytic enantioselective allylation of ketones with allyltrimethoxysilane under the influence of the TBAT catalyst [38]. Evans and coworkers have developed (S,S)-Ph-pybox-Sc(OTf)3 complex 27 as a new chiral Lewis acid catalyst and shown that this scandium catalyst promotes enantioselective addition reactions of allenyltrimethylsilanes to ethyl glyoxylate [39]. But, when the silicon substituents become bulkier, nonracemic dihydrofurans are predominantly obtained as products of [3+2] cycloaddition. 

Radhakrishnan and coworkers reported desymmetrization of bicyclic hydrazines, derived from the [4+2] cycloaddition of fulvene 27 with diethy-lazodicarboxylate, followed by a palladium/Lewis acid-catalyzed reaction with allyl and vinyl stannane to provide the alkylidene cyclopentenes 106 and 107, respectively (Scheme 7.25) [26]. 

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