Photoinduced alkyl group exchange of ethylzinc alkoxides: X-ray crystal structure of an iodomethylzinc methoxide† André Charette,* a André Beauchemin, a Sébastien Francoeur, a Francine Bélanger-Gariépy a and Gary D. Enright b a Département de Chimie, Université de Montréal, PO Box 6128, Station Downtown, Montréal, QC H3C 3J7, Canada. E-mail: andre.charette@umontreal.ca b Steacie Institute for Molecular Sciences, National Research Council of Canada, 100 Sussex Drive, Ottawa, Ontario, Canada K1A 0R6 Received (in Cambridge, UK) 22nd November 2001, Accepted 14th January 2002 First published as an Advance Article on the web 6th February 2002 Irradiation of a solution of ethyl zinc alkoxides and CH 2 I 2 leads to clean formation of iodomethylzinc alkoxides; these intermediates are important species generated in ster- eoselective cyclopropanation processes; no alkyl group exchange is observed in the absence of irradiation; the solid- state structure of (MeO) 8 Zn 7 (CH 2 I) 6 is also reported. The Simmons–Smith reaction has evolved over the years to become a general method to perform stereocontrolled cyclopro- panation reactions. 1 The fact that a proximal basic group could direct the delivery of the methylene unit on prochiral alkenes was recognized early on and this was exploited on numerous occasions. 2 In particular, the cyclopropanation of prochiral allylic alcohols has been extensively studied and good to excellent diastereoselectivities can be obtained with cyclic and acyclic alkenes, regardless of the double bond geometry. 3 Surprisingly, while the nature of the species involved in these reactions has been highly debated, little has been unequivocally established. As a consequence, there is only scarce information on the intrinsic stereoselectivity of the various species involved in directed Simmons–Smith cyclopropanation reactions. 4 Among the possible intermediates, iodomethylzinc alkoxides are likely to be involved. These species are also invoked in the methods developed for the enantioselective cyclopropanation of allylic alcohols using chiral additives or catalysts. 5 Therefore, information on the solution and solid-state structure of iodome- thylzinc alkoxides would be highly desirable. Herein, we report a new economical route toward these species along with the X- ray structure of iodomethylzinc methoxide. Heteroatom-substituted zinc carbenoids are typically pre- pared by the deprotonation of a substrate by the corresponding bis(halomethyl)zinc [Zn(CH 2 X) 2 ]. One problem with this approach is the relative instability of these species. Indeed, they tend to decompose rapidly unless a complexing additive or low temperatures are used. 6 For that reason, we have been interested in developing an alternative approach to the preparation of heteroatom-substituted zinc carbenoids. A simple alternative would be to reverse the sequence of addition in order to perform the halogen–metal exchange between (heteroatom)ZnEt and the dihalomethane. However, this solution is not general since the rate of the halogen–metal exchange reaction depends on the nature of both species. For example, the rate of exchange of CH 2 ICl is lower than CH 2 I 2 , often resulting in lower conver- sions. Also, while the exchange between Et 2 Zn and CH 2 I 2 is very rapid in various solvents, 7 the exchange between the ethylzinc alkoxide of cinnamyl alcohol and CH 2 I 2 is extremely slow (eqn. (1)). 8 To circumvent this problem, we considered (1) using the photoinduced zinc–iodide exchange reaction devel- oped recently in our laboratories. 9 As a proof of concept, we synthesized MeOZnCH 2 I according to both methods. 10 The first approach involves deprotonation of MeOH with Zn(CH 2 I) 2 ·DME (eqn. (2)). 11 In the second method, iodome- (2) thyl zinc methoxide is generated via deprotonation of MeOH with Et 2 Zn, followed by photoinduced alkyl exchange involv- ing CH 2 I 2 (eqn. (3)). 12 The latter approach is advantageous, requiring only one equivalent of CH 2 I 2 . (3) In the photochemical approach, characteristic 1 H and 13 C NMR signals for the ‘ZnCH 2 I’ species were observed both in the solution and solid states. 13 Upon standing at room temperature, colorless crystals suitable for X-ray analysis deposited in the NMR tube. The ORTEP is represented below (Fig. 1) with selected bond lengths and angles. The carbenoid (MeO) 8 Zn 7 (CH 2 I) 6 has a centrosymmetric dicubane structure in which the central zinc atom is lying on the inversion center. 14 The Zn–C bond lengths of ca. 2.15 Å, the C– I bond and the Zn–C–I bond angles of ca. 111° are comparable † Electronic supplementary information (ESI) available: experimental section. See http://www.rsc.org/suppdata/cc/b110736d/ Fig. 1 ORTEP view of (MeO) 8 Zn 7 (CH 2 I) 6 . Ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles and torsion angles (°): I(3)–C(3) 2.152(6), I(6)–C(6) 2.152(6), I(8)–C(8) 2.145(6), Zn(1)–O(2) 2.094(3), Zn(1)–O(4) 2.107(3), Zn(3)–O(2) 2.063(3), Zn(3)–O(4) 2.035(3), Zn(3)–C(3) 1.968(6), Zn(8)–C(8) 1.964(6), O(2)–C(2) 1.436(6), O(5)–C(5) 1.434(6); Zn(3)–C(3)–I(3) 109.9(3), Zn(8)–C(8)–I(8) 112.3(3), O(2)i– Zn(1)–O(2) 180, O(2)i–Zn(1)–O(5) 98.32(13), O(2)–Zn(1)–O(5) 81.68(13), O(2)–Zn(1)–O(4) 81.65(13), O(2)–Zn(3)–O(4) 84.57(13), O(2)– Zn(3)–O(7) 83.68(14), Zn(1)–O(2)–Zn(6) 96.58(14); C(2)–O(2)–Zn(3)– C(3) 28.2(4), C(7)–O(7)–Zn(3)–C(3) 24.8(5), I(3)–Zn(3)–C(3)–O(7) 23.3(5), I(3)–Zn(3)–C(3)–O(2) 114.3(2). This journal is © The Royal Society of Chemistry 2002 466 CHEM. COMMUN. , 2002, 466–467 DOI: 10.1039/b110736d